Comparative Behavior of Poly (ethylene glycol) Hydrogels and Poly

Oct 24, 2003 - Poly(ethylene glycol) Aqueous Biphasic Systems. Jonathan G. Huddleston,* Terita K. Looney, Grant A. Broker, Scott T. Griffin,. Scott K...
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Comparative Behavior of Poly(ethylene glycol) Hydrogels and Poly(ethylene glycol) Aqueous Biphasic Systems Jonathan G. Huddleston,* Terita K. Looney, Grant A. Broker, Scott T. Griffin, Scott K. Spear, and Robin D. Rogers Center for Green Manufacturing, The University of Alabama, Tuscaloosa, Alabama 35487

The properties of covalently cross-linked poly(ethylene glycol) (PEG) hydrogels have been studied. The degrees of expansion of the hydrogels in aqueous solutions containing different types of salt and the expansion with respect to temperature have been examined. The apparent polarity of the hydrogels in these differing environments has been measured using Reichardt’s betaine dye. The uptake of hydrophobic solutes with respect to temperature and salt concentration has also been examined. We attempt to relate these observations to a consideration of the PEG molecular conformation and hydrogen-bonding ability in aqueous solution. Hydrogel expansion, apparent polarity, and solute uptake all appear to be continuous functions of temperature and salt concentration without any discontinuity typical of critical phenomena. It is suggested that phase formation in aqueous biphasic systems and miscibility gaps in PEG solutions in general result from molecular collapse and the loss of H-bonding ability, followed by the critical coacervation of preexisting molecular domains within the aqueous solution. These insights might aid in devising new theoretical and practical approaches to problems in many areas, such as separations science, pharmacology, and materials fabrication. Introduction Polymeric organic gels have found a huge range of applications. They are used as a nonconvective hydrophilic support matrix for bioelectrophoresis, principally in the form of polyacrylamide gels.1 They are used in the formation of electrochromic devices where, for example, poly(methyl methacrylate) among other polymers can be used to form the matrix.2 They are used as flow-diverting agents (e.g., acrylamides and others) in oil- and gas-recovery operations.3,4 Biologically derived gelling agents are important in the food industry, as exemplified by the recent interest in gellan gum.5 Polymeric organic gels in the form of hydrogels have been the subject of intense investigation for biomedical applications.6 Applications include use as wound dressings and artificial tissue matrixes,6 as materials for the encapsulation of cells, as matrixes for the controlled release of drugs,6-8 and as biodegradable implant materials for the delivery of proteins.9 Somewhat similar applications have been suggested in an agricultural context.10 Hydrogels also find application for their ability to absorb or retain water. Polymeric gels, solvated in many different solvents, are used as supports for synthetic chemical reactions.11 Polymeric gels have been widely used to develop biocompatible adsorbents for biomolecule purification.12 Along with applications in biopharmaceutics, gels that respond to environmental changes, so-called stimuli-responsive gels,7 have also been investigated for their potential in biomolecular recovery applications.12,14 In the biomedical area alone, the number of published papers in this area is daunting.15 Gels based on poly(ethylene glycol), or copolymers of poly(ethylene glycol), which could be of interest in many or all of the above * To whom correspondence should be addressed. Tel.: 205348-4323. Fax: 205-348-0823. E-mail: [email protected].

areas of research, have been investigated on numerous occasions.16-22 In part, this might be because PEG seems to represent a highly biocompatible polymer that is not detected by the immune system and does not seem to trigger the blood-clotting cascade.23 However, many of these matrixes contain monomers or polymers other than (poly)ethylene glycol, such as methacrylates or lactates.16-18,20 Our own interest in poly(ethylene glycol)based hydrogels arose from our development of ABEC (aqueous biphasic extraction chromatography) resins.24 These materials were based on the covalent attachment of low-molar-mass (e.g., 2 kDa) poly(ethylene glycol) to Merrifield’s reagent25 and consisted of approximately 400-µm-diameter polymeric particles, which were considered to represent a particulate alternative to liquidliquid extraction by ABSs (aqueous biphasic systems) and assumed to operate by a similar partitioning mechanism.26 ABSs are represented, in part, by the phase separation of aqueous solutions resulting from mixtures of polymers and high concentrations of kosmotropic salts and also by systems containing two polymers where the enthalpy of mixing favors phase separation into two immiscible phases above critical concentrations.27 As a result, two phases are established that differ in chemical potential and might therefore be usefully applied to liquid-liquid extraction,26 reactive extraction,28 or biphasic catalysis.29 A significant difference was observed between the behavior of ABEC resins and the analogous polymersalt ABSs in that the distribution to the polymeric phase takes place at concentrations of salt below those required for the phase separation of PEG-salt ABSs.30 It was tempting to conclude that the distribution of solutes in ABSs has nothing to do with phase separation per se, but that phase separation merely sorts the preexisting chemically separated solvent domains into physically separated phases at the macroscopic level.31 In

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other words, this interpretation suggests that the distribution of solutes in ABSs is based on partition into preexisting solvent domains below the critical point.32 However, it might equally be the case that the contrasting behavior of ABEC resins, in comparison to ABSs, simply reflects the properties of the polystyrene divinyl benzene backbone of Merrifield’s reagent and thus contributes little to, or even confounds, an understanding of the properties of ABSs. In this context, a hydrogel consisting of PEG with little or no backbone matrix would be expected to be experimentally useful, as well as conceptually and theoretically informative. In addition to this aspect, considerations of polymer solution behavior that give rise to the formation of ABSs have suggested, at the theoretical level, that dehydration of the polymer leading to polymer collapse and thus to a greater population of hydrophobic species is an important prerequisite for the phase separation of polymer-salt ABSs.33 It seems reasonable to assume that a cross-linked polymer hydrogel provides a macroscopic analogue by which the molecular-level events underlying phase separation behavior of polymer-salt mixtures can be investigated. Therefore, in this study, we examine the behavior of PEG hydrogels with respect to salt concentration, solution temperature, apparent polarity, and ability to take up hydrophobic solutes. These results can be instructively compared to the behavior of ABEC resins and other adsorbents or materials prepared with PEG surfaces and with liquidliquid PEG-salt ABSs. The properties of the PEG hydrogels that are highlighted as a consequence of this study should also be of interest to others working in a wide variety of fields from separations, through drug delivery, to electronic devices. Methods PEG hydrogels for swelling experiments in salt solutions were prepared as follows. Cross-linked PEG hydrogels were prepared by mixing 100 mg of disuccinimidylpropyl PEG (MW 3400 Da) (SPA-PEG) in 600 µL (0.049 M) of distilled H2O with 147 mg of a four-arm tetraamine PEG (MW 10 000 Da) in 600 µL (0.0245 M) of 20 mM phosphate buffer, pH 6.5 (see Figure 1). The mixture was quickly vortexed and then drawn up carefully so as to completely fill the stem of a disposable plastic Pasteur pipet, where it was allowed to cure for several hours. The hydrogel was carefully cut from the pipet to yield a cylindrical hydrogel having a diameter of ca. 4 mm and a length of ca. 80 mm prior to washing and swelling in deionized water. A variant of this procedure was used to make a second type of hydrogel by halving the concentration of both reactants. The hydrogels were equilibrated with at least three washes (25 mL) of deionized water in a capped glass vessel that was agitated by slow end-over-end rotation over a period of at least 24 h. Subsequently, the hydrogels were equilibrated in exactly the same way in various types and concentrations of inorganic salts dissolved in deionized water. In general, the concentration was increased systematically to aid equilibration, but it was observed that the same end point could be reached regardless of the approach path. After equilibration, the length of each hydrogel cylinder was measured by ruler, as this could be done with greatest ease and accuracy. Changes in hydrogel length with temperature were determined by placing the capped vessels containing the hydrogel and deionized water in a thermostated water

Figure 1. Chemistry of the preparation of the PEG hydrogels.

bath, Fisher Scientific Isotemp 1016S (Pittsburgh, PA), for the same time as before; however, the hydrogels were agitated only periodically, with several inversions by hand. PEG derivatives for cross-linked hydrogel formation were obtained from Shearwater Polymers (Huntsville, AL). Potassium phosphate and other salts were from Aldrich (Milwaukee, WI), and deionized water was prepared using a Barnstead Nanopure system (Dubuque, IA). Solvatochromic studies were performed using Reichardt’s carboxylated betaine dye obtained as a gift from Professor Christian Reichardt, The University of Marburg, Marburg, Germany. This dye was either used in solution at a concentration of approximately 5 µg/ mL or incorporated at this concentration into the hydrogel casting solution prior to mixing. The hydrogel was then cast on one face of a 3-mL quartz spectroscopy cuvette to a depth of ca. 1 mm and allowed to cure. Scanning spectroscopy was performed using a Cary 3 dual-beam spectrophotometer (Varian, Sugarland, TX) using a resolution of 0.25 nm and a signal-to-noise ratio of 5000. Analysis of the peak of maximal absorbance used the 9/10 method of Kamlett and Taft, which is described elsewhere.31 Absorption studies with respect to temperature were performed using the dye amaranth (Aldrich, Milwaukee, WI) at a concentration of 1 mg/mL in 20 mL of deionized water. The solution, containing a single piece cut from a hydrogel cast as above and having a wet weight of about 1 g, was held at a given temperature in a thermostated oven in sealed pressure tubes for 24 h. The tubes were periodically gently agitated by inversion by hand. Absorption studies with respect to salt concentration were performed using about 0.14-0.15 g of hydrogel in equilibrium over 24 h with 5 mL of amaranth dye in a solution of K2HPO4 having various concentrations. The concentration of dye was 0.25 mg/ mL. The dye concentration was calibrated and measured by spectroscopy at 521 nm using the same equipment

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as previously described. The dry weights of hydrogel samples were obtained after the samples had been dried for 48 h at 120 °C. Results The concentrations of reactants used to form the PEG hydrogel yield a stoichiometric ratio of 2 mol of crosslinker to 1 mol of four-arm amino PEG. In principle, this is sufficient to give a simple network structure, as indicated schematically in Figure 1. In reality, for stochastic reasons, an interpenetrating but somewhat incomplete network containing free amino termini and hydrolyzed succinimydyl propionate groups is likely to be the end result. Unlike many cross-linked hydrogels prepared from mixtures of monomers and cross-linkers, in this case, the concentration of hydrogel and crosslinker cannot be varied in quite the same way. The density of cross-links can be varied only by selecting different molecular weights of PEG chains for each of the reactants. This was not done here, although two different hydrogels were prepared beginning from different concentrations of reactants (see Methods). If the hydrogel were perfect, then the lengths of the polymer strands surrounding the voids in the hydrogel would be ca. 45 nm based on an ethoxide (CH2CH2O, EO) repeat length of 3.7 Å (taken from the 7/2 helical TTG (transtrans-gauche) crystal structure of ref 34) and combined polymer chains of ca. 125 EO units in length (ignoring the details of linking structures and so on). On the basis of our unquantified observations of rather slow solute diffusion (slow equilibrium) within this hydrogel, it seems possible that the water in these structures is relatively immobile, perhaps resembling confined water.35 Although only the lengths of the hydrogels, in response to the changing solvent conditions, were measured, it appeared that the diameter of the hydrogels changed in the same proportion as the length, and thus, the fractional change in length can be considered to scale with the cube root of the hydrogel volume. The fractional change in length is expressed relative to the fully swollen condition after equilibration in deionized water

fractional change in length ) L/L0

(1)

where L is the length at equilibrium under particular conditions and L0 is the length in equilibrium with pure water. The gels prepared at the maximum concentration given in the Methods section showed an almost 50-fold increase in mass (and therefore approximately in volume) on hydration from the dry state. As mentioned above, the cast hydrogels had initial dimensions of ca. 4 and 80 mm; upon hydration, these measurements increased to ca. 8 and 150 mm. The fractional change in hydrogel length with salt concentration is shown in Figure 2. It is conventional to interpret the swelling response of hydrogels in terms of the model first proposed by Flory.36 Thus, according to Maurer,37 for a hydrogel in complete contact with water, water is absorbed until phase equilibrium is reached. Equilibrium conditions are reached when the chemical potential difference between the water in the hydrogel phase and the water in the liquid phase is balanced by a contribution from the pressure arising from the mechanical properties of the hydrogel. As

Figure 2. Fractional length change of the PEG hydrogel in response to different salt types: filled symbols, hydrogel formed from 0.049 M SPA-PEG; open symbols, hydrogel formed from 0.025 M SPA-PEG.

expressed by Prausnitz,38,39 as the hydrogel swells as a result of the chemical potential difference of water in the two phases, the mechanical restraining force of the hydrogel increases until equilibrium is reached. Increasing the concentration of the polymers (as here, but also increasing the concentration of cross-linkers, as in other studies) leads to increased mechanical rigidity and entanglement of the hydrogel. Thus, there is a larger pressure difference between the hydrogel phase and the liquid phase at the same chemical potential difference between the phases. This might serve as an explanation for why the weaker (prepared from the less-concentrated hydrogelling mixture) of the two hydrogels appears to shrink to a greater extent than the stronger (see Figure 2). In reality, this might reflect the fact that the former hydrogel was more expanded initially, so that the result shown in Figure 2 is obtained simply because, as a convention, we set L0 to be the fully expanded condition of the individual gel. The fact that the two different types of PEG gel behave in so similar a manner during swelling implies that the cross-linking reaction proceeded almost to completion. Otherwise, a much stronger dependence of swelling ratio on concentration at preparation would be anticipated if considerable quantities of free amino termini had remained at the end of the reaction, given that this quantity would itself be highly concentration-dependent.40 This also means that such free amino groups as do exist have little effect on the equilibrium swelling properties of these gels under the conditions employed. The equilibrium relationship governing solventswollen hydrogels has been described by Prausnitz,38 following Flory,36 as

∆πsw ) ∆πmix + ∆πel + ∆πion

(2)

where ∆πsw is the pressure within the swollen hydrogel and ∆πmix is a Flory-Huggins term arising from the free energy of mixing of the polymer and solution that expresses any tendency toward the liquid-liquid unmixing of the polymers from the aqueous phase. The latter is generally considered a minor term.38 For a PEG polymer system where liquid-liquid immiscibility in water is a well-known feature,33 this will not be so. ∆πel is the contribution due to the elastic deformation of the hydrogel, and ∆πion is a Donnan equilibrium term expressing the contribution due to ions and any fixed charges on the hydrogel. This latter term can be thought

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to be nonexistent for a PEG hydrogel without fixed charges; however, there will be an electrostatic contribution to the free energy of mixing term.37 In light of the formal description just given, a number of observations can be made on the results shown in Figure 2. Increasing the salt concentration for all of these salts causes a decrease in the length and volume of the hydrogels. The extent of this hydrogel collapse is dependent on the salt type. It seems reasonable to assume that such a collapse is driven at the molecular level by dehydration and molecular collapse of the crosslinked PEG polymer chains. It appears that this collapse can be driven only so far before a limiting degree of collapse is encountered, i.e., the rate of collapse with salt concentration approaches an asymptotic minimum. By rate of collapse, we mean the fractional change in hydrogel length with salt concentration. This limiting degree of collapse is broadly similar for all of the salts that approach this limit. However, there is sufficient variation in this minimum degree of collapse to suggest that it could also be salt-type-dependent. On the other hand, such a dependence would be hard to distinguish from a true universal limit with rates of approach that were salt-type-dependent on the basis of the evidence in Figure 2. The rate of collapse with salt concentration is dependent on the type of salt. In broad terms, it is dependent on the free energy of hydration of the anion, although there are some surprises. Thus, the rate of collapse is higher for potassium phosphate than for ammonium sulfate. In turn, the rate for ammonium sulfate is higher than that for sodium chloride and so on. An increase in the concentration of some salts brings about the maximum degree of collapse, whereas for others, this does not happen. This is because some salts have reached their solubility limits, and higher concentrations cannot be achieved in this experiment at room temperature, e.g., NaCl. It was observed that, as the hydrogel approached the asymptotic minimum length, it appeared, by visual inspection, to go through a critical phase transition, taking on an opaque milky appearance very similar to the opalescence of critical and abovecritical solutions of PEG and ABS-forming salts. This occurred well before the asymptotes shown in Figure 2 were reached. For instance, a hydrogel in 0.4 m (NH4)2SO4 was perfectly transparent, whereas in 0.84 m (NH4)2SO4, the hydrogel was opaque. Visually, there appeared to be slight differences between the behaviors of the hydrogel at different concentrations, but this possibility was not studied in detail. However, it was noted that the critical opalescence of the hydrogel appeared to be associated not with phase separation of the polymer of the hydrogel, but rather with the exclusion of salt solution droplets within the polymeric hydrogel. It was observed that the hydrogels clouded from the outside toward the inside. Often, a transparent core could be observed within an opaque annulus. It was also observed that the hydrogels would eventually become perfectly transparent again given sufficient equilibration time (ca. 24 h) and that this transparency would also develop from the outside inward, leading to a cloudy core and transparent annulus before the hydrogel ultimately cleared completely. These observations suggest that this critical behavior was due to drops of salt phase being exuded within the polymer hydrogel phase. Such behavior seems quite analogous to spinodal decomposition in the liquid-liquid phase separation of PEG-salt ABSs.

It is interesting to note that the above behaviors, asymptotic approach to a limiting length and clouding, were also observed for KSCN solutions. We were not previously aware that KSCN was a phase-separating salt and had assumed, from all we knew of its partitioning behavior and the free energy of hydration of the anion, that KSCN was a typical chaotropic salt and would not be likely to salt out PEG at any achievable concentration close to room temperature. The behavior observed in Figure 2 prompted us to reexamine these assumptions and to find that, at sufficiently high salt concentrations, KSCN will form an ABS with PEG (molar mass 2kDa) at room temperature. The critical point of this critical mixture lies in the region of ca. 42% w/w KSCN and 10% w/w PEG 2000. In part, this behavior is due to the high solubility of KSCN.41 It is likely that almost all inorganic salts, provided their aqueous solubility limits are not exceeded, will salt out PEG at room temperature. However, one should note the case of CaCl2 shown in Figure 2. In this case, the salting out of PEG at room temperature can hardly be anticipated. The behavior of this salt is remarkable in that hardly any collapse of the hydrogel is observed. We had previously assumed that the salting out of PEG was largely dependent on the free energy of hydration of the anion. This is hardly tenable when the behavior of the PEG hydrogel in CaCl2 is compared to its behavior in NaCl, as shown in Figure 2. It seems likely that the coordination ability of the cation, the solubility of the anion in the environment of the PEG chains, and perhaps also the spatial implications for the system entropy of ionic multivalency are also important in determining the salting-out behavior of PEG in the presence of different salts. Figure 2 indicates that the PEG hydrogel collapses to a minimum fractional length structure that is ca. 0.3-0.4 of the original length. The coiled PEG structure prepared from stretched crystals from a quenched PEG melt mentioned earlier have a 7/2 helical structure, giving an end-to-end chain length of this molecule (dimethoxyhexaethylene glycol) of about 20.4 Å. Of course, this might not represent a reasonable aqueous structure. Nevertheless, a fully extended PEG molecule of this size would not be of very much greater length (+3-4 Å). If the PEG hydrogel collapse is regarded as a reasonable analogue for events at the molecular level, then collapse of this molecular structure to about 6-8 Å seems implicit. This value seems to be greater than would be implied by discussions in the literature focusing simply on changes to trans and gauche conformations about the C-C and C-O bonds. Simple molecular mechanics simulations seem to suggest that this size could be achieved only by a highly collapsed random coil or loose globular structure. The Flory model of polymer gel swelling given above also sheds light on the distribution of the salts between the hydrogel phase and the solution phase in these experiments, although such distributions were not specifically measured. Swelling of the hydrogel by the influx of water is resisted by the mechanical elasticity of the hydrogel, as noted previously. A component of the pressure inside the hydrogel is due to these elastic forces. If the salt concentration outside the hydrogel is raised, then the osmotic pressure is raised, and the pressure inside the hydrogel must increase. This pressure increase can be achieved by mechanical collapse, effectively increasing the polymer concentration, or by

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the influx of salt ions to balance the outside osmotic pressure. It can fairly be concluded that, if the hydrogel collapses, it is due to exclusion of salt ions from the PEG domains of the hydrogel. If the hydrogel does not collapse, the concentrations of ions inside and outside the hydrogel are similar. The degree of collapse is proportional to the degree of exclusion of the ions. That this is so might also be deduced by reflecting on the nature of the PEG-salt ABS phase diagram, where, above the critical point, the solution splits into a highconcentration salt phase and a high-concentration polymer phase containing much less salt.27 It seems also to be implicit in Figure 2, therefore, that, even before phase separation, a free solution of PEG and salt organizes itself into polymer domains relatively depleted in salt and salt domains relatively depleted in polymer. Many of the salts represented in Figure 2 never attain sufficient concentration to promote the limiting degree of gel collapse (a supracritical state) but still promote lesser degrees of gel collapse and therefore salt exclusion. It also seems from the polymer collapse observed here that such polymer domains would also be relatively dehydrated compared to their state in pure water or at relatively low salt concentration. This is quite paradoxical given that these salt-depleted polymeric domains are always completely surrounded by an aqueous medium. As is well-known42 and as we have discussed in a previous publication,27 an increase in the salt concentration serves to lower the cloud point of PEG solutions. Solutions of poly(ethylene glycol) in pure water are characterized by a solubility gap33 in which the polymer phase separates above a lower critical solution temperature (LCST) into polymer-rich and polymer-poor phases. As the temperature of such a system is further increased, the phase separation is marked by an increasing divergence of the phase compositions.27 This is thought to be due to the increasing dehydration of the polymer, its tendency to adopt hydrophobic conformations, and its incompatibility with the normal H-bonded structure of water.33 As the temperature is further raised, the compatibility increases once again because of the consequent increase in entropy, and miscibility is again found above the upper critical solution temperature (UCST).33 Increasing the salt concentration lowers the LCST closer to room temperature, and for some salts above critical concentrations, phase separation can be induced at room temperature. Increasing the temperature and increasing the salt concentration are demonstrably interchangeable in the induction of phase separation.27 We can therefore reason that an increase in the temperature of the aqueous environment of the present PEG hydrogel should lead to polymer collapse. Figure 3 shows that this is indeed what happens as the solution temperature is increased. Unfortunately, it is not possible to explore very high temperatures with the present hydrogel because the cross-links are subject to hydrolysis, which increases above the normal boiling point of water. This feature of these hydrogels is considered an advantage in biomedical applications, as it adds a measure of biodegradability. Note that, in Figure 3, polymer collapse with temperature appears to be a continuous function of temperature and commences very close to room temperature. The collapse of the polymer through the influence of increased salt concentration is believed to influence the polymer’s polarity. The polymer is believed to become more hydrophobic, and its hydrogen-bonding ability

Figure 3. Fractional length change of the PEG hydrogel in response to temperature. Symbols as for Figure 2.

Figure 4. ET(30) parameter calculated for Reichardt’s betaine from the response of the betaine dye (O) in free solution and (b) incorporated in the PEG hydrogel.

(compatibility with water) is believed to be reduced.33 Such changes seem to be confirmed by solvatochromic studies we have reported on the polarity and H-bonding ability of PEG solutions and the phases of ABSs.31,33 Solvatochromic dyes appear to indicate that PEG phases are generally of the same polarity as water and have the same H-bond basicity (H-bond-accepting/electrondonating) level as bulk water.33 Perhaps more importantly in the context of partition in ABSs, these properties were indicated by the solvatochromatic dyes to be the same in both phases and thus unlikely to influence solute distribution.33 Figure 4 shows the effect of temperature on the long-wavelength absorption band of Reichardt’s betaine dye in free solution (200 mM K2HPO4) and when the dye is incorporated in the PEG hydrogel bathed in the same solution. Increasing the temperature can be seen to have only a weak effect on the ET(30) parameter calculated from the peak maximum of the long-wavelength absorption band when the dye is in free solution. However, when the dye is

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Figure 5. ET(30) parameter calculated for Reichardt’s dye from the response of the betaine dye in solutions of 2% w/w PEG and different concentrations of K3PO4 in response to an increase in temperature.

incorporated in the hydrogel, polymer collapse and conformational change lead to an increased change in the ET(30) parameter, indicating in a general sense a decrease in polarity. In fact, similar changes have been observed in dry polymers hydrating in air streams of controlled humidity,43 and such changes seem to be most strongly related to a decrease in hydrogen-bond-donor ability in this phase.32 An exactly analogous dehydration and decrease in H-bond-donor ability is found for PEG polymer in free solution, which is driven either by an increase in the salt concentration or by an increase in the temperature. This is illustrated in Figure 5, where the effects of increasing the temperature and K3PO4 concentration in a solution of PEG 2000 are shown. Both increasing the temperature and increasing the salt concentration lead to a reduction in the calculated ET(30) parameter, which indicates a reduction in the H-bond-donation ability of these solutions. It is noticeable that the changes in “polarity” indicated by Reichardt’s betaine dye for both the free polymer and the hydrogel are continuous functions of temperature, begin immediately on changing the temperature, and do not indicate the existence of any critical condition necessary for the onset of these changes in polarity. Through the application of a linear free energy relationship (LFER) in the form of Abraham’s generalized solvation equation, we have shown that these changes of phase hydrogen-bonding ability (which we here associate with changes in polymer conformation) are associated with increased solubility of “hydrophobic” solutes.44 This seems to be especially the case for solutes with weak H-bond-accepting ability, given that H-bond bases would have a strong preference for the phase supplying the complementary H-bonding function, in this case, the salt phase of the ABS. This might also be important in the inclusion or exclusion of certain anions in or from the PEG phase. The preference of many solutes other than strong Lewis bases for the polymeric phase is maintained for the PEG hydrogel, although we have not developed a specific LFER for this material. The distribution of a water-soluble food coloring dye to the PEG hydrogel under the influence of temperature and salt concentration illustrates this solute preference and is shown in Figures 6 and 7, respectively. A number of features of the data shown in Figures 6 and 7 deserve further comment. There is no salt concentration or temperature at which the uptake of dye falls to zero. In part, this is

Figure 6. Uptake of the dye amaranth by the PEG hydrogel in response to an increase in temperature.

Figure 7. Uptake of the dye amaranth by the PEG hydrogel in response to an increase in the concentration of K2HPO4.

because the expression of the results in terms of the dry weight of hydrogel and passive uptake, diffusion of dye, into the initially dye-free hydrogel, results in an apparently significant uptake. In part, there might indeed be some small preference of the dye for the polymeric phase under these conditions. In addition, neither in the case of partition by temperature increase nor in the case of partition by salt concentration increase does there appear to be a critical onset of distribution to the PEG phase. It is true that, in the case of temperature, distribution seems to be an increasing function of temperature. Nevertheless, increasing both the temperature and the salt concentration leads to increased distribution to the hydrogel. All of this, the decrease in polarity and the increase in solute distribution to the hydrogel phase, neither of which represents a discontinuous function, is similarly reflected in the fact that there is no critical region of hydrogel collapse (see Figure 2). Under some conditions, hydrogel collapse is very rapid, such as in the presence of potassium phosphate at moderate concentrations. However, this is only true by comparison to other salt types, and no sudden discontinuity in the collapse rate with concentration is observable. In all cases, fractional polymer length is a continuous function of salt concentration. It seems possible to conclude, therefore that, polymer conformation, phase polarity (understood as changes in Hbonding ability) and ability to solubilize certain so-called hydrophobic solutes is likewise a continuous function

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of temperature and/or salt concentration in the counterpart liquid-liquid ABSs. None of these changes seem to follow a path leading to a critical phase transition. The only critical phase transition that seems to occur in these systems is the splitting of the aqueous medium into two differently structured phases. It therefore seems likely that these phases existed prior to the critical phase transition but were masked by the existence of sufficient numbers of “free” water molecules to preclude the necessity of forming a phase interface. This might explain certain other features of phase partitioning in these systems, such as why phase separation is marked by no thermochemical events. It also might make more appealing models of phase separation based on considerations of the molecular geometry involved.45 Conclusions It seems inescapable that the properties of PEG that lead to aqueous phase separation and the formation of a useful liquid-liquid extraction system are continuous functions of temperature and salt concentration. This conclusion leads us to propose the existence of solvent domains that exist in PEG/salt solutions at concentrations below those required for phase separation. Such domains lead to the occurrence of cosolvent effects in PEG solutions. It seems that temperature and salt concentration lead to polymer collapse and dehydration, which leads to a reduction in H-bonding ability, which causes changes in the solubility properties of the aqueous solution, leading ultimately to phase separation. This concept of the existence of solute domains might allow the freedom to consider different approaches to the separation and absorption of molecules and particles and the design of novel materials and devices for separations or solute-delivery purposes. Many different formats for the distribution of solutes or their absorption sequestration and slow and controlled release might be considered while still allowing their design to be informed by the conceptual simplicity of a partitioning step. Examples include the use of hydrogels as separating agents or solid solvents with which it might be possible to combine porosity with partition to achieve useful and interesting separations. In addition, thermally driven separations, for instance, resembling the thermally driven adhesion and release of biological cells,46 ought to be possible. Such separations might be attractive for environmental reasons as inputs and waste could, in principle, be limited to heat alone. Acknowledgment This research was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Research, U.S. Department of Energy (Grant DE-FG02-96ER14673). Literature Cited (1) Righetti, P. G.; Gelfi, C. Electrophoresis gel media: The state of the art. J. Chromatogr. B. 1997, 699, 63. (2) Byker, H. J. Electrochromics and polymers. Electrochim. Acta 2001, 46, 2015. (3) Vossoughi, S. Profile modification using in situ gelation technologysA review. J. Pet. Sci. Eng. 2000, 26, 199. (4) Moradi-Araghi, A. A review of thermally stable gels for fluid diversion in petroleum production. J. Pet. Sci. Eng. 2000, 26, 1. (5) Banik, R. M.; Kanari, B.; Upadhyay, S. N. Exopolysaccharide of the gellan family: Prospects and potential. World J. Microbiol. Biotech. 2000, 16, 407.

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Received for review April 22, 2003 Revised manuscript received August 4, 2003 Accepted September 11, 2003 IE030351X