Rheology o he lamella liquid-c ys alline phase in
polye hoxyla ed alcohol/wa e /hep ane sys ems
By F. Co dobés1*, J.M. F anco2 and C. Gallegos2
1 Depa amen o de Ingenie ía Química. Facul ad de Química. Uni e sidad de Se illa.
c/P o . Ga cía González s/n. Se illa 41012, Spain. co [email protected]
2 Depa amen o de Ingenie ía Química. Facul ad de Ciencias Expe imen ales. Uni e sidad
de Huel a. Campus Uni e si a io del Ca men, 21071, Huel a. Spain.
[email p o ec ed], [email p o ec ed]
RESUMEN
Reología de la ase líquido-c is alina lamina en sis e-
mas alcohol polie oxilado/agua/hep ano.
En es e abajo se han es udiado las p opiedades eológicas
de una ase líquido-c is alina lamina con enida en un sis ema al-
cohol polie oxilado/agua/hep ano, median e ensayos iscoelás i-
cos lineales, es aciona ios y ansi o ios. El e ec o de dis in as
a iables como la composición de ensioac i o y hep ano sob e
dichas p opiedades eológicas ha sido analizado. El espec o me-
cánico ob enido de la ase lamina mues a en odos los casos
una egión «pla eau» en el in e alo de ecuencias es udiado así
como mayo es alo es de los pa áme os iscoelás icos pa a a-
lo es in e medios de la concen ación de ensioac i o y hep ano,
indica i o de un mayo desa ollo del en amado elás ico es uc u-
al de dicha ase líquido-c is alina. Los ensayos de lujo es aciona-
io y ansi o io pusie on de mani ies o la exis encia de es uc u as
inducidas po cizalla. En las cu as de iscosidad se ob u ie on
di e en es egiones de a iación po encial de la iscosidad en e
a la elocidad de cizalla y, po an o, dis in os alo es de índices
de lujo. Las modi icaciones de la mic oes uc u a de la ase líqui-
do-c is alina lamina causadas po la cizalla, ue on con i madas
median e el uso de mic oscopía de luz pola izada en un sis ema
de cizalla que inco po a un mic oscopio óp ico. Las modi icacio-
nes es uc u ales pa ecen es a muy in luenciadas po la eloci-
dad de cizalla y, gene almen e, la aplicación de ele adas
elocidades de cizalla p o oca el alineamien o de las bicapas de
ensioac i o y la apa ición de la clásica ex u a de «oily s eaks»
de la meso ase lamina . La p esencia de esículas se de ec a en
sis emas con al o con enido en hep ano, como indica la ex u a
de ec ada po mic oscopía de luz pola izada.
PALABRAS-CLAVE: C is al líquido - Meso ase lamina -
Reología - Viscoelas icidad - Viscosidad.
SUMMARY
Rheology o he lamella liquid-c ys alline phase in
polye hoxyla ed alcohol/wa e /hep ane sys ems.
Linea iscoelas ic es s as well as ansien and s eady low
expe imen s we e ca ied ou on lamella liquid c ys alline samples o
poly(oxye hylene) alcohol/wa e /hep ane sys ems. The e ec o
su ac an and hep ane concen a ions on he heological p ope ies o
he lamella mesophase was in es iga ed. The mechanical spec um
inside he linea iscoelas ic egime shows, in all cases, a
well-de eloped pla eau egion in he whole equency ange s udied.
The alues o he dynamic unc ions we e highe o in e media e
su ac an o hep ane concen a ions indica i e o a majo de elopmen
o he elas ic ne wo k in he mid ange o exis ence o he lamella
phase. T ansien and s eady low expe imen s poin ou a
shea -induced e olu ion o he lamella mic os uc u e. Di e en powe
law egions wi h di e en alues o he low index we e de ec ed in
he iscosi y e sus shea a e plo s. These shea -induced
s uc u al modi ica ions we e con i med by using pola izing
mic oscopy in an op ical shea ing cell. S uc u al modi ica ions
appea o be highly in luenced by shea a e. In gene al, applying
ela i ely high cons an shea a es, he alignmen o he bilaye s
ollowed by he appea ance o he «oily s eaks» s uc u e was
obse ed. Appea ance o shea -induced esicles occu s a high
hep ane con en , as indica es he ex u e o close-packed
monodispe se sphe uli es de ec ed by pola izing mic oscopy.
KEY-WORDS: Lamella mesophase - Liquid c ys al -
Rheology - Viscoelas ici y - Viscosi y.
1. INTRODUCTION
Aqueous and non-aqueous solu ions o nonionic
su ac an s (Tiddy, 1980; Robles-Vásquez, 1993;
Dimi o a, 1995; Co dobés, 1997) may o m di e en
lyo opic liquid-c ys alline phases as a esul o
in e ac ions among associa ion s uc u es o micella
agg ega es. These phases p esen s a ic and
dynamic p ope ies ela ed o he c ys alline
(long- ange posi ional and o ien a ional o de ,
elas ici y,...) and liquid s a es ( iscosi y,
concen a ion luc ua ions, e c.). Knowledge o he
s uc u e and dynamic p ope ies o lyo opic phases
is e y impo an om bo h undamen al and
p ac ical poin s o iew, especially conside ing hei
applica ions in he ields o de e gen s (Ba nes,
1980), cosme ics (Comelles, 1994), pha maceu icals
(Mülle -Goyman, 1987; Engs öm, 1990), emulsions
and colloidal sys ems (Pilpel, 1987; Riego, 1988;
F ibe g, 1990), e c.
The phase diag ams o polyoxye hylene nonionic
su ac an s exhibi a succession o di e en lyo opic
liquid-c ys alline phases (Mi chell, 1983; F anco,
1995; Mo os, 2001). The ype o liquid-c ys alline
s uc u e o med depends on he cu a u e adop ed
by he in e ace be ween he agg ega es and he
su ounding sol en . Lamella , hexagonal and cubic
phases a e usually shown in hese sys ems. The
lamella lyo opic phase, Lα, appea s a a he high
concen a ions when he in e ace ends o be plana ,
al hough su ac an wi h mo e hyd ophobic chains
a o s hei appea ance a low concen a ions. This
G asas y Acei es
96 Vol. 56. Fasc. 2 (2005), 96-105
phase consis o a simple one-dimensional c ys al
composed o al e na ing laye s o wa e and su ac an
bilaye s wi h polyc ys alline cha ac e is ics, i.e.
comp ised o small, mic on-sized domains wi h
in e ening g ain bounda ies (Kilpa ick, 1994). This
pa icula s uc u e con e s o his phase heological
p ope ies clea ly di e en om o he s. I is well
known ha he lamella liquid c ys alline phase
shows lowe alues o he heological unc ions han
o he s lyo opic phases de ec ed in nonionic
su ac an /wa e phase diag ams (Mo os, 2001;
Muñoz, 1991).
Se e al aspec s conce ning he s eady-s a e and
ansien low beha io s o lamella liquid-c ys alline
phase ha e been p e iously s udied by di e en
au ho s (F anco, 1995; Gallegos, 1987; Lockwood,
1987; Paasch, 1989; Roux, 1994). In hese s udies,
he impo ance o he appea ance o shea -induced
s uc u al de ec s in he heological beha io o
lamella mesophases has been poin ed ou . Fi s
in es iga ions abou he e ec o shea on he
s uc u e o a lamella phase was analyzed by Dia e
al. (Dia , 1993) in a qua e na y sys em o anionic
sodium dodecyl sul a e (SDS), pen anol, dodecane
and b ine. As shown, a mul idomain Lα phase, which
usually holds de ec s in he liquid-c ys alline
s uc u e (Laüge , 1996), is aligned by shea in o a
s a e o o ien a ed bilaye s pa allel o he wall o he
Coue e cell. A a h eshold alue o shea a e
cha ac e is ic o each sys em, he bilaye s b eak up
and a e ans o med o esicles, also called lamella
d ople s, sphe uli es, liposomes, o «onions» ( an de
Pas, 1993; an de Linden, 1996). Fo shea a es
highe han a c i ical alue, a wo-phase egion exis s
in which ex ended bylaye s a e in equilib ium wi h
esicles. A single esicle phase is eached by
inc easing shea a e, which can exhibi a high
deg ee o posi ional o de ing (Dia , 1995). Fo e en
highe shea a es he esicles a e inally
ans o med o a pe ec ly aligned lamella phase
ha is supposed o be ee o de ec s. Mo e
ecen ly, o he au ho s ha e in es iga ed he
p esence o mul ilamella esicles (MLVs) in
di e en su ac an sys ems (Weigel, 1996; Sie o,
1997). Escalan e e al. (Escalan e, 2000) ha e
obse ed ha he ansi ion om a non-pe u bed
lamella phase is i e e sible and, o a gi en
sys em, he cha ac e is ic pa ame e o he
ans o ma ion p ocess is he de o ma ion applied.
Pa al e al. (Pa al, 2001) ha e ound ha he
ansi ion o a esicle phase occu s a a c i ical s ess
which inc eases wi h su ac an concen a ion. On
he con a y, in some bina y sys ems wi h a ela i ely
high su ac an concen a ion, his ansi ion o a MLV
phase has no been obse ed (F anco, 1995;
Neme h, 1998; Laüge , 1995), p obably due o an
insu icien bending lexibili y o he memb anes
(Vinches, 1994).
On he o he hand, his o ically less a en ion has
been paid o he linea iscoelas ic beha io o
lamella liquid-c ys alline phases and only in he las
yea s he iscoelas ici y o lamella phases has been
ex ensi ely s udied. Kilpa ick e al. (Kilpa ick, 1994)
ha e s udied he linea iscoelas ic p ope ies o
di e en lyo opic phases p esen in he Cesium
n-Te adecanoa e/wa e sys em; Robles-Vásquez e
al. (Robles-Vásquez, 1993) ha e in es iga ed he
linea iscoelas ic esponse o aqueous sys ems
con aining a sodium sul osuccina e as su ac an ;
and Dimi o a e al. (Dimi o a, 1995) ha e p o ided
some linea iscoelas ic da a ela ed o phase
ansi ions among di e en liquid-c ys alline
s uc u es in he phase diag am o a polye hoxyla ed
alcohol con aining 7 mol o e hylene oxyde. Finally,
Jage -Leze e al. (Jage -Leze , 1998, 1999) ha e
s udied he ole o he chemical na u e o he su ace
in con ac wi h a lamella mesophase o unde s and
i s heological beha io . In his sense, p e ious
s udies (Co dobés, 1997) demons a ed ha mo e
elas ic esponses can be ob ained by adding a linea
chain hyd oca bon o a bina y hexagonal sys em.
Thus, de elopmen o a pla eau egion in he
equency dependence o he dynamic unc ions can
be ob ained as hep ane concen a ion inc eases. All
o hese s udies p o e he complex beha io o
lamella phases in he linea iscoelas ic egime. The
p esen esea ch is conce ned wi h he iscous and
linea iscoelas ic beha io o he lamella mesophase
in bo h bina y polye hoxyla ed alcohol/wa e sys ems
and e na y sys ems, solubilizing a linea chain
hyd oca bon. The phase diag am o polye hoxyla ed
alcohol/wa e /hep ane has been p e iously s udied
(Co dobés, 1997). This phase diag am shows a di ec
hexagonal liquid-c ys alline egion be ween 40 and
50% (w/w) su ac an and a lamella liquid-c ys alline
egion be ween 55 and 75% (w/w) solubilizing a
maximum amoun o hep ane o 30% (w/w). The
o e all objec i e o his wo k was o s udy he e ec
ha su ac an and hep ane concen a ions exe on
ansien and s eady lows and linea iscoelas ic
esponse. In addi ion o his, he e ec o shea on
he non-pe u bed lamella phase s uc u e was
analyzed.
2. EXPERIMENTAL
A poly(ehylene glycol) nonylphenyle he
(NPFE-10) om Sigma (S . Louis, U.S.A.) was used
as ecei ed. Lamella liquid-c ys alline sys ems we e
p epa ed by mixing his nonionic su ac an wi h
dis illed deionized wa e and n-hep ane o analy ical
g ade, ob ained om Pan eac (Ba celone, Spain).
Su ac an concen a ion anged om 60 o 75% (w/w)
o bina y sys ems and 50 o 75% (w/w) o e na y
sys ems con aining up o 30% (w/w) o hep ane
solubilized, inside o he zone o exis ence o he
Vol. 56. Fasc. 2 (2005) 97
lamella s uc u e in he phase diag am (Co dobés,
1997).
Th ee di e en ypes o heological
measu emen s we e pe o med in his wo k:
1) Oscilla o y es s we e ca ied ou in a con ol
s ess heome e , RS-100 om Haake (Ka ls-
uhe, Ge many) wi h cone and pla e geo-
me y (60 mm, 4o). A anspa en bell wi h
sa u a ed a mosphe e was placed a ound he
measu ing de ice in o de o p e en chan-
ges in composi ion due o hep ane o wa e
e apo a ion du ing measu emen . All he
samples had he same ecen pas his o y,
and we e le o 20 minu es in he senso
sys em be o e any measu emen . F equency
sweep es s we e ca ied ou a cons an
s ess inside he linea iscoelas ic egion in
a equency ange o 10-2-102 ad/s. P e-
iously, in o de o ob ain he linea iscoelas-
ic ange o he samples s udied, s ain
sweeps a a cons an equency o 1Hz we e
pe o med, and he alues o he c i ical
s ain,
γc, which delimi he linea iscoelas ic
egion, we e de e mined.
2) S eady-s a e low measu emen s we e pe o -
med in con olled-shea a e heome e s
RV100/CV100 o RV100/CV20N, depending
on he sample iscosi y, om Haake (Ka ls u-
he, Ge many), using a Mooney-Ewa geo-
me y (ME15). S eady-s a e low cu es we e
ob ained in a shea a e ange o 0.1-300 s-1.
3) T ansien low expe imen s we e ca ied ou
in he abo e men ioned con olled-shea a e
heome e s a di e en cons an shea a es
be ween 0.5 and 100 s-1.
All heological es s we e pe o med a 25oC and
2 days a e he sample was p epa ed. In p e ious
s udies, i was poin ed ou ha heological p ope ies
o samples was no in luenced by ime. A leas wo
eplica es o each es we e done.
Mic os uc u al obse a ions unde shea in an
op ical shea ing cell (Linkham Scien i ic Ins umen s,
U.K.) equipped wi h pola izing mic oscopy
comple ed he heological s udy o he lamella
phase. Samples we e collec ed in he shea ing cell
and hen we e submi ed o he same ansien shea
his o y inpu in he con olled-shea a e heome e s.
3. RESULTS AND DISCUSSION
a)
Linea iscoelas ici y
. As is well-known, he
applica ion o s ains lowe han he c i ical s ain
which delimi s he linea iscoelas ic egion, γc,
allows he s udy o he heological beha io o he
non-pe u bed liquid-c ys alline s uc u e, while
highe alues p oduce s uc u al modi ica ions. The
alues o γc depend o he linea iscoelas ic unc ion
selec ed o i s de e mina ion. In his wo k, he
s o age and loss moduli we e selec ed, and he
alues o γc we e calcula ed by he c osso e
be ween wo lines: G’ o G’’=cons an (a low shea
s esses) and G’ o G’’= (σn) (a highe shea
s esses). Figu e 1A shows he e olu ion o he
s o age and loss moduli wi h he applied s ain o
selec ed lamella sys ems (5% hep ane), om he
hexagonal liquid-c ys alline phase (55% NPFE-10)
up o he iso opic su ac an - ich phase (75%
NPFE-10) bounda ies. Figu e 1 B shows he linea
iscoelas ic ange o sys ems con aining 60%
NPFE-10 and di e en hep ane con en . As can be
obse ed in Tables I and II, highe alues o γc we e
ob ained om he s o age modulus excep o he
bina y sys em. When highe s ains we e applied he
e olu ion o bo h linea iscoelas ic unc ions was
clea ly di e en . The alues o G’ le down
con inuously when s ain inc eases, while he loss
modulus shows an ini ial g ow h ollowed by a u he
dec ease. Howe e , his ini ial inc ease is a unc ion
o he equency selec ed o pe o m he s ain
sweep. Thus, i has been e i ied ha he loss
modulus con inuously dec eases as he s ain
inc eases by applying a cons an equency o 10 Hz
(Co dobés, 2000. This ac has been obse ed in
o he s lamella phases s udied in he li e a u e
(Al a o, 2000).
As p e iously demons a ed (Robles-Vásquez,
1993; Robles-Vásquez, 1994) and shown in Figu es
1 and 2, lamella liquid-c ys alline phases a e
cha ac e ized by a small linea iscoelas ic ange.
The alues o he c i ical s ain, γc, which p oduces a
ce ain deg ee o des uc ion in he liquid c ys alline
s uc u e, did no p esen a signi ican dependence
wi h su ac an con en , o a cons an hep ane
concen a ion (Table I). On he o he hand, his
c i ical s ain shows minimum alues a in e media e
Figu e 1
S ain sweep es s o sys ems con aining 5% (w/w) hep ane
(A) and 60% (w/w) NPFE-10 (B).
98 G asas y Acei es
hep ane concen a ions o ixed su ac an con en s
(Table II). These minimum alues mo e o highe
hep ane con en as NPFE-10 concen a ion
dec eases. Mo eo e , liquid-c ys alline sys ems wi h
highe hep ane concen a ions show a mo e apid
dec ease o he iscoelas ic unc ions in he
non-linea egion, which can be a ibu ed o a highe
s uc u al b eakdown.
The linea iscoelas ic p ope ies o lamella
phases we e de e mined by means o equency
sweeps inside he linea iscoelas ic egion. As can
be obse ed in Figu es 2 and 3, he equency
dependence o G’ and G’’ was always simila . Thus,
almos cons an alues o G’ and a clea minimum in
G’’ can be de ec ed o mos sys ems. This
mechanical spec um co esponds o he «pla eau»
egion, which has been ela ed o he o ma ion o an
elas ic s uc u al ne wo k due o he in e ac ions
be ween liquid-c ys alline domains (Kilpa ick, 1994).
Simila esul s ha e been ound in he li e a u e o
sys ems wi h di e en ypes o su ac an s and
hyd oca bons (Al a o, 2000; Jiménez, 2000; Neme h,
1998). As can be obse ed, lamella phases a e
cha ac e ized by highe alues o s o age modulus in
a wide ange o equencies. Thus, only he sys em
con aining 50% (w/w) NPFE-10 and 5% (w/w)
Table I
Values o he c i ical s ain which delimi s he linea iscoelas ic egion o sys ems
con aining 5% (w/w) hep ane.
%(w/w) su ac an %(w/w) wa e γc in G’ γc in G’’
55 40 0.30±0.03 0.15±0.03
60 35 0.19±0.02 0.07±0.02
65 30 0.30±0.03 0.10±0.03
70 25 0.27±0.02 0.04±0.02
75 20 0.44±0.04 0.08±0.04
Table II
Values o he c i ical s ain which delimi s he linea iscoelas ic egion o sys ems
con aining 60% (w/w) su ac an
%(w/w) wa e %(w/w) hep ane γc in G’(%) γc in G’’(%)
40 0 0.29±0.03 0.42±0.03
35 5 0.19±0.02 0.07±0.02
30 10 0.14±0.03 0.07±0.03
25 15 0.30±0.04 0.23±0.04
20 20 0.35±0.03 0.09±0.03
15 25 1.11±0.05 0.20±0.05
Figu e 2
E olu ion o s o age and loss moduli as a unc ion o equency
o sys ems con aining 5% (w/w) hep ane.
Vol. 56. Fasc. 2 (2005) 99
hep ane, loca ed inside he hexagonal egion in he
phase diag am, showed a c osso e poin be ween
bo h iscoelas ic unc ions in he equency ange
s udied. The e o e, he dynamic oscilla o y p ope ies
o hexagonal phases a e cha ac e ized by he
appea ance o a e minal egion a low equencies
(Co dobés, 1997), which was no de ec ed in he
lamella sys ems.
As can be obse ed in Figu e 2, an inc ease in
su ac an concen a ion leads o an inc ease in he
alues o he linea iscoelas ic unc ions up o a
maximum alue (i.e. 70% NPFE-10 o sys ems
con aining 5% hep ane) and hen dec eases. This
e ec can be clea ly obse ed in Table III in which he
alues o he pla eau modulus, GoN, calcula ed as he
G’ alue o which he loss angen was minimum
(Co dobés, 1997), a e shown. These maximum
alues we e ound a lowe su ac an concen a ions
as hep ane con en inc eases, p obably due o he
p oximi y o he hexagonal phase bounda ies
(Co dobés, 2000). These esul s indica e a highe
deg ee o de elopmen o he liquid-c ys alline
s uc u e as a consequence o he solubiliza ion o
hep ane (Kilpa ick, 1994). These maximum alues o
GoN a in e media e su ac an con en coincide wi h
minimum alues o loss angen . On he o he hand,
he p oximi y o he phase ansi ion is cha ac e ized
by minimum alues o he pla eau modulus, which
dec ease by inc easing he amoun o hep ane
solubilized. This beha io can be ela ed o a change
in he ex u e o he lamella phase when i is
obse ed in he pola iza ion mic oscope. Thus,
Neme h e al. (Neme h, 1998) s udying a non-ionic
su ac an /wa e sys em, obse ed a ypical ex u e
o «Mal ese C osses» a su ac an concen a ions
p e ious o he maximum alues in he iscoelas ic
unc ions. A e wa ds, he oily s eaked- ype
mic os uc u e was obse ed. This change o ex u e
indica es a change in he o ien a ion o he domains
(Pla z, 1995), which could be ela ed wi h he
de ia ion ound in he end o he s o age modulus
(see Table III). These obse a ions a e in good
ag eemen wi h he indings o o he au ho s
(Dimi o a, 1995; Thad os, 1995).
Figu e 3 shows he e olu ion o he s o age and loss
moduli as a unc ion o equency o a ixed NPFE-10
con en and di e en hep ane concen a ions. As can
be obse ed, ini ially, he highe he solubiliza ion o
hep ane, he highe he alues o he iscoelas ic
unc ions. This occu s in a hep ane concen a ion o
up o a ound 15%. S a ing om his poin , a decline
occu s p obably due o he p oli e a ion o de ec s in
he liquid-c ys alline s uc u e in he p oximi y o he
phase ansi ion ha induces mino elas ic
p ope ies. Mo eo e , he bina y sys em shows a
highe endency o a c osso e poin be ween he
Figu e 3
E olu ion o s o age and loss moduli as a unc ion o equency
o sys ems con aining 60% (w/w) su ac an .
%(w/w) su ac an %(w/w) hep ane G
No
(Pa)
25 55
±
5
50 20 40
±
3.5
5300
±
10
10 550
±
10
15 520
±
15
20 380
±
25 120
±
5
55
30 110
±
4
088
±
5
5
1020
±
6
10 1280
±
20
15 950
±
15
20 550
±
10
60
25 185
±
5
0570
±
10
51350
±
20
10 1270
±
15
15 950
±
12
65
20 400
±
8
0670
±
9
5 1520
±
18
70
10 1350
±
15
0800
±
6
75 5 1200
±
12
Table III
Values o he pla eau modulus o lamella sys ems
100 G asas y Acei es

s o age and loss moduli a low equencies, lowe
de elopmen o he pla eau egion and highe alues
o he slope o G’ s. ω. The pla eau modulus eaches
a maximum alue o a concen a ion o 10% o
hep ane indica i e o a majo de elopmen in he
elas ic ne wo k. These maximum alues mo e o
lowe hep ane con en as NPFE-10 concen a ion
inc eases (Table III).
b)
Viscous low beha io
. The low beha io o
lamella liquid-c ys alline sys ems was de e mined in
a ange o shea a es comp ized be ween 0.1 and
300 s-1 (Figu es 5, 6 and 7). The low p ope ies a e
a he simila o all he sys ems s udied, al hough o
highe and in e media e alues o shea a es,
iscosi y cu es depend on composi ion. Up o 10 s-1,
he e olu ion o appa en iscosi y as a unc ion o
shea a e is cha ac e is ic o a shea hinning luid,
showing, in gene al, a powe -law dec ease in
iscosi y. Fo highe alues o shea a es, low
cu es a e g ea ly dependen on a composi ion
showing shea hinning, New onian o e en shea
hickening beha io s. Thus, di e en egions o
po en ial dependence o iscosi y can be obse ed
when shea a e is inc eased up o 300 s-1. The
expe imen al da a can be i ed o a powe law in
each egion, de ined as:
whe e n is he low index and k is he consis ency in-
dex de ined o each po en ial egion.
Two di e en egions o po en ial decline o
iscosi y we e ound o bina y sys ems (Figu e 4),
de ined by low indexes «n» and «m», espec i ely,
«n» o alues o shea a e lowe han a c i ical
shea a e, γ1, which delimi s he change in he powe
law endency, and «m» o alues comp ized
be ween
γ1 and he maximum shea a e achie ed.
Simila alues o «n» (a ound -0.6) we e ound o all
he bina y sys ems s udied, excep o he sys em
wi h he lowes con en in su ac an (60% NPFE-10),
which p esen s a alue o «n» equal o –0.77. This
sys em is close o he hexagonal phase ansi ion. In
he second egion o po en ial dec ease, he same
e ec was ound (see Table IV). This shea - hinning
beha io has been ela ed o he o ma ion o
o de ed bylaye s uc u es being aligned in he low
di ec ion (Laun, 1992; Mon al o, 1998). Mo eo e ,
Meye e al. (Meye , 2000) ound di e en low
egimes depending o shea a e o wo lyo opic
and he mo opic sys ems using a con ol s ess
heome e . These au ho s p opose ha he i s
egime can be associa ed o he p esence o «oily
s eaks» aligned in he low di ec ion and de ec ed by
pola izing mic oscopy as elonga ed objec s. The
second egime was cha ac e ized by he p esence o
a shea -induced onion s a e. In his case, he
iscosi y cu es can be ela ed o he exis ence o
bylaye o «oily-s eaks» s uc u es aligned in he
low di ec ion. The di e en alues o low indexes
ound can be explained on he basis o he esis ance
ha he bylaye s oppose o low. I mus be
emphasized ha alues o «m» a e always lowe
han «n» (Table IV), p obably due o he highe
shea -induced alignmen o he bylaye s in he low
di ec ion achie ed as shea a e inc eases.
Mo eo e , he ex ension o he i s egion o po en ial
decay o iscosi y, de ined by he alues o γ1, is
highe o he sys ems wi h he highes su ac an
con en . As a consequence, i can be deduced ha
sys ems wi h lowe su ac an concen a ion and
close o he ansi ion phase egion, a e mo e
sensi i e o he o ma ion o s uc u es induced by
shea low due o a highe amoun o de ec s in he
liquid-c ys alline s uc u e (F anco, 1995).
As p e iously men ioned, low p ope ies o
lamella liquid-c ys alline sys ems a e s ongly
dependen on shea a e and composi ion. Figu e 5
shows he iscous low cu es o selec ed sys ems
(1)
Figu e 4
S eady-s a e low cu es o bina y sys ems.
Figu e 5
S eady-s a e low cu es o sys ems con aining
60% (w/w) su ac an .
Vol. 56. Fasc. 2 (2005) 101
as a unc ion o hep ane con en . All he sys ems
e alua ed show he same low beha io abo e
men ioned. Howe e , he numbe o po en ial
dec ease iscosi y egions depends on he abili y o
he liquid-c ys alline phase o solubilize hep ane.
Thus, sys ems wi h in e media e concen a ions o
hyd oca bone (15 and 20% (w/w)) p esen only a
egion o po en ial dec ease. Bo h sys ems p esen
he highe ela i e elas ic p ope ies o his
su ac an concen a ion showing minimum alues o
he loss angen . The es o lamella liquid-c ys alline
sys ems show he wo egions o iscosi y dec ease.
A mo e complex beha io can be obse ed in
Figu e 6 o sys ems con aining 50 and 55% (w/w)
su ac an and maximum con en o hyd oca bone
solubilized. Th ee di e en egimes can be obse ed.
Fi s , a shea - hinning egion, a low shea a es,
whe e he sample beha es as a pseudoplas ic luid,
cha ac e ized by he o ma ion o o de ed bilaye s
s uc u es being aligned in he low di ec ion (Laun,
1992; Mon al o, 1998). In he second egion,
shea - hickening o New onian low egimes can be
obse ed, depending on he composi ion, when a
c i ical shea a e, γ1, is eached. This could be
explained on he basis ha , a his poin , some o he
bilaye s begin o close, esul ing in he o ma ion o
esicles. The ansi ion be ween bo h low egimes is
aken o be an indica ion o mic os uc u al changes.
The hi d egime, abo e a c i ical shea a e, γ2, is
cha ac e ized by ano he shea - hinning egion.
Escalan e e al. (Escalan e, 2000) ela ed his
shea - hinning beha io wi h he alignmen o new
mul ilamella esicles o med du ing shea . Howe e ,
Roux e al. (Roux, 1994) iden i ied he las egime
wi h a New onian beha io , ela ed o he up u e o
he esicles and he o al alignmen o he bilaye s.
In he second egime, o 55% NPFE-10, an
inc ease in iscosi y was obse ed in he sys em
con aining 25% (w/w) hep ane. This inc ease was no
ound o a highe amoun o hep ane (30%), in which
a New onian zone can be de ec ed. The same
inc ease in iscosi y can be obse ed o sys ems
con aining 30% hep ane bu only 55% NPFE-10 A
simila beha io has been ound in a qua e na y
sys em o he anionic sodium dodecylsul a e (SDS),
pen anol, dodecane, and b ine (Roux, 1994). These
au ho s obse ed ha a ansi ion o a lamella o a
esicle phase could be discon inuous, when a jump
in he iscosi y is p oduced (∆η ≠ 0), o con inuous,
when a New onian egion is obse ed (∆η = 0). In his
case, an inc ease in iscosi y alues akes place in
sys ems wi h low alues o he su ac an /wa e a io,
which can be ela ed o a highe appea ance o
shea -induced esicles.
Figu e 6 also p o ides in o ma ion on he possible
e e sibili y o s uc u al changes occu ing in he
sys em. The e ec o an upwa d/downwa d shea
a e p og am o 30 minu es can be obse ed in
Figu e 6. Thus, shea a e was inc eased up o 300
s-1 o 5 min, his maximum shea a e was
main ained o 10 min and hen was dec eased o 15
min. The sys em wi h lowe su ac an /wa e a io
(50% NPFE-10, 30% hep ane) clea ly shows
i e e sibili y, as he signi ican highe alues o
iscosi y ob ained by educing shea a e indica es
(solid symbols) in compa ison o hose obse ed by
inc easing shea a e (open symbols). This is due o
he ac ha he esicles o med a e s able in ime
Table IV
Values o he low indexes o po en ial dec ease o iscosi y o bina y sys ems
%(w/w) su ac an %(w/w) wa e n m
60 40 -0.77 -0.50
65 35 -0.57 -0.36
70 30 -0.63 -0.38
75 25 -0.60 -0.38
Figu e 6
S eady-s a e low cu es o selec ed sys ems con aining maxi-
mum hep ane con en . Solid symbols co espond o low cu es
ob ained a e an upwa d/downwa d shea a e p og am.
102 G asas y Acei es
and he alues o iscosi y o he esicle phase a e
highe han ha o he lamella phase o s acked
bylaye s (Escalan e, 2000). Howe e , his beha io
changes by inc easing he su ac an /wa e a io.
Thus, he sys em wi h highe su ac an /wa e a io
(55% NPFE-10, 30% hep ane) showed a dec ease
in iscosi y alues, p obably due o he o al
alignmen o he bylaye s in he low di ec ion and,
consequen ly, he des uc ion o he esicles o med
du ing he upwa d shea a e amp, as sugges ed by
Pa al e al., (Pa al, 2001). The sys em wi h 55%
NPFE-10 and 25% hep ane p esen s in e media e
cha ac e is ics o he wo di e en beha io s
p e iously desc ibed.
c)
Shea -induced s uc u al obse a ions.
As
p e iously shown, shea low causes an impo an
e ec on he s uc u al modi ica ions o lamella
liquid-c ys alline phases. In his sec ion
shea -induced s uc u al changes a e s udied by
means o combined heological and pola izing
mic oscopy obse a ions. As p e iously desc ibed
(F anco, 1995), he e olu ion o ansien iscosi y o
simila sys ems consis s o an ini ial g ow h o
iscosi y (s ess o e shoo ), due o he iscoelas ici y
o he liquid-c ys alline phase, ollowed by a
dec ease in iscosi y down o an equilib ium o
s eady-s a e alue (Figu e 7). These esul s can be
explained by he pa ial alignmen o he lamella
bylaye s in he di ec ion o he low ield, and inally,
o al alignmen o he bylaye s occu s, and
consequen ly, an equilib ium iscosi y is eached.
Howe e , o sys ems shea ed a in e media e alues
o shea a e (be ween 2 and 10 s-1 ) a sligh inc ease
in iscosi y (s ess unde shoo ) akes place ela ed o
he de elopmen o mechanically induced de ec s in
he s uc u e o he lamella phase (F anco, 1995;
Pa al, 2001). As can be obse ed, his sligh
inc ease in iscosi y s a s a lowe imes as shea
a e inc eases and disappea s o he highes alues
o shea a e (100 s-1).
Figu e 8 shows he e olu ion o he lamella
ex u es du ing a ansien es a wo-selec ed shea
a es. Ini ially, applying a shea a e o 2 s-1 (Figu e
8A), he cha ac e is ic mosaic ex u e o he
non-o ien ed lamella phase is de ec ed. The
p oli e a ion o elonga ed s uc u al uni s in he low
di ec ion is obse ed la e on. As can be obse ed,
some ypical «Mal hesian c osses» a e always
p esen in he shea ed lamella ma ix. The p esence
o «Mal hesian c osses» has been ela ed o he
exis ence o shea -induced s uc u es in lamella
phases such as onions, esicles o sphe uli es
(Pa al, 2001; Goldszal, 1995). Howe e , he
appea ance o hese new s uc u es is no
gene alized. As can be obse ed in Figu e 8A, a
in e media e imes an incipien oily s eak ex u e
appea s, and inally a o al alignmen o he bylaye s
occu s. Op ical mic og aphs ob ained when he
lamella phase is shea ed a 10 s-1 show he same
sequence o liquid-c ys alline ex u es (Figu e 8B).
Howe e , in his case, he mosaic ex u e was clea ly
ollowed by he «oily s eaks» ex u e a in e media e
imes and, inally, he o al alignmen o his ex u e
was again obse ed. As can be seen in Figu e 9, he
shea a e exe s an impo an in luence on he ex u e
o he lamella phase. Thus, he di e en ex u es
obse ed a he end o he expe imen s a e a he
simila o un-p eshea ed samples when low alues o
shea a e we e applied (i.e. 0.5 s-1), in spi e o he
alignmen s o he lamella uni s. On he con a y, an
inc ease in shea a e p omo es he de elopmen o
he oily s eaked ex u e and inally, o highe alues
o shea a es (i.e. 100 s-1), he o al alignmen o he
lamella c ys al is ob ained (Figu e 9).
As can be obse ed in Figu e 10, he ansien
low beha io o a lamella e na y sys em wi h a
ela i ely high hep ane concen a ion (30% (w/w)) is
qui e di e en o ha p e iously desc ibed o bina y
sys ems. Thus, an ini ial inc ease in iscosi y wi h
ime, ollowed by a sligh dec ease o iscosi y was
always obse ed. Ne e heless, a s ess unde shoo
was no de ec ed in any case. This ansien shea
Figu e 7
T ansien es s o a bina y sys em con aining 70% (w/w) su ac-
an .
Figu e 8
E olu ion o lamella ex u es wi h ime o a bina y sys em (70%
(w/w) su ac an ) a di e en shea a es (A: 2 s-1, B: 10 s-1).
Vol. 56. Fasc. 2 (2005) 103
low causes mic os uc u al changes in he
liquid-c ys alline phase, which ha e been ela ed in
he li e a u e o he g adual de elopmen o esicles
o «onions» in he lamella ma ix (Roux, 1994;
Escalan e, 2000). The p esence o hese new
s uc u es has been de ec ed using se e al
echniques, being mic oscopy (pola izing and
elec on mic oscopy) he mos widely used. The
s uc u al changes a e e lec ed in he op ical
mic og aphs shown in Figu e 11. The ex u es
de ec ed a ela i ely high shea a es, i.e. 40 s-1, in
conco dance wi h o he s ound in he li e a u e (Dia ,
1993; Meye , 2000; Mülle , 1999), seem o indica e
he p esence o close-packed monodispe se
sphe uli es showing a homogenous g anula ex u e.
Fu he mo e, he ex u e o he sys em shea ed a
100 s-1 becomes mo e egula , which can be ela ed
o lowe cha ac e is ic sizes o he s uc u al uni s
(Mülle , 1999).
Rheological da a and mic oscopy images used in
his s udy seem o con i m he exis ence o esicles
in he lamella shea -induced ex u e o sys ems
o med by a non-ionic su ac an and a ela i ely
highe con en o a linea hid oca bu e, bu do no
p o ide in o ma ion abou esicles o ma ion. In
o de o ob ain in o ma ion abou his p ocess,
echniques such as small angle ligh and neu on
sca e ing should be used. Se e al au ho s ha e
epo ed di e en mechanisms o shea induced
esicle o ma ion depending on ac o s such as
su ac an s uc u e and concen a ion bu also on
hei ini ial shea induced mo phology (Rich e ing,
2001). The e o e, a mo e de ailed in es iga ion o his
p ocess is necessa y.
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Figu e 9
Lamella ex u es obse ed a he end o ansien expe imen s
o a bina y sys em (70% (w/w) su ac an ). A: un-p eshea ed
sample, B: shea ed a 0.5 s-1, C: shea ed a 5 s-1, D: shea ed a
10 s-1, E: shea ed a 100 s-1.
Figu e 10
T ansien es s o a e na y sys em con aining 50% (w/w) su ac-
an and 30% (w/w) hep ane.
Figu e 11
Lamella ex u es obse ed a di e en shea a es (A) 40 s-1
and (B) 100 s-1) o a e na y sys em (50% (w/w) su ac an
and 30% (w/w) hep ane).
104 G asas y Acei es