Composites of Lyotropic Lamellar Systems and
Micro-Particles
Von der Fakultät für Naturwissenschaften
Department Chemie
der Universität Paderborn
zur Erlangung des Grades eines
Doktors der Naturwissenschaften
−Dr. rer. nat.−
genehmigte Dissertation
von
Shahram Shafaei
aus Tabriz (Iran)
Paderborn 20.07.2007
Die vorliegende Arbeit wurde in der Zeit von Januar 2004 bis Juni 2007 im Fachgebiet
Physikalische Chemie am Department Chemie der Fakultät für Naturwissenschaften
der Universität Paderborn unter Anleitung von Prof. Dr. Claudia Schmidt angefertigt.
Referent: Prof. Dr. Claudia Schmidt
Korreferent: Prof. Dr. Wolfgang Bremser
Eingereicht am: 29.06.2007
Mündliche Prüfung am: 20.07.2007
Acknowledgements
There are many people, in different ways, who have contributed to the realization of
this thesis.
First of all, I wish to express my thanks to my supervisor Prof. Dr. Claudia Schmidt,
who gave me the opportunity to obtain my PhD at Paderborn University. Thank you
very much for your scientific guidance, invariable support, patience and always being
open for discussions.
Prof. Dr. W. Bremser, for scientific insight and open discussions about micro-particles.
Dr. A. Imhof, from Utrecht University, for providing the PMMA micro-particles.
Prof. Dr. U. Olsson, who gave me the opportunity to work in the Department of
Physical Chemistry 1 at Lund University.
Bruno Medronho from Coimbra and Lund University for scientific curiosity, fruitful
collaboration, it was a real pleasure to work with you! I am grateful to have you as a
friend.
Dr. H. Egold and Prof. Dr. Marsmann for NMR measurements.
Dr. S. Herres-Pawlis for reading my thesis.
Prof. Dr. N. Sinyavsky for helping with analysis of the NMR data with the Mat.Lab
program.
Houman Shirzadi, for helping with analysis of the data using the Origin program.
I want to thank everyone involved at the department of Physical Chemistry:
Prof. Dr. H. Kitzerow, Dr. K. Hiltrop for helping to do the SAXS measurements,
Richard Szopko, for the invaluable help to install Windows XP and other administrative
works, Dr. S. Benning and Dr. A. Hoischen, for their support in our department, Dr. T.
Kramer, for giving me interesting ideas about synthesis of the nano-particles, Gisela
Jünnemann, for helping to synthesize the particles, I also learnt a lot about German
culture from her. Claudia Stehr helped me in performing AFM measurements, Frau
Koralewicz, for the fast handling of bureaucratic works.
I would like to take the opportunity to thank these friends who also helped in one way
or another: Y. Wang, M. Blaschke, Dr. L. Majoros, M. Perez, G. Ar, H. Matthias, K.
Hillmann, S. Lages, F. Klama, H. Behling and R. Nouroozi.
Frank Imgrund, my best friend, who was cheering me up while in Germany.
Finally, I extend my warmest thanks to my family, who always provided me with love
and encouragement and supported me through all the years.
Presentations and Publications
• B. Medronho, S. Shafaei, M. G. Miguel, U. Olsson, C. Schmidt, “From Layers to
Onions and Vice-versa: Continuous versus Discontinuous Shear-Induced
Transformations of the L
α
Phase”, Poster, International Soft Matter Conference,
Eurogress Aachen, Germany, 1-4 October 2007.
• B. Medronho, S. Shafaei, R. Szopko, M. G. Miguel, U. Olsson, C. Schmidt, “Shear-
induced structural transformations of the lyotropic lamellar phase: Continuous or
discontinuous transition?”, Poster, 21th Conference of the European Colloid and
Interface Society, Geneva, Switzerland, 10-14 September 2007.
• S. Shafaei, B. Medronho, C. Schmidt, M. G. Miguel, U. Olsson, “Transition from
Lamellar phase to Multi-lamellar Vesicles and Vice-Versa”, Poster, 6th Annual
Surface and Colloid Symposium, Lund, Sweden, 15-17 November, 2006.
• S. Shafaei, C. Schmidt, “Rheo-NMR Study of the Shear-Induced Formation of
Multi-lamellar Vesicles in Lamellar System Filled with Micro Particles”, Poster, 26th
Annual GDCh Meeting on Magnetic Resonance “Novel Applications of
Magnetic Resonance to Condensed Matter”, Tübingen, Germany, 28-30
September 2006.
• S. Shafaei, M. Blaschke, C. Schmidt, “Influence of Spherical Particles on the
Shear-Induced Orientation States of a Lamellar Phase”, Poster, 3rd Zsigmondy-
Kolloquium, Berlin, Germany, 6-7 April 2006.
• S. Shafaei, C. Schmidt, “Mixtures of Colloidal Spheres and Lamellar Surfactant
Phases”, Poster, 42th Meeting of the German Colloid Society, Aachen, Germany,
24-28 September 2005.
• S. Shafaei, C. Schmidt, “Hybrid Systems of Bilayer Membranes and Colloid
Particles”, Oral, NMR Spring Workshop, Pottenstein, Germany, 12 March 2006.
• B. Medronho, S. Shafaei, C. Schmidt, M. G. Miguel, U. Olsson, “Transition from Lα
phase to MLVs and Vice-Versa: Coexistence versus Incomplete Transformation”,
in preparation
• S. Shafaei, C. Schmidt, “Identification of the nematic phase in the lecithin/water/n-
decane system”, in preparation
• S. Shafaei, C. Schmidt, “Influence of Micro-Particles on the Shear-Induced
Formation of Multi-lamellar Vesicles in Lamellar System”, in preparation.
Content
Content
1 Introduction ....................................................................................... 1
2 General Aspects of Surfactants and Colloidal Particles ............... 6
2.1 Surfactants and Lyotropic Liquid Crystals ................................................. 6
2.1.1 Surfactants........................................................................................................................6
2.1.2 Surfactant Aggregates in Solution..................................................................................7
2.1.3 The Lamellar Phase........................................................................................................12
2.1.4 The Nematic Phase.........................................................................................................13
2.1.5 Vesicles ...........................................................................................................................14
2.1.6 Lamellar Phase under Shear..........................................................................................15
2.2 Colloids ......................................................................................................... 16
2.2.1 Types of Colloids............................................................................................................16
2.2.2 Charge and Steric Stabilization.....................................................................................18
2.2.3 Emulsion Polymerization...............................................................................................19
2.3 Techniques ................................................................................................... 20
2.3.1 2H-NMR ............................................................................................................................20
2.3.1.1 Quadrupole Echo .....................................................................................................23
2.3.1.2 2H NMR of Lyotropic Liquid Crystals ........................................................................24
2.3.2 Rheology .........................................................................................................................25
3 Experimental.................................................................................... 28
3.1 Materials........................................................................................................ 28
3.1.1 Sample Preparation........................................................................................................30
3.1.2 General Procedures of Particle Synthesis....................................................................32
3.1.2.1 Synthesis of SiO2 Particles.......................................................................................32
3.1.2.2 Synthesis of PMMA Particles ...................................................................................32
3.1.2.3 Synthesis of Melamine-Formaldehyde Particles.......................................................34
3.2 Methods......................................................................................................... 35
3.2.1 Polarizing Microscopy....................................................................................................35
3.2.2 2H NMR ............................................................................................................................35
3.2.3 Rheology .........................................................................................................................36
3.2.4 SAXS................................................................................................................................37
3.2.5 Other Techniques ...........................................................................................................38
4 Phase Behaviour of the Lecithin Organogel................................. 39
4.1 Identification of Phase Transitions by Polarizing Microscopy ............... 40
Content
4.2 Identification of Phase Transitions by SAXS ............................................ 44
4.3 Identification of Phase Transitions by 2H NMR......................................... 49
4.4 Identification of Phase Transitions by Rheological Experiments .......... 53
4.5 Phase Diagram ............................................................................................. 55
5 Synthesis of Colloidal Particles..................................................... 58
5.1 Experimental Description............................................................................ 59
5.2 Synthesis and Characterization of SiO2 Particles .................................... 59
5.3 Synthesis and Characterization of PMMA Particles................................. 60
5.4 Synthesis and Characterization of Melamine-Formaldehyde Particles . 62
6 Influence of Micro-Particles on the Lα Phase under Shear.......... 66
6.1 Experimental Description............................................................................ 66
6.2 Results and Discussion............................................................................... 67
6.2.1 Influence of Micro-Particles on Shear-Induced Orientation States of the Lamellae..67
6.2.2 Influence of Micro-Particles on MLV Formation Process and Vesicle Size ...............71
6.2.2.1 The Results of 2H NMR............................................................................................71
6.2.2.2 The Results of Rheology..........................................................................................81
6.2.2.3 The Results of Polarizing Microscopy ......................................................................87
6.2.3 Influence of Micro-Particles on Vice Versa Shear-Induced of MLVs Formation........89
7 Conclusions and Outlook............................................................... 94
7.1 Conclusions.................................................................................................. 94
7.2 Outlook.......................................................................................................... 97
Appendix ................................................................................................ 98
A.1 The System of AOT/brine ....................................................................................................98
A.2 Characterization of PMMA Particles.................................................................................100
A.3 The System of Lecithin/water/n-decane..........................................................................102
A.4 Calibration of SAXS...........................................................................................................103
Abbreviations ...................................................................................... 104
References ........................................................................................... 107
Introduction
1
1 Introduction
The self assembly of amphiphilic molecules (surfactants) in solution has been the
subject of intensive research in the past decades. It is well known that aqueous
solutions of amphiphilic molecules tend to form different types of aggregates such as
spherical aggregates, rod-like aggregates, packed bilayers (lamellar or Lα phase) and
various other liquid crystalline structures. The richness of these phenomena is used in
many applications, including foods, animal feeds, pharmaceuticals, cosmetics,
detergents and mineral processing.1
Surfactants at high concentrations in water often form lamellar phases with bilayers
packed together to give a regular interlayer spacing as shown in Figure 1.1. The
bilayers are separated from each other by water layers. The lamellar phase can be
stabilized by a low spontaneous curvature of the water-oil interface2, by long-range
repulsive interaction forces such as electrostatic repulsion resulting from ionic
charges, or by thermal fluctuations of the membranes.3
Figure 1.1 Representation of a stack of lamellar aggregates separated by water channels.
The phase behaviour of lyotropic liquid crystals has been investigated in numerous
amphiphilic systems.4 Phase transitions, as indicated in the phase diagram, can be
observed by variation of the temperature or the concentration, and by the addition of a
co-surfactant or a salt.5-10 Shear flow can also induce transitions to other structures.11
The lecithin, water and oil system is a representative example forming different types
Introduction
2
of lyotropic liquid crystals. The isothermal quasi-ternary-phase diagram and the phase
properties of several systems of the type lecithin, water and oil have been investigated
by Angelico and co-workers.12 These authors published a still incomplete study of the
lecithin/water/n-decane system; a complete investigation of the phase transitions
especially in the multiphase region close to the decane corner and the Lα phase in the
phase diagram is still missing. Therefore, it is one of the aims of this thesis to provide
some more details about the phase behaviour of the lecithin/water/n-decane system.
The behaviour of lamellar phases under shear has been investigated in different
amphiphilic systems and followed by various techniques in recent years.13-15 Shear
flow is known to have a strong influence on the structure and orientation of lamellar
phases. Different orientation states can be characterized by specifying the orientation
of the layer normal (director) with respect to the direction of velocity, the direction of
the velocity gradient and the vorticity direction, respectively.16 The three principal
orientations of the layer normal, that is, if the director points parallel to the neutral
(vorticity) direction, the velocity direction and the velocity gradient direction are
denoted as perpendicular, transverse, and parallel, respectively. In lyotropic surfactant
lamellar phases the parallel13,15,17-20 and perpendicular15,18,20,21 orientations as well as
mixtures of the parallel and perpendicular20,22 orientations have been observed. In
addition, a unique shear-induced defect structure, consisting of close-packed, almost
monodisperse, multi-lamellar vesicles, also called “onions”, first investigated by Roux
and co-workers13 was found for many different systems. The size distribution of shear-
induced multi-lamellar surfactant vesicles depends on the applied shear rate (see
Figure 1.2). Escalante et al.23 interpreted their rheology results as an irreversible
transformation to onions with the increase of shear rate. Panizza et al.24 found two
different regimes that differ in the size of the onions that depends on the initial and
final shear rates. Müller and collaborators16 reported reversibility in non-ionic
surfactant systems. Medronho et al.25 investigated the reversibility in the onion size
distribution during stepwise cycling of the shear rate.
Different orientations of the lamellar n-decane-based lecithin organogel under shear
have been found by Burgemeister26 and Blaschke.27 The orientation diagram shows
different states including parallel and perpendicular orientations as well as onions,
depending on shear rate, composition and temperature.
Introduction
3
Figure 1.2 Schematic process of the different orientation of bilayers on the lamellar phase
under shear. Not only different orientation of layers in the Couette cell, but also the size
distribution of shear-induced MLVs depends on the applied shear rate.
For the investigation of the influence of micro-particles on the lamellar phase, the
synthesis of spherical particles is necessary. The preparation of spherical particles via
heterophase polymerization has developed into a mature technique in macromolecular
chemistry.28 The control of particle size, size distribution and shape, or the
incorporation of functional groups in the particles or on their surfaces is a central topic
in preparative colloid chemistry. The result is an ever-growing number of new
functional materials. The accessibility of highly monodisperse colloids within a tunable
size range is a crucial prerequisite for the generation of photonic band gap materials.29
During the last decade, dye-labelled spherical particles have been of interest for
industrial applications as well as academic research. They are used as calibration kits
for optical methods, e.g., confocal microscopy30 or flow-cytometry.31 Their widespread
use is based on the stability of the particles and the dyes. Furthermore, their well
determined shape, size and narrow distribution is advantageous.
The equilibrium phase behaviour of mixtures of lyotropic bilayers (not onion) and small
spherical particles has been investigated. A low concentration of magnetic32 or
nonmagnetic solid particles with a size smaller than 20 % of the bilayer spacing can be
incorporated into the lamellar phase.33,34 The behaviour of mixtures of lyotropic
lamellar phases and spherical particles of micrometer size under shear was
investigated by Poon and co-workers.35 Depending on the size of the particles, their
volume fraction, and the stage during the preparation process at which the particles
Introduction
4
are added, they have found different structures of the composite, such as stuffed
onions in which the particles are encapsulated at the centre of the onions, decorated
onions in which, as well as replacing the onion core, the particles decorate the
polyhedral lattice of edges between the onions, and so-called onion-particle alloys.
The latter are formed when the particles are added late in the shearing procedure, in
which case the onions remain intact and the particles reside entirely in the interstitial
regions between them. They found that the rheological properties of a lamellar phase
in the presence of a small amount of submicron-size solid particles are not very
different from those of the pure lamellae.36
The addition of colloidal particles can modify the rheological properties of a variety of
complex fluids.37,38 Zapotocky and et al.39 reported that the addition of a small volume
fraction of colloidal particles to lamellar systems can efficiently control their rheology
and they obtained optical visualization of both the lamellae orientation and the
structure of any defects induced by the addition of colloidal particles. Basappa et al.40
reported that the addition of spherical micro-particles to a lamellar system enhanced
the moduli (storage and loss) and retarded the decay of the defect network under
shear treatment.
The structure of the lamellar phase with a layer spacing of several nanometers is not
compatible with spherical particles of micrometer size. In mixtures, the particles prefer
defect regions of the lamellar phase as can be seen in Figure 1.3. One question
addressed in this study is whether defects of the layer structure of lyotropic lamellar
phases generated by adding micro-particles may enhance the nucleation of vesicles
and lead to faster transformation to this orientation state of the Lα phase under shear.
Figure 1.3 Representation of influence of micro-particles on the lamellar phase.
This thesis is divided in seven chapters: Following this introduction, in the second
Defects of Lα phase
Bilayer structure (Lα phase)
+
Micro -particles
nm
μm
Introduction
5
chapter the theoretical aspects related with the aggregation of surfactants in solution
and the different orientations of the lamellar phase under shear, a brief review of
colloids, as well as the background of the different techniques used for the
characterization of the samples are described. The materials and utilized chemicals,
sample preparation techniques and a brief outline of the experiments and the
employed techniques are presented in the third chapter. Chapter four contains the
salient results of the temperature-dependent phase behaviour at one specific
composition of the n-decane based lecithin organogel system. The fifth chapter
summarizes the results of the characterization of the nano- and micro-particles which
have been synthesized. In the sixth chapter the influence of micro-particles on the
lamellar phase and its behaviour under shear is discussed. The conclusions of this
thesis and a brief outlook are given in the seventh chapter.
Gernral Aspect of Surfactants and Colloidal Particles
6
2 General Aspects of Surfactants and Colloidal Particles
2.1 Surfactants and Lyotropic Liquid Crystals
2.1.1 Surfactants
Surfactants (SURFace ACTive AgeNTS) are amphiphilic molecules which combine
two distinctive parts; one part is solvent soluble and the other part is insoluble. For the
most commonly used solvent, water, the two parts of the surfactant molecule are
referred to as hydrophilic (water-loving) and hydrophobic (water-hating). The
hydrophilic part (the head), is a polar group which can be charged or uncharged. The
hydrophobic part (the tail) is constituted by one or more hydrocarbon chains, linear or
branched, with varying length. The number of chains, their length and the degree of
branching are important parameters which determine the physico-chemical properties
of amphiphiles.41
Surfactants are usually classified according to the charge of the headgroup. In this
way, it is common to divide them into cationic, anionic, non-ionic, and zwitterionic.42
Cationics. The most important surfactants in this class are the amines, which can be
obtained in the primary, secondary or ternary forms. Practically all metals, minerals,
plastics, fibers, cell membranes, etc. are negatively charged, allowing for the use of
these surfactants as anticorrosion agents, dispersants, antistatic agents, bactericides,
etc.
Anionics. Sulphates, sulphonates, carboxylates and phosphates represent the majority
of the polar groups found in anionic amphiphiles. These are the most used surfactants
mainly because of their cheap and easy manufacture. They are also used in most
detergent formations.
Non-ionics. These surfactants contain an uncharged head group, usually polyetheric
or polyhydroxylic. They are used in liquid and solid detergents and in a variety of
other industrial applications, being particularly useful in the stabilization of oil-water
emulsions.
Zwitterionics. In this class, the surfactant head group is constituted by two charged
groups of opposite signs. The ammonium group usually forms the positively charged
group, while the negative group is often phosphate or carboxylate. Since these
surfactants exhibit excellent dermatological properties and low eye irritation they are
Gernral Aspect of Surfactants and Colloidal Particles
7
often used in shampoos and other cosmetic products. Lecithin (1,2-diacyl-sn-glycero-
3-phosphocholine) is a representative example of this category of surfactants. It is
made up of a mixture of natural phospholipids account for more than 50 % of the lipid
matrix of biological membranes in most organisms.
In Table 2.1 examples of surfactants belonging to the different classes are given.
Table 2.1 Examples of surfactants from the different described classes.
Surfactant type Symbol Example
Cationic
Amine: R4N+X-
Anionic Sulphonate: RSO3-Na+
Non-ionic Ethylenoxide: RO(CH2CH2O)nH
Zwitterionic Lecithin: RPO4-CH2CH2NR3+
2.1.2 Surfactant Aggregates in Solution
The hydrophilic-hydrophobic nature of amphiphilic molecules leads to their self-
assembly into a variety of structures in aqueous solution. The energy of the
amphiphile is lowest when it can find a place at the interface; therefore such molecules
are also referred to as surfactant. The simplest surfactant aggregate is the micelle.
Micelles are formed at low concentrations of surfactant molecules. They can have the
shape of a sphere, ellipsoid or rod, depending on the surfactant parameter (see
below). The concentration at which micelles start to form is called the critical micelle
concentration, or CMC, and is an important characteristic of a surfactant. The critical
micelle concentration at a fixed temperature is observed as amphiphile concentration
increases from very low values. The precise location of the CMC depends on the
technique used to measure it. Many physical properties exhibit abrupt changes at the
CMC, as illustrated in Figure 2.1. The most widely used technique to obtain the CMC
is surface tension measurement.4,42
Gernral Aspect of Surfactants and Colloidal Particles
8
Figure 2.1 Schematic representation of some physical properties which exhibit a sudden
change or discontinuity near the critical micelle concentration (CMC). Reproduced from Ref.
[42].
Figure 2.2 Temperature dependence of surfactant solubility in the region of the Krafft point.
Reproduced from Ref. [42].
Gernral Aspect of Surfactants and Colloidal Particles
9
Another important characteristic of surfactants is the Krafft temperature, Tk. The
solubility of a surfactant varies with temperature. Most surfactants are highly water-
soluble at high temperatures, but a temperature decrease implies a decrease in
solubility. At a certain point the surfactant precipitates from the solution as a hydrated
crystal in which the hydrocarbon chains are ordered and densely packed. The
solubility of the surfactant as a function of temperature is called the Krafft boundary.
For many surfactants the crystallization appears in the concentration range of the
CMC. A commonly used term is the Krafft point, which can be understood as the
intersection between the Krafft boundary and CMC curve. The solubility of the system
below the Krafft point is determined by the solubility of the surfactant monomers,
whereas the solubility of the system above the Kraft point is determined by the
solubility of the micelles, which is much higher (see Figure 2.2).42
One of the intriguing properties of oil-water-surfactant systems is their ability to self-
assemble into a large variety of complicated structures. All these can be understood
from the simple fact that the hydrophilic head of the amphiphile tries to avoid contact
with hydrocarbons, either oil or the hydrophobic tail of other surfactants, whereas the
hydrophobic tail tries just the opposite.
The hydrophobic effect which is mainly entropic in origin43 leads to a variety of
different aggregates. In many cases, the molecules form globular, rod-like or worm-like
micelles, hexagonal phases, sponge phases and membranes, i.e., monolayers and
bilayers (see Figure 2.3). However, this hydrophobic effect does not explain why the
surfactants associate in well characterized structures with particular types or shapes of
aggregates. In this aggregation process there are several contributions: the
hydrophobic interactions, in which the water-water interactions are more favorable
than those between water and the alkyl chains; the head group repulsions due to
hydration or steric hindrance and, for ionic surfactants, also due to electrostatic
repulsions, and finally packing considerations. System variables such as the surfactant
concentration, temperature and the presence of salt contribute to the formation of
different aggregates.
Gernral Aspect of Surfactants and Colloidal Particles
10
Figure 2.3 Several micellar structures that can be formed by surfactants. a) spherical micelle,
b) cylindrical micelle, c) threadlike or wormlike micelles, d) hexagonal phase, e) sponge phase,
f) lamellar phase, g) uni-lamellar vesicles, and i) multi-lamellar vesicles (MLV).
The surfactant number or surfactant parameter, Ns, (also called critical packing
parameter, CPP) directly relates the structure of the amphiphilic molecule to the
aggregate architecture. Ns is obtained as44
Ns =
0
la
υ
, (1)
where
υ
and l represent the volume and length of the hydrocarbon chain,
respectively, and 0
a is the area per head group. The volume of a surfactant
hydrocarbon chain (in nm3) is obtained by44
υ
= 0.027(nC + nMe), (2)
where nC is the number of carbon atoms in the chain and nMe the number of methyl
groups. The length of the fully stretched alkyl chain (in nm) is calculated as44
l = 0.15 + 0.127 nC. (3)
a)
b)
c)
e)
d)
f)
g)
i)
Gernral Aspect of Surfactants and Colloidal Particles
11
The area per head group, 0
a, is not determined by the actual size, but by the
electrostatic repulsions between the head groups. Usually, the trends in the changes
of the head group area are sufficient for the interpretation of data when using the
surfactant parameter model.
The surfactant parameter is a measure of the balance between the hydrophobic and
the hydrophilic parts of the amphiphile molecule and so it relates the properties of the
amphiphiles to the preferred curvature of the aggregate. A small value of Ns means
that the head group is large, which leads to highly curved aggregates (spherical
micelles). If the surfactant presents a larger Ns value, by addition of salt, or increasing
of the surfactant alkyl chain, the aggregates present a lower curvature, going from
spheres to rod-like micelles, and finally, when Ns ≈ 1, to planar (lamellar) structures
(see Figure 2.4). For surfactant parameters above unity reverse structures are more
favorable.
Another way to analyse the surfactant aggregate structure is by using the curvature
concept explicitly. In this approach the mean curvature, H, is used instead of the
surfactant number. The mean curvature is defined as42
H = )
11
(
2
1
21 RR + (4)
Where R1 and R2 are the principal radii of curvature for a surface. For a sphere, R1 =
R2 = R and the mean curvature is equal to H = 1/R; for a cylinder, H = 1/ (2R) (R1 = R,
R2 = ∞), and for a planer bilayer, H = 0.
Figure 2.4 Illustration of the mean curvature, H, and the surfactant parameter, Ns, for three
common surfactant aggregate shapes: the sphere, the cylinder and the bilayer. Reproduced from
Ref. [44].
H = 1/R 1/R
>
H
>
0 H = 0
Gernral Aspect of Surfactants and Colloidal Particles
12
2.1.3 The Lamellar Phase
Aqueous solutions of many surfactants are known to form lamellar liquid crystalline
phases with bilayers packed together to give a regular interlayer spacing, as shown in
Figure 2.5 (a). Such phases can be formed in mixtures of surfactant with water and
primary n-alkyl alcohols and forms the main matrix of biological membranes that
contain phospholipids as lyotropic compounds. The ordered bilayer structure is formed
by amphiphilic molecules disposed in bidimensional infinite layers, delimited by water
layers, all of them having a parallel disposition. The lamellar phase exhibits a rich
structural polymorphism which, as for any system of condensed matter, is controlled
by thermodynamical variables (concentration, temperature, pressure) the variations of
which induce symmetry changes of the organizations at phase transitions.45
Figure 2.5 Lamellar structures: a) bilayer, b) rippled bilayer; arrows indicate water layer
location. Reproduced from Ref. [45].
The lamellar phases can be stabilized by low spontaneous curvature of the water-oil
interface2, by the composition of long-range (10-500 nm) repulsive interaction forces
such as electrostatic repulsion resulting from ionic charges or steric repulsions
resulting from thermal fluctuations of the membranes.3 The lamellar phase can be
swollen in some cases by adding either oil or water, until the separation of the
monolayers reaches several thousand angstrom.46,47 In this case, the lamellar phase
is stabilized by the undulation of the surfactant sheets. Undulation forces dominate
when the membranes are flexible or, in terms of the bending modulus
κ
, when
κ
~ kBT,
and electrostatic forces are absent or screened out. Uncharged bilayers of single
chain surfactants and co-surfactants with a short chain length are rather flexible and
the bilayers are therefore not flat but rippled. The hydrophilic and hydrophobic parts of
Gernral Aspect of Surfactants and Colloidal Particles
13
the molecule are balanced and the surfactant parameter has a value of 1. The lamellar
phase is birefringent when viewed between crossed polarizers. It is less viscous than
the hexagonal and cubic phases and may even flow in certain systems. The properties
of lamellar phases can be of importance in many areas: such as the formulation of
concentrated detergents, properties of foods and behaviour of cosmetic preparations.
2.1.4 The Nematic Phase
Certain solutions of lyotropic surfactants present nematic structures, similar to
thermotropic nematics, which orient themselves in an external magnetic field or under
mechanical stress and show typical Schlieren textures when observed in polarized
light. Lyonematic structures were discovered by Lawson and Flaut.48 Many ionic
ternary solutions and some nonionic surfactants in binary aqueous solution form
nematic structures in a relatively narrow concentration/temperature range. Surfactants
that give nematic phases usually have the polar head group of the type SO4¯, CO2¯,
N(C5H10)4 + or N (CH3)4 +, their alkyl tails have more than eight carbon atoms.
Aggregates of the nematic phase are finite and anisotropic, due to the one-
dimensional ordering characterized by the director n
r
, and present an important
translational disorder. Three types of nematic phases are implied, the nematic discotic
phase ND, the nematic calamitic phase NC and the nematic biaxial phase NBX (see
Figure 2.6). The discotic phase is formed by planar disc micelles and is related to the
lamellar phase. Micelles may be built like rounded bricks or ruler shaped, rather than
circular discs. The calamitic phase is formed by rod-like micelles, and is related to the
hexagonal phase. In a concentration/temperature representation, the biaxial nematic
phase, if present, always occurs between ND and NC phases.
Figure 2.6 Representation of nematic phases: a) discotic micelles shaped like rounded bricks,
b) schematic of a discotic micelle showing distribution of amphiphilic molecules, c) rod-like
calamitic micelles, d) schematic of a cylindrical micelle showing distribution of amphiphilic
molecules. Reproduced from Ref. [45].
(a) (b) (c) (d)
Gernral Aspect of Surfactants and Colloidal Particles
14
Lyotropic nematic structures are located between the well ordered phases (lamellar,
cubic, and hexagonal) and the completely disordered phases (micellar isotropic
solution). 45,49
2.1.5 Vesicles
Vesicles typically occur for phospholipid and other double-tailed surfactants. There are
two types of vesicles. Unilamellar vesicles consist of a single bilayer sphere enclosing
aqueous solution. Multi-lamellar vesicles have an onion structure as shown in Figure
2.7. They are made of several uni-lamellar vesicles formed one inside the other in
diminishing size, creating a multi-lamellar structure of concentric spheres separated by
layers of water. Vesicles are of great industrial importance as encapsulating agents for
the controlled release of drugs, as micro-reactors for artificial photosynthesis (among
other reactions), and as substrates for a variety of enzymes and proteins and
perfumes in formulations.
Figure 2.7 Vesicle structures: a) uni-lamellar vesicle, b) multi-lamellar vesicle (onion).
Vesicles, or synonymously liposomes, are one type of self organizing structure of
amphiphiles in water.44 The chemical structures of vesicle-forming amphiphiles show a
great variety. Some of these substances are isolated from natural products like egg
yolk,50 while other chemicals are synthetically accessible.51 One of the general ways to
form phospholipid vesicles is by the method of sonication of an aqueous dispersion of
the lipid.
Vesicle systems are usually not a distinct thermodynamic phase but a non-equilibrium
state of the lamellar phase. If extra energy is dissipated in a lamellar system, the
bilayers start to separate from each other, they are cut in small pieces, start to bend
and eventually close to form vesicles.52,53 Often vesicles are formed by dispersing
lamellar bilayers where this dispersion may take place by dilution or by the input of
a) b)
Gernral Aspect of Surfactants and Colloidal Particles
15
external energy. It should be noted that the size distribution of the vesicular dispersion
is strongly affected by the method of preparation.42
Shear has a profound influence on the formation of vesicles. As mentioned above, in
order to observe the formation of vesicles, it is often necessary to have an external
force acting on the planar lamellar bilayers. Such shear-induced transitions from
stacked bilayers to vesicles have been subject of a large number of investigations.54-57
It has been shown that planar lamellae which are originally present become first
oriented by the shear field at low shear rate, while the transformation to vesicles takes
place at high shear rate. Moreover, it has been observed that the size of the onions
can scale with the inverse of the square root of the shear rate. The transformation of
planar lamellae to vesicles does not necessarily depend on the shear rate, but in many
situations it is controlled by the strain.19,58,59
2.1.6 Lamellar Phase under Shear
The influence of shear on the structure and orientation of complex fluids has attracted
much interest in recent years.56,60 Especially samples with lamellar phases can
undergo a variety of morphological transitions, which have been characterized in
terms of an orientation diagram.17 The three principal states of orientation, in which the
normal of the layer is parallel to the vorticity (neutral), velocity or velocity gradient axis,
are shown in Figure 2.8. The different orientations of the lamellar phase in a flow field
are called perpendicular (a), transverse (b), and parallel (c). 15,61,62
Figure 2.8 Schematic representation of the simple orientations of a lamellar phase in a flow.
The different states are labelled as (a) perpendicular, (b) transverse, (c) parallel, and onions.
neutral direction
velocity direction
gradient
direction
(b) (c)
(a)
n
r
IIV
r
n
rII V
r
∇
n
rII
Z
r
Gernral Aspect of Surfactants and Colloidal Particles
16
Recently, there was a report on two transitions from parallel-to-perpendicular-to-
parallel orientation.63,64 In addition to these orientation states, close-packed
monodisperse multi-lamellar vesicles, called “onions”, can be found in many systems
of lyotropic lamellar phases.14,16,18-20,24,61,65-73 The detailed mechanism of onion
formation remains unclear at present but there are theoretical investigations for the
mechanisms of onion formation.74-79 Up to now, steady state structures under shear
are well studied for a variety of surfactant systems.54,56 Nevertheless, the transition
between different steady state structures, and even from the equilibrium structure at
rest, remains elusive.
2.2 Colloids
The word "colloid" was derived from the Greek "kolla" for glue, as some of the original
organic colloidal solutions were glues. This term was first coined in 1862 to distinguish
colloids from “crystalloids” such as sugar and salt. In the early 19th century, Michael
Faraday showed that when a strong beam of light passes through a colloidal solution,
it is scattered. This method to study colloids was further developed by John Tyndall
and became known as the "Tyndall effect".
In general, a colloid or colloidal dispersion is a heterogeneous mixture that visually
appears to be a homogeneous solution. A heterogeneous mixture is a mixture of two
phases whereas a solution is one phase. In a colloid, the dispersed phase is made of
tiny particles or droplets that are distributed evenly throughout the continuous phase.
There are no strict boundaries on the size of colloidal particles, but they tend to vary
between 10-9 m to 10-6 m in size. Due to colloidal particles being so small, their
individual motion changes continually as a result of random collisions with the
molecules of the dispersion medium. This random, zig-zagging movement is called
Brownian motion after the man who discovered it. This motion helps to keep the
particles in suspension. Colloidal particles are found naturally in blood, bones and food
products as well as in industrial applications like paints and pharmaceutical products.80
2.2.1 Types of Colloids
The particles are normally distributed in a dispersion medium, where both the particles
and the medium can be in the gas, liquid or solid state. Such mixtures are called
colloidal dispersions. Table 2.2 lists examples of various types of such colloids. The
classical research in colloid science was performed on liquid solutions containing solid
particles.
Gernral Aspect of Surfactants and Colloidal Particles
17
Table 2.2 Different types of colloidal dispersions and the usual classification according to the
original states of their constituent parts.
Dispersing medium Dispersed phase Name
Solid Solid Solid sol
Solid Liquid Gel
Solid Gas Solid foam
Liquid Solid Sol
Liquid Liquid Emulsion
Liquid Gas Foam
Gas Solid Solid aerosol
Gas Liquid Aerosol
Amphiphiles in solution are known as association colloids. Macromolecules in solution
can also have sizes of 1 nm or larger. These can be classified as macromolecular
colloids.
There are two main ways of forming a colloid; reduction of larger particles to colloidal
size or condensation of smaller particles, e.g. molecules, into colloidal particles. This
latter approach generally makes use of chemical reactions such as hydrolysis or
displacement. Laboratory and industrial methods make use of several techniques.
A method of forming an aerosol is to tear away a liquid spray with a gas jet. The
process can be enforced by separating the liquid into droplets with electrostatic
repulsions, done by applying a charge to the liquid.
Emulsions are usually prepared by vigorously shaking the two constituents together,
often with the addition of an emulsifying agent, e.g. a surfactant such as soap, in order
to stabilize the product formed. Polymeric particles which are prepared easily by
emulsion polymerization, form a nearly monodisperse suspension of colloidal spheres.
Colloids are often purified by dialysis, a very slow process, where the aim is to remove
a large part of any ionic material that may have accompanied their formation. A
membrane is selected that will not allow colloid particles to pass through but will let the
solvent and ions permeate through. The method relies on diffusion, osmosis and
ultrafiltration.42,80
Gernral Aspect of Surfactants and Colloidal Particles
18
2.2.2 Charge and Steric Stabilization
Colloidal particles in a fluid dispersion medium always show Brownian motion and
hence collide with each other frequently. The stability of colloids is thus determined by
the interaction between the particles during such a collision. There are two basic
interactions: one being attractive and the other repulsive. When attraction dominates,
the particles will adhere to each other and finally the entire dispersion may coalesce.
When repulsion dominates, the system will be stable and remain in a dispersed state.
Stabilization serves to prevent colloids from aggregating. Steric stabilization and
electrostatic stabilization are the two main mechanisms for colloid stabilization.
Electrostatic stabilization is based on the mutual repulsion of electrical charges.
Different phases generally have different charge affinities, so that a charge double-
layer forms at any interface. Small particle sizes lead to enormous surface areas, and
this effect is greatly amplified in colloids. In a stable colloid, the mass of a dispersed
phase is so low that its buoyancy or kinetic energy is too little to overcome the
electrostatic repulsion between charged layers of the dispersing phase.
The surface of a colloidal particle can develop a charge through a number of
mechanisms. For example, ionization of surface acid or base groups in aqueous
solution can create a charged surface. Ion adsorption or desorption or
adsorption/desorption of ionic surfactants leads to the development of an electrical
double layer in many colloidal dispersions. The charge on the dispersed particles can
be observed by applying an electric field on a concentrated electrolyte. In this case all
particles migrate to the same electrode and therefore must all have charge of the
same sign.
Steric stabilization of colloidal particles is achieved by attaching (grafting or
chemisorption) macromolecules to the surfaces of the particles (Figure 2.9). The
stabilization due to the adsorbed layers on the dispersed particle is generally called
steric stabilization. Steric stabilization has several distinct advantages over
electrostatic stabilization. First, the interparticle repulsion does not depend on
electrolyte concentration, in contrast to charge stabilized colloids where the electric
doublet layer thickness is very sensitive to ionic strength. Second, steric stabilization is
effective in both non-aqueous and aqueous media, whereas charge stabilization is
usually exploited in aqueous solutions. Finally, steric stabilization operates over a wide
range of colloid concentrations, in contrast to charge stabilization which is most
effective at low concentrations. The most effective steric stabilizers are block or graft
Gernral Aspect of Surfactants and Colloidal Particles
19
copolymers where one type of block is soluble in the dispersion medium and the other
is insoluble so that it attaches to the colloid particles.42
Figure 2.9 Schematic representation of the steric stabilization of a colloidal particle. The
contact of colloidal particles is prevented by attached long-chain molecules.
2.2.3 Emulsion Polymerization
A typical conventional emulsion polymerization recipe will consist of water, emulsifier,
monomer and initiator. Water serves as both a transport and heat transfer medium. An
emulsifier is a hydrocarbon chain with one hydrophobic end and one hydrophilic end. If
the concentration of emulsifier is high enough, the hydrophobic ends of several
emulsifiers (usually about 50 to 100) form aggregates known as micelles. The
emulsifier serves as a stabilizer for polymer particles and monomer droplets. The
hydrophobic ends will attach to the particles while the hydrophilic ends will remain in
the water phase. The charges on these emulsifiers form what is known as an electrical
double layer which prevents the particles from coagulating. In other words, the
emulsifier serves to keep the particles suspended in the water. The micelles can also
be the location of particle nucleation. Monomer is present in the reaction in the form of
large droplets. These droplets act as a reservoir of monomer. The monomer in the
droplets diffuses through the water phase and into the micelles due to thermodynamic
reasons. Initiator is added to the reaction mixture and dissociates into two radicals in
the presence of heat. The initiator radicals are extremely reactive and readily react
with any monomer in the water phase. The monomer in the water phase continues to
add to the radical until the chain grows long enough such that its solubility in water is
exceeded. The oligomeric radical chain (multiple monomeric units) is now hydrophobic
enough to enter a polymer particle or enter a micelle to nucleate a new particle. This is
known as micellar nucleation. The conventional emulsion polymerization will occur in
three stages. The first stage involves the nucleation (birth) of polymer particles. This
can occur by either micellar or homogeneous nucleation. The second stage involves
Gernral Aspect of Surfactants and Colloidal Particles
20
the growth of the particles until the monomer droplets disappear. The third and final
stage begins with the disappearance of the monomer droplets and continues until the
end of the reaction.42,81
2.3 Techniques
2.3.1 2H-NMR
Deuteron NMR spectroscopy is an extremely powerful technique for investigating
molecular order and dynamics. The motivation for acquiring 2H NMR spectra of solids
and liquid crystals is to determine the local electric environment of the deuteron and to
use this to obtain information about molecular order and dynamics. Many NMR active
nuclei have a spin I greater than ½ and are termed quadrupolar (e.g. 14N, 2H). The
deuterium nucleus has a spin 1, which means that in the presence of a magnetic field
there are three quantized energy levels with magnetic quantum numbers, +1, 0, and
−1. The NMR experiment consists of causing transitions between these levels by the
application of energy in the radiofrequency (rf) range. Since the deuterium nucleus is
quadrupolar and there is a nonspherical charge distribution at the nucleus. The
interaction of the quadrupole moment eQ with the electric field gradient (EFG) at the
nucleus causes a substantial perturbation of the Zeeman splitting (Hz = 46 MHz) at a
magnetic field of 7 T. This perturbation is so large that the other NMR nuclear spin
interactions, such as the scalar J coupling, the chemical-shift anisotropy (Hσ = 0.5
KHz) and the dipole-dipole interaction (HD = 10 KHz), are negligible. Therefore,
deuterium solid-state NMR spectra are dominated completely by the quadrupole
coupling (HQ = 250 KHz). The time independent Hamiltonian operator for deuterons in
a static magnetic field, Bo, is given by:
H = Hz + HQ+ Hσ + HD (5)
The Zeeman Hamiltonian is:
Hz = - ωoIz. (6)
The Larmor frequency ωo =
γ
Bo is expressed in rad s-1, and Iz is the operator of the z
component of the deuteron spin angular momentum.
The combined Hamiltonian of the Zeeman and quadrupolar interaction is:
Ho = Hz + HQ (7)
The NMR frequencies of the two transitions are given by:
ω = ω0
±
δ
(3cos2
θ
-1 -
η
sin2
θ
cos2
γ
) (8)
where
δ
is 3/8 times the quadrupolar coupling constant e2qzzQ/ħ (expressed in rad s-1)
Gernral Aspect of Surfactants and Colloidal Particles
21
and the asymmetry parameter η of the electric field gradient tensor is defined as
η
= (qyy – qxx)/qzz (9)
The asymmetry parameter is usually zero for C−D bonds, meaning that the electric
field gradient tensor is axially symmetric. In addition, the z axis of the electric field
gradient tensor is along the C−D bond direction. The polar angles
θ
and
γ
specify the
orientation of the magnetic field with respect to the principal axes system of the
electric field gradient tensor.
If
η
is taken as zero (qxx = qyy), the NMR angular frequencies of the two transitions are
given by:
ω = ω0
±
δ
(3cos2
θ
-1). (10)
The energy diagram and the allowed transitions for these frequencies are shown in
Figure 2.10. The quadrupole term lifts the twofold degeneracy of transitions between
the Zeeman levels at the Larmor frequency,
υ
0, and results in two transitions at
frequencies,
υ
0 ±
υ
Q, with:
2
υ
Q = 2 ωQ /2
π
= ¾(e2qzzQ/h) [(3cos2
θ
- 1)
−
η
sin2
θ
cos
γ
] (11)
Fast molecular motions on the NMR time scale average out the quadrupolar
interactions and the time-averaged quadrupolar Hamiltonian becomes zero if the
motion is isotropic. This occurs in an isotropic liquid, where only the Zeeman splitting
with a single line is obtained. If the molecular motions have correlation times slower
than the NMR time scale, which is given by the inverse of the width of the spectrum
(i.e. the quadrupole splitting), the quadrupolar interaction does not vanish. This leads
to a splitting in the NMR spectrum. The two resonance lines are then separated by a
frequency difference.82,83
The magnitude of the observed splitting reflects not only the quadrupolar averaging
due to fast but non-isotropic reorientational motions, which results in a reduction of the
splitting. It has also an angular dependence, which opens the possibility to gain
information of the relative orientation of the molecules with respect to the magnetic
field. This is particularly simple if
η
= 0, which is found, for example, in uniaxial liquid
crystalline phases. In this case motional averaging results in a residual quadrupole
interaction with
η
= 0.84
Gernral Aspect of Surfactants and Colloidal Particles
22
Figure 2.10 The energy level diagram for a deuteron (spin 1) in a magnetic field. On the left
the allowed transitions in the absence of a quadrupole interaction are indicated and the single
line NMR spectrum is sketched. On the right the allowed transitions in the presence of a
quadrupole interaction give a two line spectrum, with a separation of 2
υ
Q by the first order
quadrupole interaction. Reproduced from Ref. [82].
Figure 2.11 The deuterium NMR powder lineshape of orientations results in an isotropic
distribution. The discontinuities in the lineshape correspond to the (
θ
,
γ
) values indicated.
Reproduced from Ref. [82].
In the uniaxial case the splitting is dependent only on the angle
θ
between the
principal z-axis of the EFG tensor and the axis of the external static magnetic field Bo.
If a single crystal or an oriented system is rotated in the magnetic field, different
resonance lines are predicted for each orientation. For a single deuteron with the
principal z-axis of the EFG tensor along the magnetic field direction (
θ
= 0), the
spectrum consists of two sharp lines, while for a polycrystalline power, which has an
θ
Gernral Aspect of Surfactants and Colloidal Particles
23
isotropic distribution of angles
θ
and
γ
, the spectrum is a superposition of doublets
and the overall line shape is the characteristic powder pattern. As shown in Figure
2.11 for the general case of
η
≠ 0 three singularities occur in the spectrum
corresponding to specific orientations: for
θ
= 0° at ± 3/4 (e2qzzQ/h), for (
θ
= 90°,
γ
= 0)
at ± 3/8 (e2qzzQ/h)(1 +
η
) and for (
θ
= 90°,
γ
= 90°) at (± 3/8(e2qzzQ/h)(1 -
η
). For
η
= 0
the latter two singularities coincide. 82,84
2.3.1.1 Quadrupole Echo
The deuterium NMR lineshapes are very broad and the free-induction decay signal
therefore dies away very rapidly. This decay is very fast, so that most of the signal is
lost during the receiver dead time. The standard procedure for avoiding distortions due
to this dead time is to use a quadrupole echo pulse sequence (QE), 90x-τ-90y, and
record the echo signal occurring at time τ after the second pulse. The term “echo”
refers to an increase in the NMR signal after a certain time interval, following the
application of an intense radiofrequency pulse to an ensemble of spins in a constant
magnetic field. The creation of an echo usually requires the application of two exciting
pulses. The first pulse at t = 0 generates coherence which defocuses under the
influence of an inhomogeneous interaction, e.g. caused by an inhomogeneous
magnetic field or a quadrupole interaction. A second pulse, applied at t = τ, inverts the
accumulated effects of the inhomogeneous interaction and initiates the refocusing
which leads then to an echo at t = 2τ. The pulse sequence is shown in Figure 2.12.
The crosshatched areas schematically represent the time required for the receiver to
recover.82,85
Figure 2.12 The quadrupole echo pulse sequence, used to refocus inhomogeneously broadened
lines, such as the solid-state deuterium NMR powder pattern. Reproduced from Ref. [85].
Gernral Aspect of Surfactants and Colloidal Particles
24
2.3.1.2 2H NMR of Lyotropic Liquid Crystals
Deuteron NMR is a standard method for investigating the structure and dynamics of
membranes, phospholipid bilayers and related model system. The 2H NMR spectrum
of a liquid crystalline anisotropic phase containing D2O confined in the water layer
exhibits a quadrupolar splitting characteristic of the water molecules. Despite the rapid
motion of water molecules, a small degree of orientational order remains enforced by
the orientation of the anisotropic liquid crystalline phase. Spectra of isotropic phases
like, for example, cubic liquid crystalline phases exhibit zero splitting. Both lamellar
and hexagonal phases produce splittings. Figure 2.13 shows the powder patterns
obtained for an isotropic phase (iso), a hexagonal phase (hex), and a lamellar phase
(lam). The lamellar and hexagonal spectrum can be distinguished because the 2H
powder pattern for the hexagonal phase of amphiphilic rods has a splitting that is
roughly half of that for the corresponding lamellar phase.86 This is due to lateral
diffusion about the rod generating one more axis of symmetry for further averaging of
the quadrupolar interaction.
Figure 2.13 Illustration of the powder patterns obtained for isotropic (iso), hexagonal (hex),
and lamellar (lam) liquid crystalline phases.
The diffusion about this extra axis provides one more term ½ (3cos2
α
-1) to the
interaction averaging. In this case
α
= 90° (the angle between the surface normal and
the cylinder axis), so the term counts as ½, making the quadrupolar splitting for
hexagonal array of cylinders smaller in absolute magnitude by a factor of 1/2
Gernral Aspect of Surfactants and Colloidal Particles
25
compared to that of the lamellar system. 87
2.3.2 Rheology
Rheology (from the Greek words “ρείν ”, flow, “λογια ”, study) is the science of the
deformation and flow of matter. It provides information on the mechanical response to
a dynamic stress or strain. Stress (σ) is the force per unit area and so has the units
Nm−2 or Pa. Strain (γ) is the deformation of the sample, for example, its relative
change in length, and is dimensionless. A schematic of a shear deformation is shown
in Figure 2.14, where the liquid is filled between parallel plates. The bottom plate is
fixed but the top plate is moved at a speed vx in the x direction. The shear stress is
proportional to the velocity gradient dy
dvx:
dy
dvx
ησ
= , (12)
where
η
is called the coefficient of viscosity, and is expressed in kg m−1 s−1 or Pa s.
We can define dy
dvx
=
γ
& as the shear rate in units of s−1. The shear stress is:
γ
η
σ
&
=. (13)
Figure 2.14 Schematic of a shear experiment between parallel plates. The top plate is moved at
constant speed vx, causing a steadily increasing strain. Reproduced from Ref. [42]
In principle there are two categories of materials, either Newtonian fluids where the
viscosity is independent of the shear rate or non-Newtonian fluids where the viscosity
is a function of shear rate. Many everyday materials like paint or yogurt get thinner as
the shear rate is increased, this is termed shear thinning. Some things become thicker
with increasing shear rate and are shear thickening (e. g. whipped cream). Other
substances are plastic above a certain shear stress, meaning that the fluid does not
Gernral Aspect of Surfactants and Colloidal Particles
26
exhibit plastic flow until a yield stress, σo, is reached. Such fluids are called Bingham
fluids and the stress is given by 42
0
σγησ
+= &. (14)
In contrast to liquids, solids have an elastic response to applied stress or strain, at
least for small deformations. At low strains, the stress is proportional to the strain
(Hooke’s law) and independent of the deformation rate. One of the most important
characteristics of soft materials is the dependence of their mechanical behaviour on
the rate of deformation. Since at low rates of deformation most soft materials exhibit
viscous behaviour whereas at high rates of deformation they behave elastically, they
are called viscoelastic materials.
The viscoelasticity of soft materials is probed via several types of experiments. In
dynamic or oscillatory tests the complex modulus of a sample at small angles of
deformation is determined. The tests are called microscale experiments compared to
macroscale tests like rotational or viscometry tests. The complex modulus describes
the total resistance of the sample to oscillatory shear:
γσ
&
*
G=; similar is the resistance
to flow in rotational tests (
γ
η
σ
&
=). The complex modulus G* is:
G* = G'+iG" (15)
where G' is the elastic or storage modulus and G" is the viscous or loss modulus and
tan
δ
= G" / G' is the phase angle or loss angle.
A structured system will gain energy from the oscillatory motion as long as the motion
does not disrupt the structure. This energy is stored in the sample and is described by
the elastic modulus. The magnitude of the storage modulus is depending on the
number of interactions between the ingredients in the sample. The strength of each
interaction G' is the sum of all interactions multiplied with the strength in the
interaction. The higher the number of interactions, the higher is G'. The stronger the
interaction the higher is the G' value. G' is a measure of the structure of the sample.
Oscillatory shear will also create motion between the ingredients of the sample, i.e.
friction. This friction will cause energy to be lost as viscous heating. The loss modulus
describes the part of the energy which is lost as viscous dissipation. The loss modulus
is only related to the number of interactions but is virtually independent of the strength
of the interaction. The greater the number of interactions in which friction can be
created, the larger is G". The G" value is a measure of the flow properties for the
sample in the structured state. The viscosity associated with the loss modulus is the
Gernral Aspect of Surfactants and Colloidal Particles
27
dynamic viscosity
η
' = G"/ω, where ω is the angular frequency of the imposed
oscillating stress or strain.42,88
The phase angle tan
δ
is associated with the viscoelasticity of the sample. A low value
in tan
δ
or
δ
indicates a higher degree of elasticity (more solidlike). The phase angle
δ
can be used to describe the viscoelastic properties of a sample as shown in
2.2.
Table 2.2 Overview of the relation between phase angles and the viscoelastic properties of
samples.
δ
= 90
°
G* = G" and G'= 0 viscous sample
δ
= 0
°
G* = G' and G"= 0 elastic sample
0
°
<
δ
< 90
°
viscoelastic sample
δ
> 45
°
G" > G' semi-liquid sample
δ
< 45
°
G' > G" semi-solid sample
Experimental
28
3 Experimental
The purpose of this chapter is to describe the materials and experimental
techniques used for this study. The description of materials will include the
lecithin system, the investigation of which has formed the basis of this work, and
furthermore many other materials used for the synthesis of particles. The major
experimental techniques which have been used for this work, including rheo-
polarizing microscopy, rheo-2H NMR, and rheological measurements will be
described.
3.1 Materials
The pseudo-ternary surfactant system investigated in this thesis consists of
lecithin as a surfactant, water as solvent and n-decane as oil. The surfactant
soybean lecithin (Epikuron 200) was supplied by Degussa Bioactives AG. It
consists of 92 % soybean phosphatidylcholine (PC) and other minor compounds
which are listed in Table 3.1. The average molecular weight is 772 g mol-1. n-
Decane (C10H22) of 95 % purity and a molecular weight of 142.29 g mol-1 was
obtained from Merck and D2O was purchased from Deutero GMBH with 99.96 %
purity and a molecular weight of 20.028 g mol-1.
Table 3.1 Composition of soybean lecithin “Epikuron 200”.
Chemical name Percentage
Phosphatidylcholine
Min. 92 %
Lyso-phosphatidylcholine Max. 3 %
Other Phospholipids Max. 2 %
Water Max. 0.8 %
Oil Max. 2 %
α-Tocopherol Max. 0.2 %
The influence of micro-particles on the flow behaviour of the lamellar phase was
Experimental
29
studied on samples of the lecithin system, mixed with spherical particles. The
PMMA particles used were surface-modified with poly(12-hydroxystearic acid)
and labelled with the fluorescent dye (Rodamine RITC) and had a diameter of 2
µm. These particles were provided by Dr. Imhof, Utrecht University.89
For the synthesis of SiO2 and PMMA nano-particles and melamine-formaldehyde
micro-particles the chemicals listed in table 3.2 were used.
Table 3.2 Specification of chemicals used for the synthesis of nano- and micro-particles
Chemical Formula Purity Supplier
Ammonia
NH3+H2O
-
Lancaster
Ethanol C2H5OH ≥ 96 % Lancaster
Tetraethyl orthosilicate C8H20O4Si ≥ 98 % Lancaster
Methyl methacrylate C5H8O2 > 99 % Lancaster
Glycidyl methacryalt C7H10O3 ≥ 95 % Lancaster
Ethylen glycol dimethacrylate C10H14O4 ≥ 98 % MERCK
Aluminum oxide Al2O3 - Woelm Pharma
Toluene C6H5CH3 > 99 % MERCK
Potassium peroxodisulfate K2S2O8 ≥ 98 % MERCK
Sodium sulphite Na2SO3 ≥ 98 % Lancaster
n-Dodecylamine C12H27N ≥ 98 % Lancaster
2,4,6-Triamino-1,3,5-triazine C3H6N6 ≥ 99 % Fluka
Tetramethylammonium hydroxide C4H13NO - MERCK
Formaldehyde CH2O - MERCK
Hydrogen peroxide H2O2 - Fluka
Sodium bis (2-ethylhexyl) sulphosuccinate C20H37O7S - SIGMA
Sodium Chloride NaCl - MERCK
Experimental
30
N P
O
O
O
O
O
O
O
O
Figure 3.1 Phosphatidylcholine (PC), the main component of the lecithin used.
3.1.1 Sample Preparation
For all experiments of this work, two different types of samples were prepared,
on the one hand, samples of lecithin/D2O/n-decane forming an Lα phase, and on
the other hand samples of the same system, but mixed with spherical particles of
PMMA. The samples were prepared by adding the desired mass of lecithin to the
solutions of D2O and n-decane. For the preparation of samples with particles, the
micro-particles of PMMA were added to the desired amount of n-decane and the
composite was mixed using an ultrasonic bath for 1 min, followed by the addition
of the surfactant and D2O. All samples were mixed by slow rotation on a
Staudinger wheel for two days at 40 °C. The air bubbles introduced in the mixing
process were subsequently removed by brief centrifuging, in the case of samples
without particles. The samples were stored in darkness. The compositions of all
samples used are listed in table 3.3.
Experimental
31
Table 3.3 Composition of the samples of lecithin/decane/D2O system, mL: mass of
lecithin, mD: mass of D2O, mn: mass of n-decane mp: mass of particles and nw: molar ratio
of D2O/ lecithin.
Sample mL/g mD/g mn/g mp/g nw
1
0.9998
0.1423
0.8555
0.0020
5.496
2 0.9998 0.1348 0.8442 0.0201 5.197
3 0.9997 0.1155 0.8221 0.0597 4.453
4 0.9507 0.1368 0.8220 0.1000 5.547
5 0.9879 0.1408 0.8520 0.0403 5.494
6 0.9701 0.1398 0.8376 0.0810 5.555
7 1.0131 0.1436 0.8679 0.0202 5.464
8 1.0145 0.1445 0.8583 - 5.490
9 0.9981 0.1423 0.8560 - 5.496
10 0.9500 0.1356 0.8084 0.1010 5.501
11 1.9962 0.2847 1.7109 - 5.498
12 0.9917 0.1422 0.8492 0.0201 5.527
13 1.9969 0.2857 1.7122 - 5.515
14 0.9494 0.1364 0.8147 0.1000 5.538
15 0.9910 0.1434 0.8496 0.0201 5.578
16 1.9978 0.2967 1.738 - 5.729
17 2.9943 0.4270 2.9949 - 5.546
18 1.9963 0.2878 1.7117 - 5.557
19 1.0004 0.1428 0.8569 - 5.502
20 1.0005 0.1427 0.8574 0.1010 5.498
21 2.0041 0.2079 1.7927 - 3.999
22 0.9996 0.0828 0.9172 - 3.193
23 0.8001 0.0730 0.7259 - 3.517
24 1.0007 0.0958 0.9034 - 3.690
25 1.0000 0.1038 0.8996 - 4.001
Experimental
32
26 0.4984 0.0554 0.4402 - 4.285
27 1.0007 0.1170 0.8938 - 4.507
28 0.9993 0.1296 0.8752 - 4.999
29 0.9999 0.1424 0.8570 - 5.494
30 0.9993 0.1572 0.8641 - 6.064
31 1.0603 0.1423 0.8445 - 5.998
32 1.0001 0.1687 0.8315 - 6.502
33 1.0001 0.1813 0.8185 - 6.988
3.1.2 General Procedures of Particle Synthesis
3.1.2.1 Synthesis of SiO2 Particles
A solution of ammonia (195 g, 25 % in water, Riedel-de Haën) and ethanol (1000
g, 99.9 %) was stirred for 10 minutes at room temperature to give a
homogeneous solution. Tetraethoxysilane (13.1 g, 99 %, ABCR) was carefully
added to this solution and then the mixture was stirred for 24 hours at room
temperature. After a few more minutes, the transition to a turbid white
suspension occurred regularly. The solvent was removed, and the residue
dissolved in ethanol, washed with ethanol (3 x 30 ml). The particles were
redispersed after centrifugation (for 15 minutes at 3000 rotations per minute) in
four total steps.
3.1.2.2 Synthesis of PMMA Particles
In a three-necked round bottom flask (250 ml), 140 ml of distilled water were
heated to 80 °C and degassed with a gentle stream of argon during 1 hour. A
mixture of 16 g of methyl methacrylate (MMA, 99 % Lancaster), 10 wt % (2 ml)
glycidyl-methacrylate (functionalized comonomer) and 10 wt % (2 ml) ethylene
glycol-di-methacrylate (cross-linker) were destabilized using basic aluminum
oxide (Al2O3). The mixture was added to 16 g of toluene. The resulting solution
was added to the degassed water and stirred for 10 minutes at 80 °C to give an
Experimental
33
emulsion. The polymerization was initiated by adding a solution of 0.8 g of
potassium peroxodisulfate in 20 ml of degassed water. The progress of the
polymerization was monitored by observing the interference colour of a dried
sample taken from the reaction vessel (see Table 3.4). After 40 to 60 minutes the
reaction was stopped by fumigating with air for at least 5 min. Then the reaction
vessel was opened and heated up to 80 °C and stirred for at least 1 hr. Stirring
was continued to remove the toluene and unreacted monomers at ambient
temperature overnight. Particles of large size were removed by centrifugation for
10 min at 3000 rpm. Further purification was performed by repeated (4 times)
centrifugation for 120 min at 3000 rpm, decanting of the solvent, and
resuspension of the particles in distilled water after each cycle.
The surface of the particles was chemically modified by stirring the resulting
aqueous particle suspension at 50 °C for 24 hr with the dissolved nucleophile. In
the case of dyes, 10 mg and in the case of dodecylamine 500 mg were added.
The unreacted nucleophile was removed by repeated centrifugation and washing
with water, until no reactant was found in the water phase. The principle of the
preparation of modified PMMA particles is shown in Figure 3.2.
Table 3.4 The colour of PMMA particles as a function of reaction time.
Time/min Colour of particles Remark
0
Colourless
Start of reaction
5 Violet-blue -
10 Green-yellow Centre was light blue
15 Green-red -
20 Red (deep red) -
30 White-red -
40 red The reaction was stopped
Experimental
34
H
2
CCH
3
OO
O
O
OO
O
O
Glycidyl-methacrylat
Ethylenglycoldimethacrylate
Methyl methacrylate
H
2
O+K
2
S
2
O
3
O
Figure 3.2 Flow diagram of reaction steps.
3.1.2.3 Synthesis of Melamine-Formaldehyde Particles
In a three-necked round bottom flask (250 ml), 0.63 g (0.05 ml) of 2,4,6-triamino-
1,3,5-triazine (melamine) and 0.05 ml of tetramethylammonium hydroxide 25 %
were added to 50 ml of distilled water. The mixture was stirred for 10 – 12 min at
70 °C until all of the melamine was dissolved. A solution of 2.43 g (0.3 mol) of
formaldehyde was added to the mixture after stirring for 60 min at 70 °C. When
0.3 ml of hydrogen peroxide (30%) were added to the mixture to start the
polymerization the colour changed to white. Stirring was continued for 26 min at
70 °C, then the solvent was removed and the residue was washed with distilled
water and centrifuged for 30 min at 4500 rpm.
Nucleophile
Modified particles
Nucleophile:
dodecylamine,
Rhodamine 6G,
Coumarin 120.
Epoxy-functionalized particles
Experimental
35
3.2 Methods
3.2.1 Polarizing Microscopy
The microscopy experiments were performed in transmission mode under
crossed polarizers using a Leitz ORTHOLUX 2 POL microscope. Different lenses
with L10, L25 and L32 magnification were used. The pictures were recorded with
a JVC TK-C1381 colour video camera. Steady-state shear flow conditions were
generated using a CSS450 Linkam shear cell. Images in the velocity-vorticity
plane are taken with crossed polarizers oriented at 45° to the flow direction. The
samples were placed between parallel plates with diameter 15 mm and 500 µm
gap. A heating/cooling rate of 0.5 °C/min was used for temperature scans, in
order to allow time for temperature equilibration.
3.2.2 2H NMR
For the NMR experiments under shear a cylindrical Couette cell of 14 and 15 mm
inner and outer diameter respectively, was used. This cell is integrated into an
NMR probe for a superconducting magnet. Its axis is aligned parallel to the
external magnetic field. Shear is applied by rotating the outer cylinder with an
external motor located below the NMR magnet. A photograph and a schematic
drawing of the experimental setup for the rheo-NMR experiments are shown in
Figure 3.2. The spectra were recorded on a Tecmag Apollo NMR spectrometer at
the resonance frequency of 46.073 MHz for deuterons. A quadrupole echo
sequence with a pulse separation of 60 µs was used. With the saddle-shaped
NMR coil surrounding the shear cell, pulse durations of 22 µs for a 90° pulse
were obtained. Typically, 128 scans were accumulated for each spectrum, and a
recycle delay (last delay) of 500 ms was used. The temperature of the sample
was controlled to an accuracy of ±0.2 °C using an airflow system. The
temperature of the air was regulated by using a HAAKE Phoenix II P1 thermostat
filled with Silicon oil.
Experimental
36
Figure 3.2 Experimental setup for rheo-NMR experiments.
For the investigation of the phase behaviour of the lecithin system by NMR the
samples were filled in 4 cm long glass tubes of 5 mm diameter which were
sealed with a Teflon plug. The tubes were inserted into a home-built goniometer
probe such that the tube axis was perpendicular to the static magnetic field. The
sample can be rotated about the tube axis allowing for the measurement of
orientation-dependent spectra. Spectra were measured with a quadrupole echo
pulse sequence, and 128 scans were averaged before Fourier transformation.
The sample was pre-equilibrated in the isotropic phase at ca. 60 °C for every
measurement. Slow cooling from the isotropic to the anisotropic phase was
sufficient to achieve a uniform alignment of the Lα phase by the magnetic field.
3.2.3 Rheology
The rheology experiments were performed in Lund University with a Physica
UDS 200 rheometer using a cone and plate cell made of stainless steel
(MK20/M, 1º cone angle). The instrument is equipped with a temperature control
unit that was calibrated to give a temperature in the sample chamber within 0.1°C
of the set value. Evaporation of the solvent was reduced by a solvent trap on the
top of the cell. All samples were pre-sheared at 35 °C to align the Lα phase.
The viscoelastic properties, i.e. the dynamic moduli, were determined by small
frequency oscillatory tests where the linear regime was previously verified by
stress sweep tests. Shear controlled experiments at a constant shear rate were
∇v
v
Magnetic field Bo
Couette Cell
Bo
1 cm
Experimental
37
performed as a function of time in order to follow the kinetics of MLV formation. A
temperature sweep (0.5 °C/min) at a constant shear stress and frequency was
used to determine the transition from the lamellar to nematic phase.
3.2.4 SAXS
SAXS measurements on lecithin samples were performed in Lund University with
a Kratky compact small angle system equipped with a position sensitive detector
(OED 50 M from M Braun, Graz Austria) containing 1024 channels of a width of
53.6 µm. Cu Kα radiation with a wavelength of 1.54 Å was provided by a Seifert
ID-300 X-ray generator operating at 50 kV and 40 mA. To minimize background
scattering, the camera volume was kept under vacuum. A 10 µm thick Ni filter
was used to remove Kβ radiation and a 1.55 mm W filter was used to protect the
detector from the primary beam. The distance between sample and detector was
277 mm. The samples from the phase diagram determination were used. The
sample holder was a 1 mm quartz capillary and samples were transferred to the
capillary using a syringe. The samples were transferred at high temperatures
(isotropic phase) and then cooled very slowly into the anisotropic phases. A
Peltier element controlled the temperature within ± 0.1 °C.90
SAXS measurements for sample 13 of lecithin/D2O/n-decane were carried out at
Paderborn University with approximately the same equipment as described
above. In this case, the SAXS setup was calibrated using cholesterylmyristate as
a reference (see appendix, Figure A.8).91
The measurements give intensity versus channel number as primary data. For
the physical interpretation the channel number has to be converted to a
scattering vector q. The known constants of wavelength (λ), sample-detector
distance (l), and channel width (cw) are listed in Table 3.4. The scattering vector
of a peak is given by: q = 2π cw Δc /λ l, where Δc = cp – cpb with the channel
numbers cp and cpb of the current peak and the primary beam, respectively.
Experimental
38
Table 3.4 The constant parameters of SAXS, required for the conversion of the channel
number to a scattering vector.
SAXS measurement λ / nm cw /μm l / mm
Lund University
0.1542
53.6
277
Paderborn University 0.1542 53.3 230
3.2.5 Other Techniques
The particle characterization was performed with a scanning microscope (Model
Alpha, WiTec). The instrument was operating in pulsed force mode AFM using
silicon cantilevers (pointprobe-FM, Nanosensors). All samples were prepared on
glass substrates.
Light scattering experiments were performed with a model 5000e compact
goniometer system (ALV-Laser Vertriebsgesellschaft), which allows the
simultaneous recording of static and dynamic light scattering (SLS/DLS). A
Nd:YAG laser with 200 mW operating at a wavelength of 532 nm was used as a
light source. Cylindrical quartz glass cuvettes with an outer diameter of 20 mm
served as scattering cells. The scattering intensity was observed under 13
different scattering angles reaching from 30° to 150°. All measurements were
performed at 25 °C. The particles were characterized via SLS/DLS in water and
toluene solutions. All suspensions were filtered with a 1.2 µm PET syringe filter
(Mecherey-Nagel).29,92
Phase Behaviour of the Lecithin Organogel
39
4 Phase Behaviour of the Lecithin Organogel
Extensive research on lyotropic liquid crystals over the last few decades has
allowed the phase diagrams, the most common structures, and the specific
properties of these nanostructured materials to be established in great
detail.41,49,93-95 Systems formed by lecithin, water and oil are representative
examples of this category of materials and are of interest for a multitude of
different applications such as dispersion technology, cosmetics, pharmaceutical
products, food and encapsulation systems.96-102
Figure 4.1 Phase diagram for the system lecithin/water/decane at 25 °C with single and
multiphase areas which has been obtained by Angelico and co-workers. Phase
abbreviations are as follows: L2, reverse micellar solution phase, Lα, lamellar phase and
H2 reverse hexagonal phase. Reproduced from Ref. [12].
Phase Behaviour of the Lecithin Organogel
40
The isothermal quasi-ternary-phase diagrams and the phase properties of
several systems of the type lecithin, water, and oil (where the oil is cyclohexane,
isooctane or n-decane) have been investigated by Angelico and co-workers (see
Figure 4.1).12,103 As can be seen in the phase diagram of the system
lecithin/water/n-decane, several liquid crystal phases and emulsions exist.
Angelico et al. found that upon addition of water to the lecithin/n-decane system,
first a gel is formed. When more water is added, pure decane is expelled and the
system undergoes a phase separation, resulting in a coexistence of gel with pure
oil (low lecithin content). At high lecithin content a lamellar phase is observed. As
shown in Figure 4.1, a large part of the phase diagram is occupied by this
lamellar phase. In addition, a coexistence of reverse micelles, water and
spherulites close to a stable Winsor II phase equilibrium are seen in a large
range of water and decane concentration. The bold arrow in Figure 4.1 indicates
the region where a reverse hexagonal phase was found.
As reported in Reference 12 the multiphase region close to the decane corner
and the gel phase of the Lα + gel, was not investigated in detail. Therefore, it is
the aim of the present work to provide some more details of the phase behaviour
of the system lecithin/water/decane. The shear-induced orientation states of the
lamellar phase of this system have been investigated by Burgemeister and later
on by Blaschke26,104 Blaschke104 observed two distinct splittings in the 2H NMR
spectra during temperature dependence experiments, indicating that a second
liquid crystalline phase exists close to the Lα phase. Following his work a specific
region of the phase diagram (lamellar phase) will be explored and the phases
and phase properties at various temperatures will be investigated, extracting the
characteristic parameters of different structures by using polarizing microscopy,
SAXS, 2H NMR and rheological measurements.
4.1 Identification of Phase Transitions by Polarizing Microscopy
Polarizing microscopy is commonly used to detect the birefringence phenomena
and to distinguish different types of mesophases by their characteristic textures.
If the sample is anisotropic and shows strong birefringence, the intrinsic coloured
Phase Behaviour of the Lecithin Organogel
41
texture based on the structure of the phase can be observed through crossed
polarizers. In contrast with this, dark regions of the texture mean that there is no
birefringence (isotropic structure) or a rather weak birefringence.
In this thesis, the phase behaviour of lecithin/D2O/n-decane system with sample
nw = 5.5 have been characterized by polarizing microscopy. The lamellar phase
develops a typical oily streak texture which occurs only in lamellar phases, as
can be seen in Figure 4.2. This texture was obtained after loading a fresh sample
with 50 % lecithin and nw = 5.5 (sample 9 in Table 3.2) between the plate-plate
shear cell with 500 µm gap, at 16 °C.
Figure 4.2 Light microscope textures of the system lecithin/D2O/n-decane with nw = 5.5.
The oily streak texture characteristic of lamellar phase was observed between crossed
polarizers at 16 °C. Bar = 100 μm
The phase penetration experiment involves placing the pure lecithin between two
microscope slides and contacting it with an aqueous phase as shown in Figure
4.3. This experiment also was performed for the system of AOT/brine (see
appendix, Figure A.1). The metastable myelin state was obtained upon contact of
the water with lecithin. The figure on the right indicates that the tubules are
twisted. This myelin state was only observed for a short time before the typical
lamellar phase appeared. The myelinic figures represent classical myelinic
Phase Behaviour of the Lecithin Organogel
42
instabilities at the interface. These instabilities were first observed in lipids by
Virchow in 1854 and are the subject of several studies.105-110 There is no
complete understanding of the myelin formation process; we do know that the
myelin textures of water-lipid appear as a consequence of the swelling by
increasing water contact and dissolution of the bulk lamellar phase. The physical
reason why a lamellar phase should swell in this bizarre fashion still remains
unknown.
Figure 4.3 An optical microscope texture of the system lecithin/D2O was taken between
crossed polarizer and shows a typical contact experiment. Pattern (a) shows the myelin
texture observed after contacting water with lecithin at the interface. Bar = 100 μm.
Pattern (b) shows large magnification of tubules which are twisted. Bar = 50 μm.
An attempt to identify phase transitions for a sample of lecithin/D2O/n-decane
with nw = 5.5 increasing the temperature by using polarizing microscopy was
made. When microscopically observed between crossed polarization filters
different textures characteristic of lamellar, nematic and isotropic phases are
found, but a clear distinction between pure anisotropic phases and a two-phase
region was not possible. The temperature dependent experiments were
performed by loading a fresh sample of lecithin/D2O/n-decane into the plate-plate
cell with 500 µm gap of the CSS450 Linkam shear cell connected to a polarizing
microscope. The sample temperature was controlled by a heating/cooling rate of
0.5 °C/min. Figure 4.4 shows a sequence of optical micrographs of different
textures observed with increasing of temperature. Two phase transition
b
a
Phase Behaviour of the Lecithin Organogel
43
temperatures (39°C and 50 °C) were observed. As will be shown later, the lower
transition temperature at 39 °C corresponds to the phase transition from Lα to ND
+ Lα. At 50 °C there is the transition from the anisotropic region to the isotropic
phase.
Figure 4.4 The evolution of the liquid crystalline structures of lecithin/D2O/n-decane
with nw = 5.5 as imaged by polarizing microscopy with increasing temperature. (a)
Optical micrograph of Lα phase at 25 °C. (b) Pattern of the biphasic region Lα + ND at 39
°C. (c) Same system at 40 °C. (d-e) Coexistence of anisotropic and isotropic phases at 46-
47 °C. (f) Isotropic phase at 50 °C. The width of the bar on the images corresponds to
150 µm.
a b
c
f
d
e
Phase Behaviour of the Lecithin Organogel
44
4.2 Identification of Phase Transitions by SAXS
Small angle X-ray scattering (SAXS) has been recognized as a powerful
technique for the study of phase transitions in liquid crystalline phases. The
samples 13, 17 with nw = 5.5, and 21 with nw = 4.0 of the lecithin/D2O/n-decane
system were used. X-ray small angle diffraction patterns are recorded during
slow cooling after pre-equilibrating of the samples in the isotropic phase at
around 60 °C. As the temperature is decreased from 60 to 16 °C, the SAXS
pattern of sample 21 with nw = 4 shown in Figure 4.6 does not change. Only a
small decrease of the intensity at the Bragg peak was observed at high
temperatures. The sharp peak indicates the existence of the lamellar phase in
this temperature range.
The experiment was repeated for sample 17 with nw = 5.5 and the results were
compared with the SAXS results of a similar sample (13) which have been
obtained in the Department of Chemistry of Paderborn University. As shown in
Figure 4.7 different SAXS patterns compared to Figure 4.6 are obtained.
At 60 °C an isotropic phase is expected showing one single broad peak. The
observed structure of the scattering function at high temperature could result
from a superposition of an isotropic and nematic peak. At lower temperatures,
three features can be recognized. The one at the highest q value is assigned to
the lamellar phase. The origin of the broad pattern with multiple peaks at lower q,
however, remains unclear. A similar feature has been observed by Angelico et
al.12 for the system lecithin/iso-octane/water. Demixing may have occurred during
the sample preparation, resulting in a non-equilibrium multi-phase sample.
This experiment was also carried out for sample 13 with similar composition (nw =
5.5), using the SAXS technique at the University of Paderborn. In this case, the
sample was heated to 60 °C in the isotropic phase, the temperature was slowly
decreased to 30 °C, and then measurements were performed at increasing
temperature. As shown in Figure 4.8 two peaks were obtained between 30 to 49
°C. The sharp line indicates the lamellar phase and the second broad peak at
lower q shows the presence of another phase. This second phase probably is a
nematic phase. The two phases coexist over a large range of temperatures.
Phase Behaviour of the Lecithin Organogel
45
0,0 0,2 0,4
q / A -1
0,0 0,2 0,4
q / A -1
0,0 0,2 0,4
q / A -1
Figure 4.6 SAXS diffraction patterns obtained by scanning temperatures for sample 21
(nw = 4) of the system lecithin/D2O/n-decane in Lund University.
T / °C
48
49
50
51
52
55
57
60
T / °C
40
41
42
43
44
45
46
47
T / °C
16
25
30
32
34
36
38
39
Phase Behaviour of the Lecithin Organogel
46
0,0 0,2 0,4
q / A -1
0,0 0,2 0,4
q / A -1
0,0 0,2 0,4
q / A -1
Figure 4.7 SAXS diffraction patterns obtained by scanning temperatures for sample 17
(nw = 5.5) of the system lecithin/D2O/n-decane in Lund University.
T / °C
48
49
50
51
52
55
57
60
T / °C
40
41
42
43
44
45
46
47
T / °C
16
25
30
32
34
36
38
39
Phase Behaviour of the Lecithin Organogel
47
0,0 0,2 0,4
q / A -1
0,0 0,2 0,4
q / A -1
Figure 4.8 SAXS diffraction pattern obtained by scanning temperatures for sample 13
(nw = 5.5) of the system lecithin/D2O/n-decane at Paderborn University. Two different
peaks in the SAXS pattern give evidence of the coexistence of Lα and ND phases.
At 54 °C there is only the single broad peak of the nematic phase. Its low
intensity could be due to a coexistence with the isotropic phase which may have
lower scattering intensity. SAXS measurements give structural information about
the distance between the lattice planes. Table 4.1 shows the average distances
between bilayers and discs in the lamellar and nematic phases for samples with
T / °C
43
41
39
34
30
T / °C
54
52
49
47
45
Phase Behaviour of the Lecithin Organogel
48
nw = 4 and nw = 5.5, respectively. The results obtained for the measurement in
Lund (Figures 4.6 and 4.7) and Paderborn (Figure 4.8) are shown. For the former
experiment the exact position of the primary beam is not known any more.
Therefore, two calculations for two different values for the channel of the primary
beam were performed. The results for a beam position at channel 236 are closer
to the ones obtained in Paderborn, for which a calibration had been carried out.
Therefore, the correct values are 60 and 95 Å for the lamellar and nematic
phase, respectively. The average distance between the discs in the nematic
phase is larger compared to the bilayer period in the lamellar phase.
Table 4.1 The average period of the bilayer and discs in lamellar and nematic phases for
samples with nw = 5.5 and nw = 4 of the system lecithin/D2O/n-decane obtained by SAXS
measurements.
Measurement Place Primary beam D /Å
Sample nw = 5.5 Sample nw = 4
Lα ND Lα
119 35.6 46 31.4
Lund University
236 72.2 133.6 57.8
Paderborn University - 60 95 -
Phase Behaviour of the Lecithin Organogel
49
4.3 Identification of Phase Transitions by 2H NMR
The NMR spectrum from the deuterated water is dominated by the interaction of
the deuteron quadrupole moment with the electric field gradient at the nucleus. In
anisotropic liquid crystalline samples, the quadrupole interaction generates an
NMR spectrum with two peaks of equal intensity, while for an isotropic LC phase
or a solution a single sharp peak is obtained because of the rapid and isotropic
molecular motions which average the interactions to zero.111,112 The deuterium
NMR technique is thus applicable in phase behaviour studies of amphiphile-
water systems such as the system of AOT/brine (see appendix, Figure A.2) and
the lecithin system investigated here. For a system containing more than one
phase, the 2H NMR spectra are superimposed unless there is fast 2H exchange
between the phases. For the mixture of lamellar and nematic liquid crystalline
phases, two splittings can then be observed with the one originating from the
lamellar phase being of larger magnitude than that obtained for the nematic
phase.
In the following, the 2H NMR study of the phase behaviour of lecithin/D2O/n-
decane is presented. Sample 11 was filled in 4 cm long glass tubes of 5 mm
diameter, which were sealed with a Teflon plug and epoxy glue. The tube was
inserted in a goniometer probe with the tube axis perpendicular to the static
magnetic field. Spectra were obtained with a quadrupole echo pulse sequence,
typically 128 scans were averaged. To obtain well-aligned lyotropic phases, the
sample was pre-equilibrated in the isotropic phase at temperatures above 54 °C
before cooling. Slow cooling from the isotropic to the anisotropic phase is
sufficient to achieve uniform alignment of the anisotropic phase by the magnetic
field. In figure 4.9 spectra as a function of the temperature are shown. The
evolution of the phases is clearly seen: spectra with splittings characteristic of an
anisotropic phase exist from 24 °C up to 52 °C indicating a large amount of the Lα
phase. A small shoulder on the inner side of the right peak of the spectra at low
temperature indicates the existence of another phase (probably a nematic one).
An additional spitting is clearly seen at 44 °C and above, showing the co-
existence of two anisotropic phases. The two anisotropic phases coexist with an
Phase Behaviour of the Lecithin Organogel
50
isotropic phase at 50 to 52 °C.
To elucidate the structure of the second phase which appears to exist in larger
amount at elevated temperature, a rotation experiment, in which the line shape is
measured in dependence of the sample orientation with respect to the magnetic
field, was carried out. Changing the sample orientation by rotating it by 90° about
an axis perpendicular to the magnetic field, one observes a change of the line
shape. The new line shape corresponds to the superposition of doublets
according to the distribution of director orientations in the sample. For a lamellar
phase, after rotation the spectrum extends over the range of frequencies
corresponding to the range of director orientations from θ = 90° to 0°. To
distinguish Lα and ND phases by the rotation experiment, the relaxation time of
the samples needed to realign in the initial orientation was measured.
Rotation experiments carried out at two different temperatures, 30 and 49°C, for
the presented system are shown in Figure 4.10. At 49 °C (right side of Figure
4.10), where a large fraction of nematic phase is expected, the first spectrum
recorded after rotation of the sample by 90°, exhibits a line shape, which shows
different orientations of the director with respect to the magnetic field. All
orientations from 0° (edges of the shoulders) to 90° (maximum peaks) occur. The
line shapes change in a way indicating that a partial relaxation towards the initial
state occurs on the time scale of the rotation of the samples which takes about
30 s. This experiment was repeated at 30°C for the same sample. As can been
seen in the left side of Figure 4.10 the line shapes immediately after rotation
shows an additional splitting twice as big as the initial splitting but also intensities
at an frequencies between the peaks of the outer doublet. This indicates a
distribution of orientations of the normal vectors in a plane parallel to the
magnetic field. Such a distribution is expected for a lamellar phase, the directors
of which are aligned in the plane perpendicular to the magnetic field before the
sample rotation and in a plane parallel to the magnetic field after sample rotation
if no realignment occurs during the rotation. The relaxation time of director
alignment for this sample with a large amount of lamellar phase at this
temperature was found to be larger than 138 min. These results indicate that
Phase Behaviour of the Lecithin Organogel
51
the
-1 0 1
Δν / KHz
Δν / KHz
-1 0 1
-1 0 1
Δν / KHz
Figure 4.9 2H NMR spectra obtained by scanning temperatures for sample 11 (nw = 5.5)
of the system lecithin/D2O/n-decane. For the lamellar phase (Lα) a quadrupole splitting
∆ν is observed (spectra at 24 to 40 °C), whereas an additional nematic phase (ND) gives
an additional doublet with a small splitting at 42 to 49 °C. At 50 °C and 52 °C anisotropic
and isotropic phases coexist. The single peak at 54 °C shows an isotropic phase.
T/°C
34
32
30
28
26
24
T/°C
46
44
42
40
38
36
T/°C
54
52
50
49
48
47
Phase Behaviour of the Lecithin Organogel
52
-1 0 1
Δν / Hz
-1 0 1
Δν / Hz
Figure 4.10 2H NMR spectra obtained before and after a sample rotation by 90° at two
different temperatures 30 °C (left) and 49 °C (right) for sample 11 of the system
lecithin/D2O/n-decane. The relaxation of the director orientation at 49 °C is faster
compared to 30 °C.
the relaxation at higher temperature, where a large amount of the nematic phase
is present, is much faster due to the lower viscosity than the relaxation at low
temperature with a large amount of the lamellar phase.
indicate that the
t / min
138
113
81
43
21
6
2
0
t / min
45
30
25
20
10
6
2
0
Phase Behaviour of the Lecithin Organogel
53
4.4 Identification of Phase Transitions by Rheological Experiments
The rheological signatures of the various liquid crystalline phases were
determined using a Physica UDS 200 rheometer in stress-controlled mode. The
measurement cell had cone-and-plate geometry with a cone angle of 1°. To
determine the phase transition temperatures sweeps were performed for a
sample of lecithin/D2O/n-decane with nw = 5.5 (sample 17) and a heating rate of
0.5 °C/min. The stress amplitude was 1 Pa (linear regime) and the angular
frequency 1 rad/s.
Measurement of the storage and loss moduli as a function of temperature reveals
different regions. Figure 4.10 shows an example of a temperature scan of G' and
G" for the system lecithin/D2O/n-decane. At 60 °C G' is lower than G" which
indicates that the system is more liquid (existence of isotropic phase). By slow
cooling of the sample to about 50 °C, the storage and loss moduli increase, G"
remaining larger than G'. This is consistent with a sample consisting of some
amount of lamellar phase in coexistence with a large portion of other phases
(isotropic and nematic phases). Further decrease of the temperature to around
42 °C causes an approach of G' and G" and a crossing point between G' and G",
at about 40 °C. Below 40 °C the sample is more elastic than viscous. At about 37
°C a discontinuity in the slopes of both G' and G" is observed. This may be the
temperature, above which the lamellar phase coexists with nematic one in this
sample. The behaviour is reversed as the temperature increases, in agreement
with the changing proportions of lamellar and nematic phase observed by 2H
NMR and other techniques.
This experiment was repeated by scanning the temperature from 25 to 60 °C for
a similar sample as shown in Figure 4.11. After equilibrating the sample in the
isotropic phase, the temperature was lowered to 25 °C and the measurement
was started upon increasing temperature. As shown in Figure 4.11 curves
qualitatively similar to those of Figure 4.10 were obtained.
Phase Behaviour of the Lecithin Organogel
54
30 35 40 45 50 55 60
10
100
1
G' & G''/ Pa
T / oC
(G')
(G'')
Figure 4.10 Storage, G', and loss, G", moduli as a function of temperature for the system
lecithin/D2O/n-decane with nw = 5.5. The measurement was performed with decreasing
temperature. The vertical lines indicate the temperatures were phase transitions where
observed by 2H NMR.
20 25 30 35 40 45 50 55 60 65
10
100
G' & G'' / Pa
T /
o
C
(G')
(G'')
Figure 4.11 Storage, G', and loss, G", moduli as a function of temperature for the system
lecithin/D2O/n-decane with nw = 5.5, measured at increasing temperatures from 25 to 60
°C.
Lα + ND
I
ND
+
Lα
+
I
Phase Behaviour of the Lecithin Organogel
55
4.5 Phase Diagram
The temperature dependence of many properties of surfactant solutions provides
a convenient way to examine phase structures as a function of temperature. The
phase transition temperatures of the system lecithin/D2O/n-decane in a small
region of the phase triangle were obtained by 2H NMR measurements. The
phase transitions were determined for a limited number of samples, 22−33 as
listed in Table 3.3. The samples were prepared according to the standard
procedure which is described in section 3.2. Figure 4.12 shows the complete
pseudo ternary phase triangle of lecithin/D2O/n-decane at 25 °C (top) and a sub-
section of the phase triangle showing our samples investigated (bottom).
Figure 4.13 shows the transition temperatures between different regions of
lamellar, nematic, biphasic and isotropic phases as a function of the mole ratio of
deuterium per lecithin (nw). Three different types of phase transitions are
obtained: a) a lamellar phase to an isotropic one via a narrow two-phase region
at low concentration of water (nw ≤ 4), b) a lamellar phase to the coexistence of
the lamellar and nematic phases and c) the phase transition from this biphasic
region to the isotropic phase, passing a three-phase region at high
concentrations of water (nw > 4).
The curves shown in Figure 4.13 were obtained by analyzing the quadrupole
splittings of 2H NMR spectra. Addition of more water (D2O) to a system of
lecithin−n-decane/D2O increases the temperature of the transition to the isotropic
phase. A large part of the phase diagram is occupied by a lamellar phase. The
nematic phase could not be distinguished for samples with nw ≤ 4. For sample
with nw > 4 above a certain temperature two different anisotropic phases, a
lamellar one and a nematic one, could be distinguished. Because the quadrupole
splitting of these two phases are very similar at low temperatures the location of
the boundary between the regions of pure Lα and Lα + ND remains somewhat
uncertain.
Phase Behaviour of the Lecithin Organogel
56
0.42 0.43 0.44 0.45 0.46 0.47 0.48
0.04
0.05
0.06
0.07
0.08
0.09
0.10 0.48
0.49
0.50
0.51
0.52
0.53
0.54
D2O(wt %) n - Decane(wt %)
Lecithin(wt %)
Figure 4.12 Phase diagram for the system lecithin/water/decane at 25 °C with single and
multiphase areas which has been obtained by Angelico and co-workers (top)12. The
samples with different compositions investigated by 2H NMR are mapped in the phase
diagram as shown at the bottom.
Phase Behaviour of the Lecithin Organogel
57
3,03,54,04,55,05,56,06,57,07,5
15
20
25
30
35
40
45
50
55
60
T / oC
nw
Figure 4.13 Temperature dependence of phase equilibria as a function of mole ratio of
water to lecithin (nw) for the system lecithin/water/decane. The curves indicate the phase
transitions from Lα to biphasic Lα + ND to Lα + ND + isotropic to isotropic.
Lα
Iso
ND + Lα
ND + Lα+ Iso
Synthesis of Colloidal Particles
58
5 Synthesis of Colloidal Particles
A colloidal dispersion consists of colloidal particles dispersed in a medium. Both
the particle and the dispersion medium can be in the gas, liquid or solid state.
Colloidal dispersions are found in food, paint, foam, toothpaste etc. One specific
type of colloidal dispersions is a sol, where solid particles are dispersed in a
liquid. The liquid can be either polar or non-polar depending on the nature of the
colloidal particles.80
The stability of a colloidal dispersion is determined by the interactions between
the colloidal particles in the dispersion medium. As the colloidal particles are
continuously moving around in the medium, they will sooner or later meet. If the
colloidal particles attract each other, aggregates will form. An aggregation of
colloidal particles that is reversible is termed flocculation, while an irreversible
aggregation is called coagulation. Separation of the colloidal particles from the
dispersion medium concentrates the particles, by sedimentation at the bottom or
by creaming at the top, depending on the density of the particles compared to the
dispersion medium. On the other hand, if the particles do not attract each other,
the dispersion remains stable. Commonly, a difference between
thermodynamically stable and kinetically stable dispersions is made, where a
kinetically stable dispersion is only stabilized by an energy barrier between the
particles. Stabilization of a colloidal dispersion is created either by electrostatic
repulsion between the colloidal particles or by steric stabilization.42
The preparation of spherical particles via hetero-phase polymerization has
developed into a mature technique in macromolecular chemistry.28 The control of
the particle size, size distribution and shape, or the incorporation of functional
groups in the particles or on their surfaces is a central topic in preparative colloid
chemistry. The result is an ever-growing number of new functional materials. The
accessibility of highly monodisperse colloids within a tunable size range is a
crucial prerequisite for the generation of photonic band gap materials.29 Dye-
labelled spherical particles113 are used as calibration kits for optical methods e.g.
confocal microscopy30 or flow-cytometry.31
Synthesis of Colloidal Particles
59
Three different kinds of spherical particles were synthesized throughout this
work. Silica particles were prepared by condensation polymerization of
tetraethoxysilane following Stöber.114 Epoxy-functionalized acrylate colloids were
synthesized according to the basic procedure of a surfactant-free emulsion
polymerization which has been reported by Zentel and coworkers,115,116 and the
surface of these particles was modified by dodecylamine and dyes.113 Finally
micrometer-sized spherical melamine-formaldehyde resin particles were
prepared according to a slightly modified procedure as described in a patent by
Merck GmbH.117
5.1 Experimental Description
The materials used for the experiments in this section have been previously
described in Chapter 3. All solvents were of technical grade, if not noted
otherwise. Particle characterization was performed with a combination of
scanning probe microscopy (Model Alpha, WiTec), light scattering, light
microscopy and 13C NMR techniques. Centrifugations were done using an
Eppendorf centrifuge 5810 at speeds that never exceeded 200 rpm.
5.2 Synthesis and Characterization of SiO2 Particles
Monodisperse colloidal silica can be prepared by a simple procedure from
tetreaethoxysilane in alcoholic solutions:
Si (OC2H5) 4 + 4 H2O Si (OH) 4 + 4 C2H5OH (1)
Si (OH) 4 SiO2 + 2 H2O (2)
Hydrolysis, eq. (1), and condensation, eq. (2), of the monomers are base-
catalyzed by ammonia, which also provides the particles with negative stabilizing
surface charge. The general procedure for the synthesis of monodisperse silica
particles is described in section 3.1.2.1.
Monodisperse, spherical particles were obtained by condensation polymerization.
Figure 5.1 shows a nearly hexagonal periodic pattern of the SiO2 particles which
NH3
EtOH
NH3
EtOH
Synthesis of Colloidal Particles
60
was obtained by AFM (WiTEC type α-SNOM) indicating their spherical shape
and size uniformity. Diameters in the range from 380 to 410 nm were realized.
Figure 5.1 AFM picture of the monodisperse silica particles. The particle diameter was
determined to be between 380 and 410 nm.
5.3 Synthesis and Characterization of PMMA Particles
The basic procedure is a surfactant-free emulsion polymerization, described in
detail in the section 3.1.2.2. Particle synthesis is carried out by emulsion co-
polymerization of MMA, cross-linker and epoxy-functionalized comonomer. In
Figure 5.2 AFM pictures of a sample of epoxy-functionalized particles and of the
same particles after modification with dodecylamine are shown. It can clearly be
seen that a hexagonal close packed structure is formed, which is typical for
monodisperse spheres. The particles retain their size and shape after surface
modification. Light scattering experiments confirmed the particle size and narrow
size distribution, with a polydispersity index PDI ≤ 1.05 for the epoxy-
functionalized PMMA particles and PDI > 1.2 for the surface-modified sample.
The results of the particles characterization are summarized in Table 5.2. The
Synthesis of Colloidal Particles
61
values for the ratio of radius of gyration, Rg, and hydrodynamic radius, Rh, of the
particles exceed the expected hard sphere value of around 0.86. To verify the
incorporation of the epoxy-functionality, 13C-NMR spectra of redissolved particles
were recorded showing the expected signals in the epoxy regime at 65 ppm.
Since no signals occur in this part of the spectrum for poly (methyl methacrylate),
no disturbing signals from the main polymer component have to be considered.
13C-NMR measurements were also carried out on the particles after surface
modification with dodecylamine, but the amount of bound nucleophile is too small
to be detectable by NMR.
Figure 5.2 Topography of dried samples obtained by AFM measurements. The pictures
show (a) epoxy-functionalized PMMA particles and (b) PMMA with dodecylamine.
Table 5.2 Results of particle characterization: AFM measurements were performed on
dried samples, SLS/DLS was carried out in water and toluene. E-PMMA1: PMMA with
epoxy groups, D-PMMA1: epoxy-functionalized PMMA particles surface-modified with
dodecylamine.
Sample Rsolid [AFM]
(nm)
Rg [SLS]
(nm)
Rh [DLS]
(nm)
PDI Rg/Rh Solvent
E-PMMA1
180
189
220
1.05
0.86
Water
D-PMMA1 186−197 177 222 1.2 0.80 Toluene
a b
Synthesis of Colloidal Particles
62
5.4 Synthesis and Characterization of Melamine-Formaldehyde Particles
Amino resins, also called aminoplast resins, are major cross-linking agents for
baked thermosetting coatings. The amino resins most commonly used are
derived from melamine ( 2,4,6-triamino-1,3,5-triazine).
The first step in the synthesis of melamine-formaldehyde (MF) resins is
methylolation, the reaction of melamine with formaldehyde under basic
conditions. With excess formaldehyde, the reaction can be driven to form
predominantly hexamethylolmelamine (1), where R is H (see Figure 5.3). A
mixture of partially methylolated derivatives, including species such as
symmetrical trimethylolmelamine (2), is formed with less than the stoichiometric 6
mol of formaldehyde per 1 mol of melamine.
N
N
N
N
CH2ORROH2C
N
CH2OR
CH2OR
N
ROH2C
CH2OR
N
N
N
N
CH2ORH
N
CH2OR
H
N
H
CH2OR
(1) (2)
Figure 5.3 The chemical structure of hexamethylol melamine (1) with R = H and
trimethylolmelamine (2) with R = H.
The probable mechanism for the base-catalyzed reaction of melamine with
formaldehyde is shown in Figure 5.4. The first step involves the nucleophilic
attack by the amino group on the electrophilic C of formaldehyde, which is
facilitated by the removal of a proton from N by the base (B−). This is followed by
proton transfer from the resulting B−H to the negative oxygen atom, which yields
the methylolated product and regenerates the base catalyst. Both steps are
reversible.
Synthesis of Colloidal Particles
63
−NH2 + H2C=O + B− −NH−CH2−O− + B−H
−NH−CH2−O− + B−H −NH−CH2−OH + B−
−NH−CH2−O− + H2C=O −NH−CH2−O−CH2−O−
−NH−CH2−O−CH2−O− + B−H −NH−CH2−O−CH2−OH + B−
Figure 5.4 The probable mechanism for the base-catalyzed reaction of melamine with
formaldehyde.
Studies of the kinetics of the reaction indicate that the presence of one methylol
group on N deactivates the group for a second reaction by a factor of 0.6. On the
other hand substitution on one amino group has little effect on the reactivity of
the other amino groups. These kinetic factors favor formation of the symmetrical
trimethylolmelamine (TMMM). However, the preference is not strong enough to
overcome the thermodynamic tendency to produce mixtures of products. Thus, at
equilibrium, the reaction of 6 mol of formaldehyde with 1 mol of melamine yields
a mixture of products including all levels of methylolation and free
formaldehyde.118
Details of the synthesis of M-F particles are given in section 3.1.2.3. As for the
other types of particles the characterization was performed with scanning probe
microscopy (SPM) and light microscopy. The SPM was operating in pulsed force
mode using silicon cantilevers (pointprobe-FM, nanosensors). All samples were
prepared on glass substrates. NMR spectra were recorded on a Bruker AMX 300
spectrometer. Chemical shifts are reported in ppm from tetramethylsilane (TMS).
Dispersed melamine-formaldehyde micro-particles in water and aggregated
particles as observed by light microscopy are shown in Figure 5.5 (a) and (b),
respectively. The micro-particles attract each other, and after 24 hours
aggregation occurs. This aggregation is reversible which means that a
flocculation of the M-F particles occurred. In Figure 5.6 a dispersion of M-F
micro-particles of M-F in n-dodecane is shown. The micro-particles can easily be
resuspended in apolar liquids such as n-dodecane by mild sonication. The
pictures shown were observed between parallel polarizers immediately after
Synthesis of Colloidal Particles
64
sonication (a) and after 24 hours (b), respectively.
Figure 5.7 shows an AFM image of the M-F particles indicating their spherical
shape and size uniformity. Diameters in the range from 1 to 1.8 μm were
measured.
Figure 5.5 Pictures obtained by light microscopy of the micro-particles of melamine-
formaldehyde in water. Pattern (a) shows a dispersion of monodisperse M-F particles in
water. Pattern (b) shows the aggregation of M-F particles in water after 24 hours. Bar =
100 μm.
To verify the mechanism of M-F particles, 13C NMR and FTIR spectroscopy were
performed. An ether bridge in the micro-particles was found at 78 pmm in 13C
NMR and 1160 cm-1 to confirm the mechanism of M-F reaction.
Figure 5.6 Pictures obtained by light microscopy of the micro-particles of melamine-
formaldehyde in n-dodecane. Patterns show the dispersion of monodisperse particles in n-
dodecane after dissolving the particles and mild sonication (a) and after 24 hours (b),
respectively. Bar = 100 μm.
a b
a b
Synthesis of Colloidal Particles
65
Figure 5.7 Topography of a dried sample of M-F particles obtained by AFM, covering a
5 × 5 µm square.
Influence of Micro-Particles on the Lα Phase under Shear
66
6 Influence of Micro-Particles on the Lα Phase under Shear
Liquid crystalline phases used as solvent for colloidal particles provide new
methods to control the spatial organization of colloidal systems. Well defined
anisotropic structures and new phases have been recently observed in several
systems such as mixtures of rodlike viruses and colloid,119 molecular nematic or
cholesteric emulsions39,120,121 and suspensions of latex particles in discotic
micellar nematics.122-125 The behaviour of lamellar phases under shear is a
subject that has been receiving a lot of attention in recent years.54-57 In fact,
many different shear effects have been reported such as transformation from
lamellar phase to MLVs, changes in lamellar orientation,15,61,62 reduction in
lamellar spacing126 and transitions from MLVs to unilamellar vesicles.67 The most
interesting shear effect is the formation of MLVs in the presence of micro-
particles. In the mixture, topological defects and distortions of layers around the
particles are generated due to the incompatibility of particle size (μm) and the
interlamellar space (5 nm). These defects also generate inter-particle elastic
forces that govern the stability and the ordering of the particles.38
The aim of this work is to clarify whether defects in the layer structure of lyotropic
lamellar phases generated by the incorporation of micro-particles enhances the
nucleation of vesicles under shear. Regarding our goal, we have studied the
kinetics of the MLV formation, the influence of spherical micro-particles on the
shear-induced orientation states and the reversibility of size changes of the
MLVs.
6.1 Experimental Description
The system investigated is the pseudo-ternary mixture of soybean lecithin in D2O
and n-decane. The PMMA particles used were provided by Dr. A. Imhof from
Utrecht University. They are surface-modified with poly(12-hydroxystearic acid)
and labelled with the fluorescent dye (Rodamine RITC) and had a diameter of 2
µm. The particles synthesized in this work either were too small to be observed
by light microscopy or could not be well dispersed in the liquid crystal. Some of
Influence of Micro-Particles on the Lα Phase under Shear
67
them showed aggregation and poor miscibility with a lamellar phase (see
appendix, Figure A.5). The D2O to lecithin mole ratio was kept constant at nw ≈
5.5 for all experiments. The composition of the samples is shown in Table 3.3.
Samples 9, 16, 17 and 19 (without particles) and samples 10, 14, 15 and 20 (with
particles of PMMA) have been used. These samples were all prepared according
to a standard procedure as described in section 3.1.1.
The influence of micro-particles on the shear-induced orientation states in a
lamellar system is studied by polarizing microscopy, deuterium rheo-NMR
spectroscopy and rheological experiments. Detailed descriptions of these
methods are given in chapter 3.
6.2 Results and Discussion
This section is divided into three subsections. These subsections correspond to
groups of experiments that have been conducted to examine: (1) influence of
micro-particles on the shear-induced orientation states of the lamellae, (2)
influence of micro-particles on the formation process and size of MLV formation
and (3) influence of micro-particles on the reversibility of shear-induced formation
of the MLVs.
6.2.1 Influence of Micro-Particles on Shear-Induced Orientation States of the
Lamellae
This subsection presents results from experiments in which the different
orientation states of the lamellar phase of lecithin/water/n-decane under constant
shear rate is investigated at various temperatures for two samples, one without
particles and another one with 5 % particles. From the comparison of these
results the influence of micro-particles on the shear-induced orientation states
will be understood.
Figure 6.1 shows deuteron NMR spectra of the lamellar phase during shear. The
spectra reflect the sequence of orientation states with increasing temperatures
for a sample with nw = 5.5 (nw is mole ratio of D2O/Lecithin). The spectra were
recorded at a constant shear rate of 20 s-1 in the steady-state. For each series of
NMR measurements the samples were heated up to temperatures slightly above
Influence of Micro-Particles on the Lα Phase under Shear
68
the transition into the isotropic phase. After alignment of the samples in the
magnetic field by slow cooling through the biphasic regime of the
isotropic/lamellar phase transition, the temperature was set to the initial value of
the series, a spectrum at rest (
γ
& = 0 s-1) was measured and measurement of the
temperature series under shear was started. The samples were sheared for 30
minutes at each temperature before measurements were started. The first
spectrum depicted in Figure 6.1 (bottom left) corresponds to the magnetically
aligned lamellar phase, which shows a director orientation perpendicular to the
magnetic field. At T = 14 °C, upon the start of shear (20 s-1) in the Couette cell
the NMR line shape changes to a broad featureless spectrum which indicates the
formation of multi-lamellar vesicles. The line shapes do not change until 18 °C
but the line width of the spectra becomes smaller during the shear. With further
increase of the temperature, a gradual transformation to a new line shape with a
doublet splitting is observed. At 22°C the line shape resembles a powder pattern,
which indicates that layers with all directions are present in the sample. At 24 °C
a doublet grows from the outer edges of the powder line shape which indicates a
preference of the director orientation parallel to the magnetic field, that is, a
preference of the perpendicular orientation in the flow field. At 34 °C the layers
show the best alignment with their normal axes parallel to the vorticity axis
(parallel to magnetic field) in this so-called perpendicular orientation. This type of
alignment gives rise to a doublet with almost twice the initial splitting. With
increasing temperature once more the inner splitting is observed in the spectra
showing a change to the parallel (a) orientation. With increasing temperature,
however, the splitting decreases and finally vanishes due to transition to the
isotropic phase at 40 °C.
Influence of Micro-Particles on the Lα Phase under Shear
69
-2 0 2
Δν / KHz
Δν / KHz
-2 0 2 -2 0 2
Δν / KHz
Figure 6.1 Deuteron NMR spectra obtained at different temperatures (30 min for each
temperature) for sample 19 (containing no particles) with nw = 5.5 of the system
lecithin/D2O/n-decane at a constant shear rate of 20 s-1. The first spectrum (bottom left)
was obtained at rest.
Deuteron NMR spectra of the lamellar phase of lecithin/water/n-decane during
shear (20 s-1) are shown in Figure 6.2 for a sample with surfactant concentration
of 50 wt %, 43 wt % n-decane and 7 wt % D2O mixed with 5 % micro-particles of
PMMA (sample 20). The experiment was carried out in the same way as for the
sample without particles. By increase of the temperature during constant shear
rate for the sample with particles, different orientations of the director are
observed as well. As can be seen in figure 6.2 at low temperatures a state of
shear-induced MLV occurs. The spectra between 34 °C to 38 °C exhibit just
T / °C
22
20
18
16
14
14
T / °C
34
32
30
28
26
24
T / °C
56
44
42
40
38
36
at rest
Influence of Micro-Particles on the Lα Phase under Shear
70
shoulders corresponding to the perpendicular flow orientation which indicates
that only a small amount of the layer normals is parallel to the magnetic field, in
contrast to the sample without particles. It might take more than 30 min for
complete alignment or the state of perpendicular orientation does not exist in the
presence of particles.
-2 0 2
Δν / KHz
Δν / KHz
-2 0 2 -2 0 2
Δν / KHz
Figure 6.2 Deuteron NMR spectra obtained at different temperatures (30 min for each
temperature) for sample 20 with nw = 5.5 of the system lecithin/D2O/n-decane at mixed
with 5 % micro-particles of PMMA at a constant shear rate of 20 s-1. The first spectrum
(bottom left) was obtained at rest.
A comparison of the single broad spectra in Figures 6.1 and 6.2 shows that the
MLVs were formed between 14 °C to 24 °C for a sample with 5 % particles, while
T / °C
22
20
18
16
14
14
T / °C
34
32
30
28
26
24
T / °C
56
44
42
40
38
36
at rest
Influence of Micro-Particles on the Lα Phase under Shear
71
they exist between 14 °C to 18 °C for a sample without particles. This means that
the temperature range where MLVs exist is larger for a sample containing
particles, which indicates that the stability of MLVs (in comparison to planar
layers) is increased when particles are present.
As can be seen in Figure 6.2 the perpendicular orientation was not observed
during shear for a sample with 5 % particles, however, it appears clearly for a
sample without particles at 34 °C. One may conclude from these results that not
only a change in temperature but also adding micro-particles to an Lα system,
changes the orientation state of the layers during shear.
At last, the temperature of the transition to the isotropic phase under shear for
the two samples of Figures 6.1 and 6.2 is different. For the sample without
particles it is around 40 °C while it is around 44 °C for the sample with 5 %
particles. If we consider that the composition of the lamellar system, the duration
of shear and the temperature applied for the two samples are similar, it is
possible that additional micro-particles increase the stability of MLVs under
shear.
6.2.2 Influence of Micro-Particles on MLV Formation Process and Vesicle Size
In this section, we will present the results from experiments, in which the kinetics
of MLV formation is investigated by using 2H rheo-NMR technique and rheology
measurement. The following of these results the size dependence of MLV as a
function of time at various shear rates will be presented.
6.2.2.1 The Results of 2H NMR
Deuteron NMR spectra of the lamellar phase of lecithin/water/n-decane during
shear are shown in figure 6.3 for two samples, one without particles and the
other one with 5 % particles. The spectra were recorded at 16 °C at a constant
shear rate
γ
& = 1 s-1. The number at each pair of spectra gives the shear strain
γ
=
γ
&t. The spectra at the bottom of each column were obtained at zero shear rate
after aligning the samples in the magnetic field by slow cooling through the
biphasic regime of the isotropic/lamellar phase transition. The quadrupole
Influence of Micro-Particles on the Lα Phase under Shear
72
splittings of the doublet spectra are 1191 Hz, and 1075 Hz for samples without
particles and with 5 % particles, respectively. As can be seen from Figure 6.3,
the transition from the aligned lamellar phase (characterized by the quadrupole
splitting) to MLVs (broad single peak) started with a clear decrease of the
quadrupole splitting until it vanishes at
γ
= 12540 and
γ
= 2640 for samples
without and with particles, respectively. The line widths (full width of half height)
for the samples as mentioned are 1198 Hz and 1104 Hz.
Figure 6.4 shows the quadrupole splitting, Δ
υ
, of the NMR spectra of the
samples without particles and with 5 % particles as a function of time under a
constant shear rate of 1 s-1. In this plot, the transition regions from lamellar to
MLVs are characterized by a rapid decrease of Δ
υ
for the sample with 5 %
particles. However, it decreases slowly for a sample without particles. In this
diagram, tc is defined as the time at which formation of vesicles has proceeded to
the point where the splitting cannot be recognized any more. A comparison of the
two curves in figure 6.4 shows that the formation of MLVs for a sample with 5 %
particles is faster compared to the sample without particles.
The NMR spectra were recorded also at shear rates of 2, 5, 10, 20, 50 s-1 at
constant temperature (16°C). The results of these experiments are in agreement
with the results from the experiments at a shear rate of 1 s-1 at 16 °C. Figure 6.5
depicts the critical times where the splitting has vanished as a function of the
shear rate for samples without and with 5 % particles. A comparison of the two
curves in figure 6.5 shows that the addition of the particles is found to accelerate
the formation of multi-lamellar vesicles compared to the system without particles.
Influence of Micro-Particles on the Lα Phase under Shear
73
Figure 6.3 Deuteron NMR spectra obtained at different deformations for samples 9 (left)
and 10 (right) of the system lecithin/D2O/n-decane at a constant shear rate of 1 s-1 and T =
16 °C.
36960
18480
12540
3960
2640
1980
1320
660
300
0
Δν = 1198 ΗΖ
Δν = 1104 ΗΖ
.
Deformation
γ = γ t
With 5 % particles of PMMA Without particles
-5 0 5 -5 0 5
ν / KHz ν / KHz
Influence of Micro-Particles on the Lα Phase under Shear
74
0 10000 20000 30000 40000 50000 60000
0
200
400
600
800
1000
1200
tC = 12540 s
tC = 2640 s
Δν
/ Hz
t / s
Sample With 5% Particles
Sample Without Particles
Figure 6.4 Quadrupole splitting as a function of time for the spectra shown in figure 6.3.
tc is the critical time when the splitting can no longer be determined.
Figure 6.5 Critical time tc as a function of shear rate for samples 9 and 10 of the system
lecithin/D2O/n-decane at T = 16 °C.
0 1020304050
0
50
100
150
200
250
tc / min
sample without particles
sample with 5 % particles
γ
&/ s-1
Influence of Micro-Particles on the Lα Phase under Shear
75
In Figures 6.6 and 6.7 the different line shapes of NMR spectra at various shear
rates (1, 2, 5, 10 s-1) for samples without particles and with 5 % particles are
presented. The numbers at each pair of spectra give the shear strain
(deformation). At rest the samples are aligned with the director perpendicular to
the magnetic field, resulting in the doublet spectra seen in the first spectra
(bottom). Upon applying shear, the director orientation changes and the NMR
line shapes change to the single broad peaks characteristic of MLVs. Only the
spectra obtained at large strain are shown in Figures 6.6 and 6.7. A close
inspection of the spectra in Figures 6.6 and 6.7 indicates that not only the shear
rate but also the presence of micro-particles can change the size of the multi-
lamellar vesicles. This is seen even better in Figures 6.8 to 6.11, in which the line
widths of the spectra in Figures 6.6 and 6.7 are shown as a function of time and
strain.
Figure 6.8 shows the full line width at half height, LW, of the NMR spectra of the
sample without particles as a function of time at different shear rates. In this plot,
the region I is characterized by a rapid decrease of LW. The beginning of region II
can be identified as the point where the slope of the curves becomes zero. Within
region I, LW decreases, until in region II a constant plateau is reached, indicating
that the MLVs have reached their steady-state size. The decrease of the line
width to lower values at higher shear rates reflects the dependence of the
steady-state MLV size on the shear rate, well known from other lamellar systems.
A comparison of the curves in figures 6.8 demonstrates not only that the MLVs
are smaller at higher shear rates but also that the transition to the steady-state
with MLVs of constant size occurs faster at high shear rates.
Influence of Micro-Particles on the Lα Phase under Shear
76
-
505
Δν / KHz
-5 0 5
Δν / KHz
-5 0 5 -5 0 5
Without Particles 5 % particles Without Particles 5 % particles
(Shear rate 1 s-1)(Shear rate 2 s-1)
Δν / KHz
Δν / KHz
Figure 6.6 Deuteron NMR spectra obtained at different deformations for samples 9 and
10 of the system lecithin/D2O/n-decane at shear rates of 1and 2 s-1 and T = 16 °C. Spectra
at rest and at large stain are shown.
33000
30000
24000
18000
13200
0
60000
48000
36000
24000
12000
0
γ γ
Influence of Micro-Particles on the Lα Phase under Shear
77
-5 0 5
Δν / KHz
-5 0 5
5 % particles
Without Particles
5 % particles
Without Particles
(Shear rate 10 s-1)
(Shear rate 5 s-1)
Δν / KHz
Δν / KHz
-5 0 5 -5 0 5
Δν / KHz
Figure 6.7 Deuteron NMR spectra obtained at different deformations for samples 9 and
10 of the system lecithin/D2O/n-decane at shear rates of 5 and 10 s-1 and T =16 °C.
240000
180000
120000
60000
33000
0
γ
165000
150000
90000
60000
30000
0
γ
Influence of Micro-Particles on the Lα Phase under Shear
78
0 200 400 600 800
400
600
800
1000
1200
L W / Hz
t / min
Shear rate (s-1)
1
2
5
10
20
50
Figure 6.8 Full line width at half height of the NMR spectra of sample 9 of the system
lecithin/D2O/n-decane as a function of the time at T = 16 °C.
0 200 400 600 800
400
600
800
1000
1200
L W / Hz
t / min
Shear rate (s-1)
1
2
5
10
20
50
Figure 6.9 Full line width at half height of the NMR spectra of sample 10 (with 5 %
particles) as function of the time at T = 16 °C.
I
II
Influence of Micro-Particles on the Lα Phase under Shear
79
Figure 6.9 depicts the line width LW of the NMR spectra of the sample with 5 %
particles as a function of time at different shear rates. As in Figure 6.8 the
transition to the steady-state of MLVs is identified by a sharp decrease of LW. A
comparison with Figure 6.8 shows that the plateau values reached are lower in
the presence of particles, indicating that the size of the MLVs is smaller.
In Figures 6.10 and 6.11 the same data as in Figures 6.8 and 6.9 are plotted but
as a function of strain instead of time. Although there is no perfect scaling with
the strain it becomes obvious that strain and not time is the important parameter
for the MLV formation. The comparison of the curves in Figures 6.8 and 6.9 or in
Figures 6.10 and 6.11 shows that in the presence of particles the size of onions
is smaller compared to the system without particles under the same conditions.
There are energetic reasons for this: the stability of multi-lamellar vesicles can be
described as an equilibrium between large strain energies (drag force) at the
interstices between onions which are strongly deformed and large curvature
energies (thermodynamic force) of bilayers in the core. If we consider that the
presence of a particle in the onion core reduces the elastic part of the stress,
equilibrium with the drag force can be reached at a smaller onion size. However,
this is rather speculative.35,73
Influence of Micro-Particles on the Lα Phase under Shear
80
0 2000 4000 6000 8000 10000 12000
300
400
500
600
700
800
900
1000
1100
1200
1300
Lw / Hz
γ
Shear rate (s-1)
1
2
5
10
20
50
Figure 6.10 Full line width at half height of the NMR spectra of sample 9 of the system
lecithin/D2O/n-decane as a function of the deformation at T = 16 °C.
0 2000 4000 6000 8000 10000 12000
300
400
500
600
700
800
900
1000
1100
1200
1300
LW / Hz
γ
Shear rate (s-1)
1
2
5
10
20
50
Figure 6.11 Full line width at half height of the NMR spectra of sample 10 (with 5 % wt
micro-particles of PMMA) of the system lecithin/D2O/n-decane as a function of the
deformation at T = 16 °C.
Influence of Micro-Particles on the Lα Phase under Shear
81
6.2.2.2 The Results of Rheology
The kinetics of onion formation from the lamellar phase and the influence of
micro-particles on the formation of MLVs as shear proceeds can be investigated
by using rheological techniques. In this section flow experiments (transient
viscosity η(t) under constant shear rate), and viscoelastic properties (storage and
loss moduli, G' and G") of the lyotropic lamellar phase composed of
lecithin/D2O/n-decane for a sample without particles and a sample with PMMA
micro-particles will be presented. Detailed descriptions of the equipment and the
sample preparation used for these experiments are given in chapter 3.
In Figure 6.12 the steady-state viscosity as a function of shear strain at constant
shear rate of 5 s-1 and constant temperature of 16 °C is shown. After pre-
shearing the sample for 10 min at 5 s-1 and 35 °C for alignment of the lamellar
phase, the sample was cooled to 16 °C and a constant shear rate of 5 s-1 was
applied for 1 hour. During the pre-shear (not shown) the viscosity for the three
samples 14, 15 and 17 (see Table 3.3) decreases indicating the alignment of the
lamellar phase. Figure 6.12 shows the viscosity as a function of strain for
samples 14(A), 15(B) and 17(C) which indicates that the orientation or the
structure of the Lα phase changes. In the beginning, the viscosities of the
samples are ηC> ηB >ηA which indicates that by adding micro-particles of PMMA
to the lamellar phase the system becomes more viscous. The constant plateau
is reached at 1250 s for sample (C), while for sample (B) and sample (A) a
plateau or maximum indicating the formation of MLVs is reached at later times.
The decrease of the curves in the end region of the diagram for samples (C) and
(B) indicates that the size of MLVs becomes smaller.
A comparison of the curves for the three samples in Figure 6.12 indicates not
only that adding 5 % micro-particles of PMMA to a lamellar system accelerates
the formation of MLVs compared with samples containing only 1 % or no
particles but also that the size of MLVs become smaller resulting in lower
viscosity, if the particles are incorporated into the core of the onions.
Influence of Micro-Particles on the Lα Phase under Shear
82
0 5000 10000 15000 20000
1
10
1
η
/ Pa.s
γ
C 5 % Particles
B 1 % Particles
A without Particles
Figure 6.12 Viscosity versus deformation for samples 17 (A), 15 (B) and 14 (C) of the
system lecithin/D2O/n-decane at T = 16 °C under constant shear rate
γ
& = 5 s-1.
Figure 6.13 shows the viscosity of samples with lecithin/water/n-decane filled
with 5 %, 1 % and without micro-particles as a function of strain at constant shear
rate of 1 s-1 and constant temperature of 16 °C. In this plot, the viscosity for
sample 14 (C) is higher than for the other samples. Upon application of shear to
an aligned lamellar phase, a steep increase of the viscosity is observed which is
evidence of the reorganization of the lamellar phase into MLVs. A gradual
increase of viscosity at about 1800 s might indicate the formation MLVs. With
continued shear a constant plateau can be reached which is not shown in the
diagrams.
A comparison of the curves for the three samples in figure 6.13 shows that the
viscosity of the sample with 5 % particles is higher than for the other samples,
and also the formation of MLVs for samples with particles is faster than for the
sample without particles. Those results, at low shear rate (1 s-1) are in agreement
Influence of Micro-Particles on the Lα Phase under Shear
83
with results at high shear rate (5 s-1).
0 2000 4000 6000 8000 10000 12000
1
10
100
1000
η
/ Pa.s
γ
A without particles
B 1 % particles
C 5 % particles
Figure 6.13 viscosity versus deformation for samples 17 (A), 15 (B) and 14 (C) of the
system lecithin/D2O/n-decane at T = 16 °C under constant shear rate
γ
& = 1 s-1.
Dynamic or oscillatory tests were also performed to study the influence of
particles on the viscoelastic properties of the lamellar phase and multi-lamellar
vesicles formed by lecithin/D2O/n-decane for two samples, one without particles
and another one with particles of PMMA. Three experiments are performed at
different temperatures of 16, 25, 35 °C. First a pre-shear treatment was applied
at 35°C during 10 minutes (lamellar phase orientation). Then the temperature
was decreased and MLVs were formed at 16 °C, where a constant shear rate of
5 s-1 was applied. Before any dynamic test, a stress sweep test was done at the
desired temperatures in order to check the linear viscoelastic regime (both for the
oriented lamellar phase and MLVs). Finally, the frequency sweep test was done
in a frequency range between 10 and 0.1 Hz.
Figure 6.14 shows the typical frequency scans (mechanical spectrum) for the
oriented lamellar phase at different temperatures for sample 17. The values of G'
and G" at various temperatures show different behaviour when the frequency
Influence of Micro-Particles on the Lα Phase under Shear
84
increases. At all temperatures G' is larger than G" showing the elastic character
of the sample. Both G' and G" decrease with increasing temperature. At 16 °C
(temperature were the MLVs are stable) a considerable difference between G'
and G" is observed. MLVs present a dramatic elastic behaviour (solid material).
When the frequency is increased, both storage and loss moduli increase for all
temperatures as shown in the diagram.
-10123456789101112
100
200
300
400
500
600
G' & G''/ Pa
ν / Hz
Temperature oC
G' - 16
G'' - 16
G' - 25
G''- 25
G' - 35
G'' - 35
Figure 6.14 Storage, G', and loss, G", moduli as a function of frequency for sample 17
(without particles) at 16, 25, 35 °C and constant shear stress amplitude of 1 Pa.
At low temperature (16°C), MLVs are stable and due to their volume fraction,
they occupy all the space. We do not have data to confirm this, but the spherical
shape is probably deformed to a more polyhedral array to fill all the available
space as observed in related systems. The MLVs space disposition increases
the magnitude of the storage modulus. At higher temperatures (25 and 35 °C),
MLV are no longer stable and the zero curvature structure, planar lamellae in
coexistence with a nematic phase, appears. As we saw, the elastic modulus is
dramatically lower at 35 °C compared to the MLV solid system.
Influence of Micro-Particles on the Lα Phase under Shear
85
024681012
0
200
400
600
800
1000
1200
G' & G''/ Pa
ν/ Hz
Temperature o C
G' - 16
G'' - 16
G' - 25
G'' - 25
G' - 35
G'' - 35
Figure 6.15 Storage, G', and loss, G", moduli as a function of frequency for sample 14
(with 5 % particles) at 16, 25, 35 °C and constant shear stress of 1 Pa.
The influence of particles on the viscoelastic properties of the system was also
studied. The G' and G" were determined as a function of various temperatures
for a sample of lamellar phase formed by lecithin/D2O/n-decane filled with 5 %
micro-particles of PMMA. Note that the viscoelastic properties have a strong
dependence on the orientation states of the bilayers in the lamellar phase mixed
with spherical micro-particles. The results are shown in figure 6.15. They clearly
show that at 16 °C where MLVs or stuffed onions are formed the value of G' is
higher than G" in the whole frequency range. Moreover, G' is almost three times
higher in the doped system. Similar results can be observed at 25 °C and 35 °C.
The doped planar lamellae or coexistence of the lamellar phase with nematic one
present a higher elastic behaviour when compared to the system without
particles. The latter can be explained by an increase of structural defects in the
lamellae where different layers can be connected giving rise to the observed
elastic behaviour. In the former case (MLVs), the increase of elasticity is related
Influence of Micro-Particles on the Lα Phase under Shear
86
to the increase of the number density of MLVs caused by the particles (higher
polydispersity).
Table
6.1 summarizes the G' and G" values at a constant frequency of 10.3 Hz for the
lecithin/water/n-decane system with and without particles. At 16 °C MLVs are
stable and, on the other hand, at 25 and 35 °C planar lamellae or coexisting
phases (lamellar and nematic) are stable.
Table 6.1 Values of the storage, G', and loss, G", moduli at various temperatures and
constant frequency of 10.3 Hz for two samples, one without particles and the other one
with 5 % micro-particles of PMMA.
With particles Without particles
T / °C G'/Pa G"/Pa G'/Pa G"/Pa
16
943
652
361
225
25 269 251 261 189
35 152 124 90.8 80.3
Influence of Micro-Particles on the Lα Phase under Shear
87
6.2.2.3 The Results of Polarizing Microscopy
The polarized microscopy experiments were performed in transmission mode in
the Linkam plate-plate shear cell for samples 4 and 8 of the system
lecithin/D2O/n-decane.
The lamellar phase develops a typical oily-streaks texture at rest. This texture
occurs only in lamellar phases. An example for sample 4 is shown in Figure 6.16
(a). This texture was obtained after loading the fresh sample in the plate-plate
shear cell with 500 µm gap, at 16 °C. The PMMA micro-particles aggregated at
the network nodes are clearly seen in Fig 6.16 (b).
An example of the multi-lamellar vesicle texture observed by polarizing light
microscopy in the system lecithin/D2O/n-decane with nw = 5.5 is shown in Figure
6.17. The sample was sheared at 5 s-1 for 20 min and diluted by D2O. The
structure of a mixture of lecithin/D2O/n-decane with PMMA micro-particles
observed under shear of 1 s-1 at 16 °C is shown in Figure 6.18. The fine
dispersion of the micro-particles can be recognized, but the portion of particles in
the onion core or around the onions can not be distinguished. Therefore, this
microscopic texture may represent a phase with stuffed onions or decorated
onions. A similar feature has been observed by Arrault et al.36 for the system
AOT/ brine mixing with polystyrene spheres.
Figure 6.16 Light microscope textures of the system lecithin/D2O/n-decance with nw =
5.5. The lamellar oily-streaks texture of a sample containing PMMA micro-particles,
observed between crossed polarizers (a) and parallel polarizers (b) at 16 °C. The particles
aggregated at the network nodes are shown by the circles. Bar = 100 μm.
a b
Influence of Micro-Particles on the Lα Phase under Shear
88
Figure 6.17 A vesicle texture is observed for system of lecithin/D2O/n-decance with nw =
5.5 taken in polarized light. The texture was observed after 20 min with a shear rate of 5
s-1.The sample was diluted by D2O. The arrows show the MLVs. Bar = 100 μm.
Figure 6.18 Optical micrograph of lecithin/D2O/n-decance with nw = 5.5 and 5 % wt of
PMMA. The sample was sheared at 1 s-1 for 1 hour at 16 °C. Bar = 100 μm.
Influence of Micro-Particles on the Lα Phase under Shear
89
6.2.3 Influence of Micro-Particles on Vice Versa Shear-Induced of MLVs
Formation
In this section the stability of shear-induced MLVs and the reversibility of the MLV
size changes under shear for a lamellar phase of lecithin/D2O/n-decane and for
the same system but including micro-particles will be presented.
Figure 6.19 shows the NMR spectra of D2O in the lamellar phase during stepwise
cycling of the shear rate. Spectra were recorded at shear rates of 1, 5, 10, 20,
50, 100, 50, 20, 10, 5 and 1 s-1, for a sample with nw = 5.5 at T = 16 °C. The
samples were sheared for 25 min at each shear rate. At rest the sample is
aligned with the director perpendicular to the magnetic field by slow cooling
through the biphasic regime of the isotropic/lamellar phase transition. Upon
increasing the shear rate (left column of Figure 19.6), a gradual transformation to
a new line shape is observed: The splitting decreases while, at the same time, a
powder-like spectrum develops, the features of which become less distinct. At
last, the splitting coalesces and a broad single peak appears. The single broad
peak first observed at
γ
& = 10 s-1 can be assigned to the state of close-packed
vesicles. Upon further increase of the shear rate, the total line width of the
spectra becomes smaller which indicates that the vesicles become smaller. The
right part of Figure 6.19 shows the spectra during the decrease of the shear rate
from 100 to 1 s-1. The total line width of the spectra at first decreases when the
shear rate is reduced to 50 s-1 but then it slightly increased by decreasing the
shear rate. After the spectrum at
γ
& = 1 s-1 had been recorded the shear was
stopped and the sample kept at T = 16 °C. After one day an NMR spectrum was
measured. This spectrum, which can be seen in the last row of Figure 6.19,
shows that a splitting reappears, indicating a coexistence with an aligned Lα
structure. In Figure 6.20 the line width of the spectra during the cycle of shear
rates is shown. The line width decreased by increasing the shear rate while it is
only very slightly increased by decreasing the shear rate as shown in Figure
6.20. It is possible, however, that the MLVs will return to their initial size when
shear is applied for long times.
Influence of Micro-Particles on the Lα Phase under Shear
90
-2 0 2
Δν / KHz
-2 0 2
Δν / KHz
Figure 6.19 Deuteron NMR spectra obtained at different shear rates (25 min for each
shear rate) for sample 19 with nw = 5.5 of the system lecithin/D2O/n-decaneat at T = 16
°C.
γ
&
/ s
-1
100
50
20
10
5
1
relaxed state ≈ 24h
after shear stop
100
50
20
10
5
1
0
γ
&/ s-1
Influence of Micro-Particles on the Lα Phase under Shear
91
0 20406080100
300
350
400
450
500
550
600
650
Lw / Hz
Shear Up (Sample without Particles)
Shear Down (Sample without Particles)
Figure 6.20 The full line width, Lw, of the NMR spectra shown in figure 6.19 as function
of vice versa shear rate at a constant T = 16 °C.
The experiments were repeated for a sample with lecithin/D2O/n-decane mixed
with 5 % PMMA micro-particles under the same conditions. We thus address the
question whether the spherical micro-particles have an effect on the size and
stability of MLVs during stepwise cycling of the shear rate. As can be seen in
Figure 6.21 the formation of MLVs now occurs already at
γ
& = 5 s-1. The line
width of the spectra is shown in Figure 6.22. With increasing shear rate up to 100
s-1 the line width of the spectra becomes smaller, in agreements with other
systems. Interestingly, with decreasing shear rate from 100 to 1 s-1 the line width
of the spectra becomes broader which indicates an increase in the size of the
MLVs. The NMR measurement recorded one day after shear was stopped at T =
16 °C, shows a similar result as for the sample without particles. As can be seen
for the last spectrum of Figure 6.21 the NMR line shape becomes broader after
one day which shows that the size of MLVs has changed.
γ
&/ s-1
Influence of Micro-Particles on the Lα Phase under Shear
92
-2 0 2
Δν / KHz
-2 0 2
Δν / KHZ
Figure 6.21 Deuteron NMR spectra obtained at different shear rates (25 min for each
shear rate) for sample (20) with nw = 5.5 of the system lecithin/D2O/n-decane at mixed
with 5 % PMMA micro-particles at a constant T = 16 °C.
100
50
20
10
5
1
0
100
50
20
10
5
1
relaxed state ≈ 24 h
after shear stop
γ
&/ s-1
γ
&/ s-1
Influence of Micro-Particles on the Lα Phase under Shear
93
0 20406080100
300
350
400
450
500
550
600
650
Lw / Hz
Shear Up (Sample with 5 % Particles)
Shear Down (Sample with 5 % Particles)
Figure 6.22 The full line width, Lw, of the NMR spectra shown in figure 6.21 as function
of vice versa shear rate at a constant T = 16 °C.
γ
&/ s-1
Conclusions and Outlook
94
7 Conclusions and Outlook
7.1 Conclusions
The present work consists of a study of the lamellar phase in the pseudo-ternary
system formed by lecithin/D2O/n-decane. The investigation of the phase
behaviour as a function of the temperature in a narrow range of compositions
and of the influence of spherical micro-particles on the shear-induced orientation
states of the lamellar phase, in particular, on the formation of multi-lamellar
vesicles were the main research topics. The major conclusions of this thesis are
summarized below.
We have investigated a previously unidentified phase in the organogel system
lecithin/D2O/n-decane by using different techniques including polarizing
microscopy, 2H NMR, SAXS, and rheological measurements on samples
containing 50 w/w % lecithin and 5.5 mole water per mole lecithin. Polarizing
microscopy shows different optical textures as a function of temperature. The
typical oily-streaks of the lamellar phase are found at 16 °C. At temperatures
above 25 °C the textures are consistent with a coexistence of a lamellar phase
with a discotic nematic phase. An isotropic phase is observed above 50 °C. The
existence of these phases is confirmed by 2H NMR spectroscopy. A second
doublet in the NMR spectra appears as a shoulder at 24 °C and its peaks grow
on increasing temperature. This additional doublet indicates the coexistence of
the lamellar phase with another liquid crystalline phase. To obtain information on
the rotational viscosity of the LC phases rotation experiments were performed by
2H NMR. The relaxation time required for the director realignment by the
magnetic field, following a 90° rotation of the sample, by which a non-aligned
state is generated was measured at two different temperatures. At 45 °C
realignment occurs fast due to a low viscosity, but at 30 °C orientational
relaxation is slow. This can be explained by a large amount of lamellar phase at
low temperature and a large amount of nematic phase at high temperature. The
Conclusions and Outlook
95
coexistence of two phases is confirmed by the presence of two peaks in the
SAXS pattern. The additional broad peak that grows as temperature is increased
occurs at lower scattering angle (q vector) than the narrow peak of the Lα phase.
Finally, the rheology results show the existence of three different regions: (i) a
region of coexistence of a large amount of lamellar phase and a small amount of
nematic phase from 25 to around 37 °C, where the storage modulus (G') is larger
than the loss modulus (G") and the system becomes semi-solid (highly elastic)
consequently, (ii) a region from around 37 to around 50 °C, where G" is larger
than G' and the system becomes semi-liquid (low viscous), which means that the
amount of nematic phase has increased, and (iii) the existence of the isotropic
phase above 50 °C, where G" >> G'. The second anisotropic phase, coexisting
with the lamellar phase thus has been confirmed by different experimental
techniques. It was expected to be a discotic nematic phase and not a calamitic
nematic one because of its proximity to the Lα phase. This was proven by the 2H
NMR line shapes following a 90° rotation of aligned samples.
Several types of particles, based on SiO2, PMMA, and M-F, were synthesized
and characterized using AFM, polarizing microscopy, light scattering and 13C
NMR techniques. We have successfully synthesized monodisperse spherical
particles of SiO2 with diameters of 380 to 410 nm by the base-catalyzed
condensation of monomers (Si(OH)4) with ammonia.
Monodisperse colloidal particles of PMMA with diameters of 260 to 850 nm were
obtained by the reaction of MMA, a crosslinker and an epoxy-functionalized
comonomer. The size of the particles was controlled by observing the
interference colour of samples taken from the reaction vessel and dried on a
glass plate. The investigation of the reaction with amino-substituted compounds
confirms that functional components can be covalently attached to the surface of
particles.
Larger micro-particles of melamine formaldehyde were realized with diameters of
1 to 1.8 μm as determined by AFM. The size of the particles depends on the kind
of solvent, initiator, temperature and reaction time of the process. Flocculation
was observed for the M-F particles; however, the micro-particles can be
Conclusions and Outlook
96
resuspended in apolar liquids such as n-dodecane by mild sonication and the
suspension is stable for 24 h.
The influence of spherical particles on the shear-induced orientation states of the
lecithin organogel system was studied using the rheo-2H NMR technique.
Different orientation states of the lamellar phase were observed when the
temperature was increased from 14 to 56 °C at a constant shear rate of 20 s-1.
Without particles the following sequence of orientation states occurs with
increasing temperature: MLVs, parallel and perpendicular orientations. When
micro-particles were added the sequence of orientation states is roughly the
same. However, the perpendicular orientation was not detected in the particle-
doped system. An additional and important observation is the fact that the
presence of particles indeed stabilizes the temperature range of multi-lamellar
vesicles. It is increased from ΔT = 2 °C (without particles) to ΔT = 10 °C (with
particles).
Concerning the shear-induced MLVs we conclude that the presence of particles
increases the rate of onion formation. We think that the local order of the
lamellae is greatly disrupted by adding large external objects such as micron-
sized particles. The flow of this defected lamellar system is irregular and tumbling
processes may occur, speeding up the onions formation. This conclusion is
supported mainly by the rheo-2H NMR data where the broad singlet spectrum
(characteristic fingerprint of these MLVs systems) was observed earlier when
particles are present, comparing with the case without particles. Moreover, the
rheological flow curves also show that the steady state is reached faster (less
strain units needed) when the system is doped with particles. Concerning the
dimensions of the shear-induced MLVs it seems that the presence of particles
decreases the size of onions. This conclusion is supported by a clear decrease of
the line width of the MLV broad singlet spectrum and also from the observation of
the rheological mechanical spectra where the elastic modulus drastically
increases when particles are added. The elastic modulus, G', can be related to
the number density of onions and so an increase of this parameter reflects an
increase of the number of onions and consequently a decrease of their size.
Conclusions and Outlook
97
7.2 Outlook
The phase diagram of the system lecithin/D2O/n-decane, especially in the region
of the lamellar and its neighbouring phase, is still incomplete. Therefore, the
elucidation of the phase transition temperatures of this system for various
samples with different amount of oil and different mole ratio of water per lecithin
will be interesting to determine the boundary of the lamellar phase with the
discotic nematic one and to identify possible other phase transitions in this
region.
Using particles of different size, e.g., nano-particles or other kinds of micro-
particles, also at high volume fraction, mixed into different types of lyotropic liquid
crystalline systems such as non-ionic surfactant solutions will be a large research
field to address the question of how these particles have an effect on the size,
stability and formation of MLVs under shear.
Furthermore, the influence of the concentrations of the different components,
lecithin, water and oil, on the shear-induced orientation states of the lamellar
phase, in particular, on the formation of multi-lamellar vesicles, which has been
investigated only partly, is an interesting research topic worthwhile to be
completed in the future.
Appendix
98
Appendix
A.1 The System of AOT/brine
Figure A.1 Optical micrograph of a contact preparation of AOT (centre) and brine (15 g/l
of NaCl) observed between crossed polarizers. The different textures correspond to
myelin (a), lamellar phase (b), cubic phase (c), hexagonal phase (d). Bar = 10 μm.
Δν/Hz
Figure A.2 2H NMR spectra for a sample with 25 w/w % AOT in brine (15 g/l of NaCl).
For the lamellar phase (Lα) a quadrupole splitting ∆ν is observed. At 22 to 26 °C the
lamellar phase and the isotropic one coexist. The single peak at 15 °C shows the pure
isotropic phase.
a
b
c
d
15 °C 22 °C 24 °C
26 °C 28 °C 30 °C
Appendix
99
Δν/Hz
Figure A.3 2H NMR spectra obtained at different shear rates (15 min for each shear rate),
given to the right of each spectrum in s-1 for a sample with 25 w/w % AOT in brine (15
g/l of NaCl) at 30 °C.
3 5
7 10
20
0
Appendix
100
A.2 Characterization of PMMA Particles
Table A.1 The different sizes of PMMA particles obtained by AFM measurements.
* Surface was modified with dodecylamine
Sample
Size of epoxy-
functionalized
PMMA (nm)
*Size o
f
modi
f
ied
PMMA (nm) Comment-Remark
PMMA1 400−600 400−850
PMMA2 320−340 500−1400
PMMA3 270−300 400−800 The surface functionalized
groups were analyzed by FT-IR
PMMA4 290−320 260−410
Synthesis of PMMA without
Cross-linker, the surface
functionalized groups were
analyzed by FT-IR
PMMA5 aggregation aggregation
Synthesis of PMMA without
functionalized comonomer
PMMA6 270−350 370−800
The surface functionalized groups
were analyzed by FT-IR and
NMR-13C/MAS
PMMA7 - -
AFM was detected, PMMA was
modified at different conditions,
the surface functionlazed groups
Were analyzed by NMR-13C/MAS
Appendix
101
0.00E+000 2.00E+014 4.00E+014 6.00E+014 8.00E+014 1.00E+015
9
10
11
12
13
14
15
ln(ΔR/Kc)
q2
0.00E+000 2.00E+014 4.00E+014 6.00E+014 8.00E+014 1.00E+015
8
9
10
11
12
13
14
15
ln(ΔR/Kc)
q2
Figure A.4 Scattering curves as a function of scattering vector q for samples of epoxy-
functionalized PMMA particles before (left) and after (right) surface modification with
dodecylamin.
Figure A.5 Miscibility of spherical PMMA particles of a few hundred nanometres
diameter in the lamellar phase of the system AOT/brine. Left: image obtained by
confocal microscopy of the particles labelled with Rhodamine 6G (360 nm) in lamellar
phase of 25 w/w % AOT in brine (15 g/l of NaCl) at 25 °C. Right: image obtained by
polarizing microscopy of the particles labelled with Coumarin 120 (280 nm) in the
similar system at 28 °C. Bar = 100 μm. Aggregation is observed in both cases.
Rg = 168 nm
Rh = 220 nm
Rg/ Rh = 0.76
PDI ≈ 1.05
Rg = 240 nm
Rh = 340 nm
Rg/ Rh = 0.71
PDI ≈ 1.2
Appendix
102
A.3 The System of Lecithin/water/n-decane
Figure A.6 Quadrupole splitting of NMR spectra as function of temperature for samples
7, 5, 6 and 4 (see Table 3.3) with 1 %, 2 %, 4 % and 5 % particles of PMMA (2 μm)
respectively, and sample 29 without particles of the system lecithine/D2O/n-decane.
Figure A.7 Full line width at half height of the NMR spectra of samples 7, 5, 6 and 4 (see
Table 3.3) with 1 %, 2 %, 4 % and 5 % particles of PMMA (2 μm) respectively, and
sample 27 without particles of the system lecithine/D2O/n-decane as function of
temperature.
10 15 20 25 30 35 40 45 50 55
-100
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
Δν / Hz
Temperature / oC
% 1
% 2
% 4
% 5
without particles
Lα
Lα+ N
Iso
∆
υ
10 15 20 25 30 35 40 45 50 55
-20
0
20
40
60
80
100
120
140
Lw
Temperature / oC
% 1
% 2
% 4
% 5
without particles
Lα+ N
Lα
Iso
∆
υ
Appendix
103
A.4 Calibration of SAXS
0 200 400 600 800 1000 1200
0
2000
4000
6000
8000
10000
12000
119190
316
448
717
cm100
--
0 200 400 600 800 1000 1200
0
2000
4000
6000
8000
10000
12000
192
318
452
715
cm102
--
0 100 200 300 400 500 600 700 80
0
-1
0
1
2
3
4
5
n
Mean cm100 - cm102
Linear Fit
Y = -0.4381 + 0.00762 X
c0 = 57.5
cc = c - c0
65.76 k = 0.00762; k = 1.159 * 10-4
d = 665 / cc nm
Figure A.8 X-ray scattering of cholesterylmyristate (top) used for calibrating the Kratky
camera and calibration curve (peak order (n) as a function of channel number (Cn))
(bottom).
Cn Cn
Cn
Intensity Intensity
Abbreviations
104
Abbreviations
AFM Atomic force microscopy
AOT Aerosol-OT
0
a Area per head group
B0 Static magnetic field
CMC Critical Micelle Concentration
CPP Critical Packing Parameter
V
r
∇ Velocity gradient
Δν
Quadrupole splitting
δ
Quadrupolar coupling constant
δ
Phase angle
EFG Electric field gradient
eQ Quadrupole moment
EtOH Ethanol
FT Fourier transform
G' Elastic or storage modulus
G" Viscous or loss modulus
γ
Strain or deformation
γ
& Shear rate
H Mean curvature
h hour(s)
HD Hamiltonian of dipole-dipole interaction
Hσ Chemical-shift Hamiltonian
Hz Zeeman Hamiltonian
hex. Hexagonal phase
iso. Isotropic phase
κ
Bending modulus
Abbreviations
105
kB Boltzmann constant
l Length of the hydrocarbon chain
lam. Lamellar phase
Lα Lamellar phase
Lw Line width
Iz Operator for the total z component of I
M-F melamine-formaldehyde
MMA Methyl methacrylate
MLV Multi-lamellar vesicle
η Asymmetry parameter
η Viscosity
n
r Director
nc Number of carbon atoms
NC Nematic calamitic phase
ND Nematic discotic phase
Nex Nematic biaxial phase
nMe Number of methyl groups
NMR Nuclear magnetic resonance
Ns Surfactant number or Surfactant parameter
nw Mole ratio of water to lecithin
PDI Poly dispersity index
PMMA Poly methyl methacrylate
ω Frequency
ωo Larmor frequency
υ
Volume of the hydrocarbon chain
R1, R2 Principal radii of curvature
Ref. Reference
Rg Radius of gyration
Rh Hydrodynamic Radius
rpm Rotation per minute
SAXS Small angle X-ray scattering
Abbreviations
106
σ
Shear stress
0
σ
Yield stress
t time
tc critical time
TMMM trimethylol melamine
T Temperature
Tk Krafft Temperature
τ Time between two pulses
V
r Velocity
vx Velocity in the x direction
Z
r
Vorticity or neutral axis
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