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Effects of Hofmeister Anions on the Aggregation Behavior of PEOPPOPEO Triblock Copolymers Branden A. Deyerle and Yanjie Zhang* Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, Virginia 22807, United States ABSTRACT: The effects of a series of Hofmeister anions on the phase behaviors of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEOPPOPEO) triblock copolymer were investigated with an automated melting point system. Well hydrated anions and poorly hydrated anions interacted with the polymer differently and further affected the phase transition of the polymer. Poorly hydrated anions worked through changing the interfacial tension at the polymer/aqueous interface and in enhancing the polymer hydration by ion binding. The phase transition of the polymer in the presence of well hydrated anions correlated directly to the hydration entropy of the anions. As a consequence, the polymer showed a two-step phase transition in solutions containing poorly hydrated anions while displayed a single-step phase transition in the presence of well hydrated anions. The mechanisms of how ions interact with the polymer and further modulate its phase behaviors were discussed.
’ INTRODUCTION The Hofmeister series was first introduced in 1888 by Franz Hofmeister, who was studying the ability of ions to precipitate proteins out of solution.1,2 Over the years there have been numerous aqueous phenomena that have been found to follow the Hofmeister series.331 Examples include micelle formation,14 enzyme activity,1113,32 protein folding,22 protein crystallization,33 proteinprotein interactions,23,29 and phase behavior of macromolecules.2628 A typical order for ability of anions to affect the physical behavior of aqueous processes is as follows:3437 CO3 2 > SO4 2 > S2 O3 2 > H2 PO4 > F > Cl > Br ∼ NO3 > I > SCN There are two distinct groups that make up this series: well hydrated anions and poorly hydrated anions. Well hydrated anions on the left, sometime referred to as kosmotropes, tend to salt biomacromolecules out of solutions and increase the stability of biomacromolecules toward their folded natural state. Poorly hydrated anions on the right, called chaotropes, tend to make biomacromolecules more soluble in water and promote their unfolding. Chloride ion is usually considered to be the dividing line between these two types of behaviors. Although the Hofmeister series is a general recurring trend in aqueous solutions, the molecular-level mechanisms of the Hofmeister effects have remained elusive for over 120 years. It was originally believed that these ions affected the hydrogen-bonding network in bulk water and further influenced the solutes in salt solutions.34 Namely, well hydrated ions were thought to enhance the water structure in bulk solutions and poorly hydrated ions were thought to interfere with structure. However, recent studies revealed that the change in bulk water structure probably plays little, if any, role in the Hofmeister series.36,38,39 This led to a hypothesis that the direct interaction r 2011 American Chemical Society
between ions and macromolecules is the key to understanding the Hofmeister series.3,4,26,30,31,4042 It was believed that the ions could affect the protein/polymer solution outside the first hydration shell of the polymer, but it was found that only the first hydration shell affected the phase behaviors.2628,30,31 In addition, the relative polarization of anions has been found to contribute to the Hofmeister series.7,4345 In some cases, the Hofmeister series is reversed when the surface charge or surface polarity changes.29,4648 Herein, we use a simple model, the temperature-induced phase behaviors of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEOPPOPEO), to further explore the nature of polymeranion interactions. We find that well hydrated and poorly hydrated anions affect the phase transition of the polymer through different mechanisms. Specifically, our studies show, for the first time, that the polymer goes through a two-step phase transition in the presence of poorly hydrated anions while exhibiting only a single-step aggregation in solutions containing well hydrated anions. PEOPPOPEO triblock copolymers are water-soluble nonionic surface-active agents. These block copolymers bear a hydrophobic block (PPO) in the middle and two hydrophilic blocks (PEO) at both ends, as shown in Figure 1a. The notation for these block copolymers starts with letters L (liquid), P (paste), or F (flake). The numbers following the letters indicate the molecular weight of the PPO block and the weight fraction of the PEO block. For example, the polymer employed in this study, L44, is a liquid at room temperature and has a PPO block with molecular weight in the order of 1200 (4 300) and 40 wt % of PEO block. The molecular weights and PPO/PEO composition ratios in these polymers can be well controlled during synthesis with optimum properties that meet Received: April 20, 2011 Revised: June 16, 2011 Published: June 20, 2011 9203
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Figure 1. (a) Chemical structure of PEOPPOPEO triblock copolymers. (b) Phase behaviors of PEOPPOPEO as a function of temperature. (c) The light scattering intensity as a function of temperature from 10 mg/mL L44 in water. The onset points of the curve were taken as the phase transition temperatures of L44. CMT represents critical micellization temperature, and CP represents cloud point.
the specific requirements in widespread industrial applications such as detergency, emulsification, pharmaceuticals, bioprocessing, and separations.4952 PEOPPOPEO block copolymers are thermoresponsive, and the aqueous solutions of the copolymers go through phase transition in two steps as a function of temperature as represented in Figure 1b. At low temperatures, PEOPPOPEO chains exist as unimers in a dilute aqueous solution. When the solution temperature reaches the critical micellization temperature of the polymer, the chains aggregate into micelles with hydrophobic PPO blocks as the core and hydrophilic PEO blocks as the corona. When the temperature increases further and goes through the cloud point, the PEO blocks become dehydrated, inducing the aggregation of micelles. These phase transitions lead to aggregate formation, turning the transparent solution cloudy. It has been reported that the phase behaviors of these polymers are very sensitive to inorganic additives;50,51,5358 however, the mechanisms of how inorganic ions interact with these polymers and further modulate their phase transitions remain unclear. There are very few studies on the effects of well hydrated anions on the phase transition of PEO PPOPEO block copolymers.56 In this paper, we use an automated melting point system to determine the mechanisms of aggregation of PEOPPOPEO resulting from the polymer’s interactions with anions. L44 is chosen for this study because it allows us to observe the effects on the anions on both steps of the phase transition simultaneously. We find that poorly hydrated anions affect the phase transition of the polymer by changing the interfacial tension at the polymer/aqueous interfaces and by binding to the hydrophobic portion of the polymer. On the other hand, well hydrated anions interact with the polymer by changing the hydration of the polymer around the hydrophilic moieties of the polymer.
’ EXPERIMENTAL SECTION Pluronic L44 (average molecular weight 2200 Da) was provided by BASF Corporation (Edison, NJ). NaSCN, NaI, NaBr, NaNO3, NaCl, NaF, NaH2PO4, Na2S2O3, Na2SO4, and Na2CO3 were purchased from Fisher Scientific Inc. Polymer and salt solutions were prepared with lowconductivity H2O produced from a NANOpure Ultrapure Water System (Barnstead, Dubuque, IA) with a minimum resistivity of 18 MΩ 3 cm.
Stock solutions of L44 at 20 mg/mL in water were prepared at room temperature. Aliquots of these solutions were mixed with water or inorganic salt solutions at the desired concentration. The final polymer concentration for all measurements was 10 mg/mL. The concentrations of inorganic salts ranged from 0 to 2.0 M. The phase transition temperatures of the polymer solutions in the presence of inorganic salts were measured in an automated melting point system (Optimelt MPA100, Stanford Research System). In a typical experiment, three capillary tubes each filled with 10 μL of 10 mg/mL polymer solution were placed side-by-side into the apparatus. The temperature was ramped from below the phase transition temperature to 100 at 2 °C/min. The heating rate does not affect the measurements significantly. Real-time images of the samples were continuously captured by a built-in camera. Digital image processing was used to determine the phase transition temperatures in the samples, and the scattering intensities of the polymer solutions were plotted against temperature. Stored images may be recalled at any time and can be played back frame-by-frame or as a movie. Figure 1c shows the scattering intensity profile of 10 mg/mL L44 in water. Phase transition temperatures were taken as the initial break points of these curves.59 The scattering intensity is low until the temperature reaches its critical micelle temperature and then increases sharply at the critical micelle temperature. The scattering intensity increases further when the polymer solution experiences the second phase transition. The measurement of the phase transition temperature at each salt concentration was repeated at least six times, and the mean value was taken. The measurements had a typical standard error of (0.05 to ( 0.10 °C.
’ RESULTS AND DISCUSSION Phase Transition Temperature of PEOPPOPEO vs Salt Concentration. Sodium salts of ten anions, SCN, I, Br,
NO3, Cl, F, H2PO4, S2O3, SO42, and CO32, were employed to investigate the specific anion effects on the phase transition temperature of PEOPPOPEO block copolymer, L44. It was found that L44 behaves completely differently in the presence of poorly hydrated and well hydrated anions. In solutions containing poorly hydrated anions, L44 showed distinct two phase transitions as illustrated by 1 M NaCl solution in Figure 2a. By obtaining phase transition temperatures for L44
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Figure 2. (a) Light scattering intensity of PEOPPOPEO in 1 M NaCl. (b) The first step of the phase transition for PEOPPOPEO against salt concentration of poorly hydrated anions. (c) The second step of the phase transition for PEOPPOPEO against salt concentration of poorly hydrated anions.
solutions varying in anion identity and concentration from 0 to 2.0 M, plots can be made of phase transition temperature with salt concentration for five poorly hydrated anions used in this study (Figure 2b,c). The first phase transition happens at lower temperature and represents the critical micellization temperature of L44, and the second phase transition indicates its cloud point temperature. The phase transition temperature exhibits a complex dependence on the nature and concentration of salts. For example, in the first step, the poorly hydrated anions show a nonlinear dependence of phase transition temperature on the anion concentration. This is most obvious for more poorly hydrated anions such as SCN, I, and NO3. On the other hand, the second step shows a linear dependence on salt concentration. When well hydrated anions are introduced into the polymer solution, only one phase transition was observed as
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shown in 0.4 M NaF in Figure 3a. Phase transition temperature decreases linearly with increasing anion concentration for all the well hydrated anions investigated, including F, H2PO4, S2O32, SO42, and CO32 (Figure 3b). Modeling of the Phase Transition Data. The modulation of the phase transition temperatures of PEOPPOPEO in the presence of salts can be explained by three kinds of interactions of ions with the polymer as presented in Figure 4. First, an anion could affect the hydrophobic hydration of the PPO and PEO blocks as shown in Figure 4a. When a small hydrophobic solute is inserted into water, the liquid water in the surrounding solvent tends to maintain a dense hydrogen-bonding network around the hydrophobic solute. The water molecules do not bond to the hydrophobic solute; instead, they form a water cage around it.6062 Hydrophobic hydration entails the formation of a cavity where the water and polymer meet to form an interface.63,64 The free energy to create this cavity is directly proportional to the interfacial tension at the interface.63,64 The interfacial tension at this polymer/aqueous interface changes in the presence of salts. If the interfacial tension rises as the salt concentration becomes larger, the free energy of cavity formation increases and the phase transition temperature of PEOPPOPEO drops accordingly. In contrast, the free energy of cavity formation is reduced if the interfacial tension is decreased as the salt concentration increases. This effect will make PEOPPOPEO molecules more soluble, and the phase transition temperature will increase. It has been suggested that the salt-induced change of interfacial tension at biomacromolecule/water interfaces may be either positive or negative depending on the particular salt.65 Second, anions could bind directly to the hydrophobic moieties of the polymer4042 (Figure 4b). If an anion can shed its hydration water and bind to the polymer, it will add extra charge to the hydrated polymer and increase the solubility of the polymer. This effect will increase the phase transition temperature of the polymer. Poorly hydrated anions usually show a strong ability to bind to the hydrophobic portion of the polymer while well hydrated anions typically do not.4042,6668 Third, anions can interact with the polymer by changing the entropy of water molecules around the hydrophilic portion of the polymer (Figure 4c). The oxygen in the polymer chain interacts with water molecules to form a hydration layer around the polymer. If a well hydrated anion approaches to this hydrophilic surface, the interaction of water molecules with the oxygen in the polymer chain will be reduced. Therefore, the polymer will be less soluble in the presence of well hydrated anions. On the other hand, the effect of poorly hydrated anions on hydration water around hydrophilic surfaces is less pronounced. It had been indicated by molecular dynamics simulation that the correlation for well hydrated anions should be with the entropy of hydration.67 The ability of an anion to polarize the hydration water around hydrophilic portion of the polymer should correlate to the anion’s ability to order water molecules around itself.69,70 This is reflected quantitatively in the entropy of hydration, ΔShydr, of each hydrated anion.29,71 Both the interfacial tension effect at hydrophobic/aqueous interfaces and the entropy effect at hydrophilic surfaces should be linearly dependent on anion concentration,2628,72 while anion binding to a hydrophobic surface is a saturation phenomenon. Based on these interactions between anions and the polymer, the modulation of the PEOPPOPEO’s phase transition by added salts was modeled by simple equations as the following. For the first step of the phase transition in the presence of poorly hydrated anions, the data were fit with an equation that includes 9205
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Figure 3. (a) Light scattering intensity of PEOPPOPEO in 0.4 M NaF. Only a one-step phase transition was observed at all anion concentrations. (b) Phase transition temperatures for PEOPPOPEO against salt concentration for well hydrated anions.
a constant, a linear term, and a Langmuir isotherm:2628 T ¼ T0 + c½M +
Bmax KA ½M 1 + KA ½M
ð1Þ
in which T0 is the phase transition temperature of PEO PPOPEO in the absence of salt and [M] is the molar concentration of salt. The constant c has units of temperature/molarity representing the linear portion of the specific anion effect on the phase transition temperature, and KA is the apparent binding constant of the anion to the polymer, while Bmax is the increase in the phase transition temperature due to ion binding to the polymer at saturation. Employing eq 1, the solid lines in Figure 2b (above) are excellent fits to the experimental data. For the second step of the phase transition in solutions containing poorly hydrated anions and that for well hydrated anions, the data were fit with a simple linear equation: T ¼ T0 + c½M
ð2Þ
where the parameters bear the same physical meanings as in eq 1. It should be noted that the c values for well hydrated and poorly hydrated anions correlate with different properties of anions. The values for KA, Bmax, and c obtained by fitting the experimental data are provided in Tables 1 and 2 along with relevant thermodynamic information for the anions. These parameters are employed to correlate the changes in phase transition temperature and the physical properties of anions as below. Poorly Hydrated Anions: Influence of Interfacial Tension and Ion Binding. Poorly hydrated anion affects the phase transition temperature of PEOPPOPEO in a complex manner (Figure 2), exhibiting a two-step phase transition. The phase transition temperature of the first step shows a nonlinear dependence on anion concentration while the second step demonstrates a linear dependence. The c value for each anion is plotted against the polarizability of the anions as shown in Figure 5. For poorly hydrated anions, c values for both steps correlate well to the polarizability of anions. On the other hand, there is no similar correlation for well hydrated anions. The polarizability of an anion is the relative tendency of its electronic shell to be distorted from its “normal” shape by an external electric field due to the presence of a nearby ion or dipole. This parameter is usually described in dimension of volume.71 It has been shown that polarizability of anions determines the distribution of anions at interfaces.7,4345 At the hydrophobic/aqueous
Figure 4. Schematic diagram of the interactions between PEO PPOPEO and the anions. (a) The hydrophobic hydration of the block copolymer is modulated by the interfacial tension at the polymer/ aqueous interface in the presence of salt. (b) The binding of poorly hydrated anions to the hydrophobic moieties (PPO block) of the polymer. (c) Well hydrated anions interfere with the hydration water around the hydrophilic moieties (PEO block) of the polymer.
interfaces, more polarizable species have a tendency to partition to the hydrophobic phase more than the aqueous phase.29,73,74 As the temperature of a PEOPPOPEO solution is increased above its phase transition temperatures, dehydration of hydrophobic groups may lead to the aggregation of the polymer. The partitioning of anions to the hydrophobic/aqueous interface modulates the interfacial tension and the thermodynamics of the polymer aggregation.20,21 It is well-known that surface tension at the airwater interface and interfacial tension at many hydrocarbon/water interfaces changes linearly with salt concentration up to moderate ionic strength.19,20,72 The PPO block is more hydrophobic than the PEO blocks in the polymer chain, so the dehydration of PPO should appear at lower temperature compared to the dehydration of PEO. This suggests that the first step phase transition of PEO PPOPEO in the presence of poorly hydrated anions involves the dehydration of PPO block, and the second step is the dehydration of PEO blocks. As can be seen in Figure 5a, the slopes of the data for Cl, Br, and NO3, c values, are negative in both steps, which indicates that increasing the salt concentration raises the interfacial tension at both PPO/aqueous and PEO/aqueous interfaces and consequently lowers the phase transition temperatures. On the other hand, the c values for I and SCN are positive in both steps, which suggests that increasing the salt concentration actually lowers 9206
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Table 1. Fitted Values of Bmax, KA, and c Abstracted from the Phase Transition Temperature Measurements in the Presence of Poorly Hydrated Anion as Well as Literature Values for Some Physical Properties of the Anions; Estimated Interfacial Tension Increments for the Anions at the Polymer/Water Interface (σPPO/water and σPEO/water) c (°C/mol) σair/water anion
a
ΔShydrb
polarizability
(mN L/(m mol))
(J/(K mol))
(1030 m3)
Bmax (°C)
KA (M1)
a
σpolymer/water (mN L/m mol)
b
1st step
2nd step
σPPO/water
σPEO/water
SCN
0.5
66
6.74
14.9
4.5
4.5
16.2
0.6
2.0
I
1.0
36
7.51
7.2
4.4
2.0
6.4
0.3
0.8
NO3
1.1
76
4.13
6.3
1.9
8.2
5.2
1.0
0.6
Br Cl
1.3 1.6
59 75
4.85 3.42
5.8 12.8
5.8 13.0
0.7 1.6
0.7 1.6
From ref 80. b From ref 71.
Table 2. Fitted Values of c Abstracted from the Phase Transition Temperature Measurements in the Presence of Well Hydrated Anions as Well as Literature Values for Some Physical Properties of the Anions σa (mN L/
ΔShydrb
polarizabilityb
(m mol))
(J/(K mol))
(1030 m3)
c (°C/mol)
F
2.0
137
0.88
29.8
H2PO4
2.3
166
5.79
34.3
S2O32
2.9
180
9.20
53.8
SO42
2.7
200
5.47
66.7
CO32
2.6
245
4.54
69.5
anion
For F from ref 80, for H2PO4 from ref 35, and CO32 and S2O32 from ref 81. b From ref 71. a
Figure 6. Plot of the residual portion of the LCST vs salt concentration curves for the poorly hydrated anions.
Figure 5. (a) c values vs polarizability of the anions for poorly hydrated anions. (b) c values vs polarizability of the anions for well hydrated anions. Polarizability values were obtained from ref 71.
the interfacial tension at the polymer/aqueous interfaces and the phase transition temperature increases. More polarizable anions have a stronger tendency to decrease the interfacial tension at the polymer/aqueous interface to make the polymer more soluble. This type of behavior is consistent with the effects of poorly hydrated anions on the interfacial tension at the oil/aqueous interfaces, where Cl is known to increase the interfacial tension, while more poorly hydrated anions such as I and SCN are known to decrease it.75,76 The interfacial tension increments at PPO/aqueous and PEO/ aqueous interfaces for the poorly hydrated anions used in this study are abstracted from the fitting parameters as reported in Table 1 by using Cl as the calibrating ion.29 It is known that the
interfacial tension increment value of Cl at the oil/water interface is approximately the same as its surface tension increment at the air/water interface.76 Here, we assume this value remains the same at the polymer/aqueous interfaces. As can be seen in Table 1, the interfacial tension increment values for the most poorly hydrated anions at the PEO/aqueous interface are more negative than those at the PPO/aqueous interface. This behavior indicates that poorly hydrated anions tend to accumulate at the interface to lower the interfacial tension. This effect at a relatively hydrophilic component/aqueous interface is more significant than that at a hydrophobic component/aqueous interface. The dielectric constants of the substances at the liquid/liquid interface have been adopted as 9207
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Figure 7. (a) c values vs hydration entropy of the anions for well hydrated anions. (b) c values vs hydration entropy of the anions for poorly hydrated anions. Hydration entropies values were obtained from ref 71.
an indicator of polarity difference between these two phases and further determine the partition of solute at the interface.7779 It is interesting to compare the interfacial (surface) tension increment values of poorly hydrated anions at different interfaces with varied dielectric constants. At the air/water interface, where the air has very low dielectric constant, all the poorly hydrated anions employed in this study have positive surface tension increment values, which means all these anions increase the surface tension.19,20,72 At the hydrophobic/aqueous interfaces in the hydrophobic collapse of thermoresponsive polymers or peptides such as poly(N-isopropylacrylamide) and elastin-like peptides, the interfacial tension changes in the presence of these chaotropic anions are well correlated with the surface tension increments at the air/water interface.2628 When relatively hydrophilic species such as PPO blocks and PEO blocks in the polymer are involved, the most poorly hydrated anions such as I and SCN lower the interfacial tension, while less poorly hydrated anions such as NO3, Br, and Cl raise it. In a system of increasing hydrophilic nature with higher dielectric constants such as lysozyme, all the poorly hydrated anions except Cl lower the interfacial tension at the interface.29 It should be noted that the phase transition temperature vs ion concentration curves for some poorly hydrated anions in the first step are not linear as shown in Figure 2b. For these anions, subtracting the linear fit (T0 + c[M]) to the curves in Figure 2b and replotting the residual data yields Langmuir-shaped binding isotherms (Figure 6). This behavior indicates that more poorly hydrated anions tend to bind more strongly to the polymer.2628 SCN showed the strongest binding followed by I and NO3. The order of Bmax value and the magnitude of the binding constants are consistent with previously reported work describing the binding of these anions to thermoresponsive polymers and peptides.2628 Br and Cl bind weakly enough to the polymer that significant deviation from linear dependence of phase transition on anion concentration is not observed. The most likely binding sites on the polymer for these poorly hydrated anions are the hydrophobic propylene groups in the PPO blocks. It has been reported by molecular dynamics simulations that anions with low charge density exhibit preferential binding to hydrophobic moieties in proteins and to hydrophobic plates in aqueous solution.4042,67 It should also be noted that poorly hydrated anions do not significantly bind to less hydrophobic ethylene groups in the PEO blocks because the phase transition temperature of the second step changes linearly as a function of poorly hydrated anions (Figure 2c). Well Hydrated Anions: Influence of Hydration Entropy. PEOPPOPEO shows a one-step phase transition in the
presence of well hydrated anions, and the phase transition temperature decreases linearly with increasing anion concentration. To determine if there is a correlation between the entropy effect around hydrophilic groups and the phase transition temperature of the polymer, the slope of the phase transition temperature in the presence of added salts (c values in eqs 1 and 2) was plotted against the hydration entropy of anions, ΔShydr (Figure 7). As seen in Figure 7a, the c values for well hydrated anions correlate well with ΔShydr. On the other hand, the c values for poorly hydrated anions do not correlate with ΔShydr (Figure 7b). This suggests that the phase transition of PEOPPOPEO in the presence of well hydrated anions is related to the dehydration of the hydrophilic groups of the polymer by changing the entropy of water around the hydrophilic moieties. In this case, the most well hydrated anions are capable of interacting with the water molecules around the hydrophilic moieties of the polymer, making these water molecules less available to the polymer. This effect makes it easier for the hydrophilic groups in the polymer to become dehydrated. Since there is only one type of hydrophilic group in PEOPPO PEO, the oxygen in the polymer chain, the collapse of the polymer appears as one step. The data shown here are consistent with the findings from other thermoresponsive polymers that well hydrated anions work primarily through perturbing the solvation water molecules around hydrophilic portion of the polymers.2628
’ CONCLUSION Poorly hydrated anions and well hydrated anions modulate the phase transition temperature of PEOPPOPEO through separate mechanisms. Poorly hydrated anions partition to the polymer/ aqueous interface and change the interfacial tension at the interface. Also, poorly hydrated anions bind to the hydrophobic moieties of the polymer. In contrast, well hydrated anions interact with the hydrophilic portion of the polymer by changing the entropy of hydration water around the polymer. The finding that poorly hydrated and well hydrated anions interact with macromolecules through separate mechanisms is consistent with previously reported studies on poly(N-isopropylacrylamide) and elastin-like peptides, and it may be a general phenomenon.2628 The proposed mechanisms provide new insights into the understanding of the specific anion effects on colloidal phenomena in aqueous solutions. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 9208
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’ ACKNOWLEDGMENT We are grateful to Gina MacDonald and Debbie Mohler for useful discussions. We acknowledge the following agencies for their support of this research: the Donors of the American Chemical Society Petroleum Research Fund (51008-UNI4), the Thomas F. and Kate Miller Jeffress Memorial Trust (J-966), the National Science Foundation (CHE-0754521)Research Experience for Undergraduates Program, and the Department of Defense ASSURE program (0851367). We thank the Department of Chemistry and Biochemistry at James Madison University and Research Corporation Departmental Development Grant (7957) for providing start-up funds to initiate this research. We also thank BASF Corporation (Edison, NJ) for supplying the Pluronic polymers for this study. ’ REFERENCES (1) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247–260. (2) Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 19–37. (3) Heyda, J.; Lund, M.; Oncak, M.; Slavicek, P.; Jungwirth, P. J. Phys. Chem. B 2010, 114, 10843–10852. (4) Heyda, J.; Vincent, J. C.; Tobias, D. J.; Dzubiella, J.; Jungwirth, P. J. Phys. Chem. B 2010, 114, 1213–1220. (5) Mason, P. E.; Heyda, J.; Fischer, H. E.; Jungwirth, P. J. Phys. Chem. B 2010, 114, 13853–13860. (6) Tobias, D. J.; Hemminger, J. C. Science 2008, 319, 1197–1198. (7) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361–6373. (8) Jungwirth, P.; Tobias, D. J. Chem. Rev. 2006, 106, 1259–1281. (9) Jungwirth, P.; Winter, B. Annu. Rev. Phys. Chem. 2008, 59, 343–366. (10) Horinek, D.; Serr, A.; Bonthuis, D. J.; Bostr€om, M.; Kunz, W.; Netz, R. R. Langmuir 2008, 24, 1271–1283. (11) Pinna, M. C.; Bauduin, P.; Touraud, D.; Monduzzi, M.; Ninham, B. W.; Kunz, W. J. Phys. Chem. B 2005, 109, 16511–16514. (12) Pinna, M. C.; Salis, A.; Monduzzi, M.; Ninham, B. W. J. Phys. Chem. B 2005, 109, 5406–5408. (13) Vrbka, L.; Jungwirth, P.; Bauduin, P.; Touraud, D.; Kunz, W. J. Phys. Chem. B 2006, 110, 7036–7043. (14) Bostr€om, M.; Williams, D. R. M.; Ninham, B. W. Langmuir 2002, 18, 6010–6014. (15) Bostr€om, M.; Deniz, V.; Ninham, B. W. J. Phys. Chem. B 2006, 110, 9645–9649. (16) Bostr€om, M.; Lima, E. R. A.; Tavares, F. W.; Ninham, B. W. J. Chem. Phys. 2008, 128, 135104. (17) Parsons, D. F.; Bostr€om, M.; Maceina, T. J.; Salis, A.; Ninham, B. W. Langmuir 2010, 26, 3323–3328. (18) Salis, A.; Parsons, D. F.; Bostr€om, M.; Medda, L.; Barse, B.; Ninham, B. W.; Monduzzi, M. Langmuir 2010, 26, 2484–2490. (19) Pegram, L. M.; Record, M. T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 14278–14281. (20) Pegram, L. M.; Record, M. T. J. Phys. Chem. B 2007, 111, 5411–5417. (21) Pegram, L. M.; Record, M. T. J. Phys. Chem. B 2008, 112, 9428–9436. (22) Broering, J. M.; Bommarius, A. S. J. Phys. Chem. B 2005, 109, 20612–20619. (23) Perez-Jimenez, R.; Godoy-Ruiz, R.; Ibarra-Molero, B.; SanchezRuiz, J. M. Biophys. J. 2004, 86, 2414–2429. (24) Leontidis, E.; Aroti, A.; Belloni, L.; Dubois, M.; Zemb, T. Biophys. J. 2007, 93, 1591–1607. (25) Leontidis, E.; Aroti, A. J. Phys. Chem. B 2009, 113, 1460–1467. (26) Zhang, Y. J.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505–14510. (27) Zhang, Y. J.; Furyk, S.; Sagle, L. B.; Cho, Y.; Bergbreiter, D. E.; Cremer, P. S. J. Phys. Chem. C 2007, 111, 8916–8924.
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