Changes in Phase Behavior from the Substitution of
Ethylene Oxide with Carbon Dioxide in the Head Group of
Nonionic Surfactants
Vivian J. Spiering,* [a] Aurora Ciapet ti, [a] Michelle Tu pinamb aL ima, [b] Dominic W. Haywar d, [a]
Laure nc eN oirez, [c] Mari e-Sousai Appavo u, [d] Reinhard Schom - cker , [b] and
Michael Gradzielski* [a]
Introduction
Nonioni cs urfactants with ethylene oxide [EO ;o re thylene
glycol (EG)] head groups are the workhorse surfactants in the
field of detergency and many other applicat ions of surfac-
tants. [1, 2] For example, in the European Union, more than 1m il-
lion tons are produced per year . [3] EO is ap etrochemical prod-
uct, and for such al arge-scale product it is interesting to sub-
stitute it by biorenew able resources. One op tion that has been
explored so far in that direction is the use of glycerol for the
head group, [4] an dw ith such surfactants also nanoemulsions
can be formed. [5] However ,i nt he field of detergenc yt hey have
not been able to subst itute conventional nonion ic surfactants,
and here researchers are still sea rching fo ra more sustainable
chemica ls olution.
In our experiments, we studied nonionic surfactants that
were modifie di nt er ms of their head group, containing differ-
ent numbers of CO 2 moieties that partly substitu te the EO
units. The use of CO 2 as ar esource in organ ic chemistry and
also, in particular ,i ts copolymeriz ation with epoxi des is at opic
of high curren ti nterest. The proces si ts elf has been known for
over 40 years but has seen substantial catalytic improveme nts
in recent years. [6–9] Application of such synthe tic schemes ,f or
instance, led to the formatio no fn anostructures from CO 2 -
based polycarbonates [10] or functional copolymers based on
CO 2 and glycidyl ethers. [1 1]
In our case, CO 2 -modif ied surfactants were obtaine db ya
double metal cyanid e( DMC)-catalyzed copolymeriz ation of a
mixture of EO and CO 2 with am ono functional alkanol as start-
ing unit [12, 13] (here :d odecanol). By doing so and var ying the
relative amount of CO 2 ,n onionic surfactants with different CO 2
content in their head group are accessible (Figure 1), which
apart from this are identical with respec tt ot heir hydrophobic
group and the length of their hydrophilic head (see Ta ble S1 in
the Supporting Information). Such surfac tants are of high inter-
Nonioni ce thylene oxide (EO)-based surfactants are widely em-
ployed in commercial applications and normally form gel-like
liquid crystalline phases at higher concentration s, rendering
their handling under such condit ions difficult. By incorporating
CO 2 units in their hydrophilic head groups ,t he consumption of
the petrochemical EO was reduced, and the tendency to form
liquid crystal sw as suppress ed completely .T his surprisi ng be-
havior was characterize db yr heology and studied with respect
to its structural origin by mean so fs mall-angle neutro ns catter-
ing (SANS). These exp eriments showe da strongly reduced re-
pulsive interaction between the micellar aggregate s, attributed
to ar educed hydratio na nd enhanced interpenetra tion of the
head groups owing to the presence of the CO 2 units .I na ddi-
tion, with increasin gC O
2 content the surfactants became more
efficient and effectiv ew ith respec tt ot heir surfac ea ctivity .
These findings are important because the renew able resource
CO 2 is used, and the CO 2 -containing surfac tants allow handling
at very high concen trations ,a na spect of enormous practical
importance.
[a] V. J. Spiering, A. Ciapetti, Dr .D .W .H ay ward, Prof. Dr .M .G radzielski
Stranski-Laborato rium f e rP hysikalische und Theoretische Chemie
Institut f e rC hemie
Te chnis che Universit - tB erlin
Straße des 17. Juni 124, 10623 Berlin (Germany )
E-mail :v .spiering@tu-berlin .de
michael.gradzie ls [email protected]
[b] M. T. Lima, Prof. Dr .R .S chom - cker
Institut f e rC hemie—T echnische Chemie
Te chnis che Universit - tB erlin
Straße des 17. Juni 124, 10623 Berlin (Germany )
[c] Dr .L .N oir ez
Laboratoir eL 8 on Brillouin (CEA-CNRS)
C.E.-Saclay ,9 11 91 Gif sur Yv ette Cedex (Franc e)
[d] Dr .M .-S. Appavo u
Forschungszentru mJ e lich GmbH
J e lich Centre for Neutron Sc ience (JCNS) at Heinz Maier-Leibnitz Zentrum
(MLZ)
Lichtenbergerstr .1 ,8 57 47 Garching (Germany)
Supporting Informati on and the ORCID ide ntificati on number(s) for the
author(s) of this article can be fou nd unde r:
http s: //doi.org/10.1002/ cssc.201902855.
T 2019 The Authors. Publish ed by Wiley -VCH Ve rlag Gm bH &C o. KGaA.
This is an ope na cces sa rticl eu nder the term so ft he Creative Commons
Attributio nN on-Commercia lN oDerivs License, which permits use and
distribution in any medium, prov ided the original work is properly cited,
the use is non-comm ercial, and no modifications or adaptations are
made.
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DOI :1 0.1002 /cssc.20 1902855

est because of the subst itution of the petrochemical EO unit
by CO 2 as ac heap and environ mentally benign resource .I na d-
dition, the CO 2 units are diesters, which typ ically can be de-
graded more easily (a point of ec ological impo rtance).
Owing to their high im portance, the prope rties of nonionic
surfactants have been studied in detail .K ey features are t heir
self-assembly behav ior in aqueous solution, whic hs hows a
marked temperatu re dependence, and at highe rc oncen tra-
tions, they form liquid crystal line (LC) phases. [1, 14] The type and
location of thes eL Cp hases in the phase diagram de pends in a
systematic fashion on the molecular architectur eo ft he non-
ionic EO surfactants. [14, 15] Ty pic ally ,h exagonal an dc ubic phases
are formed, which exhibit marked gel-like properties (with
shear moduli G 0 in the range of 10 4 –10 6 Pa). [16, 17] This behavior
is often problematic because for many formulation st he forma-
tion of highly viscous phases during the preparatio np rocess is
am ajor nuisance and therefo re has to be avoided or circum -
vented .T his often mean so ne has to work in application /for-
mulatio nw ith accordingly diluted surfactants from the very
beginning, which leads to substantially higher logistic costs
and volumes, thereby having an egative ecological impact.
In our work, we were now interested in how CO 2 -containi ng
surfactants compar et ot heir conventiona ln onionic counter-
parts. Accordingly ,w es tudied as eries of dodec yl surfactants in
which the amount of co ntained CO 2 units was systematica lly
varied [i.e. ,C
12 (CO 2 ) x EO n with x in the range of 0–3.1 and n =
8.2–14. 0; see Figure 1a nd Ta ble S1 in the Supporting Inform a-
tion] .H ere it should be noted that the distrib ution of the CO 2
in the head group is statistical (see Figur eS 1i nt he Supporting
Information). The synthesis an db asic properties have been de-
scribed before, [13] including their hydroph ilic–lipophilic balance
and behavior in the formation of emulsi ons, whic hs howe da
similar property spectrum as their conventional analogues . [18]
Results and Discussion
Phase behavior and surface activity
Ak ey prope rty in the characterization of new surfactant si s
their surface activity ,a nd it allows to determine the critical mi-
celle concentration (cmc). The surfac et ension data of all four
CO 2 -containing surfac tants look quite similar (Figure 2), with a
cmc value of 0.053 mmol L @ 1 for the surfactant with the high-
est CO 2 content [C 12 (CO 2 ) 3.1 EO 8.2 -OH] and 0.175 mmo lL @ 1 for
the surfactant without CO 2 (C 12 EO 14.0 -OH). Surf ace tension mea-
surement data for the individu al surfactants are shown in Fig-
ure S2 in the Supporting Informa tion. From this change in cmc
values, one may conclude that the surfac tants become more
hydrophobic with increas ing CO 2 content (Figur eS 3, left in the
Supporting Informa tion). The obtained values are lower com-
pared with other nonionic C 12 surfactants at 25 8 C, such as n -
dodecyl- b - d -glucoside with 0.19 mmol L @ 1[ 19] or C 12 EO 5 -OH with
0.064 mmol L @ 1 , [20] thereb yi ndicating ah igher extent of hydro-
phobicity .T he cmc, the hea dg roup areas ( a 0 ), and the Gibbs
free energy of micellization ( D G mic )o ft he differen ts urfac tants
are summarized in Ta ble 1. The surface tension of the CO 2 sur-
factants at 25 8 Ca tt he cmc was found to be betwe en 34.4
and up to 35.9 mN m @ 1 .T his is in the expected range for C 12 EO j
nonio ni cs urfactants, [21–23] but it is interesting to note that sur-
face tension becomes lower with increas ing CO 2 content ,t hat
is, the surfactants become more effective.
Furthermore, it was observed that the head group area de-
crease sw ith increasing CO 2 content (Figure S3 in the Support-
ing Information). This decrease can be explained by an en-
hanced hydrophobic ity and ad ecreased hydration. D G mic was
found to be linearly proportional to the CO 2 content .P erform-
ing al inear regression fit to the gradi ent (Figure S3, right in the
Supporting Information) reveals at ransfer en ergy of ( @ 0.37 :
0.06) kT per CO 2 unit, independent of temperature .T his value
is substantially smaller than the transfer energy of aC H
2 group
in ah ydrophobic chain ( &@ 1.2 kT), but nonetheless large
enough to significant ly influence the micel li zation process. It
was previously found that the introductio no fh ydrophobic
propylene oxide (PO) units into the hydrophilic chain of C 12 EO 8
Figure 1. General st ructural formula of the CO 2 -containin gn onio nic dodecyl
surfactants inve st igate di nt hi sw ork ;i np articula rw es tudied :C
12 EO 14.0 -OH,
C 12 (CO 2 ) 0.6 EO 13.3 -OH, C 12 (CO 2 ) 1.3 EO 11 .4 -OH, C 12 (CO 2 ) 1.5 EO 11 .6 -OH, and
C 12 (CO 2 ) 3.1 EO 8.2 -OH.
Figure 2. Surfac et ension s as af unction of conce nt ratio nf or
C 12 EO 8.2 (CO 2 ) 3.1 OH, C 12 EO 11 .6 (CO 2 ) 1.5 -OH, C 12 EO 14 (CO 2 ) 1.3 -OH, C 12 EO 13.3 (CO 2 ) 0.6 -
OH, and the reference sample C 12 EO 14.0 -OH at 25 8 C.
Ta ble 1. Summary of the surface tens io nm easurements. [a]
Surfactant cmc
[mmol L @ 1 ]
s CMC
[m n m @ 1 ]
a 0
[nm 2 ]
D G mic
[kJ mol @ 1 ]
C 12 (CO 2 ) 3.1 EO 8.2 -OH 0.053 34.4 0.55 @ 34.3
C 12 (CO 2 ) 1.5 EO 11 .6 -OH 0.091 35.9 0.65 @ 33.0
C 12 (CO 2 ) 1.3 EO 11 .4 -OH 0.099 35.5 0.67 @ 32.8
C 12 (CO 2 ) 0.6 EO 13.3 -OH 0.1 18 35.9 0.69 @ 32.4
C 12 EO 14 . 0 -OH 0.175 40.1 1.07 @ 31.4
[a] Critical micell ec oncentration (cmc), surface tension at the cmc ( s CMC ),
head grou pa rea ( a 0 ), and Gibbs free ene rg yo fm icellizatio n( D G mic ).
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surfactants gives rise to at ran sfer energy of @ 0.25 kT/P Ou nit
at 25 8 C. [24] The effect of adding PO and CO 2 units is theref ore
similar ,i nb oth cases facilitating the form ation of micelles ,b ut
more pronounced for the case of CO 2 incorporat ion. For a
compa ri son, the transfer energy per EO unit is + 0.16 kT , [25] and
thus the EO unit is caus ing as mall contribution disfavo ring mi-
cellization.
In summary ,t he CO 2 -containin gs urfactants are having a
lower cmc, which mean sf or many application st hat one can
reduce the requir ed amount, and they also show lo wer surface
tension values above the cmc, as typically wished for as urfac-
tant. Accordingly ,s ubstitution of EO by CO 2 in the head group
leads to more efficient and effective nonionic surfactants.
Rheological behavio r
Most interesting is the behavior at high concentration sa sf or
conventional nonionic surfactants of this type (e.g. ,C
12 EO 8 [14]
or C 12 EO 12 [26] )o ne observes with increasing concentr ation
above approximately 25–30 wt %f irst ac ubic phase and at still
higher concentration ah exagonal phase is formed, both of
which are highly viscous and gel-like in appearance. In con-
trast, the CO 2 -containing non ionic surfactants show av ery dif-
ferent phase behavior depicted in Figur e3 .F or surfac tants
containing more than one CO 2 unit per molecule in their hy-
drophilic head group ,L Cp hases are no longe rf ormed. The as-
signment of the differen tL Cp ha ses was done based on polari-
zation microscopy (the cubic ph ase is isotro pi c, whereas the
hexagonal phase shows typical fan-like birefringence textures,
see Figure S4 in the Supporting Information) and was corrobo-
rated by the small-angle neutron scattering (SANS) results (see
Figure S5 in the Suppo rting Information).
The absence of the LC phases for highe rC O
2 content is not
only interesting for fundamental soft ma tter science but is also
largely altering the flow behav ior of such system s, an dt hereby
the way they can be handled in terms of applic ation. The CO 2 -
containing surfactants with more than one CO 2 unit per mole-
cule are alway sr ather low-viscosity Newtonian fluids
( h < 1P as ), whereas co nventional nonionic surfac tants form
highly viscous gels with ay ield stress. This effect of largely
changed flow behavior is quantified by the viscosity curve s
shown in Figur e4 a, which direct ly compar et he surfactant
witho ut CO 2 and the one containing 3.1 CO 2 units
[C 12 (CO 2 ) 3.1 EO 8.2 -OH] at different concen trations. The latter is in
the concentratio nr ange from 45 to 65 wt %a lway sa Newtoni-
an liquid with av iscosity of approximately 0.4–0.8 Pa s, where -
as for the equivalent conv entional surfactant without CO 2 a4 –
6o rders of magnitude highe rv iscosity (no finite zero-s hear vis-
cosity is seen, corresponding to having ay ield stress) and ar e-
ductio nw ith increasin gf requency (corresponding to shear
thinning) is observed. The storage modulus G ’ as af unction of
frequency obtained from oscillatory rheolo gical measurements
for the different surfac tants at ag iven concentr ation of
65 wt %( Figure 4b )s hows for C 12 EO 14.0 -OH constant values of
approximately 2 V 10 4 Pa s( rather stiff gel ;t hese values depend
somewhat on the concentration as shown in Figure S6 ai nt he
Supporting Informa tion), which quantifies the gel-lik ep rope r-
ties of these samples. Alr ead yf or the surfactant with an aver-
age of 0.6 CO 2 units per molecule, the value is reduced by two
Figure 3. Phase diagram at 25 8 Ca saf unction of the surfa ctant concentra-
tion and the number o fC O
2 units containe di nt he hydrophili ch ead group
(with isotropic L 1 -phase, cubic phase, and hex agona lp hase).
Figure 4. Rheological parameter sf or different surfactant samples in the
high-concentrati on regime of 45–6 5w t% obt ained at 25 8 Cf rom oscillation
measurements at as hear stress of 0.5 Pa .( a) Magnitude of the complex vis-
cosity j h * j as af unction of angular frequen cy for the CO 2 -richest surfactant
( x = 3.1 )a nd the reference sam ple without CO 2 ( x = 0) ;( b) Storage modulus
G ’ as af unctio no fa ngular frequenc yf or the differen ts urfa ctants at constant
concentration of 65 wt %( measurement system :c one-plate st ainless-steel
geome tr yr adius :4 0m m, gap size 150 m m).
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orders of ma gnitude. It is still rather constant ,t hereby confirm-
ing the gel properties, but as am uch softer gel. In contras t, for
the surfactants conta ining more than one CO 2 group the
values are smaller by four to six orders of magnitude .A ctually ,
the val ues given in Figure 4b for the samples with x > 1a re
not really mean ingful because the viscou sc omponent of the
comple xs hear modulus G * is largely dominating ,a ss hown in
Figure S6 bi nt he Supporting Informa tion.
Small-angle neutron scattering
Apparently ,t he flow and phase behavior of the concentrated
nonionic surfactant solutions are largely changed by the incor-
poration of the CO 2 moietie si nto their head gr oups. To eluci-
date this interesting phenomen on further ,w es tudied in detail
the structure of the self-assembled aggregates formed here
and their structu ral ordering. This was done by SANS experi-
ments ,w hich confirm that at ah igher concentration no LC or-
dering is present, and especi ally no cubic phases are observed,
which typically show pronounce dg el-like behavio r. [27] Often
cubic phases are manifested in the scattering pattern sb y
spikes on the isotrop ic correlation peak, [27–31] as seen for
C 12 EO 14.0 -OH and C 12 (CO 2 ) 0.6 EO 13.3 -OH (show ni nF igur e5 af or
samples with 50 wt %; for the scatte ring patterns at other con-
centration ss ee Figur eS 7i nt he Supporting Information). In
contra st ,t he samples with higher CO 2 content show isotrop ic
scattering rings. When looking more closely at the radially
averaged correlatio np eak at ag iven co ncentration of 50 wt %
(Figure 5b ), one notice st hat the peaks becom ei ncreasingly
wider and less shar pw ith increas ing CO 2 content .T his indi-
cates am uch lower degree of ordering, which generally could
be attributed to less prono unced repulsive interaction sb e-
tween the micelles. At the sa me time, the peak positio nm oves
somewhat towards smaller q values, which indicates as mall
micellar growth with increasin gC O
2 content in the head
group.
SANS measurements at al ower concentration of 1w t%
showe ds imilar scattering curve s( Figure S8 in the Supporting
Information) that prove that globula rm icelles of similar size
(radius of g yration of 2.4–2.7 nm, Ta ble S3 in the Supporting In-
formation )a re alway sf ormed, irrespectiv eo ft he CO 2 co ntent
of the surfactant. Aq uantitative analysis of the SANS data
shows that there is some increase in size with increasin gc on-
tent of CO 2 (T able S3 in the Supporting Informa tion), in agree-
ment with the shift of the correlation peak at highe rc oncen-
tratio n. For spherical micelles ,t his corresponds to ar eduction
of the head group area ( a 0 )f rom 0.85 nm 2 for the pure EO sur-
factan tt o0 .62 nm 2 for C 12 (CO 2 ) 3.1 EO 8.2 -OH (T able S3 in the Sup-
porting Information). These values are in good agreement with
the data from surface tension measurements (T able 1). Ar a-
tional explanation of this observation would be that with in-
creasin gC O
2 content a 0 is reduced because of al ower extent
of hydration. SANS measur ements over al arge concentration
range up to 65 wt %( see Figur eS 9i nt he Supporting Informa-
tion) show that the size of the micelles is only slight ly affected
by the change of concen tration (i.e. ,o ne has similar aggre-
gates present), but in the case without or with little CO 2 in the
head group leading to LC stiff phases ,w hereas for more than
one CO 2 unit in the head groups the fluid state wa sr etained
up to highest concen tratio ns. Apparen tly ,t he interaction and
the extent of orderi ng at high co ncentrati on change largely
upon the incorpora ti on of CO 2 into the hea dg roups .T oq uan-
tify the effective inter action between the micelles as af unction
of the CO 2 content of the different surfactants, we furt her ana-
lyzed the scattering data in the thermodynamic limit (i.e. , q !
0). The determ ined I (0) exp values as af unction of the volu me
fraction f (here considering the “dry” volum ef raction f result-
ing from the surfactant only) are given in Figure 6a nd show
the expected passing through am aximum at ag iven f . This
can be explained such that the intensity first increas es linearly
with the number of dispersed particles, but with increa sing
concentration, they become increasingly ordered (thereby sup-
pressing fluctuations responsible for the scattering). This effect
is quantitatively descri bed by the structur ef actor S (0), whic h
then leads to ar eductio ni ns cattering intensity .T od escribe
the experimentally observed scattering behav ior ,w ee mployed
the hard-sphere model according to Carnahan–Starling [32]
[Eq. (1)]:
S 0 ðÞ
@ 1 ¼ 1 þ 2 @ hs
ðÞ
2 @ 4 @ 3
hs þ 4 @ 4
hs
1 @ @ hs
ðÞ
4 ð 1 Þ
Figure 5. (a) Comparison of 2D scatt ering patterns of the nonionic surfac -
tants with different CO 2 co ntent in their head groups for ac onstan tc oncen -
tratio no f5 0w t% .( b) Radially averaged intensity curves for the same data
(indicatin gt he marke dly sharper peaks of the gels). The curves are scaled on
the y -axis by the followin gm ultiplicato rs :( CO 2 ) 3.1 ·1 ;( CO 2 ) 1.5 ·4 ;( CO 2 ) 1.3 ·12 ;
(CO 2 ) 0.6 ·64 ;( CO 2 ) 0 ·240.
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in which f hs is the solvated volum ef raction effective responsi-
bility for the hard-spher ei nteraction, and f hs = B f (1 @ A f ), in
which B quantifies the extent of hydration of the aggregates
(i.e. ,t he amount of water stron gly bound to the hea dg roup)
that has to be considered when describing the micelles as
hard spheres. A is account ing for the effect of “softness ”,
which means the exten tt ow hich aggregates can interpene-
trate at higher concentr ation, thereby effectively reduci ng the
volum ef raction of the system .O nt his basis, one can calculate
directly the observed intensity I (0) accord ing to Equati on (2):
I ð 0 Þ¼ @ ? D ð SLD Þ 2 ? V ? S ð 0 Þ ð 2 Þ
which is the monodisper se approximation, in which D (SLD )i s
the difference of the scatterin gl ength densities of (dry) mi-
celles an dD
2 O, and V the volum eo ft he dr ya ggre gates.
Lookin gi nd etail at the experimental data shown in Figure 6
one observe sa marked increas ei ni ntensity with increasing
CO 2 content and at the same time, the relative reduction of in-
tensity beyond the maxim um becomes much less pron ounced.
The higher intensity arises from larger aggre gates with increas-
ing CO 2 content, as already seen from the SANS curves taken
at 1w t% (Figure S8 and Ta ble S3 in the Suppo rting Informa -
tion). Describing these data with Equatio ns (1) and
(2) shows that with increasin gC O
2 content there is
less hydration of the head groups (smaller B )a nd
therefore al ower effective volume fraction (data
summarized in Ta ble 2). The hydration number H of
water molecules per surfac tant molecule is such that
for each EO group one has approximately two water
molecules, where as one can assign nominally zero
water molecules per CO 2 group. More important ly ,
the aggregates become much softer ,a se videnced
by the substantial in crease of A ,w hich mean st hat at
higher concentration the effective volu me fraction of
the aggregates does not increase much. Thi si n-
crease at high concentration is seen even more clear-
ly in Figure S1 0i nt he Supporting Information, in which we
normalized the data with respect to the maximu mo f I (0) and
the volume fraction f max at which the maximum is located.
This mean st hat the CO 2 -containing micelles are much more in-
terpenetrating (see Figure 7), and thereby much less repulsive
and unable to form ordere d( LC) phases.
Accordingly ,s imilar micelles are present, but the incorpora-
tion of the CO 2 moietie si nto the hydr ophilic hea dg roups
leads to as ubstantial alteration of the interaction potential
and renders them much softer .T his app arently arises firstly
from the lower extent of hydration of the head groups and
secondl yf rom attractive interactions owing to the presence of
CO 2 units that allow for interpenetration of the hydrophilic
corona of the surfactant s( see Figure 7) Polyethylen eo xide
(PEO) is know nt ob es trongly hydrated, [33, 34] but this tendency
is reduced by the presence of CO 2 units. Such ar eduction of
the hydration is concomitan tt oar educed a 0 ,w hich in turn ex-
plains the f ormation of larger aggre gates based on simple ge-
ometry (as the radiu so fas pherical micell es ho uld be given as
R = 3 v h / a h ,w ith v h being the molecu lar volume of the hydro -
phobic part of the surfactant). In addition, it appea rs that CO 2
units of neighboring micellar head group coronas are les sr e-
pulsively interac ting with each other ,p resum ably because of
their strong dipole moment coupled with the hig hp olarizab ili-
ty of the CO 2 group. In summary ,t he interac tion potentia lb e-
tween the nonionic micelles becomes much les sr epulsive, and
such “soft spheres” then accordingly do not form highly or -
dered LC phases with their correspondi ng gel-like properties
(Figure 7).
Figure 6. I (0) exp for all CO 2 surfa ctants from SANS data depe nd ing on the dry
volume fraction f . Line sa re I (0) th considering the static structure factor S (0),
which was calculated from the volum ef raction f hs of the swolle na ggre-
gates with ac ertain amoun to fw ater (described by B )a nd the softnes sp a-
rameter A that describe st he extent of interpe ne tratio no ft he aggregates
[Eq. (1)] .
Ta ble 2. Parameters from the fits shown in Figu re 5f or I (0) exp from SANS
data for volume fractions up to 0.35. [a]
Surfactant AB HM
w
[kDa]
N agg a 0
[nm 2 ]
C 12 (CO 2 ) 3.1 EO 8.2 -OH 0.55 1.5 11 7.2 55.4 80.5 0.62
C 12 (CO 2 ) 1.5 EO 11 .5 -OH 0.52 1.5 81 9.7 35.4 46.4 0.74
C 12 (CO 2 ) 1.3 EO 11 .4 -OH 0.50 1.6 32 3.2 29.6 39.8 0.86
C 12 (CO 2 ) 0.6 EO 13.3 -OH 0.37 1.6 42 5.2 22.8 28.7 0.94
C 12 EO 14.0 -OH 0.01 1.7 22 8.8 30.9 38.5 0.85
[a] A , B ,h ydratio nn umber H (molecule so fw ater per surfactan tm ole-
cule), molecular weight M w ,a ggr egatation number N agg ,a nd head group
area a 0 .
Figure 7. Illustratio no ft he packing conditio ns prevailing in the case of nonio nic surfac-
tants without an dw ith incorp orated CO 2 units in the head group. Ah igher CO 2 content
reduce sh ydration ,r educin gt he repulsiv ei nteractions ,a nd thereby allow sf or interpe ne-
tration of the hydrophilic mic elles shells.
ChemSusChem 2020 , 13 ,6 01 –6 07 ww w. chemsuschem .org T 2019 The Authors. Publishe db yW iley-VCH Ve rlag GmbH &C o. KGaA, We inheim 605
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Conclusions
In this work, we studied the effect of replacing petrol-base d
ethylene oxide (EO) in the head gro ups of nonio ni cs urfactants
by the renewable res ource CO 2 .T he presence of CO 2 in the
head group reduces the critical micellar concen tration (cmc)
and the surfac et ension above the cmc (i.e. ,t he surfactants
become more efficient and effective). Most interestingly ,o ur
study shows that the phase behavio ro fn onionic surfactants is
fundamentally altere db ys ubstituting CO 2 moieties into the
hydrophilic EO-based hea dg roups. Alread yt he incorpor ation
of one CO 2 group per molecu le suppresses the formation of
gel-like liquid crystal line phases completely .T his is not only
fundamentally av ery interesting observ at ion but also one of
high practical importance ,f acilitating enormously the ha ndling
of these surfactant sa th igher concentrations, which is typically
required in almos ta ll application sa ts om es tage. It should also
be noted that this behavior is unique for CO 2 incorporation ;
for instance, the incorporation of the hydrophob ic propylene
oxide (PO) unit, even at am uch higher percentage of substitu-
tion, still leads to the formation of ag el-like phase (e.g. ,s hown
for C 12 EO 4 PO 5 ). [35]
Scattering exper im ents show that the origin of this surpris-
ing behavior is as ubstantial dehydration of the head groups
owing to the presence of the CO 2 moieties and ac onsiderably
reduced repulsive interaction and interpenetration between
the head groups .T ogether ,t his causes the repulsiveness of the
interactions to become too weak to allow ah ighe ro rdering of
the micelles, and therefore no liquid crystal line phases are
formed.
This is an interesting exam pl eo fh ow af undamental invest i-
gation of the structura la nd phase behavior of as urfactant
system allow sf or as ystematic understanding based on the
molecular architecture. Being able to work at any concentra-
tion allows for more compa ct shipm ent, thereby reducing sub-
stantially the ecologic al impact of the logistics for this large-
scale commodity product. Th is information can be applie dd i-
rectly in surfactant science for formu lation at high concen tra-
tions. In addition, these CO 2 -containing surfactants contri bute
to the aim of more sustainab le chemistry as the petrol based
EO units are replaced by CO 2 (by up to 25 %). Furthermore,
even less surfactant has to be employed for typical applica-
tions because the CO 2 incorporation also reduces the cmc,
owing to the fact that the CO 2 group sr ender the surfactant
more hydrophobic ;t he transfer energy for micellization is
changed by @ 0.37 kT per CO 2 unit. Finally ,a bove the cmc the
CO 2 -containing surfactants also show lower surface tension
values (i.e. ,t hey are more eff ec tive surfactants). For all these
reasons, and with this simple synthetic approach, it can be ex-
pected that CO 2 -containin gn onionic surfac tants will become a
very promising alternative in the surfactant market.
Experimental Section
Surface tension measurements were performed with ad u-No e y
ring on aD CA Tt ensiometer (Data Physics) at 25, 30, 40, and 50 8 C.
The temperature was maintained by ac irculating thermostat with
prior heating of the surfactant solution in an oven and an equili-
bration time of 20 min. The surface tension was measured until the
value remained constant for ag iven period, as defined by the mea-
surement software (typically 350 s). The measured average surface
tension was determined for each in ac oncentration range from
10 @ 5 up to 0.02 wt %a nd plotted as af unction of logarithmic con-
centration. The cmc was determined by calculating the point at
which the surface tension reaches ac onstant plateau value. In ad-
dition, the surface tension measurements were used to determine
the surface excess concentration ( G )a nd the head group area ( a 0 )
using the Langmuir–Szyszkowski isotherm [36] [Eq. (3)]:
s ¼ s 0 @ RT G ? ln ð 1 þ Kc Þð 3 Þ
in which s is the surface tension [N m @ 1 ], s 0 the surface tension of
water (72.8 mN m @ 1 at 298 K), R the ideal gas constant
(8.314 JK @ 1 mol @ 1 ), T the temperature, G the surface excess concen-
tration, K the absorption constant, and c the concentration.
The head group area a 0 is related to the surface excess concentra-
tion by the Avogadro constant, N A ,b yE quation (4):
a 0 ¼ð G ? N A Þ @ 1 ð 4 Þ
According to the phase-separation model [37] and the mass-action
model, [38] the standard Gibbs free energy of micellization per mole
of monomer ( D G mic )i sg iven by Equation (5):
D G mic ¼ RT ? ln c cmc ð 5 Þ
in which x cmc is the mole fraction of surfactant in aqueous solution
at the cmc (assuming ideal behavior).
Rheology measurements were performed with aB ohlin Gemini 200
HR nano rheometer (Malvern Instruments), using ac one-plate ge-
ometry (radius :4 0m m, gap size 150 m m, stainless-steel geometry).
In steady-state conditions, the applied shear rates varied from
0.00014 to 50 s @ 1 .T oa nalyze the viscoelastic behavior of these
samples, oscillatory measurements were performed with the same
instrument and fixtures. As train amplitude sweep was performed
at ac onstant angular frequency of 6.3 Hz varying the amplitude of
the shear stress at 0.01–15 Pa to identify the linear viscoelastic
regime. Based on these measurements af requency sweep at a
shear stress of 0.5 Pa and angular frequencies varying from 0.6–
314.2 Hz was performed to determine the storage modulus G ’ and
the loss modulus G “. The reference surfactant was analysed with a
MCR301 rheometer (Anton Paar) under the same measurement
conditions.
SANS was measured at the Heinz Maier-Leibnitz Zentrum (MLZ) in
Munich, Germany ,o nt he Instrument KWS-1 [39] and at the Labora-
toire L 8 on Brillouin (LLB) in Saclay ,F rance, on the instrument PA XY .
KWS-1 was operated at aw avelength of 5 a and as ample-to-de-
tector distances (SDD) of 1.5 to 20 m, to cover a q -range of 0.015–
4n m
@ 1 .S ample transmissions were measured at aS DD of 8m .F or
all scattering data, the intensities were divided by the correspond-
ing transmission and sample thickness (1 mm), corrected for the
empty cell, and normalized with respect to the scattering of a
1m ms ample of light water ,a ccording to standard procedures. [40]
The incoherent background was determined by aP orod plot. The
SANS measurements performed at PA XY were performed at a
wavelength of 4 a for SDDs of 1.2 and 5m ,a nd aw avelength of
12 a for an SDD of 6m ,t hereby covering a q -range of 0.02–
6n m
@ 1 .
ChemSusChem 2020 , 13 ,6 01 –6 07 ww w. chemsuschem .org T 2019 The Authors. Publishe db yW iley-VCH Ve rlag GmbH &C o. KGaA, We inheim 606
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Acknowledgements
This work has been carried out within the project “Dream Re-
source ’’ (033RC0 02 C). The project is funde db yt he German Feder-
al Ministry of Educ ation and Research (BMBF) within the funding
priority ”CO 2 Plus—Stoffl iche Nutzung von CO 2 zur Ve rbess erung
der Rohstoffbas is“. Covestro Deuts chland AG is much thanke df or
providing th es urfactants and M. Sebas tian and M. Weinkraut for
input regarding this manuscri pt. In addition, we are gratefu lt o
the LLB and JCNS for allocati ng SANS beam times. This work is
based upon experime nts performed at th eK WS-1 instrument op-
erated by JCNS at the Heinz Maier-Leibnit zZ entrum (MLZ),
Garchin g, Germany and at the PA XY instrument at the Labora-
toire L 8 on Brillouin (LLB, CEA-CNRS). Additionally ,w ew ould like
to thank Profes sor Auhl for the use of the MCR30 1R heometer.
Conflict of interest
The authors declare no confli ct of interest.
Keywords : carbon dioxide · liquid cryst als · low viscosity ·
nonionic surfactants · small-angle neutron scatte ring
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Manuscript received :O ctober 16, 2019
Revise dm anuscript receiv ed :N ovember 22, 2019
Accep te dm an uscript online :N ovember 25, 2019
Ve rsion of record online :D ecemb er 30, 2019
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Why institutions use Plag.ai for originality review, entry 99

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