Comparison of Temperature Effects on the Salting Out of Poly

Center for Green Manufacturing, Department of Chemistry, and Alabama ... can be attributed to the lower cloud point temperatures and probable conforma...
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Ind. Eng. Chem. Res. 2010, 49, 2371–2379

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Comparison of Temperature Effects on the Salting Out of Poly(ethylene glycol) versus Poly(ethylene oxide)-Poly(propylene oxide) Random Copolymer Meghna Dilip,†,‡ Scott T. Griffin,†,§ Scott K. Spear,| He´ctor Rodrı´guez,⊥ Christiaan Rijksen,⊥,# and Robin D. Rogers*,†,⊥ Center for Green Manufacturing, Department of Chemistry, and Alabama Institute for Manufacturing Excellence, The UniVersity of Alabama, Tuscaloosa, Alabama 35487, and QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s UniVersity of Belfast, Belfast BT9 5AG, Northern Ireland, U.K.

In common with many poly(ethylene glycol)s (PEGs), a poly(ethylene oxide)-poly(propylene oxide) random copolymer (EO-PO, with EO/PO ) 1:1.3) of molecular weight 3320 g · mol-1, known as Sunbright-3320 (SB-3320), can be salted out of aqueous solution using kosmotropic salts such as (NH4)2SO4 to form aqueous biphasic systems (ABSs) comprising upper polymer-rich and lower salt-rich aqueous phases. The effects of temperature on these liquid/liquid ABSs and related solid/liquid resin-based systems, where one end of the polymer has been covalently attached to a solid support, have been studied. Distribution ratios of NH499TcO4 were determined radiometrically at various polymer and salt concentrations and temperatures. SB-3320grafted poly(styrene) resins exhibit opposite effects of variable-temperature partitioning compared to the SB3320-based ABS. However, the results are complicated because of the conformational changes that are possible for the SB-3320 polymer. Enthalpy and entropy changes were found to be temperature-dependent. The differences observed between the distribution of 99TcO4- in EO-PO-based systems versus PEG-based systems can be attributed to the lower cloud point temperatures and probable conformational changes for the EO-PO systems. 1. Introduction Aqueous biphasic systems (ABSs) are liquid/liquid (l/l) systems formed when two aqueous polymer solutions are combined (polymer/polymer ABSs) or when an aqueous polymer solution is mixed with certain inorganic salts (polymer/ salt ABSs) above a critical concentration or temperature.1,2 Kosmotropic (water-structuring) salt solutions can also salt out chaotropic (water-destructuring) salts such as hydrophilic ionic liquids, leading to the formation of a third kind of ABS (salt/ salt ABSs).3-7 However, controversy regarding the interpretation of the interaction of salt ions and water has recently arisen and is the object of active dispute.8-10 The driving force of phase separation in an ABS, although not completely understood, might be due to water structuring and the partial dehydration of the solutes within the two phases.2,11 ABSs are particularly attractive as separating media because both of the phases are mainly composed of water.2 Further, the components of many ABSs are generally nonvolatile, nontoxic, and considered environmentally benign, thus making ABSs attractive as potentially “green” media for separations.4-7,11-15 Several biological applications of ABSs have been reported using these systems, owing to the gentle and nondenaturing character of their phases.16-19 Separations of metal ions, organic molecules, radiochemicals, nanoparticulate matter, and minerals * To whom correspondence should be addressed. Tel.: +1-205-3484323. Fax: +1-205-348-0823. E-mail: [email protected]. † Center for Green Manufacturing, Department of Chemistry, The University of Alabama. ‡ Current address: Department of Chemistry, Worcester State College, Worcester, MA 01602. § Current address: Cytec Industries Inc., 1937 West Main Street, Stamford, CT 06904. | Alabama Institute for Manufacturing Excellence, The University of Alabama. ⊥ The Queen’s University of Belfast. # Current address: Cambridge Major Laboratories Europe BV, Vliesvenweg 1, 6002 NM Weert, The Netherlands.

have been previously demonstrated in ABSs.20-31 However, to gain wider acceptance in industry, the fundamental parameters governing partitioning in these systems need to be further understood. This work investigates one such parameter, temperature, and the differences in its effect on poly(ethylene glycol) (PEG) versus poly(ethylene oxide)-poly(propylene oxide) (EO-PO) random copolymer ABSs. A group of water-soluble, nonionic, linear copolymers composed of poly(ethylene oxide) (EO) and poly(propylene oxide) (PO) spontaneously separate into two phases in aqueous media at a particular temperature.32 These copolymers belong to the category of “thermoseparating” polymers, defined as polymers that phase separate from water above a particular temperature specific to that polymer, known as the cloud point, resulting in a lower water-rich phase and an upper polymerrich phase. The same phenomenon is also observed in analogous simple polymers, such as PEG, as will be discussed later. One such thermoseparating polymer is Sunbright-3320 (SB3320), an EO-PO random copolymer (EO/PO ratio ) 1:1.3) with an average molecular weight of 3320 g · mol-1 and a cloud point of 70 °C for a 40% w/w aqueous solution, as reported by the manufacturer and reconfirmed experimentally in our laboratory (see sections 2 and 3). SB-3320 can additionally be classified as a “smart” polymer, a term applied to macromolecules that can undergo both a reversible and rapid change in microstructure (hydrophilic to hydrophobic) through the alteration of an environmental parameter (temperature in this case).29,32 This characteristic has been utilized in several separations, termed generically as cloud-point extractions, with the added advantage of easy recycling of the polymer.33-39 In addition to thermoseparation from water at the cloud point, SB-3320 can also be salted out with kosmotropic salts to form ABSs, a feature in common with PEG. The cloud point of SB3320 (70 °C for a 40% w/w solution) is much lower than that observed for many PEGs (which are thermoseparating polymers as well), where the cloud points can range from 185 °C for

10.1021/ie901268m  2010 American Chemical Society Published on Web 02/02/2010

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lower-molecular-weight PEG to around 95 °C for PEG of molecular weight 20000 g · mol-1.40 The addition of a kosmotropic salt to solutions of thermoseparating polymers (including PEG) is known to decrease the cloud point even further, as well as the amount of polymer needed for phase separation.41-43 SB3320 thus might be preferred over PEG in separation applications because of lower operating concentration and temperature ranges needed for polymer recovery and reuse. Although separations can be achieved with ABSs, the systems suffer from a few operational disadvantages such as difficult recovery of extracted solutes, loss of biphasic components as a result of their presence in the opposite phases, and long separation times.44 To overcome these disadvantages a solidsupported resin form can be developed by attaching one end of the water-soluble polymer to a poly(styrene)-divinylbenzene cross-linked copolymer resin bead (Merrifield resin). These resins have been named aqueous biphasic extraction chromatographic resins, or ABEC resins, and have been reported to behave similarly to their l/l counterparts, with complementary applications being reported.13,19,45-52 Some dissimilarities, however, have been reported between ABSs and ABECs based on the PEG polymer. For example, solutes can partition to ABECs below the critical concentration necessary to support biphase formation in solution or perhaps more generally at low salt concentration.53 ABSs and ABECs also differ in their response to temperature, as illustrated by the temperature dependencies of their distribution ratios: whereas PEG-based ABSs exhibit increased partitioning with increasing temperatures, uptake to the ABEC resins decreases.54 In this article, we compare the temperature-dependent saltingout properties of the EO-PO copolymer SB-3320 with those of the PEG polymer methyl poly(ethylene glycol) (MePEG5000), in both l/l ABSs and solid-liquid (s/l) ABEC analogs (ABEC-SB3320 and ABEC-5000). Using a technetium tracercontaining salt as the partitioning probe, similarities and differences can be readily observed. 2. Materials and Methods 2.1. Chemicals. SB-3320 copolymer with an EO/PO ratio of 1:1.3 and a molecular weight of 3200 g/mol was obtained from Shearwater Polymers (now Nektar Therapeutics, Huntsville, AL). Ammonium sulfate and other salts were purchased from Aldrich (Milwaukee, WI). ABEC-SB3320 [SB-3320 covalently attached to poly(styrene)-divinylbenzene cross-linked beads, 200-300 mesh] was obtained from EIChroM Industries (Darien, IL). All water was deionized using a commercial deionizer and polisher (Culligan, Northbrook, IL). The technetium tracer (as NH499TcO4 in aqueous 0.1 M NH4OH) was obtained from Amersham Life Science, Inc. (Arlington Heights, IL) and used as received. Technetium-99 activities were measured in Ultima Gold Scintillation cocktail with a Packard Tri-Carb 1900 TR scintillation analyzer (Packard Instrument Co., Inc., Meriden, CT). 2.2. Phase Diagrams. The phase diagrams were constructed using turbidimetric titration, also known as cloud-point titration, described in detail elsewhere.55 All solutions were equilibrated at the desired temperature, before and during the construction of the phase diagram, using a temperature-controlled water bath (Neslab RTE 110, Neslab Instruments Inc., Newington, NH). The binodals were fitted using mathematical methods reported by Merchuk et al.56 2.3. Measurement of Cloud Points. Equal volumes (4 mL each) of stock solutions of (NH4)2SO4 of increasing molarity (1.5, 2.0, 2.5, and 3.0 M) and 40% w/w SB-3320 were mixed,

vortexed, and allowed to phase separate. After equilibration overnight at room temperature, 1 mL of the top phase (rich in SB-3320) was disengaged and placed in a temperature-controlled water bath at 0 °C and allowed to equilibrate at that temperature for 30 min. The temperature was then raised in 1 °C increments, and the first instance where cloudiness was observed was noted as the cloud point. The cloud point of pure aqueous SB-3320 was also determined in this way and matched that reported by the manufacturer (70 °C). 2.4. Liquid/Liquid Partitioning. The pertechnetate anion (TcO4-) is chaotropic (∆Ghyd ) -251 kJ · mol-1)57 and has been shown to preferentially partition to the more hydrophobic (less polar) phase in PEG/salt and salt/salt ABSs.7 It is a convenient probe of phase behavior in ABSs, as it has been shown that the value of the distribution ratio is sensitive to the solvent properties of the two phases, increasing with increasingly divergent phases. In addition, the long-lived, weak β-emitter 99Tc can be easily studied radiochemically at near-infinite dilution. Studies of the partitioning of pertechnetate can thus provide valuable information about the nature of the phases in the ABS and the thermodynamics driving partitioning. To study the partitioning of 99TcO4- in the l/l ABSs, the following procedure was used: Equal aliquots of 40% (w/w) polymer solution and salt stock solution of varying molar concentration were mixed. (Note that the total concentration of each of the phase-forming components in the ABS systems was approximately one-half the value for the stock solutions used to prepare the ABSs, as equal volumes of aqueous polymer and salt solutions were mixed.) Each system was vortexed for 2 min to mix the phases and then subjected to 2 min of centrifugation (1000g) to promote phase separation. The systems were allowed to equilibrate at experimental temperatures using a Neslab RTE-110 water bath. A tracer quantity of NH499TcO4 (10 µL, ca. 0.035 µCi · µL-1) was then added to each system, which was then vortexed for 2 min and centrifuged (1000g) for 2 min to separate the phases. The systems were then maintained at the experimental temperature until samples could be removed for radioanalysis. Equal aliquots (100 µL) of the top and bottom phases were pipetted out for radioanalysis. Because equal aliquots of each phase were analyzed and the activity of the tracers is directly proportional to its concentration, the distribution ratios (D) were determined as D)

Ap As

(1)

where Ap is the activity of the upper polymer-rich phase and As is the activity of the lower salt-rich phase, both in counts per minute. The distribution ratios reported here are the averages of at least two measurements and are typically accurate to (5%. 2.5. Batch Uptake. To ensure a constant water content of the resins in all experiments, ABEC-SB3320 was pretreated using the same procedures as described for ABEC-5000 (MePEG-5000 attached to the polymer beads) in earlier works.51,57 Approximately 3 g of resin was placed in a 50-mmdiameter Bu¨chner funnel containing a Whatman #2 Qualitative Filter disk. The resins were washed by passing several portions of deionized water over the resin and then treated with air that was bubbled through deionized water. The resins were then stored in deionized water until use. The hydrated mass of the resin was converted to its dry-weight equivalent using a dryweight conversion factor (dwcf) obtained by gravimetric analysis (as a quotient between the dry weight of the resin and the hydrated weight of the resin).

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The batch contacts were carried out as follows: The radiotracer (NH499TcO4, 10 µL, ca. 0.035 µCi · µL-1) was added to 2.2 mL of the solution of interest and then mixed, and a 100 µL aliquot was removed for analysis. For each batch uptake, 0.015-0.025 g of resin was contacted with 1 mL of the spiked solution (contact volume) and then placed in a temperaturecontrolled water bath. The mixture was magnetically stirred at the desired temperature for 45 min. Samples for radioanalysis were withdrawn by first inserting a pipet tip with a 0.45-µm filter attached to the end to remove the solution from the resin. After the solution had been filtered, a 100 µL aliquot was removed for radioanalysis. The dry-weight distribution ratios (Dw) were calculated using eq 2: Ai - Af V Dw ) Af m × dwcf

(2)

where Ai is the activity of the solution before contact with the resin, Af is the activity of the solution after contact, V is the contact volume (in mL), m is the mass of resin (in g), and dwcf is the dry-weight conversion factor defined above. The dwcf values for the ABEC-5000 and ABEC-SB3320 resins obtained and used in Dw calculations were 0.1200 and 0.4634, respectively. The comparison of these two values is consistent with the greater hydrophobicity exhibited by the random copolymer resins, due to the presence of propylene oxide units in their structure, which is a potentially tunable variable. The Dw values reported here are the averages of at least two measurements and are typically accurate to (5%. 2.6. Thermodynamic Study: Theoretical Fundamentals. The distribution of a solute such as TcO4- between phases in an ABS and partitioning onto ABEC resins can be thermodynamically represented using the following well-known thermodynamic equation ln K )

∆S° ∆H° + RT R

(3)

where K is the equilibrium constant; R is the ideal gas constant; T is the absolute temperature; and ∆H° and ∆S° denote, respectively, the standard change of enthalpy and standard change of entropy accompanying the partitioning process. In this equation, K is equivalent to the distribution ratio (D) if the phase ratio is the unity. When ∆H° is invariant to temperature, a linear van’t Hoff plot (ln K vs 1/T) can be obtained to evaluate the thermodynamic constants ∆H° and ∆S° from the slope (-∆H°/R) and the intercept (∆S°/R), respectively. The relative compositions of the phases in equilibrium can be different at different temperatures (see below), but we believe that such changes are small enough within our temperature ranges, that eq 3 can still provide useful information. It is our understanding that the original work by van’t Hoff, from which the equation was derived, had a degree of complexity similar to the systems we report here (transport across semipermeable membranes within aqueous media).58 We would point out that even the addition of solutes in small amounts causes some variation of the compositions of coexisting phases in an equilibrated ABS. Nonlinear van’t Hoff plots signify temperature-dependent thermodynamic parameters.59-61 These thermodynamic parameters can be analyzed by fitting the partitioning data to the quadratic equation

ln K' ) a +

c b + 2 T T

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(4)

and obtaining the parameters a, b, and c.62,63 Various thermodynamic parameters can be obtained using Kirchhoff’s relations, as illustrated in eqs 5-754 ∆H° ) -R

∂(ln K) 2c ) -R b + 1 T ∂ T

(

()

(

∆S° ) R a ∆C◦p )

c T2

)

2Rc T2

)

(5)

(6)

(7)

In addition, of course, ∆G° can be subsequently calculated by means of the equation ∆G° ) ∆H° - T∆S°

(8)

3. Results and Discussion 3.1. Effect of Temperature on the SB-3320/(NH4)2SO4 ABS Phase Diagram. The binodals representing the boundary between the monophasic (to the left) and the biphasic (to the right) regions for the SB-3320/(NH4)2SO4 ABS at 10 and 40 °C are plotted in Figure 1, along with the binodals for the MePEG-5000/(NH4)2SO4 and PEG-2000/(NH4)2SO4 ABSs for comparison.54,64 In all three systems, the binodal shifts toward lower salt concentrations (to the left in the diagram) as the temperature increases.25,54 It is also clear that the system with the copolymer SB-3320 requires less salt to induce phase separation than the other systems, at any of the given temperatures. This is also consistent with the lower cloud point for SB-3320 solutions, compared to those containing PEG-2000 or MePEG-5000. Figure 2 expands the data for the SB-3320/(NH4)2SO4 ABS, providing a more detailed analysis of the temperature effect. Although compositions of coexisting phases were not determined, the shifting of the binodals with temperature allows for a valuable analysis toward a better understanding of the system. (This permits a suitable comparison of the different systems as a function of temperature, at least in relative terms.) As expected, the binodals do consistently shift to the left as the temperature is increased in the range of 10-50 °C. This is similar to the behavior exhibited by ABSs based on PEG.25,54 3.2. Effect of Different Kosmotropic Salts on the SB3320/Salt ABS Phase Diagram. The position of the binodal and phase divergence are also influenced by the nature of the kosmotropic salt used to form an ABS. Phase diagrams of aqueous SB-3320 with solutions of three different kosmotropic salts commonly used in the generation of ABSs [(NH4)2SO4, K3PO4, and Na2CO3] were constructed at the same temperature (25 °C). The results in Figure 3, plotted versus the molal concentration of the salts, show that less salt is required to salt out SB-3320 (the binodal curves are shifted to the left, closer to origin) in the order PO43- < CO32- < SO42-. It has been shown, for the salting out of PEG, that the salt’s anion plays a more determinant role than the cation.56 The salting-out strength of the anion depends on its relative kosmotropicity (typically its position in the Hofmeister series), and this can be determined by the ion’s Gibbs free energy of hydration (∆Ghyd).65 Thus, to form an ABS with a given amount of polymer, a lower concentration of K3PO4 (∆Ghyd of PO43- ) -2773 kJ · mol-1)

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Figure 1. Experimentally determined phase diagrams for several polymer/ (NH4)2SO4 ABSs at 10 °C (closed symbols) and at 40 °C (open symbols). Polymers: MePEG-5000 (circles, s), PEG-2000 (triangles, - · -), and SB3320 (squares, - - -). The MePEG-5000 data are adapted from ref 54. The PEG-2000 data are adapted from ref 64.

Figure 2. Experimentally determined phase diagrams for the SB-3320/ (NH4)2SO4 ABS at several temperatures: (9) 10, (∆) 20, (b) 30, (0) 40, and (1) 50 °C. The inset shows a closeup of a section of the binodal curves fitted to Merchuk et al.’s equation.56 From right to left: (- - -) 10, ( · · · ) 20, (- · -) 30, (s) 40, and (- · · -) 50 °C.

Figure 4. Distribution ratios (D) for 99TcO4- as a function of temperature for the SB-3320/(NH4)2SO4 ABS, using different concentrations of the (NH4)2SO4 stock solution mixed with an equal volume of 40% polymer solution: (9) 1.5, (b) 2.0, (2) 2.5, (1) 3.0, and (() 3.5 M. Regressions are provided as a guide to the eye. For comparison, data for the MePEG-5000/ (NH4)2SO4 ABS are plotted in the inset (adapted from ref 54).

Figure 5. Change in cloud point of the SB-3320-rich phase of the SB3320/(NH4)2SO4 ABS with respect to aqueous SB-3320 polymer with no salt present, as a function of the molarity of the (NH4)2SO4 stock solution used to prepare the ABS by mixing with an equal volume of 40% polymer solution.

3.3. Effect of Temperature on the Distribution of TcO4- in l/l ABSs and Influence of the Kosmotropic Salt Concentration on the Cloud Point. The distribution ratios of 99TcO4- (as the NH4+ salt) in SB-3320/(NH4)2SO4 ABSs were studied as a function of temperature, and the results are plotted in Figure 4, with an inset plotting those previously reported for the MePEG-5000/(NH4)2SO4 ABS for comparison. In the systems prepared with the lowest amounts of salt [stock salt solutions of 1.5 and 2.0 M (NH4)2SO4], the distribution ratios for 99TcO4- increase with temperature, as observed for similar PEG/salt ABSs. However, this behavior changes as higher concentrations of salt are used. The midrange salt concentration (2.5 M stock solution) produces an ABS in which pertechnetate partitioning is almost invariant to temperature, whereas the highest salt concentrations (3.0 and 3.5 M stock solutions) actually produce ABSs in which the distribution ratios decrease with increasing temperature. To further define the relationship between increasing kosmotropic salt concentration and increasing temperature, cloud points of the individual top (polymer-rich) phases of the SB-3320/(NH4)2SO4 ABS were determined experimentally. A plot of the differences between the cloud point of SB-3320 (70 °C) and that of the ABS top phases, containing SB-3320 and a small quantity of (NH4)2SO4, is shown in Figure 5. As the concentration of salt in the stock solution used to generate the ABS is increased, the concentration of (NH4)2SO4 in the equilibrated polymer-rich phase also 99

Figure 3. Experimentally determined phase diagrams for SB-3320/salt ABSs, at 25 °C, with different concentrations of kosmotropic salts: (9) K3PO4, (2) Na2CO3, and (b) (NH4)2SO4. Solid lines correspond to the binodal curves fitted by means of Merchuk et al.’s equation.56

is needed than of Na2CO3 (∆Ghyd of CO32- ) -1315 kJ · mol-1) or (NH4)2SO4 (∆Ghyd of SO42- ) -1090 kJ · mol-1), for which the highest concentration of salt is required.66 It is noted that the addition of salt here has an effect similar to that of increasing temperature in an ABS formed utilizing SB-3320, consistent with results reported previously for ABSs formed using PEG.25 Just as an increase in temperature results in less salt being needed for biphase formation, increased salt concentration results in biphase formation at lower temperatures. Therefore, to tune the formation of an ABS with SB-3320, one can use salt, temperature, or a combination of the two.

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Figure 6. Dry-weight distribution ratios (Dw) for 99TcO4- as a function of temperature with ABEC-SB3320 (filled symbols), using different concentrations of the (NH4)2SO4 solution contacted with each resin: (`) 0.5, (9) 1.5, (b) 2.0, (2) 2.5, (1) 3.0, and (() 3.5 M. Splines are drawn as a guide to the eye. For comparison, data for ABEC-5000 are plotted in the inset (adapted from ref 54).

Figure 8. van’t Hoff plots for the SB-3320/(NH4)2SO4 ABS (top, black symbols) and ABEC-SB3320 (bottom, gray symbols), for different concentrations of (NH4)2SO4 in the solutions used for forming the ABS or for contacting with the ABEC resin: (`) 0.5, (9) 1.5, (b) 2.0, (2) 2.5, (1) 3.0, and (() 3.5 M. Figure 7. Comparison of the distribution ratios (D) and dry-weight distribution ratios (Dw) for 99TcO4- in the SB-3320/(NH4)2SO4 ABS (black symbols) and ABEC-SB3320 (gray symbols), as a function of temperature, for different concentrations of the (NH4)2SO4 stock solutions used to prepare the ABS or contacted with the resin: (`) 0.5, (9) 1.5, (b) 2.0, (2) 2.5, (1) 3.0, and (() 3.5 M.

increases. As expected, this causes a decrease in the cloud point compared to that of the pure polymer. There is a sharp drop in the variation of cloud points between the systems prepared using 2.5 and 3.0 M (NH4)2SO4 stock solutions. It is pertinent that this is the same salt concentration at which a difference in the trend of pertechnetate distribution ratios as a function of temperature (Figure 4) was observed. Currently, there are three main established hypotheses to explain the phenomenon of thermoseparation of polymers based on ethylene oxide units. Kjellander and Florin explained thermoseparation in terms of the changing structure of water,67 whereby the water structure is decreased and the dielectric constant drops rapidly with increasing temperature, so that water becomes a poorer hydrophilic solvent with reduced solubility for the polymer.68,69 A second hypothesis by Goldstein is based on the hydrogen-bonding environment of the solution,70 where the hydrogen bonds are broken as the temperature is raised, increasing polymer-polymer interactions and hence inducing phase separation. The third hypothesis, which we prefer, suggests that nonpolar conformers are thermodynamically favored and begin to dominate the properties of the polymer at higher temperatures, making the interactions with water less favorable.71 This hypothesis is the most accepted, based on the work of Karlstro¨m.40,69,72,73 The results presented here are an indication of a conformational change of the SB-3320 polymer in the ABS at higher salt concentrations, specifically above 2.5 M (NH4)2SO4.

3.4. Effect of Temperature on the Uptake of 99TcO4- to ABEC-SB3320 and ABEC-5000 Resins. Dry-weight distribution ratios (Dw) to ABEC-SB3320 were determined by contacting the resin with a kosmotropic salt solution containing NH499TcO4. It is thus important to make a distinction in the salt concentrations reported here for the l/l and resin studies. For each l/l ABS, distribution ratios were calculated by mixing the aqueous salt solution containing 99TcO4- with an equal volume of aqueous polymer solution. Contacting two liquid phases leads to dilution, and because equal volumes of the two phases were used, final salt concentrations can be approximated as one-half of the original stock solution concentration. On the other hand, for the polymer-bound resin studies, the salt concentrations reported are those making direct contact with the resin. The local concentration of SB-3320 or salt at the polymer could be quite high, or the polymer could be bound in such a way as to make it inaccessible for partitioning. These effects are difficult to estimate, and comparison of ABSs and ABEC can present some unusual difficulties. Nonetheless, such comparisons often provide valuable information, and for PEG, ABS behavior can be used to predict resin performance. Figure 6 shows the effect of temperature on the Dw values of pertechnetate from five different (NH4)2SO4 concentrations to ABEC-SB3320 resin. The data for ABEC-5000 (Figure 6, inset) reveal that the Dw values increase with increasing salt concentration and decrease linearly with increasing temperature, exhibiting similar slopes across all salt concentrations.54 Conversely, the Dw values obtained using ABEC-SB3320 decrease with increasing temperature, but do not exhibit the same rate of decrease, as greater decreasing rates are found at higher salt concentrations. The data also show nonlinear behavior, with interesting features appearing at various combinations of salt concentration and temperature, as described below.

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Figure 9. Temperature dependence of the change in (a) standard enthalpy (∆H°), (b) standard entropy (∆S°), (c) standard Gibbs free energy (∆G°), and (d) 99 standard molar heat capacity (∆C°), TcO4- in the SB-3320/(NH4)2SO4 ABS (black symbols, solid lines) and ABEC-SB3320 (gray p in the partitioning of symbols, dashed lines) using different concentrations of the kosmotropic salt (NH4)2SO4 to form the ABS or to contact the resin: (hexagons) 0.5, (squares) 1.5, (circles) 2.0, (upward triangles) 2.5, (downward triangles) 3.0, and (diamonds) 3.5 M. Straight segments between points of the same series are drawn just as a guide to the eye.

The Dw values of the lowest salt concentration (0.5 M) are relatively flat, similar to those for ABEC-5000, but with a slightly more pronounced decrease at 40 °C. The series for the salt concentrations 1.5 and 2.0 M decrease more rapidly at 30 °C, whereas the data for 2.5 and 3.0 M show this behavior at 20 °C. For the highest salt concentration, 3.5 M, the Dw values decrease even from the lowest experimental temperature at a higher rate than for any of the other solutions. This change in trends between the MePEG-5000 and SB-3320 ABEC resins is attributed to the fact that the cloud point of MePEG-5000 is over 100 °C, whereas the cloud point of SB-3320 is as low as 70 °C. The SB-3320 polymer, although tethered at one end, potentially undergoes conformation change at a specific temperature, equivalent to the cloud point in an l/l system. This conformational change is hypothesized to be responsible for the differences observed in ABEC-SB3320 as compared to ABEC5000. 3.5. Comparison of SB-3320-Based ABSs and ABECSB3320. In addition to the changes observed between systems based on different polymers (MePEG-5000 and SB-3320), differences in the distribution of 99TcO4- with temperature between the l/l SB-3320/(NH4)2SO4 ABS and the analogous s/l ABEC-SB3320 were observed (Figure 7). In SB-3320-based ABSs, an increase of D with increasing temperature is observed for at least the lower salt concentrations, whereas in the corresponding ABEC resins, the Dw values consistently decrease as the temperature is increased. This is the most observable difference in the behavior of the l/l ABS and supported ABEC systems based on the SB-3320 copolymer. Differences in the behavior between ABSs and ABECs for PEG have been explained on the basis of the anchoring of the polymer to the surface of the resin bead, where the local concentrations of polymer and salt can be much higher than in

the corresponding ABS.44,53,54 The biphasic microdomains of high component concentrations that are generated can act as upper critical solution temperature (UCST) environments, in contrast to the lower critical solution temperature (LCST) behavior usually characterizing these systems, thus explaining the reverse trends observed. 3.6. Thermodynamics of 99TcO4- Partitioning. Figure 8 shows van’t Hoff plots for SB-3320/(NH4)2SO4 ABS (ln D vs 1000/T) and ABEC-SB3320 (ln Dw vs 1000/T) with different concentrations of kosmotropic salt. Although a reasonably linear trend is observed for some of the data series, for others, clear nonlinear behavior is evident. This implies that the enthalpy and entropy of partitioning in these systems depend on the temperature, in contrast to both MePEG-5000-based ABSs and ABEC-5000 systems, where data showed linear behavior, indicating no variation of entropy and enthalpy with temperature.54 To deal with the nonlinearity of the data for the SB-3320based systems, eq 4 was used to fit them, in order to derive the temperature-dependent parameters. Figure 9 shows the resulting plots of ∆H°, ∆S°, ∆G°, and ∆C°p versus temperature, calculated with eqs 5-8, for the SB-3320 ABS (black symbols) and the ABEC-SB3320 system (gray symbols). Whereas no trend in enthalpy or entropy with respect to the concentration of kosmotropic salt was previously observed for similar MePEG-5000-based ABSs, here, the SB-3320-based ABS exhibits a decrease in both enthalpy and entropy (with a few exceptions, as described below) with increasing concentrations of (NH4)2SO4. At lower salt concentrations, the enthalpy and entropy increase with increasing temperature; however, starting at the series for which a 2.5 M (NH4)2SO4 stock solution was used, both thermodynamic parameters start decreasing with temperature. The most drastic decrease is observed for the

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system made using the 3.0 M (NH4)2SO4 stock solution, which again suggests a change in polymer conformation at this point. For ABEC-3320, ∆H° and ∆S° show essentially no temperature dependence at the lowest salt concentration (0.5 M); however, for the other salt concentrations, the slopes become increasingly negative with increasing salt concentration, but switch to positive at the highest concentration (3.5 M). The enthalpy and entropy values in the ABEC systems are mainly negative, indicating that the partitioning of 99TcO4- in these systems is predominately enthalpically driven. This effect increases with increasing temperature for most of the salt concentrations tested, but the positive slope of the series with the 3.5 M stock solution indicates that, in this case, the enthalpic factor is less relevant as the temperature increases, whereas entropic effects become more important. The Gibbs free energies of partitioning (∆G°) of 99TcO4- are negative for both ABSs and ABECs, indicating spontaneity of separation in all cases. The ABS plots are mostly linear and have a negative slope, with the strong exception of the 3.0 M (NH4)2SO4-based system. The ABEC plots show that spontaneity decreases with increasing temperature. In addition, at the lowest temperature, it can be observed that the spontaneity of the system is strongly influenced by the kosmotropic salt concentration, with ∆G° values increasing in absolute value from -11 to -18 kJ · mol-1 as the (NH4)2SO4 concentration increases. As the temperature is increased to 50 °C, ∆G° for the different salt concentrations appears to converge. The changes in molar heat capacity (∆C°) p for the ABSs studied are near zero ((0.2 kJ · mol-1 · K-1) and show little variation with temperature, except for the case of the system made as a result of contact with a 3.0 M (NH4)2SO4 stock solution, where the heat capacity change is clearly more negative (-1.5 kJ · mol-1 · K-1 at 10 °C) and increases with temperature. This change is near the salt concentration at which the SB3320 polymer undergoes a marked change in the cloud point (Figure 5) and likely is linked to a conformational change of the polymer. In the ABEC plots, the values of ∆C°p are mostly larger in absolute value and negative (except for the series at the highest salt concentration), with positive slopes in the salt concentration range from 0.5 to 2.0 M; for higher concentrations, the slopes level off and eventually become negative at 3.5 M, where the ∆C°p values switch to positive. Again, the data are consistent with a change in the thermodynamics driving the separation of 99TcO4- at ca. 3.0 M (NH4)2SO4, likely as a result of polymer conformation changes. 4. Conclusions Less salt is needed to induce ABS formation with SB3320 than MePEG-5000, consistent with the lower cloud point of SB-3320. However, the effect of temperature on the distribution of 99TcO4- in SB-3320/(NH4)2SO4 ABSs and on analogous ABEC-SB3320 resins reflects increasing system complexity over traditional PEG/salt systems because of the lower temperatures at which conformational changes occur in EO-PO random copolymers. The LCST of SB-3320 decreases with higher concentrations of kosmotropic salt added to induce phase separation, and we believe that this behavior can be attributed to the ability of thermoseparating polymers to undergo conformational changes to more hydrophobic structures induced by either temperature, salt concentration, or both simultaneously. We had previously observed that PEG-grafted poly(styrene) resins exhibited opposite effects of variable-temperature partitioning compared to l/l PEG-based ABSs.54 This was

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rationalized on the basis that, within the ABEC resin, the local concentration of the polymer is much higher than in the l/l system. Similar opposite trends in variable-temperature partitioning were observed when comparing SB-3320-based ABSs and ABEC-SB3320 resin; however, the results are more complicated because of the additional contribution of conformational changes that are possible for the SB-3320 polymer at much lower temperatures than PEG. At low salt concentrations, ABEC-SB3320 resembles ABEC-5000, that is, they have similar slopes of Dw versus temperature. At higher salt concentrations; however, the uptake behavior of ABEC-SB3320 drops rapidly, and the slopes are steeper, again suggesting a change in the conformation within the EO-PO random copolymer. van’t Hoff plots for the SB-3320 ABS and ABEC-SB3320 indicate that the enthalpy and entropy changes are temperaturedependent in these systems (contrary to PEG/salt ABSs). The Gibbs free energies of partitioning of 99TcO4- are negative, indicating spontaneity of separation in all cases. The trends in the courses of the thermodynamic parameters ∆H°, ∆S°, ∆G°, and ∆C°p as a function of temperature change dramatically at the threshold value of 3.0 M (NH4)2SO4 solution. A conformation change of the polymer can explain the variation in the trends and the differences in thermodynamics observed in these systems at the aforementioned salt concentration. Acknowledgment This research was supported by the Division of Chemical Sciences, Geoscience, and Bioscience, Office of Basic Energy Sciences, and Office of Science, U.S. Department of Energy (Grant DE-FG02-96ER14673). Literature Cited (1) Albertsson, P.-Å. Partition of Cell Particles and Macromolecules, 3rd ed.; Wiley: New York, 1986. (2) Rogers, R. D., Eiteman, M. A., Eds. Aqueous Biphasic Separation: Biomolecules to Metal Ions; Plenum Press: New York, 1995. (3) Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Controlling the Aqueous Miscibility of Ionic Liquids: Aqueous Biphasic Systems of Water-Miscible Ionic Liquids and Water-Structuring Salts for Recycle, Metathesis, and Separations. J. Am. Chem. Soc. 2003, 125, 6632. (4) Dilip, M.; Venkateswaran, P.; Palanivelu, K. Removal of textile dyes from textile dye effluent using TBAB based aqueous biphasic systems. J. EnViron. Sci. Eng. 2005, 47, 176. (5) Rodrı´guez, O.; Madeira, P. P.; Macedo, E. A. Gibbs Free Energy of Transfer of a Methylene Group in Buffer + Ionic Liquid Biphasic Systems. Ind. Eng. Chem. Res. 2008, 47, 5165. (6) Wu, B.; Zhang, Y.; Wang, H. Phase Behavior for Ternary Systems Composed of Ionic Liquid + Saccharides + Water. J. Phys. Chem. B 2008, 112, 6426. (7) Bridges, N. J.; Rogers, R. D. Can Kosmotropic Salt/Chaotropic Ionic Liquid (Salt/Salt Aqueous Biphasic Systems) be Used to Remove Pertechnetate from Complex Salt Waste. Sep. Sci. Technol. 2008, 43, 1083. (8) Holzmann, J.; Ludwig, R.; Geiger, A.; Paschek, D. Pressure and salt effects in simulated water: Two sides of the same coin. Angew. Chem., Int. Ed. 2007, 46, 8907. (9) Mancinelli, R.; Botti, A.; Bruni, F.; Ricci, M. A.; Soper, A. K. Hydration of sodium, potassium, and chloride ions in solution and the concept of structure maker/breaker. J. Phys. Chem. B 2007, 111, 13570. (10) Tobias, D. J.; Hemminger, J. C. Getting specific about specific ion effects. Science 2008, 319, 1197. (11) Chen, J.; Spear, S. K.; Huddleston, J. G.; Holbrey, J. D.; Rogers, R. D. Application of polyethylene glycol-based aqueous biphasic reactive extraction to the catalytic oxidation of cyclic olefins. J. Chromatogr. B 2004, 807, 145. (12) Huddleston, J. G.; Willauer, H. D.; Griffin, S. T.; Rogers, R. D. Aqueous Polymeric Solutions as Environmentally Benign Liquid/Liquid Extraction Media. Ind. Eng. Chem. Res. 1999, 38, 2523.

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ReceiVed for reView August 12, 2009 ReVised manuscript receiVed December 26, 2009 Accepted December 30, 2009 IE901268M