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Langmuir 1997, 13, 3948-3952
Partitioning of Polymer and Inorganic Colloids in Two-Phase Aqueous Polymer Systems Steven M. Baxter, Peter R. Sperry,* and Zhenwen Fu Rohm and Haas Company, 727 Norristown Road, Spring House, Pennsylvania 19477 Received December 2, 1996. In Final Form: March 10, 1997X The well-known phenomenon of partitioning of bioparticles in two-phase aqueous polymeric systems is extended to a variety of polymer (acrylic latex) and inorganic (TiO2) colloidal particles. It is demonstrated that particle partitioning into a particular phase is controlled in rational ways by particle surface chemistry, viz., hydrophobicity, functional groups such as hydroxyl and carboxyl, and pH effects, in relation to the chemical nature of the water soluble polymers (WSPs) that comprise the phase-separated system. WSPs include poly(ethylene oxide), poly(N-vinylpyrrolidone), dextran, and a hydrophobe-terminated water soluble polyurethane based on polyethylene oxide. Mixtures of colloidal particles can be separated by selective partitioning.
Introduction Two-phase aqueous polymer systems, notably those based on poly(ethylene oxide) (PEO) + dextran, have long been used to selectively partition and separate biological systems such as those containing cells, proteins, DNA, and the like.1 We were motivated to explore the possibility that it is likewise feasible to selectively partition and separate a variety of nonbiological colloids familiar to the aqueous latex polymer industry, viz., synthetic polymer colloids made by emulsion polymerization of vinyl monomers, and dispersions of inorganic pigments, particularly titanium dioxide, which is extensively used as an opacifying agent. With respect to polymer colloids, there is a large selection of vinyl monomers that afford easily accessible variations in polarity of the polymer particle as a whole, e.g., via copolymers ranging from the relatively nonpolar and hydrophobic styrene and isobornyl methacrylate to the relatively polar and hydrophilic ethyl acrylate. Surface functionality can be further modified by incorporating highly polar groups such as hydroxyl and carboxyl by copolymerization with, e.g., hydroxyethyl methacrylate and acrylic or methacrylic acid, respectively. With the acid monomers, pH adjustment affords yet further control of surface polarity. Choice of polymerization surfactant will also affect the surface chemistry of the particles. With respect to inorganic pigments such as TiO2, the coatings and related industries use a variety of low molecular weight anionic polyelectrolyte salts at levels on the order of 1 wt % based on solids content as dispersing and stabilizing agents. A common class of dispersant is based on acrylic or methacrylic acid as either homopolymers or copolymers with other acrylic monomers. These polyelectrolytes adsorb more or less strongly on the amphoteric inorganic pigments and are believed to function usually by a combination of charge and steric contributions. There are many possibilities for selection of immiscible pairs of water soluble polymers. We report on experiments with three phase-separated water soluble polymer (WSP) pairs (Table 1) that demonstrated a range of behavior with respect to colloidal particle partitioning: the archeX
Abstract published in Advance ACS Abstracts, July 1, 1997.
(1) (a) Albertsson, P. Partition of Cell Particles and Macromolecules; Wiley-Interscience: New York, 1986. (b) Walter, H., Brooks, D. E., Fisher, D., Eds. Partitioning in Aqueous Two-Phase Systems: Theory, Methods, Uses, and Applications to Biotechnology; Academic Press, New York, 1985. (c) Abbott, N. L.; Blankschtein, D.; Hatton, T. A. Bioseparation 1990, 1, 191-225. (d) Abbott, N. L.; Blankschtein, D.; Hatton, T. A. Macromolecules 1993, 26, 825-828.
S0743-7463(96)02065-3 CCC: $14.00
typal PEO/dextran system, PEO/poly(N-vinylpyrrolidone) (PVP), and PEO/HEUR. HEUR denotes hydrophobically modified ethylene oxide urethane block copolymers that have become widely used as associative thickeners in aqueous latex polymer formulations such as coatings.4 On a weight basis, HEURs typically have 95% or greater ethylene oxide polymer content, but the attached hydrocarbon hydrophobe groups control rheological behavior through self-micellization and strong adsorption on colloidal particles such as latex polymers. Albertsson1 proposed arrangement of a number of water soluble polymers on a scale of relative hydrophobic/ hydrophilic character. In this arrangement dextran is more hydrophilic than polyethylene oxide. It is reasonable that the HEUR polymer would be more hydrophobic yet than unmodified PEO. PVP was not on Albertsson’s scale, but we speculate that it would, like dextran, be relatively more hydrophilic than PEO. Theoretical Basis of Partitioning While our principal interest is in the phenomenological exploration of partitioning behavior as affected by the variables noted above, we note that there has been some theoretical development of partitioning concepts, stemming from the biochemical interests. Albertsson1 developed an elementary theory of particle partitioning wherein the relative energies of a particle in the two phases were calculated from the interfacial tensions between the particle and the polymer solutions. However, the interfacial tensions between polymer solutions and colloidal particles are neither easily measurable nor readily calculable. Alternatively, Baskir has more recently adapted the ideas of Flory-Huggins theory of polymer solution thermodynamics to provide a conceptually useful model for partitioning, based on a spherical lattice around a protein (or colloid) with a homogeneous surface.2 Sites on the lattice can be occupied either by solvent (water) or segments of the soluble polymers A and B. Two FloryHuggins-type interaction parameters, χAP and χBP, characterize the enthalpic interactions between polymer segments and the protein/colloid surface, while a statistical mechanical approach accounts for entropic effects. χAP is really a measure of the degree of soluble polymer A’s adsorption onto the particle surface. In the absence of adsorption, there will be a depletion layer around the particle where the soluble polymer segment concentration drops to zero because of the conformational restrictions (2) Baskir, J. N.; Hatton, T. A.; Suter, U. W. J. Phys. Chem. 1989, 93, 969-976.
© 1997 American Chemical Society
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Table 1. Water Soluble Polymers polymer
grade
nominal MW
PEO
abbr poly(ethylene oxide)
PVP
poly(N-vinylpyrrolidone) dextran hydrophobically modified PEO-based polyurethane
POLYOX N-3000a POLYOX N-80 POLYOX N-10 PVP K-90 Dextran 500 ACRYSOL RM-825b
400 000 200 000 100 000 360 000 500 000 70 000
HEUR a
source Union Carbide ISP Technologies Sigma Chemical Rohm and Haas
POLYOX is a trademark of the Union Carbide Company. b ACRYSOL is a trademark of the Rohm and Haas Company.
placed on the polymer chains near the surface. The thickness of this depletion layer is on the order of the gyration radius of random coils for dilute solutions or on the order of the mesh size ξ for semidilute polymer solutions. In general, this approach predicts that particles would partition to the phase which presents the smaller depletion layer, with strong polymer adsorption being one extreme where there is essentially zero excluded volume. Note that these are the same factors which govern the depletion flocculation (i.e., phase separation) of latices by high molecular weight soluble polymers.3 Materials and Methods A. Water Soluble Polymers. The four types of polymer selected to prepare the solutions of the three incompatible WSP pairs (PEO/dextran, PEO/PVP, and PEO/HEUR) are all commercial materials and no doubt quite polydisperse (Table 1). Nominal molecular weights are those provided by the manufacturer. The partial phase diagrams (Figure 1) were obtained by mixing various known ratios of concentrated stock solutions into the twophase region, as evidenced by turbidity followed by development of two layers. The mixture was titrated with pure water to the point where there was a single phase, completely clear, with disappearance of the layers; often samples were centrifuged to accelerate the separation process. The * on each of the phase diagrams in the twophase region represents the overall mixture composition chosen to assess particle partitioning behavior, as described later. Partitioning behavior at our level of qualitative inquiry was not found to be notably sensitive to the choice of WSP pair concentration; we simply wanted the composition to be in the midrange of the two-phase region, with approximately equal phase volumes, but at not so high concentrations as to yield high viscosity and concomitantly low rates of bulk phase separation. The locations of the phase diagram binodals (Figure 1) with respect to the axes would suggest that the respective phases in our test systems have a substantially higher concentration of one polymer with respect to the other, and we do refer to the individual phases as “PEO-rich”, “PVP-rich”, etc. Certainly it would be desirable in future experiments to better quantify the degree of crossmiscibility and to establish its importance in partitioning. We also did not examine the role of ionic strength in these aqueous systems, nor did we quantify contributions to ionic strength possible from contaminants in the WSPs themselves or from the colloid test samples. Ionic strength is certainly not great, and we expect that the impact on the phase behavior of these nonionic WSPs is minimal. B. Polymer Colloids. The overall monomeric compositions (wt % of monomer charge) of the principal series of latex polymers used in the study are given in Table 2. (3) (a) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989; Chapter 10. (b) Lekkerkerker, H. N. W.; Poon, W. C. K.; Pusey, P. N.; Stroobants, A.; Warren, P. B. Europhys. Lett. 1992, 20, 559-564. (c) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: New York, 1993.
Figure 1. Phase diagrams for water soluble polymer pairs: (a) PEO (POLYOX N)/dextran 500; (b) PEO (POLYOX N)/PVP K90; (c) PEO (POLYOX N-80)/HEUR. Unit of concentration is wt % of the component. * signifies the composition of the test systems used for partitioning experiments reported in Table 3.
They were prepared by semicontinuous emulsion polymerization using ammonium persulfate initiator and, generally, using the ammonium salt of a sulfated polyethoxynonylphenol as polymerization surfactant. In one case, the latex “A5” vs “A5N” composition, a polyethoxynonylphenol nonionic soap was used in the synthesis. The latex polymers are coded A through E according to the common component monomers. The number following the letter code refers to the wt % of methacrylic acid in the composition, and the numbers in parentheses are the particle diameters for those other than the principal 100 nm particle diameter.
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Baxter et al. Table 3. Colloid Partitioning Resultsa
Table 2. Colloids Used in Partitioning Study Colloidsa
Latex (Monomer Ratios are by Weight) A5 ) 53 BA/42 S/5 MAA A5N ) 52 BA/42 S/5 MAA (ethoxylated octyl phenol surfactant) 180 nm B5 ) 50 BA/35 S/10 HEMA/5 MAA B2 ) 50 BA/38 S/10 HEMA/2 MAA B1 ) 50 BA/39 S/10 HEMA/1 MAA B2(320) ) B2 at 320 nm C1 ) 55 EHA/20 S/24 IBOMA/1 MAA C5 ) 55 EHA/20 S/20 IBOMA/5 MAA D5 ) 20 EHA/30 S/40 BMA/5 MAA D5(240) ) D5 at 240 nm D5(360) ) D5 at 360 nm D5(450) ) D5 at 450 nm E1.5 ) 15 EA/83.5 MMA/1.5 MAA at 500 nm Inorganic Colloids P ) titanium dioxide pigment, ca. 250 nm diameter (TI-PURE R-900, DuPont) P(COO-) ) dispersion prepared with 1% on solids of an acrylic acid polyelectrolyte (TAMOL 1124, Rohm and Haas) P(OH) ) dispersion prepared with 1% on solids of a hydroxylrich polyelectrolyte (TAMOL SG-1, Rohm and Haas) a BA ) n-butyl acrylate, S ) styrene, MAA ) methacrylic acid, HEMA ) 2-hydroxyethyl methacrylate, EHA ) 2-ethylhexyl acrylate, IBOMA ) isobornyl methacrylate, BMA ) n-butyl methacrylate. Prepared at ca. 100 nm diameter using the ammonium salt of a sulfated polyethoxynonylphenol as surfactant, except as noted.
The latex polymers were prepared at pH < 6, the copolymerized methacrylic acid is but little ionized. Ammonium hydroxide was used as base where the effects of neutralization to carboxylate form are examined (pH 9-10). To dramatize and photograph the partitioning and separation phenomenon, we obtained dyed latex particles from Seradyn, Inc. (Indianapolis, IN). The ca. 300 nm particles selected are described as follows: blue particles, “carboxylate-modified”, “high acid” (ca. 20 Å2 per carboxyl), abbreviation CMBL; red particles, “polystyrene”, abbreviation PSRD. C. Inorganic Particle Dispersions. We prepared aqueous pigment (P) dispersions by milling TI-PURE R-900 TiO2 with either TAMOL 1124 (1 wt % solids on solids) or TAMOL SG-1 (1.2%) as the dispersing agent. (TI-PURE is a trademark of the DuPont Company; TAMOL is a trademark of the Rohm and Haas Company.) TI-PURE R-900 is an alumina-treated grade of TiO2 of approximately 250 nm diameter and 94% titania content used in the manufacture of latex paints. The TAMOL pigment dispersants are both the ammonium salts of carboxyl functional, low molecular weight polyelectrolytes, but TAMOL SG-1 has additionally a high concentration of copolymerized hydroxyl group. The pigment dispersions are accordingly abbreviated as P(COO-) and P(OH) to reflect the presumed surface functionality imparted by the dispersants. D. Partitioning Experiments. The latex polymers or TiO2 dispersions were thoroughly mixed in small vials with the two-phase polymer test systems at low concentration (ca. 1-2 wt % colloid solids on total system weight). The samples were allowed to stand until phase separated (order of minutes to hours), and the phase preference of the colloidal particles was recorded (Table 3). In the vast majority of cases, partitioning of the particles was for all practical purposes completely into one of the phases. In all the systems discussed here, latices appeared uniformly dispersed within the preferred phase, at least for several days. Very high soluble polymer concentrations and/or molecular weights caused particle flocculation resulting in rapid sedimentation of the colloids.
POLYOX N-3000 (1.6 wt %) + Dextran 500 (3.2 wt %) PEO layer (top)
dextran layer
A50
A5-
A5NB50, B5B20, B2B10, B1C1C5D5POLYOX N-3000 (1.8 wt %) + PVP K90 (1.8 wt %) PEO layer (top) A50 A5NB5B20, B2-
PVP layer
r 100+ nm (A5-) 90- nm f B50 B10, B1-
C1C5D5(240, 360, 450)E1.5P(COO-)
P(OH) POLYOX N-80 (1 wt %) + HEUR (1 wt %)
PEO layer (top)
HEUR layer all latices examined of a selection from Table 2
a 0 superscript ) low pH, acid form; - superscript ) high pH, carboxylate form.
Results and Discussion In the PEO/dextran system, hydrophilic (e.g., highly carboxylated, base-neutralized) latices such as Latex A5, C5, and D5 tended to prefer the hydrophilic dextran phase, while more hydrophobic compositions such as Latex C1 migrated into the upper PEO layer. Partitioning was clearly affected by the polymerization surfactant used. Compositionally identical latices but with an ethoxylated octyl phenol nonionic surfactant (Latex A5N) vs an ammonium salt of a sulfated polyethoxynonylphenol anionic surfactant (Latex A5) showed opposite behavior; Latex A5N preferred the PEO-rich phase, consistent with the higher degree of ethoxylation of the surfactant. The importance of specific interactions is further exemplified by the pH-dependent partitioning behavior of the carboxylated latices. A titration curve of Latex A5 confirmed that most of the surface carboxylate is protonated at pH 6.5, and this latex partitions cleanly into the PEO-rich phase, presumably due to hydrogen bonding. At pH 9.6, surface carboxylic acid is over 80% neutralized, and the particles partition into the dextran phase. This pH effect can be overridden, however, by the presence of OH groups in the latex composition. Latex B5, containing 10% hydroxyethyl methacrylate (HEMA) comonomer but otherwise similar to Latex A5, partitioned into the PEOrich phase, independent of pH. Partitioning is evidently dominated by hydrogen bonding of HEMA and PEO. Similar behavior was observed with the analogs of latex B5 which contained only 2 and 1 wt %, respectively, of copolymerized methacrylic acid (MAA) (B2, B1). PVP is a relatively hydrophilic WSP and provided different chemical functionality than dextran. Again, the hydrophobic latex C1- tended toward the PEO phase, as in the PEO/dextran system. In the PEO/PVP system, however, the OH functional B- type latices generally preferred the PVP phase, rather than the PEO phase as in the PEO/dextran system. The amide group in poly(N-vinylpyrrolidone) is indeed a better hydrogen bond
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Figure 2. Partitioning of a 320 nm latex with 10% copolymerized HEMA in the PEO/PVP system (3 wt % each). External phase is PEO-rich. Particles are in the internal PVP-rich phase.
acceptor than the PEO ether oxygen. However, the carboxylate form of the highest MAA content latex (B5-) partitioned to the PEO phase, as in the PEO/dextran system. A rationale might be that 5 wt % MAA as carboxylate is on the same order of molar content as its companion 10 wt % HEMA in this latex and may be adequate to disrupt the presumed hydrogen bonding interaction of HEMA with PVP. Partitioning of the particle, to the PEO phase, becomes controlled by the PEO interaction with the remainder of the latex polymer backbone, viz., BA/S. Figure 2 is an optical micrograph of the phase-separated PEO/PVP system containing a 320 nm version of the 10% HEMA Latex B2 (prepared at the larger size for visibility) that demonstrates the essentially complete partitioning of the particles into the PVP phase in this case. Although particle diameter is, in general, a primary colloid chemical variable, we have not adequately assessed its role in partitioning behavior. In the PEO/PVP system as noted above, the latex B2 composition partitioned cleanly into the PVP phase both at our reference 100 nm size and at 320 nm. In the case of the latex A5 composition, partitioning at ca. 90 nm of the carboxylate form (A5-) was indistinct in the PEO/PVP pair. However, “A5” type latices of diameter approximately 100 nm up to nearly 400 nm partitioned cleanly into the PEO phase, whereas those of about 80 nm and smaller partitioned into the PVP phase. Conceivably, in the latter cases, we may not be observing particle size effects as such but the result of changes in particle surface chemistry that inevitably accompany latex particle size variations and the synthetic manipulations required to produce them. The same chemical principles that apparently control the partitioning of latex particles also appear applicable to the TiO2 pigment dispersions. We examined the partitioning behavior of the P(COO-) and P(OH) dispersions in PEO/PVP. Figure 3 contains optical micrographs of the results, summarized also in Table 3. The pigment dispersed with TAMOL 1124, P(COO-), partitioned into the PEO-rich phase, consistent with the high carboxylate content of this dispersant. On the other hand the same pigment with the hydroxyl-rich TAMOL SG-1, P(OH), partitioned into the PVP phase. The third WSP pair examined is a mixture of PEO and HEUR. All of the test latices examined partitioned without exception into the HEUR phase, a result we attribute to the well-known tendency of these hydrophobically modified WSPs to adsorb strongly onto polymer latices.4 Lastly, we established that a mixture of latices with different individual partitioning behavior could be physically separated into different layers. Thus, a mixture of latices D5(240)- and E1.5(500)- in the PEO/PVP system
Figure 3. Partitioning the TI-PURE R-900 TiO2 in the PEO/ PVP system: (a) with TAMOL 1124 dispersant (particles in PEO phase, external) and (b) with TAMOL SG-1 (particles in PVP phase, internal).
separated quite cleanly into the expected layers (see results of Table 3). Identity was derived from the significant light scattering appearance of the phases of the mixed system vs the single particle results and by verifying that each phase with its presumed type of particles repartitioned correctly in a fresh sample of the PEO/PVP mixture. The experiment was also done, with the expected result, using a mixture of pigment dispersion and latex, viz., again in the PEO/PVP system with a mixture of P(OH) and latex D5(450)- (see again Table 3 for results with the individual dispersions). Latex particle mixture separation was dramatized using a blend of the red and blue dyed latex particles obtained from Seradyn, Inc. We found that, in individual parti(4) (a) Schaller, E. J.; Sperry, P. R. Associative Thickeners; In Handbook of Coatings Additives; Calbo, L. J., Ed.; Marcel Dekker, Inc.: New York, 1992; Vol. 2, Chapter 4. (b) Schulz, D. N., Glass, J. E., Eds.; Polymers as Rheology Modifiers; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (c) Hulden, M. Water-Soluble Polymers and Surfactants in Latex Paints. Thesis, Abo Akademi University (Finland) and Institute for Surface Chemistry (Stockholm, Sweden), 1994.
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Baxter et al.
Figure 4. Partitioning and separation of latex particle mixtures: (a) 10 mL aqueous mixture of overall composition 3% PEO (POLYOX N-3000) and 5% dextran; (b) mixture from part a with 0.1 mL of 2.5% solids blue latex CMBL; (c) mixture from part a with 0.1 mL of 2.5% solids red latex PSRD; (d) mixture from part a with 0.1 mL each of 2.5% solids CMBL and PSRD; (e) 10 mL plain water with 0.1 mL each of 2.5% solids CMBL and PSRD.
HEUR associative polymer. This indeed forms a third phase, and consistent with the findings at the bottom of Table 3, both latices are drawn into the HEUR layer. This is presumably because of the strong association tendency of the hydrophobes on the HEUR polymer to adsorb on the latices, evidently overriding any other of the WSP/ latex interactions in the other phases. Some comments are in order regarding the kinetics of the phase separation and partitioning processes we have observed. In general, we find that, on a low magnification light microscopic scale (e.g., as in Figures 2 and 3), phase separation and partitioning of particles into one of the phases occurs within the time from vigorous shaking of a sample to ability to view it on the microscope stagesthat is, several seconds to a minute or so. On the other hand, aggregation and coalescence, and sedimentation of phaseseparated microdomains into clean layers is a slower process, requiring on the order of minutes to hours in the systems examined here. Phase viscosity, as controlled by molecular weight and concentration, and the density difference between phases appear particularly important, but we have not quantified these variables.
Figure 5. Latex particle mixture in a three-phase system with HEUR: 9 mL aqueous mixture of overall composition 2% PEO (POLYOX N-3000), 3.3% dextran, and 1.7% HEUR, with 0.1 mL each of 2.5% solids latices CMBL and PSRD.
tioning tests in the PEO/dextran system, the blue CMBL particles partitioned into the dextran phase, whereas the red PSRD particles partitioned cleanly into the PEO phase. A blend of the red and blue latices in plain water had a very dark, muddy color that persisted as a stable mixture, but they separated quite cleanly in the presence of the WSP mixture according to their individual partitioning behaviors. Figure 4 is a composite of photographs of the vials associated with these experiments. Figure 5 depicts a further experiment with the mixed colored particle system in the PEO/dextran system in which a third water soluble polymer was added, viz., the
Summary We demonstrate the phenomenon of colloid partitioning with a broad spectrum of polymer latices and inorganic particles in a variety of two-phase WSP systems. The application of selective partitioning to the separation of colloidal particle mixtures is shown. The partitioning behavior, i.e., phase preferences, of the particles is seen to be widely variable in plausible ways according to the surface chemical nature of the particles and the chemical composition of the WSPs. Our quasichemical interpretations of partitioning behavior are highly qualitative and speculative and leave ample room for refined experimental and theoretical treatment in future investigations. Acknowledgment. We thank Dr. Michael D. Bowe and Ms. Karen Shust for preparing a number of the latex polymers for this study, Mr. Jeff Panara for assistance with generation of the photographic images, and Dr. Patricia M. Lesko for helpful critique. LA962065E
