Particle-Induced Phase Separation in Quasi-BinaryPolymer Solutions

Martin Olsson, Fredrik Joabsson,† and Lennart Piculell*

Division of Physical Chemistry 1, Center for Chemistry and Chemical Engineering,Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Received October 15, 2003

A long-ranged attractive force was recently detected between two mica plates immersed in a quasi-binary polymer solution (Freyssingeas et al. Langmuir 1998, 14, 5877-5889). The quasi-binary polymersolution was aqueous ethyl(hydroxyethyl)cellulose (EHEC), where the EHEC had a broad polydispersity.The long-ranged attractive force in the EHEC solution could not be attributed to classical mechanismssuch as depletion or bridging. In this study, we investigated if this attractive force can give rise to instabilityeffects in mixed polymer-particle solutions. Accordingly, the effects of added particles on the phase behaviorof aqueous EHEC solutions were investigated by cloud point measurements. Aqueous EHEC solutionsphase separate on heating. Three different samples of EHEC were investigated, including hydrophobicallymodified EHEC. As colloidal particles, silica and polystyrene latex were used. The dispersed colloidalparticles lowered the cloud point temperature at low polymer concentrations for all EHEC-particlecombinations. This particle-induced phase separation is discussed in terms of surface effects.

Introduction

Mixtures of polymers and colloids are of general interestin colloid chemistry as a consequence of their wide rangeof applications. The stability of polymer-colloid mixturesis of fundamental importance and has been consideredexperimentally and theoretically in many previous in-vestigations. Two different classes of mixtures have beenstudied, namely, nonadsorbing polymers and colloids1-7

and adsorbing polymers and colloids.8-12 In the first case,a depletion layer will be formed in close vicinity to thesurfaces of the colloidal particles. If two such layerscoalesce, an attractive force between the particlesdevelops, and flocculation of the particles can occur. Inthe second case, the adsorbing polymer can give rise toattractive bridging forces between the colloidal particles,leading to flocculation; due to that a polymer strandadsorbs to more than one colloidal particle. Bridging forcestypically occur in solutions where the surfaces of thecolloidal particles are not fully covered with polymermaterial. At full surface coverage, a “steric” stabilizationof the particles occurs instead.

In addition to the above well-studied attractive mech-anisms in polymer-colloid mixtures, recent experimental

and theoretical studies have indicated that a thirdmechanism exists in polymer solutions close to phaseseparation.13-18 This third mechanism is a surface-inducedeffect, referred to as a capillary-induced phase separation(CIPS); see Figure 1.

The basic physics behind the CIPS is that a new“capillary” phase can be formed between two nearbysurfaces if the capillary phase has a lower interfacialenergy than the “reservoir” phase, even under conditionswhen the capillary phase would be unstable in the absenceof the surfaces.19 The condition for the CIPS to occur isthat the decrease in interfacial energy is larger than theincrease in bulk free energy of the capillary phase. Oncethe capillary phase is formed, an attractive force develops,since a decrease in the volume of the capillary phase willresult in a decrease in the unfavorable bulk contributionto the free energy. Recent surface force measurements

* Corresponding author. E-mail: Lennart.Piculell@fkem1.lu.se.Fax: +46 46 222 44 13.

† Present address: Camurus AB, Ideon Gamma 2, Solvegatan41, SE-223 70 Lund, Sweden.

(1) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255-1256.(2) Vrij, A. Pure Appl. Chem. 1976, 48, 471-483.(3) Gast, A. P.; Hall, C. K.; Russel, W. B. J. Colloid Interface Sci.

1983, 96, 251-267.(4) Lekkerkerker, H. N. W.; Poon, W. C. K.; Pusey, P. N.; Stroobants,

A.; Warren, P. B. Europhys. Lett. 1992, 20, 559-564.(5) Sear, R. P.; Frenkel, D. Phys. Rev. E 1997, 55, 1677-1681.(6) Warren, P. B. Langmuir 1997, 13, 4588-4594.(7) Lee, J. T.; Robert, M. Phys. Rev. E 1999, 60, 7198-7202.(8) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions;

Academic Press Inc.: London, 1983.(9) Klein, J.; Luckham, P. F. Nature 1984, 308, 836-837.(10) Dickinson, E.; Eriksson, L. Adv. Colloid Interface Sci. 1991, 34,

1-29.(11) Lafuma, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991,

143, 9-21.(12) Liu, S. F.; Legrand, V.; Gourmand, M.; Lafuma, F.; Audebert,

R. Colloid Surf., A 1996, 111, 139-145.

(13) Freyssingeas, E.; Thuresson, K.; Nylander, T.; Joabsson, F.;Lindman, B. Langmuir 1998, 14, 5877-5889.

(14) Wennerstrom, H.; Thuresson, K.; Linse, P.; Freyssingeas, E.Langmuir 1998, 14, 5664-5666.

(15) Chhajer, M.; Gujrati, P. D. J. Chem. Phys. 1998, 109, 11018-11026.

(16) Forsman, J.; Woodward, C. E.; Freasier, B. C. J. Chem. Phys.2002, 117, 1915-1926.

(17) Joabsson, F.; Linse, P. J. Phys. Chem. B 2002, 106, 3827-3834.(18) Olsson, M.; Linse, P.; Piculell, L. Langmuir 2004, 20, 1611-

1619.(19) Evans, D. F.; Wennerstrom, H. The Colloidal Domain. Where

Physics, Chemistry, Biology and Technology Meet, 2nd ed.; Wiley: NewYork, 1999.

Figure 1. A schematic picture of a CIPS in the gap betweentwo particles.

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10.1021/la035929m CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 01/22/2004

indicate that a long-ranged CIPS force arises betweentwo mica surfaces immersed in a solution of ethyl-(hydroxyethyl)cellulose (EHEC) in water.13 EHEC is anonionic “clouding” polymer that shows reversed-tem-perature-dependent phase behavior in water; i.e., anaqueous EHEC solution separates into two phases if thetemperature is raised. A similar long-ranged attractiveforce between two mica surfaces has also been shown ina ternary polymer solution containing two polymers thathave different affinity to the mica surfaces.14

The aim of this study is to investigate if the attractiveforces found in the surface force studies13,14 also manifestthemselves as a destabilization of mixed polymer-particlesolutions. The focus is on quasi-binary polymer solutions,i.e., solutions of a polydisperse polymer in water. Specif-ically, we study if the phase behavior of a quasi-binarypolymer solution is affected by dispersed colloidal particles.The polymer studied is EHEC, as in the recent surfaceforce study,13 and hydrophobically modified EHEC(HM-EHEC). To imitate the mica surfaces used in thesurface force study,13 colloidal silica is used. EHECdisplays a similar adsorption to these two surfaces.20,21

For comparison, we also study polystyrene latex particles,with a quite different surface. EHEC and HM-EHEC havean affinity to the surfaces of both types of particle, andtherefore the particles prefer to be in the concentratedpolymer phase after the macroscopic phase separation.In the accompanying paper to this investigation, thephenomenon is studied theoretically by a lattice mean-field theory.18 The latter theoretical study predicts anincrease of the two-phase region for the polymer solutionas a consequence of CIPS.

To distinguish the surface induced effects studied herefrom the two well-known destabilizing mechanisms ofdepletion and bridging, the conditions of investigation hadto be chosen properly. Here, all studies have been doneat conditions close to bulk phase separation for the polymersolutions. Furthermore, we have only studied cases wherethe polymer adsorbs to the particle. This will excludedepletion as a possible mechanism in the systems. Finally,to avoid bridging, low concentrations of the dispersedparticles have been used. Thus, the particle surfacesshould be well saturated with polymer even at the ratherlow polymer concentrations studied here.

Experimental Section

Materials. Ethyl(hydroxyethyl)cellulose (EHEC) and hydro-phobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC)were kind gifts from Akzo Nobel Surface Chemistry AB,Stenungsund, Sweden. Two different batches of EHEC were used,henceforth referred to as EHEC1 and EHEC2, with differentdegrees of substitution with ethyl and hydroxyethyl substituents.The degrees of substitution were given by the manufacturer andare presented in Table 1 as DSethyl and MSEO. DSethyl and MSEOrefer to the average number of substituents per glucose unit ofthe cellulose backbone. EHEC1 is identical to the EHEC usedin the surface force study mentioned earlier.13 HM-EHEC differs

from EHEC1 only by the introduction of a few hydrophobic graftsonto the backbone of EHEC1. The hydrophobic grafts werenonylphenol groups at a substitution degree (MShydrophob) givenin Table 1. Further, a fractionation of EHEC1 was accomplishedby heating a 2 wt % solution to 80 °C and letting the polymersolution phase separate macroscopically at this temperature intotwo clear phases, which were collected separately. The separationgave two fractions containing approximately equal amounts ofEHEC1. The two fractions were referred to as EHEC1LS andEHEC1MS, respectively.

EHEC1, EHEC2, and HM-EHEC were purified before use asdescribed elsewhere.22 The molecular weight averages Mm (massaverage) and Mn (number average) for the polymers includingEHEC1LS and EHEC1MS were determined by size exclusionchromatography (SEC) by Akzo Nobel Surface Chemistry AB.As calibration standards, pullulans of known molecular weightswere used. The determined Mm value and the polydispersity indexPI ) Mm/Mn of the polymers are presented in Table 1.

Silicaparticleswithameandiameterof100nmwerepurchasedfrom Nissan Chemicals, Japan, and obtained as a stock dispersionof 40.5 wt % particles in water. Polystyrene latex particles witha mean diameter of 350 nm were obtained from Polyscience Inc.,Warrington, USA, as a stock dispersion of 2.6 wt % particles inwater. Sodium chloride from Riedel-de Haen, Seelze, Germany,was used without any further purification. The water was ofMilliPore quality (resistivity ∼18 MΩ cm-1).

Methods. Samples were prepared by weighing the desiredamounts of polymer stock solution and water, or aqueous NaCl,directly into test tubes, which were sealed with screw caps madeof Teflon. Two identical samples were made for each studiedpolymer concentration. To one of the samples, colloidal particleswere added at a particle concentration of 0.05 wt % for silica, or0.01 wt % for polystyrene latex. All samples were equilibratedon a tilting board at room temperature for at least 12 h beforethe measurements.

The phase separation temperatures, Tp, of the solutions weretaken as the cloud points determined visually in a temperaturecontrolled water bath, where the temperature was raisedcontinuously by 0.5 °C/min. The particle concentrations werekept low not only to avoid polymer bridging in the mixed solutionsbut also to keep the turbidity of the particle dispersionssufficiently low so that it would not disturb the visual deter-mination of Tp. For each of the two different kinds of particles,the particle concentration was chosen to be close to this upperlimit. In the measurements, Tp was determined simultaneouslyfor a particle-containing sample and its particle-free referencesample in order to minimize the experimental error. By thisprocedure, the uncertainty in the particle-induced difference inTp was smaller than the reproducibility in the determination ofTp, which was within (0.5 °C.

Results

The cloud point curves of EHEC1 in water and in 20mM NaCl are shown in Figure 2. The polymer concentra-tion range was 0.1-1 wt % for all samples in this study.Above 1 wt % EHEC1, the cloud point hardly changes upto 20 wt %.23 The figure shows that 20 mM NaCl addedto the particle-free EHEC1 solutions gives a decrease ofTp by up to 5 °C in the investigated polymer concentrationrange. A similar effect of salt was seen for all EHECsamples. Experiments with added silica and latexparticles were also made both in pure water and in 20 mMNaCl. Salt screens the long-range electrostatic repulsionbetween the charged particles and should thus facilitatethe separation of a particle-rich phase. Indeed, this wasconfirmed by the experiments reported below.

Figure 3a compares the cloud point curves for EHEC1in pure water, with and without added polystyrene latexparticles. Clearly, the added particles lower the cloud point

(20) Malmsten, M.; Lindman, B. Langmuir 1990, 6, 357-364.(21) Pezron, I.; Pezron, E.; Claesson, P. M.; Malmsten, M. Langmuir

1991, 7, 2248-2252.

(22) Thuresson, K.; Karlstrom, G.; Lindman, B. J. Phys. Chem. 1995,99, 3823-3831.

(23) Joabsson, F.; Rosen, O.; Thuresson, K.; Piculell, L.; Lindman,B. J. Phys. Chem. B 1998, 102, 2954-2959.

Table 1. Degree of Substitution, Molecular Weight, andPolydispersity Index for the Polymers

polymer DSethyl MSEO MShydrophob Mm (Da) PI

EHEC1 1.0 2.0 5.4 × 105 6EHEC2 1.0 2.8 1.5 × 106 3HM-EHEC 1.0 2.8 0.008 5.1 × 105 7EHEC1MS 3.6 × 105 6EHEC1LS 7.1 × 105 3

1606 Langmuir, Vol. 20, No. 5, 2004 Olsson et al.

temperature, that is, the particles destabilize the system.In 20 mM salt (not shown), the lowering of Tp by theparticles was larger; however, it was also found that thisamount of salt was sufficient to make the particlescolloidally unstable. The particles would slowly (overnight)settle from the mixture at temperatures far below Tp. Forthis reason, all the experiments reported here for thepolystyrene latex particles refer to salt-free solutions.Figure 3b shows a similar lowering of the cloud point ofEHEC1 when silica particles were added, this time in thepresence of 20 mM NaCl. In contrast to the polystyrenelatex particles, the silica particles were colloidally stablein the polymer solution also in the presence of salt. In thesalt-free systems, the lowering of Tp by added silica wasfound to be quite small. Hence, all our reported experi-ments on silica particles refer to 20 mM salt solutions.The impact of the colloidal particles on the phase behaviorseen in Figure 3 can be summarized by two mainconclusions. First, Tp decreases for the EHEC solutions,i.e., the two-phase region for the solution increases by thedispersed colloidal particles. Second, the decrease in Tpby the dispersed colloidal particles is larger at lowerpolymer concentrations.

If an EHEC solution is kept at a temperature above thecloud point, the cloudy solution eventually separates intotwo clear phases, where one is concentrated in polymerand the other is dilute. The concentrated phase ischaracterized by a much larger viscosity than the dilutephase. After similar macroscopic phase separation ex-periments on EHEC solutions containing particles, weinvariably (for all the polymer-particle mixtures studiedhere) found that virtually all particles collected in theviscous concentrated phase. The dilute phase was trans-parent and scattered no light after the phase separationwas complete. Hence, the phase separation was of theassociative type, according to the terminology introducedpreviously by one of us.24 By contrast, a phase separationcaused by depletion is segregative; i.e., the particles andpolymers are enriched in different phases.

It is important to appreciate the fact that the particlesin the concentrated phase of the phase-separated EHEC-particle mixtures were still colloidally stable. The con-centrated phase could easily be distinguished from, e.g.,

the low-viscous sediments formed in unstable solutions ofpolystyrene latex and EHEC in 20 mM NaCl. Moreover,the thermally induced phase separation of an EHEC-particle mixture was always completely reversible, justas for the reference EHEC solution. When the temperaturewas decreased below Tp, a single solution phase with stabledispersed particles reemerged. In conclusion, the observedphenomenon is not best understood in terms of the colloidalstability of the particles. The nature of the phase separa-tion with added particles is still intrinsically a polymer-solvent phase separation, but the tendency for phaseseparation is enhanced by the added particles.

The macroscopic phase separation was used as a furthertest to confirm the particle-induced shifts in Tp, shown inFigure 3. Some sample pairs, with and without dispersedparticles, were kept during ca. 12 h at a temperature belowthe measured Tp for the reference polymer solution butabove Tp for the polymer-particle mixture. The sampleswith the particles showed a macroscopic phase separationat these conditions while the reference samples did notphase separate.

To gain a better understanding of the parameters thataffect the magnitude of the shift in Tp by dispersedparticles, further studies were done using different

(24) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41,149-178.

Figure 2. Cloud point curves of EHEC1 in water (opendiamonds), EHEC1 in 20 mM NaCl (filled diamonds), EHEC1-MS in water (triangles pointing upward), and EHEC1LS inwater (triangles pointing downward).

Figure 3. Cloud point curve of EHEC1 (a) with (filled circles)and without (open circles) dispersed polystyrene latex particlesin water and (b) with (filled circles) and without (open circles)dispersed silica particles in 20 mM NaCl. The concentrationsof particles in the solutions were 0.01 wt % of polystyrene latexand 0.05 wt % of silica, respectively.

Particle-Induced Phase Separation Langmuir, Vol. 20, No. 5, 2004 1607

samples of EHEC. Two of these samples were obtained byfractionation of EHEC1. The conditions for the fraction-ation are described in the experimental part. Cloud pointcurves for the two fractions in water are shown in Figure2. Clearly, the fractionation had resulted in one less solublefraction (EHEC1LS), with a cloud-point curve significantlydisplaced toward lower temperatures compared to that ofEHEC1, and one more soluble fraction (EHEC1MS), witha cloud point curve shifted upward by up to 8 °C. As mightbe expected, Table 1 shows that EHEC1LS was enrichedin longer polymer molecules, whereas EHEC1MS wasenriched in shorter polymer molecules. The polydispersityof EHEC1LS was significantly lower than that ofEHEC1MS.

Figure 4 presents the cloud point curves with andwithout polystyrene latex particles for the two fractionsof EHEC1 in water. The trends are the same as forunfractionated EHEC1; i.e., the two-phase region in-creases with added particles and the change in Tp is largerat low polymer concentrations. However, the shift in Tpby added polystyrene latex particles was larger for themore soluble fraction than for the less soluble fraction ofEHEC1. Dispersed silica particles in 20 mM NaCl gavesimilar effects (not shown). To exclude the possibility thatthe heating in the fractionation procedure had degradedthe polymer (which would affect the phase behavior), the

EHEC1LS and EHEC1MS fractions were mixed togetherin a control experiment, and the phase behavior of thisremixed solution was investigated. The cloud point curvesfor the remixed EHEC1 with and without salt were almostidentical to the cloud point curves for the unfractionatedEHEC1 presented in Figure 2. Moreover, dispersedpolystyrene latex or silica particles in the remixed EHEC1gave similar effects on the phase behavior as those shownin Figure 3.

The effects of dispersed particles for a different EHEC,EHEC2, were also investigated and are shown in Figure5a for polystyrene latex and in Figure 5b for silica. Silicaand polystyrene latex particles affected the phase behaviorfor EHEC2 in the same manner as was seen for EHEC1.However, the effects of the dispersed particles were smallerfor EHEC2 than for EHEC1.

Finally, the phase behavior and the influence ofdispersed particles were studied for HM-EHEC. Thehydrophobic groups are grafted on the glucose units inHM-EHEC. The hydrophobic groups give a lower Tp forHM-EHEC compared to EHEC1 as seen in Figure 6. Alsohere dispersed polystyrene latex or silica particles giverise to an increase in the two-phase region, although it isnot evident that a larger shift in Tp occurs for the lowerpolymer concentration.

Figure 4. Cloud point curve of fractionated EHEC1 in waterfor (a) EHEC1LS and (b) EHEC1MS with (filled circles) andwithout (open circles) dispersed polystyrene latex particles ata concentration of 0.01 wt % particles in the solutions.

Figure 5. Cloud point curve of EHEC2 (a) with (filled circles)and without (open circles) dispersed polystyrene latex particlesin water and (b) with (filled circles) and without (open circles)dispersed silica particles in 20 mM NaCl. The concentrationsof particles are as in Figure 3.

1608 Langmuir, Vol. 20, No. 5, 2004 Olsson et al.

Discussion

Dispersed particles have been seen to affect the phasebehavior of all the investigated quasi-binary polymersolutions. The cloud points for the various polymersolutions were lowered by up to 5 °C by the dispersedparticles. A common feature of all the investigatedpolymer-particle pairs is that the polymer adsorbs to theparticle surface.20,25,26 This results in an affinity of theparticle to the phase concentrated in polymer in phase-separated systems, and the phase separation is thusassociative, rather than segregative. Therefore, depletionis not a possible explanation to the observed increasedinstability.

To check whether bridging could contribute to theincreased instability, a parameter to consider in theexperiments is the amount of polymer per surface areafor the various polymer-particle mixtures. Previousstudies on the adsorption of EHEC to macroscopic surfacesreveal that the plateau value of the adsorption isothermis 0.4 mg/m2 for EHEC1 and 0.6 mg/m2 for HM-EHEC onsilica surfaces.26 The adsorption of EHEC2 to silica

surfaces has not been studied, but it should be similar. Inour experiments on solutions containing 0.05 wt %dispersed silica particles, the amount of polymer per silicasurface area was ca. 80 mg/m2 for the lowest studiedpolymer concentration and ca. 800 mg polymer/m2 for thehighest polymer concentration. Clearly, these proportionsshow that the experiments were carried out underconditions far above surface saturation. The same holdstrue for the systems containing dispersed polystyrene latexparticles. At a concentration of 0.01 wt % polystyrene latexparticles, our samples contained between 600 and 6000mg of polymer/m2 of polystyrene surface area. At hydro-phobic surfaces such as polystyrene, EHEC adsorbsslightly worse and HM-EHEC adsorbs slightly better thanto silica surfaces.20,25,26

Having rejected depletion and bridging, we propose thatthe effect of the dispersed particles on the phase behaviorof the polymer solutions is due to surface effects of theparticles. The interfacial free energy due to the dispersedparticles is lowered on phase separation, when theparticles are collected in the phase concentrated inpolymer. Thus, the driving force is the same as in theCIPS phenomenon observed in the surface force measure-ments done on EHEC.3 The latter conclusion is confirmedfurther by the accompanying theoretical study of CIPS inquasi-binary polymer solutions.18 In the latter study, themagnitude of the surface-induced lowering of Tp was alsocomparable to that observed experimentally in this study.

It is important to realize that the polymeric nature ofthe solutes in our study is not a requirement for CIPS orparticle-enhanced phase separation to occursalthough itmay be quite important indirectly, since both the tendencyfor phase separation and the tendency for surface adsorp-tion is enhanced in polymeric solutions. Similar effects ofparticles have indeed been found previously in simplebinary solutions. Studying a solution of an adsorbing solutein a solvent containing dispersed particles, Beysens andEsteve first showed that in a binary mixture of lutidineand water, the cloud point curve on the water-rich sideof the lower consolute point was changed under conditionswhen lutidine adsorbed to the dispersed particles.27 Thestudy by Beysens and Esteve has been followed by otherstudies on the lutidine-water system.28-32 In analogy withthe binary solution of lutidine and water, the phasediagrams of the polymer solutions studied here wereaffected by the dispersed particles on the water-rich, orthe polymer-poor, side of the lower consolute point. Thesame trend was found in the accompanying theoreticalstudy of CIPS in quasi-binary polymer solutions.18

Although a particle-enhanced phase separation wasseen in all systems studied here, there were somequantitative differences that should be discussed. Poly-styrene latex affected the salt-free systems more thansilica. However, when small amounts of salt were addedto the silica solutions, the influences of the particles weresimilar except for EHEC1. This can possibly be a resultof that the magnitude of the particle-induced effectdepends not only on the surface interaction but also onthe translational entropy loss associated with placing allthe particles in one of the phases after phase separation.

(25) Malmsten, M.; Tiberg, F. Langmuir 1993, 9, 1098-1103.(26) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17,

1499-1505.

(27) Beysens, D.; Esteve, D. Phys. Rev. Lett. 1985, 54, 2123-2126.(28) Gurfein, V.; Beysens, D.; Perrot, F. Phys. Rev. A 1989, 40, 2543-

2546.(29) Van Duijneveldt, J. S.; Beysens, D. J. Chem. Phys. 1991, 94,

5222-5225.(30) Gallagher, P. D.; Maher, J. V. Phys. Rev. A 1992, 46, 2012-

2021.(31) Gallagher, P. D.; Kurnaz, M. L.; Maher, J. V. Phys. Rev. A 1992,

46, 7750-7755.(32) Beysens, D.; Narayanan, T. J. Stat. Phys. 1999, 95, 997-1008.

Figure 6. Cloud point curve of HM-EHEC a) with (filled circles)and without (open circles) dispersed polystyrene latex particlesin water and b) with (filled circles) and without (open circles)dispersed silica particles in 20 mM NaCl. The concentrationsof particles are as in Figure 3.

Particle-Induced Phase Separation Langmuir, Vol. 20, No. 5, 2004 1609

This entropy loss, in turn, depends not only on the numberof particles but also on their charge and on added salt, viathe translational entropy of the counterions.

The change in Tp by dispersing particles in the polymersolutions has been shown to be larger at low polymerconcentrations except for the solution with HM-EHEC inwater. The larger change in Tp at low polymer concentra-tions can be understood from the fact that the systemgains more in surface free energy on phase separationfurther away from the lower consolute point promoting alarger shift in phase behavior.

In addition to the surface affinity, the molecular weightand the polydispersity of the polymer are parameters thathave been shown theoretically to influence the surface-induced phase separation of a quasi-binary polymersolution.18 Table 1 lists the molecular weights andpolydispersity indices for the investigated solutions. Thesedata show that PI is much larger for EHEC1 than forEHEC2. Therefore, the stronger influence on the phasebehavior by dispersed particles for EHEC1 than forEHEC2 seen in Figures 3 and 5 could be an effect of thelarger polydispersity. This correlation between polydis-persity and the magnitude of the particle-induced loweringof Tp holds, in fact, for the entire set of experiments (seeTable 1 and Figures 3-6): The only other sample with asignificantly reduced PI, compared to EHEC1, wasEHEC1LS, and only for this sample, the decrease in Tpwas significantly smaller than that for EHEC1. Since theeffect of polydispersity is confirmed also in the ac-companying theoretical study, we believe that it is a realeffect. In this context, we also note another interestingcorrelation found experimentally. In all the studiedsystems in Figures 3-6 it is found that the effect of addedparticles on Tp is small in phase diagrams, or regions ofa phase diagram, where the dependence of Tp on thepolymer concentration is weak. We have no explanationfor this observation at this stage, and more data wouldobviously be required to establish whether it is just acoincidence.

Finally, we wish to compare the previous surface forceresults from EHEC solutions13 with the results obtainedhere on dispersed particles. In the surface force experi-ment, attractive forces between two mica plates occurred

in EHEC1 solutions already at room temperature, whilein the present study the dispersed particles were seen toaffect the phase behavior for EHEC1 only at significantlyhigher temperatures, close to Tp. One factor contributingto this discrepancy could be that the surface-to-volumeratio is very much smaller in the surface force experiment.This means that the fractionation of the polymer could bemuch stronger in the surface force experiment. That is,the composition of the polymer quasi-component (molarmass distribution, pattern of substitution) in the capillaryphase could be quite different from that in the reservoir.A second difference to note is that the loss in translationalentropy of the particles on phase separation is absent inthe surface force experiment.

Conclusions

Effects of dispersed colloidal particles on the phasebehavior of quasi-binary polymer solutions near phaseseparation have been investigated in systems where thepolymers adsorb to the surfaces of the colloidal particles.The dispersed particles were shown to enhance the phaseseparation of the polymer solution, giving a shift in thecloud point temperature by up to ca. 5 °C. Parameterssuch as polymer polydispersity and particle charge wereseen to influence the magnitude of the effect. The resultsare interpreted in terms of a surface contribution to thefree energy of phase separation. The mechanism isdifferent from the well-known depletion and bridgingmechanisms for phase separation in mixed polymer-particle solutions. The results in this study are inqualitative agreement with theoretical results on surface-induced phase separation for adsorbing quasi-binarypolymer solutions.18

Acknowledgment. Malin Juberg at Akzo NobelSurface Chemistry AB, Stenungsund, Sweden, is grate-fully acknowledged for performing the SEC measurementsand Per Linse for valuable comments on the manuscript.This work was funded by the Centre for AmphiphilicPolymers from Renewable Resources (M.O.) and theSwedish Research Council (L.P.).

LA035929M

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