Anion Specificity in Dimethylsulfoxide-Water Mixtures Exemplified by a

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Anion Specificity in Dimethyl Sulfoxide−Water Mixtures Exemplified by a Thermosensitive Polymer Renwei Zhu,† Monika K. Baraniak,‡ Frieder Jäkle,‡ and Guangming Liu*,† †

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, P. R. China 230026 ‡ Department of Chemistry, Rutgers UniversityNewark, 73 Warren Street, Newark, New Jersey 07102, United States J. Phys. Chem. B Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/17/18. For personal use only.

S Supporting Information *

ABSTRACT: In the present work, we have investigated the anion-specific upper critical solution temperature (UCST) behavior of polymer-supported borinic acid (PBA) in dimethyl sulfoxide−water (DMSO−H2O) mixtures. An inverted V-shaped series CH3COO− < Cl− < salt-free > NO3− > ClO4− > SCN− is observed in terms of the anionspecific UCST of PBA in the DMSO−H2O mixtures. Both direct anion−polymer interactions and indirect solventmediated anion−polymer interactions are involved in the specific anion effect on the UCST behavior of PBA. The direct binding of anions to the PBA surface generates a salting-in effect on PBA, causing the UCST for the different types of anions to increase from chaotropic to kosmotropic anions due to the stronger binding of the more chaotropic anions. On the other hand, the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules also produces a salting-in effect on PBA, leading the UCST for the different types of anions to increase from kosmotropic to chaotropic anions because of the stronger capability of the more kosmotropic anions to polarize the hydrogen bonding. Thus, the dominating anion−PBA interactions change from the direct anion binding to the indirect anionic polarization of hydrogen bonding as the anions change from chaotropes to kosmotropes. The observed inverted V-shaped series suggests that the specific anion effect on the UCST behavior of PBA in the DMSO−H2O mixtures is determined by the combined effects of the binding of anions to the PBA surface and the anionic polarization of hydrogen bonding between PBA and DMSO molecules.

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INTRODUCTION

In comparison with the ion specificities in aqueous systems, specific ion effects in the mixtures of water and nonaqueous solvents have attracted much less attention, though the studies of ion specificities in solvent mixtures are related to many important applications such as energy storage, electrochromic materials, and ion exchange.6,36,37 We have previously demonstrated that the ranking of ions in terms of the ion specificities of polymers in water−alcohol mixtures remains the same at different solvent compositions.38−41 Nevertheless, the strength of specific ion effects can be amplified by the addition of alcohols, mediated by the stability of water/alcohol complexes.38−41 The ion specificities of polymers can also be observed in an alcohol system.40 Similar to water molecules, alcohol molecules can interact with polymers as either hydrogen bond donors or acceptors due to their protic organic solvent nature. In contrast, aprotic organic solvents such as dimethyl sulfoxide (DMSO) can only behave as hydrogen bond acceptors. Thus, the manner of hydrogen bonding between aprotic organic solvents and polymers should be distinct from that of the protic organic solvents, which would

Although specific ion effects have been extensively investigated in aqueous solutions and significant progress has been achieved during the last 20 years, a big challenge still remains in fully understanding the mechanisms behind such important ionic effects.1−16 In aqueous solutions, synthetic polymers have usually been employed as model systems to understand the ion specificities of macromolecules because of their relatively simple and well-defined chemical structures.6,17−34 In general, the ion specificities of polymers in aqueous solutions involve two types of different interactions, i.e., the direct ion−polymer interactions and the indirect water-mediated ion−polymer interactions.6,17,18,27 For neutral polymers, the direct binding of ions to the polymer surface generally leads to an increase in the solubility of polymers through a salting-in effect.6,17,18,35 On the other hand, the indirect water-mediated ion−polymer interactions can produce either a salting-out or a salting-in effect depending on the manner of hydrogen bonding between polymers and water molecules.18 Therefore, it is expected that the ion specificities of neutral polymers will be distinct in different solvent systems, as the manner of hydrogen bonding interactions between polymers and solvent molecules is changed from one type to another type of solvent. © XXXX American Chemical Society

Received: June 27, 2018 Revised: July 31, 2018 Published: August 7, 2018 A

DOI: 10.1021/acs.jpcb.8b06125 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 1. (a) Temperature dependence of the transmittance of PBA dissolved in DMSO in the presence of different types of anions with the tetrabutylammonium as the common cation. (b) UV spectra of PBA dissolved in DMSO in the presence of different types of anions with the tetrabutylammonium as the common cation. Here, the concentration of PBA is fixed at 1.0 mg mL−1 and the salt concentration is fixed at 0.1 M.

turbidity, using an UNICO 2802PCS UV/visible spectrophotometer with the wavelength set at 500 nm. The sample temperature was controlled using a circulating water bath with an accuracy of ±0.1 °C and was monitored by an electronic thermometer. The temperature was cooled at a rate of 1 °C/5 min during the measurements of cloud points. The cloud points were determined from the intersection of two straight lines drawn through the transmittance versus temperature curves during the initial decrease in transmittance of the PBA solutions with decreasing temperature (Figure S1, Supporting Information). Other Measurements. The interactions between anions and PBA were investigated using isothermal titration calorimetry (ITC) at 50 °C. Both salts (20 mM) and PBA (0.1 mg mL−1) were dissolved in DMSO during the ITC measurements. The data for the titration of salt solutions into DMSO and the titration of DMSO into the PBA solution were subtracted from the raw data to correct the heat of salt dilution and the interactions between DMSO and PBA. Curve fitting was performed using an ITC data analysis module to obtain the binding constants of anions according to the one-type-ofsites model.25,35,44 The ultraviolet (UV) spectra of PBA solutions in the presence of different types of salts were measured on a UNICO 2802PCS UV/visible spectrophotometer at 50 °C. The surface tensions of salt solutions were measured using a KSV CAM200 surface tension meter at ∼22 °C.

make the ion specificities of polymers in the water−aprotic organic solvent mixtures different from those in the water− protic organic solvent mixtures. Our previous study has shown that the ranking of anions according to the specific ion effect on the phase transition behavior of thermosensitive poly(N-isopropylacrylamide) (PNIPAM) in the H2O−DMSO mixtures follows the classical Hofmeister series at the low mole fraction of DMSO but is inconsistent with the Hofmeister series at the relatively high mole fraction of DMSO.42 Nevertheless, several open questions remain to be clarified. For instance, does the anion specificity of polymers exist in DMSO? What is the mechanism behind the anion specificity of polymers in DMSO? Is the anion specificity of polymers in DMSO influenced by the addition of water? These questions cannot be answered on the basis of the PNIPAM system, as PNIPAM does not have the thermosensitivity in DMSO. To answer these questions, we have employed the polymer-supported borinic acid (PBA), which has an upper critical solution temperature (UCST) in DMSO,43 as an effective model system to investigate the anion-specific phase transition behavior of PBA in DMSO− H2O mixtures.

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EXPERIMENTAL SECTION Materials. The homopolymer PBA (Mn ∼ 3.2 × 104 g mol−1, Mw/Mn ∼ 2.5) was prepared according to the procedure reported previously.43 Tetrabutylammonium nitrate [(Bu4N)NO3, 98%, Macklin], tetrabutylammonium acetate [(Bu4N)CH3COO, 98%, Macklin], tetrabutylammonium perchlorate [(Bu4N)ClO4, 98%, Macklin], tetrabutylammonium chloride [(Bu4N)Cl, 98%, Meryer], tetrabutylammonium thiocyanate [(Bu4N)SCN, 98%, Energy chemical], and tetrabutylammonium fluoride [(Bu4N)F, 1.0 M in tetrahydrofuran, Energy chemical] were all used as received without further purification. The DMSO (H2O < 50 ppm) was purchased from Macklin chemical company and used as received. No water can be detected from the DMSO by the Karl Fischer method during our experiments. In this work, the tetrabutylammonium was chosen as the common cation for all of the employed salts to increase the solubility of the salts in DMSO. Note that the tetrahydrofuran was removed under a vacuum before the preparation of tetrabutylammonium fluoride/ DMSO solution. The water used was purified by filtration through a Millipore Gradient system after predistillation, giving a resistivity of 18.2 MΩ cm. Cloud Point Measurements. Cloud points of the PBA solutions (1.0 mg mL−1) were measured by monitoring the

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RESULTS AND DISCUSSION Figure 1a shows the temperature dependence of transmittance of 1.0 mg mL−1 PBA dissolved in DMSO in the presence of different types of anions with a salt concentration of 0.1 M. In the salt-free solution, the decrease in transmittance with decreasing temperature is an indication of the UCST-type phase transition behavior of the PBA solution. It is evident that all of the added anions generate a salting-in effect on PBA, as reflected by the shifts of the transmittance versus temperature curves to lower temperatures compared with that in the saltfree solution. Moreover, the UCST-type behavior can be observed for almost all of the anions with the exception of F−. In the presence of F−, the PBA solution remains a high transmittance even after the temperature decreases to ∼12.6 °C. The further decrease in temperature will lead to a solidification of the PBA solution for F− due to the high melting point of DMSO. Thus, the F− should interact with PBA in a different manner from that for the other types of anions. B


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The Journal of Physical Chemistry B Figure 1b shows the UV spectra of PBA dissolved in DMSO (1.0 mg mL−1) in the presence of different types of anions. The maximum absorption peak at ∼282 nm in the salt-free solution can be assigned to the π−π* transition of the π system in PBA.43,45,46 In the presence of F−, the maximum absorption peak of PBA exhibits an obvious blue shift in comparison with that in the salt-free solution, whereas the maximum absorption peaks of PBA for the other types of anions are almost overlapped with those for the salt-free solution. The blue shift in the maximum absorption peak of PBA in the presence of F− indicates a strong Lewis acid−base interaction between the boron atom on PBA and the F− anion.43,45,46 The strong affinity between the F− anion and PBA gives rise to a high solubility of PBA in DMSO.43 This is why no obvious UCSTtype behavior of PBA can be observed in the presence of F− (Figure 1a). Consequently, the manner of interactions between the other types of anions and PBA should be distinct from that for F−, as no obvious shifts in the maximum absorption peaks of PBA can be observed in the presence of these types of anions. In addition, the small peak located at the left side of the maximum absorption peak of PBA in the presence of SCN− is probably attributed to the charge-transfer-to-solvent band of thiocyanate ion.47 In Figure 2, an inverted V-shaped series can be observed in terms of the anion-specific UCST of PBA in DMSO. That is,

Scheme 1. Schematic Illustration of the Mechanism of Interactions between Anions and PBAa

a

The anions can interact with PBA through both the direct binding of anions to the PBA surface and the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules.

PBA in the DMSO−H2O mixtures, as reflected by an increase in the UCST of PBA with increasing xw. Nevertheless, the observed anion specificity is independent of xw; namely, the inverted V-shaped series remains with the addition of small amounts of water. This fact suggests that the specific anion effect on the UCST behavior of PBA demonstrated here is dominated by a similar mechanism in the DMSO−H2O mixtures at the different solvent compositions. As can be seen from Scheme 1, it is expected that the boron atom should be partially positively charged due to the smaller electronegativity compared with the covalently bound oxygen atom. As a result, the added anions are anticipated to interact with this positively charged part of PBA. Moreover, we hypothesize that the strength of interactions between the anions and PBA should be anion-specific, which is tested by the ITC experiments. Figure 3 shows the ITC titration data for

Figure 2. UCST of PBA in the DMSO−H2O mixtures as a function of the volume fraction of water (xw) in the presence of different types of anions with the tetrabutylammonium as the common cation. Here, the concentration of PBA is fixed at 1.0 mg mL−1 and the salt concentration is fixed at 0.1 M.

the UCST of PBA changes following the series CH3COO− < Cl− < salt-free > NO3− > ClO4− > SCN− in DMSO. For either the same type of anion or the salt-free solution, the UCST of PBA increases with increasing volume fraction of water (xw) in the DMSO−H2O mixtures, indicating that the solubility of PBA is decreased with the addition of water. Scheme 1 shows the chemical structure of PBA, which contains the B−OH group. Thus, PBA can act as both the hydrogen bond acceptor and donor when interacting with the solvent molecules. The dissolution of PBA in DMSO is partially facilitated by the hydrogen bonding interactions between the B−OH group and the solvent molecules.43 With the addition of water molecules, the associated DMSO molecules with PBA will be substituted by the water molecules, and the interchain and intrachain aggregation will be generated by the bridging effect through the hydrogen bonding interactions of the associated water molecules.43 This will lead to a decrease in the solubility of

Figure 3. ITC titration data for the titration of different salt solutions into PBA solution at 50 °C, where DMSO is used as the solvent. Here, the molar ratio represents the ratio of anion/BA and the dashed lines are the curve fits to the experimental data.

the titration of different types of salt solutions into the PBA solution, where DMSO is used as the solvent. The direct binding of anions to the PBA surface can take place only if the change in free energy (ΔG) is negative during the binding process. It is evident that the binding of anions to the PBA surface is an endothermic process, as indicated by the positive values of the enthalpy change (ΔH) for the anion binding C


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of hydrogen bonding between PBA and DMSO, the added anions can strengthen the hydrogen bonding interactions via the anionic polarization, as shown in Scheme 1. Similar to the binding of anions to the PBA surface, the strengthening hydrogen bonding interactions between PBA and DMSO would also lead to an increase in the solubility of PBA, producing a salting-in effect on PBA. The different types of anions are expected to have distinct capabilities to polarize the hydrogen bonding between PBA and DMSO, which is correlated with the anion−dipole interactions between the anions and DMSO molecules. It is anticipated that the anions can more effectively polarize the hydrogen bonding between PBA and DMSO molecules if the anions can more strongly interact with the DMSO molecules. The strength of anion−dipole interactions between the anions and DMSO molecules can be reflected by the salt concentration (Csalt) dependence of the surface tension (γ) of DMSO. Figure 4 shows the salt concentration dependence

(Table 1). Therefore, the binding of anions to the PBA surface should be driven by the increase in entropy. More specifically, Table 1. Thermodynamic Parameters Obtained from the ITC Experiments for the Different Types of Anions anion identity

K (M−1)

ΔS (cal/mol/deg)

ΔH (cal/mol)

ΔG (cal/mol)

SCN− ClO4− NO3−

1030 858 645

14.0 13.6 13.0

77.2 63.7 42.7

−4447.0 −4331.2 −4158.2

the binding of anions may generate a disturbance on the solvation shell around the PBA chains, resulting in a release of DMSO molecules from the solvation shell accompanied by an increase in entropy.35 This is confirmed by the entropy change (ΔS) during the binding of anions to the PBA surface, as shown in Table 1. The binding of the more chaotropic anions to the PBA surface results in a higher value of ΔH, which is unfavorable for the binding of anions. However, the stronger disturbance to the solvation shell induced by the binding of the more chaotropic anions would lead to a higher value of ΔS through the release of solvent molecules from the solvation shell, which is favorable for the binding of anions. The ΔG for the anion binding increases following the anion trend SCN− < ClO4− < NO3−, suggesting that the binding of anions is actually driven by the increase in entropy and the strength of interactions between the anions and PBA should increase following the anion trend NO3− < ClO4− < SCN−. This concurs with the ranking of binding constants of the anions obtained from the fitting of the experimental data by the “one-type-of-sites” model. Obviously, the binding constants of the anions increase following the anion trend NO3− < ClO4− < SCN−, indicating the increasing affinity of the anions to the PBA surface along this anion trend. The fitting of the titration data for Cl− and CH3COO− is not achievable, probably due to the weak interactions between these two types of anions and PBA. Thus, the binding affinity of the anions to the PBA surface should increase following the anion trend CH3COO− ≈ Cl− < NO3− < ClO4− < SCN−. This anion trend can be understood on the basis of the theory of ion dispersion interactions between anions and PBA.2,48−50 A more chaotropic anion would have a stronger interaction with PBA owing to a larger polarizability in comparison with a more kosmotropic anion. A stronger binding of the anions to the PBA surface would lead to a higher solubility of PBA via the salting-in effect. Consequently, the UCST of PBA should decrease following the trend salt-free > CH3COO− ≈ Cl− > NO3− > ClO4− > SCN− if the specific anion effect on the UCST behavior of PBA is dominated by the direct anion−polymer interaction. However, this series is inconsistent with that observed in Figure 2. In fact, the anions can also interact with PBA through the solvent-mediated anionic polarization of hydrogen bonding. The DMSO molecules can interact with PBA through the hydrogen bonding interactions between the SO group and the BOH group (Scheme 1). Here, the SO group acts as the hydrogen bond acceptor, whereas the BOH group acts as the hydrogen bond donor. It has been reported that the added anions can either strengthen or weaken the hydrogen bonding interactions between polymer and solvent molecules depending on the manner of hydrogen bonding.18 For the case

Figure 4. Salt concentration (Csalt) dependence of the surface tension (γ) of DMSO in the presence of different types of anions with the tetrabutylammonium as the common cation.

of the surface tension of DMSO in the presence of different types of anions. A faster increase in surface tension with increasing salt concentration is an indication of stronger anion−dipole interactions between the anions and DMSO molecules. In Figure 4, the effectiveness of anions to increase the surface tension of DMSO increases following the anion trend SCN− < ClO4− < NO3− < Cl− < CH3COO−, suggesting that the strength of anion−dipole interactions between the anions and DMSO molecules increases following the same anion trend. In other words, the more kosmotropic anions can more effectively polarize the hydrogen bonding between PBA and DMSO molecules, thereby leading to a higher solubility of PBA with a lower UCST. Consequently, the UCST should increase following the trend CH3COO− < Cl− < NO3− < ClO4− < SCN− < salt-free if the specific anion effect on the UCST behavior of PBA is dominated by the DMSO-mediated indirect anion−polymer interaction. Again, this series is inconsistent with that observed in Figure 2. Thus, it is expected that the specific anion effect on the UCST behavior of PBA should be determined by the combination of both the direct and indirect anion−polymer interactions. From the discussions above, the UCST of PBA should increase following the trend SCN− < ClO4− < NO3− < Cl− ≈ CH3COO−< salt-free if the specific anion effect is dominated by the direct anion−PBA interaction, whereas the UCST of PBA should increase following the trend CH3COO− < Cl− < NO3− < ClO4− < SCN− < salt-free if the specific anion effect is dominated by the indirect DMSO-mediated anion−PBA interaction. It is expected that the dominating anion−PBA D


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salting-in effect generated by the anionic polarization of hydrogen bonding. KA is the binding constant for the anions, and Bmax represents the maximum decrease in UCST caused by direct binding of anions to the PBA surface. KA and Bmax are also related to the salting-in effect. Figure 6 shows the UCST of PBA as a function of Csalt in the presence of different types of anions. The dashed lines shown in Figure 6 are the curve fits to the experimental data with eq 1. Obviously, the experimental data can be well fitted using eq 1. The obtained parameters of KA, Bmax, and c are summarized in Table 2. The value of c decreases following the anion trend CH3COO− > Cl− > NO3− > ClO4− > SCN−, suggesting that the more kosmotropic anions can more effectively polarize the hydrogen bonding between PBA and DMSO molecules. A stronger anionic polarization of hydrogen bonding would lead to a higher solubility of PBA with a lower UCST. On the other hand, the value of KA increases following the anion trend CH3COO− < Cl− < NO3− < ClO4− < SCN−, indicating that the strength of direct interaction between the anions and PBA increases following this anion trend. A stronger binding of anions to the PBA surface also gives rise to a higher solubility of PBA with a lower UCST, which could be further reflected by the increase in Bmax following the anion series CH3COO− < Cl− < NO3− < ClO4− < SCN−. Clearly, both the anionic polarization of hydrogen bonding and the anion binding can produce a salting-in effect on PBA but generate opposite anion trends in terms of their respective effect on the UCST of PBA. The competition between these two types of interactions would lead to an inverted V-shape-like series, as observed in Figure 2. Therefore, the specific anion effect on the UCST behavior of PBA in the DMSO−H2O mixtures is determined by the combined effects of the direct binding of anions to the PBA surface and the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules. We have previously systematically investigated the specific anion effects on the LCST behavior of PNIPAM in the water− protic organic solvent mixtures including water−methanol, water−ethanol, water−propanol, water−ethylene glycol, and water−glycerol mixtures.38,39,41 We have found that the ranking of anions in terms of the specific anion effects in these solvent mixtures is consistent with the classical Hofmeister series. Similar to water molecules,18 the protic organic solvent molecules would mainly act as hydrogen bond donors to interact with PNIPAM. Thus, the direct anion binding leads to a salting-in effect, whereas the indirect anionic polarization of hydrogen bonding results in a salting-out effect. That is, the direct and indirect anion−PNIPAM interactions give rise to opposite effects on the solubility of PNIPAM. As a consequence, a classical Hofmeister series can be observed in the water−protic organic solvent mixtures. In contrast, the aprotic organic solvents such as DMSO only have hydrogen bond acceptors and can merely act as hydrogen bond acceptors to interact with polymers. Consequently, the DMSO molecules interact with PBA as hydrogen bond acceptors and both the direct anion binding and the indirect anionic polarization of hydrogen bonding can generate a salting-in effect. As a result, an inverted V-shape-like series can be observed in this work. In addition, it is anticipated that the specific anion effects on the phase transition behavior of thermosensitive polymers could be expanded to more protic and aprotic organic solvent systems to make the conclusion obtained here more general.

interactions should change from the direct anion binding to the indirect anionic polarization of hydrogen bonding with the change of anions from chaotropes to kosmotropes; as a result, an inverted V-shaped series CH3COO− < Cl− < salt-free > NO3− > ClO4− > SCN− could be observed when these two types of interactions simultaneously act on PBA and compete with each other. This is exactly what is observed in Figure 2. To further verify that the observed inverted V-shaped series is actually induced by the competition between the direct and indirect anion−PBA interactions, we have investigated the specific anion effect on the UCST behavior of PBA at different salt concentrations in the DMSO−H2O mixtures (Figure 5).

Figure 5. UCST of PBA as a function of salt concentration (Csalt) in the presence of different types of anions with the tetrabutylammonium as the common cation in the DMSO−H2O mixtures with a volume fraction of water of 3%.

Here, all of the experiments are performed in the DMSO− H2O mixtures with a xw value of 3%, because the UCST of PBA in DMSO is too low to be detected at high salt concentrations (Figure S2, Supporting Information). Nevertheless, the results obtained in the DMSO−H2O mixtures at the xw value of 3% can be used to understand the specific anion effect on the UCST behavior of PBA in DMSO, because of the similar mechanism behind the anion specificity (Figure 2). In Figure 5, the inverted V-shape-like series can be observed at all of the salt concentrations. Cremer et al. have studied the specific anion effects on the lower critical solution temperature (LCST) phase transition behavior of thermosensitive PNIPAM and have proposed an equation including a constant term, a linear term, and a Langmuir isotherm to address the salting-in effect generated by the direct binding of anions on the PNIPAM surface and the salting-out effect caused by destabilization of hydrogen bonding between PNIPAM and water molecules, triggered by the anionic polarization.18 The specific anion effects can be analyzed by modeling the salt concentration dependence of the LCST of PNIPAM using the above-mentioned equation.18 In contrast with PNIPAM, PBA exhibits UCST behavior and the anionic polarization of hydrogen bonding between PBA and DMSO molecules generates a salting-in effect. Therefore, the specific anion effect on the UCST behavior of PBA in the DMSO−H2O mixtures can be analyzed by modeling the salt concentration dependence of the UCST of PBA using the following equation18 T = T0 − c[M] −

Bmax KA[M] 1 + KA[M]

(1)

where T0 is the UCST of PBA in the absence of salt and [M] is the molar salt concentration. The constant c is related to the E


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Figure 6. UCST of PBA as a function of salt concentration (Csalt) in the presence of different types of anions with the tetrabutylammonium as the common cation in the DMSO−H2O mixtures with a volume fraction of water of 3%: (a) CH3COO−, (b) Cl−, (c) NO3−, (d) ClO4−, and (e) SCN−. Here, the dashed lines are the curve fits to the experimental data with eq 1.

Table 2. Parameters of c, Bmax, and KA Obtained from the Fitting of Experimental Data in Figure 6 c (°C/M) Bmax (°C) KA (M−1)

CH3COO−

Cl−

NO3−

ClO4−

SCN−

37.52 9.14 5.91

33.54 10.84 5.94

24.81 13.07 11.21

24.67 13.24 23.81

23.40 15.57 24.55

combination of these two effects gives rise to an inverted Vshaped series in terms of the anion-specific UCST of PBA in the DMSO−H2O mixtures. Consequently, an inverted Vshape-like series should be observed when both the direct and indirect anion−polymer interactions exert a salting-in effect on polymers, whereas the classical Hofmeister series should be observed if the direct and indirect anion−polymer interactions exhibit opposite effects on the solubility of polymers.

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CONCLUSION In this work, the anion specificity in the DMSO−H2O mixtures is exemplified by the anion-specific UCST behavior of PBA. The anion specificity in DMSO is almost uninfluenced by the addition of small amounts of water. The specific anion effect on the UCST behavior of PBA in the DMSO−H2O mixtures is determined by the competition between the direct binding of anions to the PBA surface and the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules. As the anions change from chaotropes to kosmotropes, the dominating anion−PBA interactions change from the direct anion binding to the indirect anionic polarization of hydrogen bonding. The binding of anions to the PBA surface causes the UCST to increase from chaotropic to kosmotropic anions. Meanwhile, the anionic polarization of hydrogen bonding between PBA and DMSO molecules leads the UCST to increase from kosmotropic to chaotropic anions. The

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b06125. Temperature dependence of the transmittance of PBA in

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the DMSO−H2O mixtures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guangming Liu: 0000-0003-2455-1395 Notes

The authors declare no competing financial interest. F


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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (21574121, 21622405, 21374110), the Youth Innovation Promotion Association of CAS (2013290), and the Fundamental Research Funds for the Central Universities (WK2340000066) is acknowledged. F.J. and M.K.B. thank the US National Science Foundation (1609043) for financial support.

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