the binary phase behavior of short-chain pdms-b-peo diblock copolymers in aqueous solutions in...

Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 37–48

The binary phase behavior of short-chain PDMS-b-PEO diblockcopolymers in aqueous solutions in dependence of the PDMS

chain length—a combined polarized optical microscopy,2H NMR and SAXS study

Guido Kickelbicka,∗, Josef Bauera, Nicola Husinga, Martin Anderssonb, Krister Holmbergb

a Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9-165, A-1060 Wien, Austriab Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 G¨oteborg, Sweden

Received 21 June 2004; accepted 12 November 2004Available online 29 December 2004

Abstract

ec ed ina centrationsa ngtha ro t rich phaseL by a mixed“ ellar phased©

K

1

ntafTisdp

d inati-hort--

wre-ith

owesen intontra-herwithaserfac-

0d

The binary phase behavior of three short-chain polydimethylsiloxane-b-poly(ethylene oxide) (PDMS-b-PEO) diblock copolymers with thompositions DMS4-b-EO12, DMS6-b-EO12 and DMS10-b-EO12 (DMS = dimethylsiloxane unit, EO = ethylene oxide unit) was investigatqueous solutions using polarizing light microscopy, deuterium NMR spectroscopy, and SAXS measurements. At low surfactant conll three diblock copolymers show a mixed water/lamellar phase (W + L�), which switches to a lamellar phase depending on the DMS lend temperature in the region of 40 wt.% for DMS4 to over 50 wt.% for DMS10. The surfactants consisting of DMS4 and DMS6 show furthen in the concentration range between 40 and 85 wt.% predominately lamellar phases. While at very high concentration a surfactan2 developed. If the DMS segment was increased to 10 units the region between 55 wt.% and approximately 75% was dominated

inverse hexagonal” and lamellar phase at higher concentration a pure “inverse hexagonal” and afterwards a surfactant rich lameveloped, respectively.2004 Elsevier B.V. All rights reserved.

eywords:Diblock copolymers; Phase behavior; Polysiloxane; Poly(ethylene oxide); Amphiphiles

. Introduction

Surfactants containing polysiloxane segments are tech-ologically important molecules used in textile manufac-

ure and in cosmetics formulations, as agricultural adjuvant,nd as paint additives[1]. Polysiloxane copolymers are sur-

ace active in a variety of aqueous and nonaqueous media.he outstanding ability of aqueous solutions of certain sil-

cone surfactants to wet and spread on very hydrophobicurfaces is well documented and these surfactants are to-ay referred to as superspreaders[2]. The aggregation andhase behavior of siloxane surfactants with different struc-

∗ Corresponding author. Tel.: +43 15880115321; fax: +43 15880115399.E-mail address:[email protected] (G. Kickelbick).

tures, e.g. ABA- or comb-type, is extensively describethe literature[1,3–6]. We have recently started to systemcally investigate the aggregation and phase behavior of schain polydimethylsiloxane-b-poly(ethylene oxide) (PDMSb-PEO) diblock copolymers[7,8]. Surfactants with narromolecular weight distribution and with high purity were ppared by a hydrosilation of a Si-H-capped PDMS block wan allyl-modified PEO segment in high purity and with lmolecular weight distribution. Aqueous solutions of thsurfactants with water showed a spontaneous aggregatiovesicles and lamellar structures at low surfactant concetions [7]. Multilamellar aggregates were formed at higconcentration. The current investigation was carried outthe aim to gain more information about the binary phbehavior of the vesicles in aqueous solution, using su

927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2004.11.011

38 G. Kickelbick et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 37–48

tants with varying length of the hydrophobic PDMS seg-ment.

2. Experimental

2.1. Phase determination by optical techniques

The typical procedure for sample preparation proceededas follows. An aliquot of surfactant was weighed on a mi-crobalance into a 5 mL test tube with a polyethylene stopperand Milli-Q deionized water was added. The balance read-ing was recorded with a precision of±0.1 mg. The sampleswere mixed using a vibromixer TK3S for at least 2 min.Afterwards they were centrifuged at 1500 rpm for 1 h at22◦C before initial observations were made. The numberof phases present in the samples was determined by opticalinspection.

Liquid crystals were identified by observing samples be-tween two crossed polarizing plastic films using an Alym-pur BH-2VMA microscope. The texture of the birefringencepattern observed in a polarized light microscope identifiedparticular liquid crystal phases[9]. Qualitative changes inviscosity were characterized by comparing flow rates uponinverting sample vials or by observing air bubbles trappedi haseb bathc hb d po-l

2

rianU iumf1 spec-t trumw resett ng as fre-q entsa ts ona ss se-q t witha ter-v n”

were indistinguishable from runs using a quadrupolar echosequence.

2.3. 2H NMR sample preparation

Samples of about 0.7 g were prepared by weighing appro-priate amounts of surfactant and D2O into glass tubes, whichwere flame-sealed. The samples were centrifuged repeatedlychanging the direction of the NMR tube in the centrifuge forabout 4 h at 22◦C and left at this temperature for one daybefore the first NMR study was made.

2.4. Small angle X-ray scattering

Small angle X-ray scattering (SAXS) experiments wereperformed on a Kratky compact small angle system equippedwith a position sensitive wire detector (OED 50 M fromMBraun, Graz) containing 1024 channels of width 53.0�m.Cu K� radiation of wavelength 1.542A was provided bya Seifert ID 3000 X-ray generator operated at 50 kV and40 mA.

3. Results and discussion

hrees cd sisa in ap t as wasc

3

udyt eights uld ber stab-l

ys-ts in ani -p rl noi e of

TS diagra[

C Conv

D 95D 96D 95

n the solutions. The temperature dependence of the pehavior was studied by keeping the samples in a waterontrolled at constant temperature (±0.3) for at least 12efore optical inspection and inspection between crosse

arizers.

.2. NMR investigations

Deuterium NMR spectra were recorded using a VaNITY/INOVA 400 spectrometer operating at a deuter

requency of 61.40 MHz. The system used a 90◦ pulse of8�s. The sample temperature was maintained by the

rometer’s variable temperature controller. Each specas recorded after the sample had equilibrated at the p

emperature for about 5 min. All data were obtained usiingle pulse sequence, without sample spinning or fielduency lock. Before Fourier transformation, 256 transit a repetition rate of 0.5 s were averaged. Experimentypical sample showed that the2H t2 relaxation rate wa

ufficiently slow to make the use of a quadrupolar echouence unnecessary and that spectra were carried ousingle pulse excitation followed by single sampling in

al delay to remove “pulse breakthrough” or “ring-dow

able 1EC analyses data for DMSn-b-EO12 diblock copolymers used in phase

omposition Mn (Mw/Mn) (PDMS) Mn (Mw/Mn) (PEO)

MS4-b-EO12 585 (1.13) 660 (1.15)MS6-b-EO12 570 (1.17) 660 (1.15)MS10-b-EO12 855 (1.21) 660 (1.15)

The binary phase behavior in aqueous solution of telected DMSn-b-EO12 low molecular weight amphiphiliiblock copolymers (Table 1) was investigated. The synthend characterization of these surfactants is describedrevious paper[8]. Keeping the PEO block length constantudy of the influence of the length of the siloxane blockarried out in the temperature range of 0–70◦C.

.1. Polarizing optical light microscopy (POM)

Polarized optical microscopy was carried out to sthe phases of aqueous samples of the low molecular wurfactants. From the obtained textures the phases cooughly assigned and qualitative phase diagrams were eished.

All samples investigated showed a lyotropic liquid cralline phase behavior at 25◦C as presented inFig. 1. Fig. 1ahows an example of the batonnets of a lamellar phasesotropic phase of DMS4-b-EO12 after 12 h aging of the samle (73 wt.% surfactant). InFig. 1b oily streaks of a lamella

iquid crystalline phase of DMS4-b-EO12 at a concentratiof 50 wt.% surfactant are observed.Fig. 1c shows a POM

mage of a sample of a lamellar liquid crystalline phas

ms8]

ersion Mn (Mw/Mn) (diblock) Yield DMS/EO ratio by1HNMR (theoretical)

1235 (1.16) 95 0.49 (0.50)1285 (1.21) 96 0.71 (0.75)1460 (1.24) 98 1.18 (1.25)

G. Kickelbick et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 37–48 39

Fig. 1. Polarizing light microscopy image of the aqueous samples at 25◦C (magnification 100) of (a) 73 wt.% DMS4-b-EO12, (b) 50 wt.% DMS4-b-EO12, (c)55 wt.% DMS6-b-EO12, (d) 80 wt.% DMS6-b-EO12, (e) 83 wt.% DMS10-b-EO12, and (f) 50 wt.% DMS10-b-EO12.

DMS6-b-EO12 at 55 wt.%. The sample reveals a focal conicaldetail at the birefringent border near an air bubble. Sample1d at 80 wt.% of the same surfactants displays a fine mosaictexture of a lamellar liquid crystalline phase. At high concen-trations of DMS10-b-EO12 in water the POM image showsa non-geometric, striated texture commonly associated with“inverse hexagonal” liquid crystalline phases (Fig. 1e).Fig. 1fshows a POM image of a 50 wt.% sample of DMS10-b-EO12in which zones of uniform extinction in combination withbatonnets of a lamellar phase in an isotropic phase can beobserved.

3.1.1. POM of the water/DMS4-b-EO12 systemFig. 2shows the phase behavior of DMS4-b-EO12 over the

entire concentration range and within the temperature range

0–70◦C. A water-rich isotropic phase (W) was found at verylow surfactant concentration (less than about 0.1 wt.%). Theconcentration at which turbidity, as a sign of aggregation orphase separation, first appears was not precisely determined.However, the onset of turbidity was very close to the criti-cal aggregation concentration (CAC) determined in an earlierwork[8]. At higher concentrations the phase behavior is dom-inated by large lamellar regions (L�), which are stable up toaround 50◦C. At elevated temperature an optically turbid,two-phase region was observed. The turbidity was due to aseparation into a surfactant-rich phase (L2) and a water-richphase (W), a well known behavior of nonionic surfactantsand often referred to as clouding. The cloud point (transi-tion from W + L� to W + L2) was roughly determined by thedisappearance of birefringence. A translucent lamellar liquid


Fig. 2. Phase diagram of the DMS4-b-EO12/H2O system. The dashed linemarks part of the boundary of the one and two phase L� regions as uncertain.The squares mark the concentrations chosen for2H NMR and the circles theSAXS experiments in combination with2H NMR.

crystalline phase was found at concentrations between around40 and 90 wt.% up to 65◦C. The dashed line inFig. 2 indi-cates that the position of the boundary between the two-phaseregion (W + L�) and the one-phase region (L�) is uncertain,which is due to the extended turbidity and to the fact thatboth sides of the boundary exhibit a lamellar texture in thepolarizing light microscopic images. Between crossed polar-izers, the samples showed bright birefringence and the L�

phase could easily be distinguished from the isotropic liq-uid phases at higher concentrations. The phase boundary athigher temperatures of the L� phase was determined by theonset and disappearance of the birefringence. At higher con-centrations there are two clearly separated phases instead ofone turbid phase, which is indicated by the phase bound-ary above 50◦C in the concentration range of 40–60 wt.%surfactant. Polarizing light microscope images of the differ-ent lamellar phases showed the typical L� extinction-crosstexture. Specific concentration and temperature points werealso investigated by2H NMR (square marks) and SAXS ex-periments (circles) as described below. Above 87 wt.% anisotropic surfactant-rich phase was found, while at low tem-peratures (<22◦C) and >95 wt.% surfactant a turbid pale yel-low paste was formed. Another very interesting feature ofthe phase behavior is the appearance of a one-phase regionin a temperature range above 40◦C. No phase separation orc ex-h

3)

i tartsa rsta gramo ep se reg

Fig. 3. Phase diagram of the DMS6-b-EO12/H2O system. The dashed linemarks part of the boundary of the two phase regions, W + L� and W + L2,as uncertain. The squares mark the concentrations chosen for2H NMR andthe circles the SAXS experiments in combination with2H NMR.

up to 40–50◦C. Between 0.1 and 50 wt.% a large opticallyturbid, two-phase region was detected. Samples from thisregion show only weak birefringence between crossed polar-izers despite the turbidity. At higher temperature, mixtures inthis range become cloudy but isotropic. The phase boundaryis somewhat shifted to lower temperatures, compared to thatof DMS4-b-EO12.

Translucent lamellar liquid crystal phases were found atconcentrations between 50 and 90 wt.% up to 50◦C. Thedashed line inFig. 3 indicates that part of the boundary be-tween the two two-phase regions (W + L� and W + L2) is un-certain. The phase boundary at higher temperatures betweenthe L� phase and the two-phase region (W + L�) was deter-mined by optical inspection of the samples. Two clearly sep-arated phases were formed instead of the turbid phase. Thisdifference is indicated by the phase boundary above 50◦Cin the concentration range of 56–65 wt.% surfactant. POMimages of the different lamellar phases showed the typicalL� extinction-cross texture. Additionally, the ‘birefringencetexture’ between two crossed polarizers was different. Above90 wt.% an isotropic surfactant-rich phase was found. At lowtemperatures (<22◦C) and >97 wt.% surfactant, a turbid paleyellow paste was formed.

3.1.3. POM of the water/DMS10-b-EO12 systemd at

v i-mA e di-a db sf ssedp e be-c ome-w inaryp

louding appeared in this region. Additionally, this phaseibited distinct birefringence.

.1.2. POM of the water/DMS6-b-EO12 systemSimilar to DMS4-b-EO12 a water-rich isotropic phase (W

s found at very low surfactant concentrations (turbidity st∼0.1 wt.%). Again the concentration at which turbidity fippears could not be precisely determined. A phase diaf DMS6-b-EO12 is presented inFig. 3. In the center of thhase diagram there is a boundary between a two-phaion (W + L�) and a one-phase region (L�), which is stable

-

Once more a water-rich isotropic phase (W) is founery low surfactant concentrations.Fig. 4shows the approxate binary phase diagram of the surfactant DMS10-b-EO12.s for the other surfactants, the major part of the phasgram at room temperature (0.1 to∼60 wt.%) is occupiey an optically turbid, two-phase region (W + L�). Sample

rom this region showed weak birefringence between croolarizers. At higher temperatures, mixtures in this rangome cloudy but isotropic, and the phase boundary is shat shifted to lower temperatures as compared to the bhase behavior of the other two diblock copolymers.


Fig. 4. Phase diagram of the DMS10-b-EO12/H2O system. The squares markthe concentrations chosen for2H NMR and the circles the SAXS experimentsin combination with2H NMR.

The translucent lamellar liquid crystalline phases werefound at concentration between 60 and 90 wt.% between30◦C and 45◦C. A two-phase region composed of a lamel-lar and an “inverse hexagonal” phase, L� and H2 respec-tively, was found between 60 and 80 wt.% surfactant. Thetwo phases could be separated by centrifugation. Althoughthe2H NMR experiments (see below) did not give proof of alamellar one-phase region, the existence of a single phase L�

region at concentrations below 60 wt.% is probable. Above90 wt.% an isotropic surfactant-rich phase (L2) was found.

3.2. Comparison of the cloud points

From the optical analysis the cloud points of the surfac-tants could be determined which are indicative for the aggre-gation of the surfactants in the solution.Fig. 5shows the cloudpoint versus concentration for the three DMSn-b-EO12 sur-factants. The order of increasing cloud point, i.e., DMS10-b-EO12 < DMS6-b-EO12 < DMS4-b-EO12, is the expected onebut the differences between the values are much smaller than

F atedD

anticipated. In the low concentration regime the cloud pointlowering effect of each additional DMS group is only oneto two degrees while an extra methylene group of regularnonionic surfactants of the alcohol ethoxylate type causesa cloud point decrease of around four degrees centigrade[10].

3.3. Investigation of liquid crystalline phases by2HNMR spectroscopy

Spectra are only explicitly shown for the DMS4-b-EO12/D2O mixtures, all other spectra are provided insupporting material.

3.3.1. DMS4-b-EO12/water systemWater-DMS4-b-EO12 mixtures were investigated using

2H NMR spectroscopy.Fig. 6shows the2H NMR spectra ofsamples of various concentrations at 25◦C. The figure revealsthat there is a distinct difference in the NMR spectra betweensamples of different surfactant concentration. In the concen-tration range 63–87 wt.% the spectra show clearly developedpowder patterns (Fig. 6d–f), i.e., at 25◦C the samples repre-sent an uniaxial liquid crystalline phase. This regime corre-sponds to the fully developed lamellar phase L�. However,the phases denoted L�1 (concentration range: 43–51 wt.%)a dia-g rns.T thec

non-b noq vis-c ases(

nc-tt dis-t n atl -e asesd on-c hasew plit-t ax-i na mi-c theq tion.

yt intiesi alsoua pectt f thep on-c

ig. 5. Comparison of the cloud points development of the investigMS4-b-EO12, DMS6-b-EO12, and DMS10-b-EO12 surfactants.

nd L�2 (concentration range: 51–63 wt.%) in the phaseram (Fig. 6a–c) do not show well-defined powder pattehere are no indications of isotropic phases (singlett inenter).

At higher concentrations (>87 wt.%), the samples areirefringent and isotropic. In this region the spectra showuadrupole splitting. In addition, the samples have lowosity, which excludes the existence of isotropic cubic phFig. 6g).

Fig. 7shows a plot of the quadrupolar splitting as a fuion of wt.% surfactant. In the DMS4-b-EO12 liquid crys-alline regions the different lamellar phases show veryinct quadrupolar splitting. Even in the two-phase regioower concentrations a small2H splitting was observed. Howver, there were uncertainties in characterizing the phue to the high viscosity and turbidity of samples in this centration range. Above 63 wt.% surfactant a lamellar pas formed, which resulted in increased quadrupolar s

ing with increasing surfactant concentration reaching a mmum at∼78 wt.% DMS4-b-EO12. Above this concentratiogain an isotropic phase, which is probably a reversedellar phase, began to form resulting in a reduction ofuadrupole splitting with increasing surfactant concentra

The quadrupole splitting,�νq, was clearly identified bhe observed powder patterns. However, due to uncertan the L�1 and L�2 regions other measurements weresed. The order parameterSb = 1/2(3 cos2 ϑD − 1) with ϑDs the time averaged orientation of the nucleus with res

o the surfactant chain axis describes the interaction oolar block with water molecules at higher surfactant centrations[11,12].


Fig. 6. Deuterium NMR spectra of various DMS4-b-EO12/D2O samples at 25◦C. (a) 40.95 wt.%, (b) 49.44 wt.%, (c) 60.41 wt.%, (d) 68.16 wt.%, (e) 78.41 wt.%,(f) 84.12 wt.%, (g) 92.2 wt.%.

Tables 2–6summarize the quadrupole splitting, the D2O(or water, W)/EO ratio and the resulting order parameter ofthe investigated samples. From the W/EO ratio it can be con-cluded that Pake powder patterns are only observed if theW/EO ratio is in the range 1–2. In this region the oxyethy-lene groups may interact via hydrogen bonds with one or twowater molecules forming stable lamellar phases. Although allsamples have been prepared on the wt.% basis the difference

in the molecular weights of H2O and D2O needs to be con-sidered. Therefore, the ‘corrected’ W/EO ratios have beenadded to the table. InFig. 6f, at a D2O/EO ratio of 0.79 cor-responding to a H2O/EO ratio of 0.88, two phases are present,namely L� and L2.

At higher water contents (W/EO > 2) the2H NMR spectraare different, which is probably due to the exchange betweenbound and unbound water (Fig. 6a–c). A W/EO ratio of 1–2


Fig. 7. Deuterium quadrupole splitting vs. concentration of DMS4-b-EO12

in D2O at 25◦C.

is necessary for the formation of lamellar structures becausea certain amount of water is required for proper hydration ofthe PEO chains and it is the hydrated chains that pack intothe liquid crystalline structure[13–15]. Assuming that thenumber of water molecules bound per polymer unitnb is atan optimum for a value between 1 and 2, each additionalwater molecule increases the fraction of unbound water and,thus, leads to averaging of the quadrupolar interaction by thefast exchange between bound and unbound water. Although

Table 2Order parameters of DMS4-b-EO12/D2O samples

Sample wt.% surfactant �νq (Hz) S/D2O (mol/mol) D

(a) 40.95 10.4 0.0137 6(b) 49.44 93 0.0194(c) 60.41 88 0.0303(d) 68.16 713 0.0425(e) 78.41 815 0.0722(f) 84.12 776 0.1053

�νq: time averaged orientation of the nucleus with respect to the surfactantparameter.

Table 3Order parameters of DMS6-b-EO12/D2O samples at 25◦C

Sample wt.% surfactant �νq (Hz) S/D2O (mol/mol) D

(a) 55.48 60 0.0221(b) 64.20 – 0.0319((

� actantp

TO

S D2

( 1.((((

�

p

the lamellar phase is still macroscopically stable, the thermalstability is reduced and the2H NMR spectra of L�1 and L�2are very different from that of L�, especially in the phasetransition areas. The change from ‘ideal’ Pake patterns to the‘exchange’ spectra starts when W/EO is around 2. Anotherinteresting observation is that at W/EO∼4 (L�1) (Fig. 6b) thespectra show two sharp and clearly separated peaks, whereasat W/EO∼3 (L�2) the peak separation disappears. The dif-ferences between the two lamellar phases may result fromrestriction of the fast and isotropic motion of the additionalunbound water molecules. Thus a W/EO ratio of 4 leads tosystems that are more ordered on the NMR time scale than aratio of 3.

The order parametersSb shown inTable 6are very small.The magnitude ofSb of L� is in the same range as observedfor long PEO polymers in the low water concentration range.Lusse and Arnold found values of∼0.001 to 0.004 depend-ing on the W/EO ratio[16]. Caboi and Monduzzi publishedvalues ofSb = 0.011 for lamellar liquid crystals of didode-cyldimethylammonium bromide/water systems[17]. Inter-estingly, the order parameters for L�1 and L�2 samples arean order of magnitude smaller than the values obtained forL�. As mentioned above, the exchange model helps to under-stand these results. Surprisingly enough, the thermal stabilityof these phases is similar to that of the L� phase. Thus, it canb n of

c) 73.13 596 0.0484d) 82.13 576 0.0816

νq: time averaged orientation of the nucleus with respect to the surfarameter.

able 4rder parameters of DMS10-b-EO12/D2O samples at 25◦C

ample wt.% surfactant �νq (Hz) S/D2O (mol/mol)

a) 30.30 73 0.0066
b) 50.59 50 0.0155 .
c) L� 70.21 523 0.0357 .

c) H2 70.21 220 0.0357 .

d) H2 82.86 332 0.0731 .

νq: time averaged orientation of the nucleus with respect to the surfactantarameter.

2O/EO (mol/mol) W/EO (mol/mol) nb Sb (×10−3)

.05 6.72 73.6 0.0354.29 4.76 52.5 0.482.74 3.05 34.0 0.441.96 2.18 24.5 3.561.15 1.28 14.9 3.960.79 0.88 10.5 3.66

chain axis[24]; nb: number of water molecules bound per polymer unit;Sb: order

2O/EO (mol/mol) W/EO (mol/mol) nb Sb (×10−3)

3.76 4.18 46.2 0.312.61 2.90 32.4 –1.72 1.91 21.7 2.961.02 1.13 13.2 2.77


O/EO (mol/mol) W/EO (mol/mol) nb Sb (×10−3)

267 14.08 153.1 0.38538 5.98 65.6 0.26

e concluded that the order is driven by the aggregatio

234 2.60 29.0 2.63234 2.60 29.0 2.21114 1.27 14.67 3.22




wt.% surfactant �νq (Hz) S/D2O (mol/mol) D2O/EO (mol/mol) W/EO (mol/mol) nb Sb (×10−3)

40.95 0 0.0137 6.05 6.72 73.6 –49.44 337 0.0194 4.29 4.76 52.5 1.7260.41 415 0.0303 2.74 3.05 34.0 2.1068.16 537 0.0425 1.96 2.18 24.5 2.8678.41 577 0.0722 1.15 1.28 14.9 2.8084.12 547 0.1053 0.79 0.88 10.5 2.58

�νq: time averaged orientation of the nucleus with respect to the surfactant chain axis[24]; nb: number of water molecules bound per polymer unit;Sb: orderparameter.


wt.% surfactant �νq (Hz) S/D2O (mol/mol) D2O/EO (mol/mol) W/EO (mol/mol) nb Sb (×10−3)

55.48 298 0.0221 3.76 4.18 46.2 1.4764.20 313 0.0319 2.61 2.90 32.4 1.5873.13 380 0.0484 1.72 1.91 21.7 1.8982.13 – 0.0816 1.02 1.13 13.2 –

�νq: time averaged orientation of the nucleus with respect to the surfactant chain axis[24]; nb: number of water molecules bound per polymer unit;Sb: orderparameter.

the chains to form bilayer structures, whereas the separationof the head group and chain regions plays a minor role. How-ever, if the W/EO exceeds the ‘critical’ value of∼4, phaseseparation is observed.

3.3.2. DMS6-b-EO12/water systemThe quadrupole splitting, the D2O (or W)/EO ratio and the

resulting order parameter of the samples have been calculatedas described above and are summarized inTable 3.

The spectrum of a sample from the L�1 region showsa smaller quadrupole splitting (�νq) than the spectrum ofDMS4-b-EO12. Although the D2O/EO ratio is about the same(∼4), the order parameterSb is smaller (0.31, compared to0.48 for DMS4-b-EO12) as a result of the smaller splitting.The spectrum from the lamellar phase L�2 is surprising inseveral ways. First of all no2H splitting is observed. Thedip at the isotropic frequency must not be mixed-up with asplitting but it may either result from inadequate set-up ofthe measurement conditions or be due to inaccurate shim-ming. Furthermore, the line shape of the D2O spectrum isremarkable. The shoulders of the spectrum indicate a lamel-lar phase structure. However, the absence of2H quadrupolesplitting of the water signal in surfactant/D2O liquid crys-talline phases does not allow for the calculation of the orderparameter. Again a W/EO ratio of about 4 (found in the L�1p hangbT bothl ori-e tioo wa-tN y-d to akep -

ber of water molecules bound per polymer unitnb has a largeimpact on the�νq value, thus influencing the order param-eter considerably. A W/EO > 2 reduces the quadrupole split-ting to zero. At higher surfactant concentrations (>88 wt.%)the reversed surfactant-rich isotropic phase L2 is found. Thecalculated order parameters (Table 3) are smaller than therelevant values found for DMS4-b-EO12 in ‘similar’ phases(Table 2). Fig. 8shows a plot of the quadrupolar spectra as afunction of wt.% surfactant.

3.3.3. DMS10-b-EO12/water systemWhereas the phase behavior for the surfactants described

above (DMS4-b-EO12 and DMS6-b-EO12) is similar in manyways the DMS10-b-EO12 surfactant shows substantial differ-ences. This may be attributed to the longer hydrophobic chainof this compound.

Up to ∼60 wt.% surfactant the phase diagram is domi-nated by an optically turbid, two-phase region. From polar-

Fi

hase) seems to lead to more suppression of the fast excetween bound and unbound water than a ratio of 3 (L�2).his behavior is particularly surprising, as the phases at

ower and higher concentration show proper anisotropicntation of D2O molecules. This implies that a W/EO raf ∼3 gives an exceptional situation with regard to the

er binding structure of the lamellar phase formed. The2HMR spectrum of the L� region reveal that, although the hrophobic block differs only by∼1.6 DMS units from thaf DMS4-b-EO12, the influence on the line shape of the Patterns is enormous (supporting material). Again, the num

e

ig. 8. Deuterium quadrupole splitting vs. concentration of DMS6-b-EO12

n D2O at 25◦C.


ized light images it is assumed that this constitutes a mixtureof a water-rich phase and a lamellar phase (W + L�) althoughthe combined ‘preliminary’ studies cannot reliably guaran-tee the existence of an ordered bilayer structure.2H NMRmeasurements were used to further characterize the structure.Measurements were performed at a concentration of 30 wt.%,i.e. in the middle of the (W + L�) region. The spectrum showstwo sharp and clearly separated peaks. The order parametersof the samples are summarized inTable 4.

Considering the order parameter of sample (a) of 0.00038the denotation of the concentration range of 0.1 to∼60 wt.%surfactant as a two phase area is highly questionable. Ad-ditionally no isotropic peak is visible, as would have beenexpected for a water/lamellar phase region. But a calculatedW/EO ratio of∼14 (resulting from�νq ∼ 73 Hz) supportsthe assumption of the region as two-phase dispersion if com-pared to the ‘magic’ W/EO ratio of 2. The very surpris-ing existence of quadrupole splitting in this concentrationrange may be caused by an incomplete motional averagingof water molecules within the relevant time range[16]. Sim-ilar quadrupole splitting has been observed in lipid bilayersuspensions by several authors[18–21]. Hey et al.[22] ob-served quadrupole splitting of PEO polymers using PEO ofhigher molecular weight (8310 g/mol). The splitting is ap-proximately 500 Hz for water contents less than 5 wt.%. Allt atiol ellarp (W).T -f y bei oft thelq g ast eser ansi-t dualab stingt rfac-t rame-t ase:Sp -m rys-t aser ra-t pure“ erseh i-t lowW byt itht liq-uD

Fig. 9. Deuterium quadrupole splitting vs. concentration of DMS10-b-EO12

in D2O at 25◦C. The dashed line marks the exact boundary of the one andtwo phase L� regions (L� and L�/H2) as uncertain.

the regime of high surfactant concentrations (W/EO < 1) anL2 phase is formed.

Fig. 9 shows the summarizing plot of the quadrupolarspectra as a function of wt.% surfactant for DMS10-b-EO12.The exact concentration range of the liquid crystalline lamel-lar region remains uncertain. Therefore the phase transition inthe low concentration range is plotted as a dashed line, whilethe transition to the two phase region, L� + H2, is determinedwith more accuracy.

3.3.4. DMS4-b-EO12/D2O mixtures at 50◦CTo investigate the role of temperature the binary phase be-

havior of DMS4-b-EO12/H2O was analyzed using2H NMRat 50◦C. As already shown with POM various anisotropiclamellar liquid crystalline phases were stable up to temper-atures of 50–70◦C. At higher temperatures a phase sepa-ration occurred. Again the phase L�1 (concentration range:40–50 wt.%) does not show well-defined powder patterns.

Fi

hese observations in connection with the high W/EO read to the conclusion, that in the 30 wt.% region the lamhase is coexisting with a surfactant-poor water phasehe same might be true for the sample with∼50 wt.% sur

actant. But the W/EO ratio (5.38) and the line shape manterpreted as a L�1 phase. This sample shows a spectrumhe two-phase region from 58 to 78 wt.%, consisting ofamellar phase L� and the “inverse hexagonal” phase H2. Theuadrupole splitting of the lamellar phase is twice as bi

he splitting found for the “inverse hexagonal” phase. Thesults supports earlier data which indicates that the trion from lamellar to “inverse hexagonal” phases is grand that a mixture of the two phases can coexist[8]. Thisehavior is predicted by the phase rule, and it is intere

o note such a coexistence region with the present suant that consists of several components. The order paers of the phases in equilibrium are similar (lamellar phb ∼ 0.00263; H2: Sb ∼ 0.00221), as shown inTable 4. Thehase transition from W + L� to L� + H2 without an interediate existence of a pure lamellar lyotropic liquid c

alline L� phase is hardly possible. Following the two-phegion (L� + H2) in the regime of high surfactant concention, powder patterns are again observed indicating ainverse hexagonal” phase which is identified as the “invexagonal” phase H2. The existence of the H2 phase is add

ionally proven by SAXS measurements (see below). The/EO ratio of∼1, which stabilizes the formed aggregate

he most efficient water–EO interaction, in connection whe high molar volume of the PDMS block, shifts theid crystalline structure from lamellar (DMS4-b-EO12 andMS6-b-EO12) to reversed structures (DMS10-b-EO12). In

ig. 10. Deuterium quadrupole splitting vs. concentration of DMS6-b-EO12

n D2O at 50◦C.


Fig. 11. Deuterium NMR spectra of a DMS10-b-EO12/D2O samples at 25 and 50◦C, 70.21 wt.% surfactant.

The spectrum of the sample of concentration 49.44 wt.%showed a remarkable line shape. However, the2H quadrupolesplitting of the water signal still indicates a lamellar phasestructure and allows for the calculation of the order parameter.The appearance of the broad signal may be indicative of theexistence of a second distinct phase. It may also emanate fromeasier rotation and exchange of unbound or weakly bound wa-ter molecules in the high water concentration range. At a con-centration of 60.41 wt.% a surprising temperature influenceon the phase transitions occurred. A phase transition froma weak birefringent one-phase region (denoted L�1 and L�)to a strongly birefringent anisotropic phase region was ob-served as the temperature was increased. The latter phase wasnot further characterized.Table 5summarizes the quadrupolesplitting, the D2O (or W)/EO ratio and the resulting order pa-rameter of the investigated samples.

3.3.5. DMS6-b-EO12/D2O Mixtures at 50◦CThe different anisotropic lamellar liquid crystalline phases

found in the binary phase diagram for DMS4-b-EO12 werestable up to a temperature range of 50–70◦C. At higher tem-peratures phase separation occurred, i.e. there was a transitionto a two-phase region (W + L�) or clouding. Due to the lowertransition temperature compared to the DMS4-b-EO12/H2Osystem, raising the temperature to 50◦C results in melting of

most of the liquid crystalline phase and hence a two-phasesystem consisting of a doublet (lamellar phase) and a singlet(liquid phase) is observed.

The quadrupole splitting, the D2O (or W)/EO ratio and theresulting order parameter of the samples have been calculatedand are summarized inTable 6. The quadrupolar splitting, andhence the values of the order parameter obtained, are smallerthan the maximum values obtained at 25◦C for the samesamples (0.003). The samples (a) to (c) lie in the two-phaseregion (W + L�) or at least close to the transition temperatureof the phase boundaries of the lamellar phase. The singletsignal coexists with the doublet of the lamellar phase.

Fig. 10shows a plot of the quadrupolar splitting as a func-tion of wt.% surfactant. The phases L�1 and L�2 undergo aphase transition into the two-phase region W + L�, whereasL� shows both a transition to W + L� and to W + L2.

3.3.6. DMS10-b-EO12/D2O mixtures at 50◦CFig. 11shows a comparison of the deuterium NMR spectra

of a DMS10-b-EO12/H2O sample at 70.21 wt.% surfactant at25◦C and 50◦C. It can be clearly seen that at lower temper-atures a typical Pake pattern of an anisotropic lamellar phaseis observed while with increasing temperature an isotropicphase develops.

TI y scatte ht,P

S

DDDDD

TC

S Inte

D 87D 60D 98D 72D (81

able 7nterplanar spacing of lamellar phases determined by small angle X-raD: polydispersity)

urfactant wt.% surfactant CalculatedMn [25]

MS4-b-EO12 49.47 1008MS4-b-EO12 78.61 1008MS6-b-EO12 55.85 1127MS6-b-EO12 73.17 1127MS10-b-EO12 82.92 1324a “Inverse hexagonal” phase.

able 8alculated surfactant and water parts of the phases formed

urfactant wt.% surfactant Density (g/dm3)

MS4-b-EO12, L� 49.47 969.9MS4-b-EO12, L� 78.61 969.9MS6-b-EO12, L� 55.85 972.7MS6-b-EO12, L� 73.17 972.7MS10-b-EO12, H2 82.92 977.5a “Inverse hexagonal” phase.

ring (all spacings refer to lamellar phases;Mn: number averaged molecular weig

Mn (SEC)[25] PD (Mw/Mn) Interplanar spacing (A)

1235 1.13 87.581235 1.17 60.391285 1.21 98.391285 1.11 72.461460 1.11 (81.33) 93.91a

rplanar spacing (A) db,s (A) db,w (A)

.58 42.66 44.92

.39 52.71 7.68

.39 54.28 44.11

.46 52.62 19.84.33) 93.91a (67.17) 52.77dh,s (14.16) 41.14dh,w


Fig. 12. SAXS patterns of selected surfactants at different concentrations.

3.4. Small angle X-ray scattering (SAXS)

All SAXS patterns were recorded at room temperature.The samples used for SAXS and the obtained interplanarspacings are summarized inTable 7. The surfactant and waterparts of the bilayer and the “inverse hexagonal” phase werecalculated and are summarized inTable 8. All SAXS patternsmerged to one graphics are shown inFig. 12, the single moredetailed spectra are shown insupporting material.

Attention should be paid to the fact, that for the DMS10-b-EO12 sample, the values for the water and surfactant partwere calculated. The water part of the interplanar spacing(in parenthesis are the values for lamellar phase calculations)refers to the diameter of the water-filled rods anddh,w isthe center-to-center inter-rod spacing. The thickness of thesurfactant layer separating the rods,dh,s, is 52.77A.

3.4.1. DMS4-b-EO12/water systemTwo concentrations were investigated by SAXS, i.e.

49.47 wt.% and 78.61 wt.%. All other measurements, includ-ing 2H NMR, failed to produce a clear identification of thedifferent lamellar structures. The measurement clearly iden-tified the phase at a concentration of 49.47 wt.% as lamellar(two peaks at 2Θ = 1.008◦ and 2.059◦) with an interplanarspacing of 87.58A.

edf t2 of6 ec-t split-ta igh-e . Al-t theS llar.T ases( heh g

of the water-containing compartments which leads to an in-crease in the interlayer spacing from 60.39A (78.61 wt.%surfactant) to a value of 87.58A (49.47 wt.% surfactant) withincreasing water content. The extended length of the blockcopolymer was calculated to∼70A. The surfactant part ofthe interlayer spacing increases with increasing surfactantconcentration from 42.66A to 52.71A.

3.4.2. DMS6-b-EO12/water systemThe SAXS measurement clearly identified the phase at a

concentration of 73.17 wt.% as lamellar via two reflectionsat 2Θ = 0.898◦ and 1.818◦ with an interplanar spacing of98.39A.

Fig. 12also shows a SAXS pattern for a sample in the L�

region at 55.58 wt.% (two peaks at 2Θ = 1.219◦ and 2.438◦).The ratio of the peak positions is 1:2, identifying the phaseas lamellar with an interplanar spacing of 72.46A.

As for DMS4-b-EO12, the phase behavior of surfactantDMS6-b-EO12/H2O system is dominated by lamellar re-gions. With increasing surfactant concentration, the inter-layer spacing is reduced from 98.39A (55.58 wt.% surfac-tant) to 72.46A (73.17 wt.% surfactant). The W/EO ratio ofthe investigated sample is reduced from∼4 to ∼2. The sur-factant part of the interlayer spacing stays constant at∼53A(54.28A and 52.62A), whereas the water part is reduced from4

3inary

p re( tur-b u-c a bi-lT rseha

giont2 se.Tf r-r mingh r-fi sot able,n fromt as ap

4

beens tudy

At 78.61 wt.% a typical SAXS pattern is obtainor the well-ordered lamellar phase L� (two peaks aΘ = 1.463◦ and 2.935◦) with an interplanar spacing0.39A. The quadrupole splitting of the relevant NMR sp

rum shows a typical powder spectrum. The quadrupoleing reaches the maximum value, a W/EO ratio of∼1 giving

strong water–PEO interaction, thus resulting in the hst anisotropic orientation of the bound water molecules

hough the2H NMR results are not absolutely conclusive,AXS measurements identify the whole region as lamehe difference in terms of appearance of the lamellar phL�1, L�2, L�) is probably due to structural differences. Tigher water content of the L�2 sample leads to a swellin

4.11A to 19.84A.

.4.3. DMS10-b-EO12/water systemContrary to the surfactants described above, the b

hase diagram of DMS10-b-EO12/H2O at room temperatu0.1 to∼60 wt.% surfactant) is dominated by an opticallyid, two-phase region (W + L�). The first anisotropic translent phases formed as a phase mixture consisting ofayer structure (L�) and an “inverse hexagonal” phase (H2).he only one-phase liquid crystalline region is a “inveexagonal” phase in the concentration range between∼78nd 90 wt.% surfactant at room temperature.

In the SAXS pattern of the pale yellow one-phase rehree reflections were detected at 2Θ = 1.086◦, 1.851◦ and.194◦. The forth peak at 2Θ = 2.825 disappeared in the noihe ratios of these peak positions are 1:31/2:2 (:71/2), identi-

ying the phase as hexagonal[23]. The center-to-center inteod spacing, calculated from the first peak position assuexagonal geometry, is 93.91A. The diameter of the watelled rods,dh,w, was calculated to 41.14A, and the thicknesf the surfactant layer separating the rods is 52.77A. Al-

hough the existence of a lamellar phase region is probo SAXS measurement have been made with samples

his region. These samples were all very turbid, whereure liquid crystalline phase should appear translucent.

. Conclusions

Summarizing the phase behavior studies, it hashown that the AB-type siloxane surfactants of this s


shows some unexpected features. First of all, there seems tobe a poor correlation between the length of the hydrophobictail and the concentration at which there is a transition froma two-phase region into a one-phase liquid crystalline region.For normal nonionic surfactants, e.g. fatty alcohol ethoxy-lates, there is a clear trend that the transition comes at a lowersurfactant concentration the longer the hydrophobic tail. Thisis not the case for the siloxane surfactants studied in this work.Secondly, it is surprising that all three surfactants give lamel-lar liquid crystalline phases, although over different concen-tration ranges. For normal nonionics it would hardly havebeen possible that surfactants of such widely different hy-drophobic tail lengths and with the same polar headgroupwould give so similar phase behavior. Even if some featuresare the expected ones, such as the occurrence of a reversehexagonal phase only for the surfactant with the longest hy-drophobic tail, the general conclusion must be that the surfac-tant phase behavior is surprisingly independent of the relativesizes of the hydrophilic and the hydrophobic block—or of thesurfactant HLB to phrase it in familiar surfactant terms. Wehave no explanation to this behavior.

It is interesting to compare the phase behavior of thesesiloxane surfactants that have the polyoxyethylene segmentattached to the end of the polysiloxane chain with that of themore common trisiloxane surfactants, the so-called super-s achedt viorf anec thep -t then ants.A solu-t ctiono er ofd d be-f

A

-s ationf in theF tionD

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, atdoi:10.1016/j.colsurfa.2004.11.011.

References

[1] R.M. Hill, in: A.T. Hubbard (Ed.), Silicone Surfactants, MarcelDekker, New York, 1999.

[2] R.M. Hill, Curr. Opin. Coll. Interface Sci. 3 (1998) 247.[3] M. Gradzielski, H. Hoffmann, P. Robisch, W. Ulbricht, B. Gruning,

Tenside Surf. Det. 27 (1990) 366.[4] M. He, R.M. Hill, Z. Lin, L.E. Scriven, H.T. Davis, J. Phys. Chem.

97 (1993) 8820.[5] R.M. Hill, M. He, Z. Lin, H.T. Davis, L.E. Scriven, Langmuir 9 (11)

(1993) 2789.[6] A. Sturmer, C. Thunig, H. Hoffmann, B. Gruning, Tenside Surf. Det.

31 (1994) 90.[7] G. Kickelbick, J. Bauer, N. Huesing, M. Andersson, A. Palmqvist,

Langmuir 19 (2003) 3198.[8] G. Kickelbick, J. Bauer, N. Huesing, M. Andersson, K. Holmberg,

Langmuir (2003) 10073.[9] S.T. Hyde, in: K. Holmberg (Ed.), Identification of Lyotropic Liquid

Crystalline Mesophases, Wiley, Chichester, 2002.[10] M.J. Rosen (Ed.), Surfactants and Interfacial Phenomena, John Wiley

& Sons, 1989.[11] K. Rendall, G.J.T. Tiddy, Chem. Soc. Faraday Trans. 1 (80) (1984)

[ ss.

[[ tise,

[ luble

[[[ 77)

[ ry

[ s. J.

[ .

[ day

[ .[ olym.

[

preaders, which have the polyoxyethylene segment atto the middle silicon atom. The physical chemical behaor a series of such surfactants with a constant trisiloxhain and with varying number of oxyethylene units inolar headgroup has been studied in detail[4]. These surfac

ants exhibited similar phase behaviors with respect toumber of oxyethylene units as normal nonionic surfactstudy similar to the one presented here, where the

ion behavior of siloxane surfactants is studied as a funf the size of the hydrophobic segment, i.e., the numbimethylsiloxane units, seems not to have been reporte

ore.

cknowledgements

We like to thank the Fonds zur Forderung der wisenschaftlichen Forschung, Austria, the Swedish Foundor Strategic Research and the European Cooperationield of Scientific and Technical Research (COST) Ac19 for their support of this work.

3339.12] M. Carvell, D.G. Hall, I.G. Lyle, G.J.T. Tiddy, Faraday Discu

Chem. Soc. 81 (1986) 223.13] K. Devanand, J.C. Selser, Macromolecules 24 (1991) 5943.14] P. Molyneux, in: F. Frank (Ed.), Water—A Comprehensive Trea

Plenum Press, New York, 1975.15] D. Eagland, in: F. Franks (Ed.), Solution Properties of Water-So

Polymers, Cambridge University Press, Cambridge, 1989.16] S. Lusse, K. Arnold, Macromolecules 29 (1996) 4251.17] F. Caboi, M. Monduzzi, Langmuir 12 (15) (1996) 3548.18] N.-O. Persson, B. Lindman, Mol. Cryst. Liq. Cryst. 38 (19

327.19] J. Ulmius, H. Wennerstrom, G. Lindblom, A. Arvidson, Biochemist

16 (1977) 5742.20] L.M. Strenk, P.W. Westerman, N.A.P. Vaz, J.W. Doane, Biophy

48 (1985) 355.21] S. Lusse, R. Knauss, A. Werner, W. Grunder, K. Arnold, Magn

Reson. Med. 33 (1995) 483.22] M.J. Hey, S.M. Ilet, M. Mortimer, G. Oates, J. Chem. Soc., Fara

Trans. 86 (1990) 2673.23] P. Mariani, V. Luzzati, H. Delcroix, J. Mol. Biol. 204 (1988) 16524] P. Cancouet, E. Daudet, G. Helary, M. Moreau, G. Sauvet, J. P

Sci. Part A: Polym. Chem. 38 (5) (2000) 837.25] U. Jorzik, B.A. Wolf, Macromolecules 30 (1997) 4713.

http://dx.doi.org/10.1016/j.colsurfa.2004.11.011

http://dx.doi.org/10.1016/j.colsurfa.2004.11.011

the binary phase behavior of short-chain pdms-b-peo diblock copolymers in aqueous solutions in...

Documents