Particle-Induced Phase Separation in Quasi-Binary Polymer Solutions

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Particle-Induced Phase Separation in Quasi-Binary Polymer 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 quasibinary polymer solution (Freyssingeas et al. Langmuir 1998, 14, 5877-5889). The quasi-binary polymer solution 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 mechanisms such as depletion or bridging. In this study, we investigated if this attractive force can give rise to instability effects in mixed polymer-particle solutions. Accordingly, the effects of added particles on the phase behavior of aqueous EHEC solutions were investigated by cloud point measurements. Aqueous EHEC solutions phase separate on heating. Three different samples of EHEC were investigated, including hydrophobically modified EHEC. As colloidal particles, silica and polystyrene latex were used. The dispersed colloidal particles lowered the cloud point temperature at low polymer concentrations for all EHEC-particle combinations. This particle-induced phase separation is discussed in terms of surface effects.

Introduction Mixtures of polymers and colloids are of general interest in colloid chemistry as a consequence of their wide range of applications. The stability of polymer-colloid mixtures is of fundamental importance and has been considered experimentally and theoretically in many previous investigations. Two different classes of mixtures have been studied, 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 the surfaces of the colloidal particles. If two such layers coalesce, an attractive force between the particles develops, and flocculation of the particles can occur. In the second case, the adsorbing polymer can give rise to attractive bridging forces between the colloidal particles, leading to flocculation; due to that a polymer strand adsorbs to more than one colloidal particle. Bridging forces typically occur in solutions where the surfaces of the colloidal particles are not fully covered with polymer material. At full surface coverage, a “steric” stabilization of the particles occurs instead. In addition to the above well-studied attractive mechanisms in polymer-colloid mixtures, recent experimental * Corresponding author. E-mail: [email protected]. Fax: +46 46 222 44 13. † Present address: Camurus AB, Ideon Gamma 2, So ¨ lvegatan 41, 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.

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

and theoretical studies have indicated that a third mechanism exists in polymer solutions close to phase separation.13-18 This third mechanism is a surface-induced effect, 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 nearby surfaces if the capillary phase has a lower interfacial energy than the “reservoir” phase, even under conditions when the capillary phase would be unstable in the absence of the surfaces.19 The condition for the CIPS to occur is that the decrease in interfacial energy is larger than the increase in bulk free energy of the capillary phase. Once the capillary phase is formed, an attractive force develops, since a decrease in the volume of the capillary phase will result in a decrease in the unfavorable bulk contribution to the free energy. Recent surface force measurements (13) Freyssingeas, E.; Thuresson, K.; Nylander, T.; Joabsson, F.; Lindman, B. Langmuir 1998, 14, 5877-5889. (14) Wennerstro¨m, H.; Thuresson, K.; Linse, P.; Freyssingeas, E. Langmuir 1998, 14, 5664-5666. (15) Chhajer, M.; Gujrati, P. D. J. Chem. Phys. 1998, 109, 1101811026. (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, 16111619. (19) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain. Where Physics, Chemistry, Biology and Technology Meet, 2nd ed.; Wiley: New York, 1999.

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

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Table 1. Degree of Substitution, Molecular Weight, and Polydispersity Index for the Polymers polymer

DSethyl

MSEO

EHEC1 EHEC2 HM-EHEC EHEC1MS EHEC1LS

1.0 1.0 1.0

2.0 2.8 2.8

MShydrophob

Mm (Da)

PI

0.008

5.4 × 105 1.5 × 106 5.1 × 105 3.6 × 105 7.1 × 105

6 3 7 6 3

indicate that a long-ranged CIPS force arises between two mica surfaces immersed in a solution of ethyl(hydroxyethyl)cellulose (EHEC) in water.13 EHEC is a nonionic “clouding” polymer that shows reversed-temperature-dependent phase behavior in water; i.e., an aqueous EHEC solution separates into two phases if the temperature is raised. A similar long-ranged attractive force between two mica surfaces has also been shown in a ternary polymer solution containing two polymers that have different affinity to the mica surfaces.14 The aim of this study is to investigate if the attractive forces found in the surface force studies13,14 also manifest themselves as a destabilization of mixed polymer-particle solutions. The focus is on quasi-binary polymer solutions, i.e., solutions of a polydisperse polymer in water. Specifically, we study if the phase behavior of a quasi-binary polymer solution is affected by dispersed colloidal particles. The polymer studied is EHEC, as in the recent surface force study,13 and hydrophobically modified EHEC (HM-EHEC). To imitate the mica surfaces used in the surface force study,13 colloidal silica is used. EHEC displays 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 have an affinity to the surfaces of both types of particle, and therefore the particles prefer to be in the concentrated polymer phase after the macroscopic phase separation. In the accompanying paper to this investigation, the phenomenon is studied theoretically by a lattice meanfield theory.18 The latter theoretical study predicts an increase of the two-phase region for the polymer solution as a consequence of CIPS. To distinguish the surface induced effects studied here from the two well-known destabilizing mechanisms of depletion and bridging, the conditions of investigation had to be chosen properly. Here, all studies have been done at conditions close to bulk phase separation for the polymer solutions. Furthermore, we have only studied cases where the polymer adsorbs to the particle. This will exclude depletion as a possible mechanism in the systems. Finally, to avoid bridging, low concentrations of the dispersed particles have been used. Thus, the particle surfaces should be well saturated with polymer even at the rather low polymer concentrations studied here. Experimental Section Materials. Ethyl(hydroxyethyl)cellulose (EHEC) and hydrophobically 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 different degrees of substitution with ethyl and hydroxyethyl substituents. The degrees of substitution were given by the manufacturer and are presented in Table 1 as DSethyl and MSEO. DSethyl and MSEO refer to the average number of substituents per glucose unit of the cellulose backbone. EHEC1 is identical to the EHEC used in the surface force study mentioned earlier.13 HM-EHEC differs (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.

from EHEC1 only by the introduction of a few hydrophobic grafts onto the backbone of EHEC1. The hydrophobic grafts were nonylphenol groups at a substitution degree (MShydrophob) given in Table 1. Further, a fractionation of EHEC1 was accomplished by heating a 2 wt % solution to 80 °C and letting the polymer solution phase separate macroscopically at this temperature into two clear phases, which were collected separately. The separation gave two fractions containing approximately equal amounts of EHEC1. The two fractions were referred to as EHEC1LS and EHEC1MS, respectively. EHEC1, EHEC2, and HM-EHEC were purified before use as described elsewhere.22 The molecular weight averages Mm (mass average) and Mn (number average) for the polymers including EHEC1LS and EHEC1MS were determined by size exclusion chromatography (SEC) by Akzo Nobel Surface Chemistry AB. As calibration standards, pullulans of known molecular weights were used. The determined Mm value and the polydispersity index PI ) Mm/Mn of the polymers are presented in Table 1. Silica particles with a mean diameter of 100 nm were purchased from Nissan Chemicals, Japan, and obtained as a stock dispersion of 40.5 wt % particles in water. Polystyrene latex particles with a mean diameter of 350 nm were obtained from Polyscience Inc., Warrington, USA, as a stock dispersion of 2.6 wt % particles in water. Sodium chloride from Riedel-de Hae¨n, Seelze, Germany, was used without any further purification. The water was of MilliPore quality (resistivity ∼18 MΩ cm-1). Methods. Samples were prepared by weighing the desired amounts of polymer stock solution and water, or aqueous NaCl, directly into test tubes, which were sealed with screw caps made of Teflon. Two identical samples were made for each studied polymer concentration. To one of the samples, colloidal particles were added at a particle concentration of 0.05 wt % for silica, or 0.01 wt % for polystyrene latex. All samples were equilibrated on a tilting board at room temperature for at least 12 h before the measurements. The phase separation temperatures, Tp, of the solutions were taken as the cloud points determined visually in a temperature controlled water bath, where the temperature was raised continuously by 0.5 °C/min. The particle concentrations were kept low not only to avoid polymer bridging in the mixed solutions but also to keep the turbidity of the particle dispersions sufficiently low so that it would not disturb the visual determination of Tp. For each of the two different kinds of particles, the particle concentration was chosen to be close to this upper limit. In the measurements, Tp was determined simultaneously for a particle-containing sample and its particle-free reference sample in order to minimize the experimental error. By this procedure, the uncertainty in the particle-induced difference in Tp was smaller than the reproducibility in the determination of Tp, which was within (0.5 °C.

Results The cloud point curves of EHEC1 in water and in 20 mM NaCl are shown in Figure 2. The polymer concentration range was 0.1-1 wt % for all samples in this study. Above 1 wt % EHEC1, the cloud point hardly changes up to 20 wt %.23 The figure shows that 20 mM NaCl added to the particle-free EHEC1 solutions gives a decrease of Tp by up to 5 °C in the investigated polymer concentration range. A similar effect of salt was seen for all EHEC samples. Experiments with added silica and latex particles were also made both in pure water and in 20 mM NaCl. Salt screens the long-range electrostatic repulsion between the charged particles and should thus facilitate the separation of a particle-rich phase. Indeed, this was confirmed by the experiments reported below. Figure 3a compares the cloud point curves for EHEC1 in pure water, with and without added polystyrene latex particles. Clearly, the added particles lower the cloud point (22) Thuresson, K.; Karlstro¨m, 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.

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Figure 2. Cloud point curves of EHEC1 in water (open diamonds), EHEC1 in 20 mM NaCl (filled diamonds), EHEC1MS in water (triangles pointing upward), and EHEC1LS in water (triangles pointing downward).

temperature, that is, the particles destabilize the system. In 20 mM salt (not shown), the lowering of Tp by the particles was larger; however, it was also found that this amount of salt was sufficient to make the particles colloidally unstable. The particles would slowly (overnight) settle from the mixture at temperatures far below Tp. For this reason, all the experiments reported here for the polystyrene latex particles refer to salt-free solutions. Figure 3b shows a similar lowering of the cloud point of EHEC1 when silica particles were added, this time in the presence of 20 mM NaCl. In contrast to the polystyrene latex particles, the silica particles were colloidally stable in the polymer solution also in the presence of salt. In the salt-free systems, the lowering of Tp by added silica was found to be quite small. Hence, all our reported experiments on silica particles refer to 20 mM salt solutions. The impact of the colloidal particles on the phase behavior seen in Figure 3 can be summarized by two main conclusions. First, Tp decreases for the EHEC solutions, i.e., the two-phase region for the solution increases by the dispersed colloidal particles. Second, the decrease in Tp by the dispersed colloidal particles is larger at lower polymer concentrations. If an EHEC solution is kept at a temperature above the cloud point, the cloudy solution eventually separates into two clear phases, where one is concentrated in polymer and the other is dilute. The concentrated phase is characterized by a much larger viscosity than the dilute phase. After similar macroscopic phase separation experiments on EHEC solutions containing particles, we invariably (for all the polymer-particle mixtures studied here) found that virtually all particles collected in the viscous concentrated phase. The dilute phase was transparent and scattered no light after the phase separation was complete. Hence, the phase separation was of the associative type, according to the terminology introduced previously by one of us.24 By contrast, a phase separation caused by depletion is segregative; i.e., the particles and polymers are enriched in different phases. It is important to appreciate the fact that the particles in the concentrated phase of the phase-separated EHECparticle mixtures were still colloidally stable. The concentrated phase could easily be distinguished from, e.g., (24) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149-178.

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

the low-viscous sediments formed in unstable solutions of polystyrene latex and EHEC in 20 mM NaCl. Moreover, the thermally induced phase separation of an EHECparticle mixture was always completely reversible, just as for the reference EHEC solution. When the temperature was decreased below Tp, a single solution phase with stable dispersed particles reemerged. In conclusion, the observed phenomenon is not best understood in terms of the colloidal stability of the particles. The nature of the phase separation with added particles is still intrinsically a polymersolvent phase separation, but the tendency for phase separation is enhanced by the added particles. The macroscopic phase separation was used as a further test to confirm the particle-induced shifts in Tp, shown in Figure 3. Some sample pairs, with and without dispersed particles, were kept during ca. 12 h at a temperature below the measured Tp for the reference polymer solution but above Tp for the polymer-particle mixture. The samples with the particles showed a macroscopic phase separation at these conditions while the reference samples did not phase separate. To gain a better understanding of the parameters that affect the magnitude of the shift in Tp by dispersed particles, further studies were done using different

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Figure 4. Cloud point curve of fractionated EHEC1 in water for (a) EHEC1LS and (b) EHEC1MS with (filled circles) and without (open circles) dispersed polystyrene latex particles at a 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 particles in water and (b) with (filled circles) and without (open circles) dispersed silica particles in 20 mM NaCl. The concentrations of particles are as in Figure 3.

samples of EHEC. Two of these samples were obtained by fractionation of EHEC1. The conditions for the fractionation are described in the experimental part. Cloud point curves for the two fractions in water are shown in Figure 2. Clearly, the fractionation had resulted in one less soluble fraction (EHEC1LS), with a cloud-point curve significantly displaced toward lower temperatures compared to that of EHEC1, and one more soluble fraction (EHEC1MS), with a cloud point curve shifted upward by up to 8 °C. As might be expected, Table 1 shows that EHEC1LS was enriched in longer polymer molecules, whereas EHEC1MS was enriched in shorter polymer molecules. The polydispersity of EHEC1LS was significantly lower than that of EHEC1MS. Figure 4 presents the cloud point curves with and without polystyrene latex particles for the two fractions of EHEC1 in water. The trends are the same as for unfractionated EHEC1; i.e., the two-phase region increases with added particles and the change in Tp is larger at low polymer concentrations. However, the shift in Tp by added polystyrene latex particles was larger for the more soluble fraction than for the less soluble fraction of EHEC1. Dispersed silica particles in 20 mM NaCl gave similar effects (not shown). To exclude the possibility that the heating in the fractionation procedure had degraded the polymer (which would affect the phase behavior), the

EHEC1LS and EHEC1MS fractions were mixed together in a control experiment, and the phase behavior of this remixed solution was investigated. The cloud point curves for the remixed EHEC1 with and without salt were almost identical to the cloud point curves for the unfractionated EHEC1 presented in Figure 2. Moreover, dispersed polystyrene latex or silica particles in the remixed EHEC1 gave similar effects on the phase behavior as those shown in Figure 3. The effects of dispersed particles for a different EHEC, EHEC2, were also investigated and are shown in Figure 5a for polystyrene latex and in Figure 5b for silica. Silica and polystyrene latex particles affected the phase behavior for EHEC2 in the same manner as was seen for EHEC1. However, the effects of the dispersed particles were smaller for EHEC2 than for EHEC1. Finally, the phase behavior and the influence of dispersed particles were studied for HM-EHEC. The hydrophobic groups are grafted on the glucose units in HM-EHEC. The hydrophobic groups give a lower Tp for HM-EHEC compared to EHEC1 as seen in Figure 6. Also here dispersed polystyrene latex or silica particles give rise to an increase in the two-phase region, although it is not evident that a larger shift in Tp occurs for the lower polymer concentration.

Particle-Induced Phase Separation

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

Discussion Dispersed particles have been seen to affect the phase behavior of all the investigated quasi-binary polymer solutions. The cloud points for the various polymer solutions were lowered by up to 5 °C by the dispersed particles. A common feature of all the investigated polymer-particle pairs is that the polymer adsorbs to the particle surface.20,25,26 This results in an affinity of the particle to the phase concentrated in polymer in phaseseparated systems, and the phase separation is thus associative, rather than segregative. Therefore, depletion is not a possible explanation to the observed increased instability. To check whether bridging could contribute to the increased instability, a parameter to consider in the experiments is the amount of polymer per surface area for the various polymer-particle mixtures. Previous studies on the adsorption of EHEC to macroscopic surfaces reveal that the plateau value of the adsorption isotherm is 0.4 mg/m2 for EHEC1 and 0.6 mg/m2 for HM-EHEC on silica surfaces.26 The adsorption of EHEC2 to silica (25) Malmsten, M.; Tiberg, F. Langmuir 1993, 9, 1098-1103. (26) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499-1505.

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surfaces has not been studied, but it should be similar. In our experiments on solutions containing 0.05 wt % dispersed silica particles, the amount of polymer per silica surface area was ca. 80 mg/m2 for the lowest studied polymer concentration and ca. 800 mg polymer/m2 for the highest polymer concentration. Clearly, these proportions show that the experiments were carried out under conditions far above surface saturation. The same holds true for the systems containing dispersed polystyrene latex particles. At a concentration of 0.01 wt % polystyrene latex particles, our samples contained between 600 and 6000 mg of polymer/m2 of polystyrene surface area. At hydrophobic surfaces such as polystyrene, EHEC adsorbs slightly worse and HM-EHEC adsorbs slightly better than to silica surfaces.20,25,26 Having rejected depletion and bridging, we propose that the effect of the dispersed particles on the phase behavior of the polymer solutions is due to surface effects of the particles. The interfacial free energy due to the dispersed particles is lowered on phase separation, when the particles are collected in the phase concentrated in polymer. Thus, the driving force is the same as in the CIPS phenomenon observed in the surface force measurements done on EHEC.3 The latter conclusion is confirmed further by the accompanying theoretical study of CIPS in quasi-binary polymer solutions.18 In the latter study, the magnitude of the surface-induced lowering of Tp was also comparable to that observed experimentally in this study. It is important to realize that the polymeric nature of the solutes in our study is not a requirement for CIPS or particle-enhanced phase separation to occursalthough it may be quite important indirectly, since both the tendency for phase separation and the tendency for surface adsorption is enhanced in polymeric solutions. Similar effects of particles have indeed been found previously in simple binary solutions. Studying a solution of an adsorbing solute in a solvent containing dispersed particles, Beysens and Este`ve first showed that in a binary mixture of lutidine and water, the cloud point curve on the water-rich side of the lower consolute point was changed under conditions when lutidine adsorbed to the dispersed particles.27 The study by Beysens and Este`ve has been followed by other studies on the lutidine-water system.28-32 In analogy with the binary solution of lutidine and water, the phase diagrams of the polymer solutions studied here were affected by the dispersed particles on the water-rich, or the polymer-poor, side of the lower consolute point. The same trend was found in the accompanying theoretical study of CIPS in quasi-binary polymer solutions.18 Although a particle-enhanced phase separation was seen in all systems studied here, there were some quantitative differences that should be discussed. Polystyrene latex affected the salt-free systems more than silica. However, when small amounts of salt were added to the silica solutions, the influences of the particles were similar except for EHEC1. This can possibly be a result of that the magnitude of the particle-induced effect depends not only on the surface interaction but also on the translational entropy loss associated with placing all the particles in one of the phases after phase separation. (27) Beysens, D.; Este`ve, D. Phys. Rev. Lett. 1985, 54, 2123-2126. (28) Gurfein, V.; Beysens, D.; Perrot, F. Phys. Rev. A 1989, 40, 25432546. (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, 20122021. (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.

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This entropy loss, in turn, depends not only on the number of particles but also on their charge and on added salt, via the translational entropy of the counterions. The change in Tp by dispersing particles in the polymer solutions has been shown to be larger at low polymer concentrations except for the solution with HM-EHEC in water. The larger change in Tp at low polymer concentrations can be understood from the fact that the system gains more in surface free energy on phase separation further away from the lower consolute point promoting a larger shift in phase behavior. In addition to the surface affinity, the molecular weight and the polydispersity of the polymer are parameters that have been shown theoretically to influence the surfaceinduced phase separation of a quasi-binary polymer solution.18 Table 1 lists the molecular weights and polydispersity indices for the investigated solutions. These data show that PI is much larger for EHEC1 than for EHEC2. Therefore, the stronger influence on the phase behavior by dispersed particles for EHEC1 than for EHEC2 seen in Figures 3 and 5 could be an effect of the larger polydispersity. This correlation between polydispersity and the magnitude of the particle-induced lowering of Tp holds, in fact, for the entire set of experiments (see Table 1 and Figures 3-6): The only other sample with a significantly reduced PI, compared to EHEC1, was EHEC1LS, and only for this sample, the decrease in Tp was significantly smaller than that for EHEC1. Since the effect of polydispersity is confirmed also in the accompanying theoretical study, we believe that it is a real effect. In this context, we also note another interesting correlation found experimentally. In all the studied systems in Figures 3-6 it is found that the effect of added particles on Tp is small in phase diagrams, or regions of a phase diagram, where the dependence of Tp on the polymer concentration is weak. We have no explanation for this observation at this stage, and more data would obviously be required to establish whether it is just a coincidence. Finally, we wish to compare the previous surface force results from EHEC solutions13 with the results obtained here on dispersed particles. In the surface force experiment, attractive forces between two mica plates occurred

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in EHEC1 solutions already at room temperature, while in the present study the dispersed particles were seen to affect the phase behavior for EHEC1 only at significantly higher temperatures, close to Tp. One factor contributing to this discrepancy could be that the surface-to-volume ratio is very much smaller in the surface force experiment. This means that the fractionation of the polymer could be much stronger in the surface force experiment. That is, the composition of the polymer quasi-component (molar mass distribution, pattern of substitution) in the capillary phase could be quite different from that in the reservoir. A second difference to note is that the loss in translational entropy of the particles on phase separation is absent in the surface force experiment. Conclusions Effects of dispersed colloidal particles on the phase behavior of quasi-binary polymer solutions near phase separation have been investigated in systems where the polymers adsorb to the surfaces of the colloidal particles. The dispersed particles were shown to enhance the phase separation of the polymer solution, giving a shift in the cloud point temperature by up to ca. 5 °C. Parameters such as polymer polydispersity and particle charge were seen to influence the magnitude of the effect. The results are interpreted in terms of a surface contribution to the free energy of phase separation. The mechanism is different from the well-known depletion and bridging mechanisms for phase separation in mixed polymerparticle solutions. The results in this study are in qualitative agreement with theoretical results on surfaceinduced phase separation for adsorbing quasi-binary polymer solutions.18 Acknowledgment. Malin Juberg at Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden, is gratefully acknowledged for performing the SEC measurements and Per Linse for valuable comments on the manuscript. This work was funded by the Centre for Amphiphilic Polymers from Renewable Resources (M.O.) and the Swedish Research Council (L.P.). LA035929M