J. Phys. Chem. B 2007, 111, 3391-3397
3391
Ion-Specific Swelling of Poly(styrene sulfonic acid) Hydrogel Ling Xu, Xin Li, Maolin Zhai,* Ling Huang, Jing Peng, Jiuqiang Li, and Genshuan Wei Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking UniVersity, 100871, People’s Republic of China ReceiVed: NoVember 20, 2006; In Final Form: January 6, 2007
Poly(styrene sulfonic acid) (PSSA) hydrogel was prepared by radiation crosslinking using methyl N,N-bisacrylamide as crosslinker. Effects of ion species and concentration on the swelling behavior of PSSA hydrogel were investigated in aqueous solution of selected anions (F-, Cl-, Br-, SCN-), cations (Li+, Na+, K+, Ca2+), and hydrophobic ions (tetramethylammonium cation TMA+, tetrabutylammonium cation TBA+, and dodecyltrimethylammonium cation TAB+). The deswelling extent of PSSA hydrogel follows anion Hofmeister series, i.e., SCN- < Br- < Cl- < F-, in solutions containing selected anions and K+ as counterion up to a concentration of 2 mol‚L-1. On the contrary, the deswelling extent of PSSA hydrogel in solutions containing selected cations and Cl- follows the sequence of Li+ < Na+ < K+ < Ca2+, which is the reverse of the Hofmeister series except Ca2+. We have discussed the effects of ions on the hydrogen bonding through SO3and phenyl ring in salt solutions at low and high concentrations. Other interactions, such as the cation-π and hydrophobic interactions, also contributed to the ion-specific swelling of PSSA hydrogel. The proposed mechanism was further elucidated by FTIR and NMR analysis. A very specific deswelling-reswelling phenomenon of PSSA hydrogel in KF solution has been observed and ascribed to the F- binding to phenyl ring through a specific interaction.
Introduction Many phenomena in colloid, polymer, and interface science that involved electrolytes show pronounced ion specificity.1 The nature of the different salts specifically affects the interaction pair potential between surfaces. Hofmeister series were established to evaluate the ionic effects on the aggregation or stabilization phenomena of particles immersed in aqueous solutions in 1888.2 The ionic effects on different aspects, for instance, polymer cloud points, protein solubility, chromatographic selectivity, critical micelle concentration, surface tension, gel-coagel transitions, molecular forces, and colloid stability, have been intensively investigated since then.3 Although the relative position of ions in different physical or chemical systems does not exactly coincide, only slight alteration appears in the characteristic rank, i.e., the
SCN- < I- < ClO4- < NO3- < Br- < ClO3- < Cl- < BrO3- < F- < SO42- anion series and the
K+ < Na+ , Li+ < Ca2+ cation series Strongly hydrated anions on the right are called salting-out ions, kosmotropes ions, or water-structure makers; while weakly hydrated anions on the left are called salting-in ions, chaotropic ions, or water-structure breakers. Generally, anions have stronger influences on polymers’ physicochemical properties than cations. The order of cations is sometimes inverted or irregular depending on polymer systems. Although the Hofmeister series is experimentally known in many fields, its mechanism remained to be fully elucidated due to a complex fusion of direct and indirect effects of ions to the solute molecule and a combination of effects on the water * To whom correspondence should be addressed.
[email protected]. Phone/Fax: 86-10-62753794.
E-mail:
structure.4,5 In systems where interactions between surfaces take place, not only the water structure around the solvated ions but also that surrounding the immersed surfaces play significant roles, and it results from ion-water, water-water, surfacewater, surface-ion, and surface-surface interactions.5 Therefore, the specific restructuring of water and the ionic distribution around polymer molecules modify their interfacial potentials. The change of potentials can be explained by a mechanism for specific exclusion or accumulation of ions at the surface. And another mechanism, based on the cooperative orientation of water molecules adjacent to the surface, has been induced by the presence of ions. The ion specificity is likely to come from a combination of both mechanisms.6 Ninham and co-workers have justified these mechanisms by dispersion force, which acts between the ion and the interface. Generally, ions have different polarizabilities with the surrounding water, so that they experience very specific dispersion potential near an interface. These ionic dispersion potentials play significant roles especially at high salt concentrations where electrostatic potentials are screened.3,7,8 In an attempt to elucidate the ionic specific effects on the volume phase transition temperature (LCST) or the swelling degree of a thermosensitive polymer gel system, poly(Nisopropyl acryl amide) (PNIPAm) was been published in 1993.9 Meanwhile, the ion-specific effects on the swelling behavior of polymer gels with various structures have been extensively studied by Satoh and co-workers.4,10-21 In these works, polymers with simple structures have been chosen so that the ion specificity became more predictable and can be simulated by microscopic model. Most polymers in aqueous systems are considered as composed of three kinds of components, i.e., ionic, polar, and nonpolar (or hydrophobic) groups. Therefore, ionic effects can be qualitatively and quantitatively estimated by summing up of the ion effects on electrostatic interaction, hydrophobic interaction, and hydrogen bonds. Dispersion forces should be also considered especially in high salt concentration.3
10.1021/jp067707d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007
3392 J. Phys. Chem. B, Vol. 111, No. 13, 2007 A simple hydrogen-bonding hydration model of different polymer side chains has been proposed, where hydrogenbonding hydration is stabilized or destabilized through the cation or anion. The model successfully interprets the ion specificities of many hydrophilic polymer gels in terms of perturbing of water electron-pair acceptance (EPA) or electron-pair donation (EPD) through ionic hydration.15 Similar ideas have been generated by Daly et al., i.e., the addition of salts to the solution provides competition for the hydrogen bonds of hydrogels, and thus the hydrophobic interactions between the hydrogel backbones become notable and the gel deswells. Kosmotropic ions interact strongly with water so that a higher extent of gel deswelling should occur.22 The ion-specificity on the deswelling or reswelling of a polymer gel is also comparable to some studies on the aggregation-stabilization of colloidal stability of protein-polymer, which is explained by hydration forces.6,23 However, many abnormal experimental facts were observed in different gel systems, implying that Hofmeister phenomena cannot be explained in terms of a single parameter or one kind of mechanism. Instead of ion-specific swelling, an unexpected super saltresistivity was found in poly(4-vinyl phenol) (P4VPh) gel, which had never been observed in other polymer gels. In other words, the swelling ratios of the gel in various inorganic salt solutions were almost constant up to their saturated concentration. The abnormal result is ascribed to stabilization of hydrogen-bonding hydrations to the phenol OH proton and to the phenol ring (π electrons) by anions and cations, respectively.24 A further investigation on the swelling of P4VPh gel in tetrabutylammonium chloride (TBACl) found that the gel remarkably deswelled with increasing salt concentrations and then sharply reswelled in a higher salt concentration region. The mechanism is suggested as following: in the highly deswollen and almost dehydrated state, phenol rings aggregated with intervening TBA+ cations, while the aggregation reswelled upon further binding with TBA+ cation.25 P4VPh gels show similar but more remarkable swelling in cationic surfactants (C10TAB and C12TAB) solution compared with that in TBACl solution. The results strongly suggest that the cationic surfactants interact with the polymer via cation-π interactions as well as hydrophobic interactions. The highly stable hydration status is attributed to the specific structure of coexistence of π electrons and acidic protons.26 The interactions involved in P4VPh gel systems, for instance, hydrogen-bonding hydrations to π electrons, cation-π interactions, and hydrophobic interactions, might significantly influence the properties of aromatic polymers. Being a typical polymer system with a phenyl ring, polystyrene (PS) latex and sulfonated PS have often been employed to study polymer-ion/water interactions in aqueous media. Although PS itself is nonionic, the latex inevitably contains small amounts of sulfate group at the chain ends so that electrostatic and hydrophobic interactions are attributed to the binding of PS and ions. The studies on the interactions between PS latex or partially sulfonated PS and cationic surfactant revealed that a similar aggregation-dissolution process with that in P4VPh systems occurred; however, the contributions of cation-π interaction were not mentioned in those publications.27,28 Thus, it is necessary to clarify the interactions between π-electron and ions (both inorganic and organic) to study the status and interactions concerning the π-electron in aromatic polymers. For this purpose, the ionspecific effects on PSSA gel in inorganic and ionic surfactant solutions are to be studied by swelling measurement, with the aid of FTIR and NMR analyses. The mechanism of the ion-
Xu et al. specific hydration via interactions in ions/phenyl ring/SO3-/ water systems is to be discussed. Experimental Materials. Sodium styrene sulfonate (SSS) was purchased from Tokyo Kasei Co. Ltd. Methyl N,N-bis-acrylamide (MBA) obtained from Beijing Chemical Co. Ltd. was used as crosslinker. KF, KCl, KBr, KSCN, LiCl, NaCl, CaCl2, tetramethyl ammonium bromide (TMABr), tetrabutyl ammonium bromide (TBABr), and other chemicals were AR products of Beijing Chemical Co. Ltd. Ionic surfactants, dodecyltrimethylammonium bromide (C12TAB), and sodium dodecyl sulfate (SDS) were purchased from Tokyo Kasei and ACROS, respectively. Preparation of PSSA Hydrogel. The solution containing 20% SSS and 1% MBA was bubbled with nitrogen for 20 min, and then the tube was sealed. A capillary with an inner diameter of 0.9 mm was set in the solution to prepare a rodtype gel. The solution was irradiated by 60Co γ rays at a dose rate of 20 Gy‚min-1 with absorbed dose in the range of 1-10 kGy. After irradiation, the gel was removed from the capillary, immersed in deionized water until equilibrium swelling was reached, and then cut to 2-3 mm length. The PSSS hydrogels (Na type) prepared by 6, 8, and 10 kGy irradiation were then changed to PSSA hydrogels (H type) by the following procedure: (1) immersing PSSS hydrogel in 0.1 mol‚L-1 HCl solution for 1 day to exchange the sulfonate group from Na type to H type; (2) removal of HCl by abundant of deionized water for several times. The completeness of ion exchange was confirmed by elemental analysis. Thus, the rodtype PSSA gels with diameter of ca. 2.7-4.0 mm and water content of 98% were kept in deionized water for further swelling investigation. Swelling Degree. Rod-type PSSA gels prepared by the abovementioned method were immersed in 10 mL of various kinds of salt solutions until equilibrium swelling was reached. The diameters of the gels before and after deswelling were determined by a microscope (×20) and referred as d0 and d, respectively. The swelling degree was defined as d/d0. FTIR Measurement. PSSA hydrogels were immersed in different swelling media until equilibrium swelling. The deswollen gels were removed from the solution and wiped off excess surface solution prior to microscope FTIR measurement. The measurements were performed at a Nicolet Magna-IR 750, using Nicolet NicPlan IR microscope attachment (resolution 4 cm-1, scan 64 times) and MCT/A detector. Elemental Analysis. Gels were removed from the solution and then dried to constant weight. The N, C, H compositions of the gel were measured by a VARIO EL elemental analyzer (Elemental, Germany). NMR Analysis. To get 1H NMR spectra with a better resolution, PSSS with Mw of 70 000 was used instead of PSSA hydrogel. PSSS and inorganic salts were dissolved in D2O to make a solution with composition of 10 mg PSSS and 2 mol‚L-1 salt. 1H NMR spectra of the samples were recorded at a Varian 300 MHz. Results and Discussion Radiation Synthesis of PSSA Gel. It is reported that pure PSSS solution primarily undergoes degradation upon γ-ray irradiation, and a soft, sticky gel mass can be formed only at a very high absorbed dose (>2500 kGy). However, the gelation dose for the crosslinking of SSS monomer or high molecular weight PSSS (Mw 106) is significantly decreased in the presence of a crosslinking agent such as N,N-methylene bis acrylamide (MBA).29,30 In this study, an optimal condition for gel formation,
Ion-Specific Swelling of PSSA Hydrogel
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3393
TABLE 1: Physical Properties of PSSA Gel as a Function of Absorbed Dose absorbed dose (kGy) 1 gel fraction (%) water content (%) diameter (mm)
2
3
5
6
8
10
0 0.34 17.8 56.9 80.0 83.1 79.2 99.9 99.0 98.5 97.4 3.338 2.760 2.798
i.e., 20 wt % SSS with the presence of 1 wt % MBA (6 mol % of SSS) was adopted.29 According to the amount of N obtained from elemental analysis, the crosslinker in feed are effectively involved in the crosslinking reaction and distributed in PSSS gel. The gelation dose is found to be 2 kGy, and the diameter of swollen gels in deionized water is stable after the absorbed dose reached 8 kGy (Table 1). For comparison with the results of P4VPh gel, PSSS hydrogel was transferred to PSSA hydrogel by ion exchange with 0.1 mol‚L-1 HCl. Ion-Specific Swelling of PSSA Gel. Swelling in Inorganic Salt Solution. Swelling degree measurement of hydrogels is found to be a simple and effective way to study the interactions involved in polymer aqueous systems.31 Investigations on the ion-specific swelling of many hydrogels reveal that these phenomena can be interpreted in terms of ion effects on the hydrogen-bonding hydration to the pertinent polar group of polymer matrix.4,15-19 For this reason, the swelling degree of PSSA gel in solutions with various ions at different position of Hofmeister series was measured and illustrated in Figure 1. The deswelling extent in each solution for gels prepared by 8- and 10-kGy irradiation is lower than that for the gels prepared by 6-kGy irradiation, which can be explained by a well-known fact that the crosslinking density of the gel is higher with high absorbed dose if the crosslinking reaction played a dominant role. In Figure 1 and the following discussions, only data obtained in the gels prepared by 8-kGy irradiation are shown, because the swelling of all the gels follow the same sequence in terms of ion species. The swelling degrees of the gel in the solutions of potassium salts with different anions and chloride salts with different cations have been shown separately to see the anion and cation specificities. The swelling profile of PSSA gel in the presence of electrolytes can be divided into two stages: deswelling stage and reswelling stage, if specific interactions are involved. The interactions in different stages are to be discussed individually. According to the ab initio calculation of the hydrogen-bonding pattern between various polymer polar groups and ions, hydrogen-bonding hydration in phenol systems has been stabilized by anions, which is different from that of poly(vinyl alcohol) (PVA).16 As described in the Introduction, the “super salt-resistivity” of P4VPh gel is ascribed to the rather stable hydrophilic structure through the cooperative effect of π hydrogen bonding and common hydrogen bonding with hydroxyl group, hence, making the different ion-specificities between PVA and P4VPh.25,26,32,33 However, the salt-resistivity swelling observed in P4VPh gel did not occur in PSSA gel. SO3H group in PSSA gel are very easy to ionize to SO3-, so that the influence of counterion binding to SO3- should be much more significant than that through the phenyl group.34 Because of the strong counterion interaction between SO3- and cations, the deswelling of PSSA gel in the presence of even a small amount of electrolyte is rather high and the ion-specificity on gel swelling is hard to be discussed in very low concentration of salt solutions. However, when counterion interaction is saturated, the ionspecificity become apparent in 2 mol‚L-1 solutions. The swelling degrees of PSSA hydrogels in 2 mol‚L-1 KF, KCl, KBr, and
Figure 1. Ion-specific swelling of PSSA gel prepared by 8-kGy irradiation. (a) anions; (b) cations.
KSCN are 0.26, 0.38, 0.40, and 0.42, respectively, which increase in the sequence of F- < Cl- < Br- < SCN-. It can be explained by the destabilization mechanism of hydrogenbonding hydration by anions.15 More significant deswelling of the PSSA gel in anions with strong hydration ability such as F- causes a more significantly destabilization than others and leads to a higher extent of dehydration of the gel. For saltingin anions such as SCN-, a slight reswelling was even observed in concentrated solution (i.e., g6 mol‚L-1). This slight reswelling may be caused by a specific binding of SCN- to the hydrophobic moieties of the polymer, which is commonly known for large anions, probably also caused by the dispersion force between ions and surface and/or the repulsive hydration forces between the PSSA backbones.6,21,23 In the case of cation specificities, one must consider effects of the counterion binding, the degree of which is significantly dependent on the counterion species (cation in the present system). When the fixed charge is a sulfate or sulfonate group, the ion specificity is known as Li+ < Na+ < K+ ≈ Cs+, in an order of stronger counterion binding. The swelling degrees of PSSA gel in 2 mol‚L-1 CaCl2, KCl, NaCl, and LiCl solutions are 0.38, 0.38, 0.43, and 0.52, respectively, increasing in the sequence of Ca2+ < K+ < Na+ < Li+. This cation specificity observed for the present PSSA gel in the lower salt concentration region seems to be explained in terms of the counterion binding. The most significant deswelling in Ca2+ may be caused by the physical crosslinking of PSSA hydrogel by the divalent cation.
3394 J. Phys. Chem. B, Vol. 111, No. 13, 2007
Xu et al.
Figure 2. (a) Illustration of hydration mode of PSSA in low water content, (b) mechanism for reswelling of PSSA gel in the presence of Li+, and (c) mechanism for reswelling of PSSA gel in the presence of F-.
In a word, in the deswelling stage of PSSA gel, the sequence for anions follows the Hofmeister series; however, the sequence for Li+, Na+, and K+ cations is in the opposite direction of Hofmeister series. PSSA gel deswells most significantly in anions with stronger hydration ability and in cations with stronger counterion binding ability to sulfonate group. The main interactions involved in aqueous systems of most polymers affected by ions include: electrostatic interaction among the polymer-charged groups and that between polymer charges and the counterions; inter- and intramolecular hydrophobic interaction among polymer’s nonpolar groups (counterion binding); hydrogen bonds between polymer’s polar groups and that between polymer and water (hydrogen-bonding hydration).21 Ion-specific swelling studies on several hydrophilic polymer gels such as poly(N-vinylpyrrolidone),13 poly(allylamine), and poly(vinyl alcohol)15 were reported, finding that these hydrogels swelled in solutions with small cations (e.g., Li+) and deswelled in solutions with small anions (e.g., F-). In hydrated status, the radial distribution functions (RDFs) for the F--H and Li+-H separations are close to zero; therefore the interactions between F-/Li+ and adjacent water molecules are strong.35 As a result, the (de)stabilization effect of F- and Li+ is stronger than that of Cl-, Na+, etc. The deswelling of PSSA hydrogel in solutions of cation series is caused by the counterion interaction between cations and SO3- which destroys the hydrogen bonding of polymer surrounding. When the counterion interaction is saturated, the ionic hydration and hydrophobic hydration should be considered. The mechanism based on the (de)stabilization to hydrogenbonding hydration by ions may successfully explain the deswelling period of the gel, i.e., hydrogel in more hydrophilic status. However, it is not sufficient to explain the reswelling of the hydrogel in some ion species, especially in F-. Some specific interaction involved with phenyl group and ambient environment should be considered. It was reported that a single coordinated phenyl radical will form a benzene-water cluster, where (H2O)n cluster cations (n > 20) may repel phenyl rings from hydrophilic ambient. When the number of water molecules surrounding the polymer is restricted, the phenyl ring can be solvated (n < 10).36 Compared with the strong hydration ability of SO3-, this kind of water cluster can be neglected from the scheme of hydrogen bond when the system contains a certain amount of water, and the phenyl ring is in hydrophobic microdomains. However, in ambient restricted water, the phenyl ring is capable to form a hydrogen bond with water through phenyl H atom or tangles above the π electron. Ions can affect the solvation of phenyl ring.17,24 The mechanism for the reswelling stage of PSSA gel in the presence of Li+ or F- is illustrated in Figure 2. When the salt concentration is higher, the effect of the electrostatic interaction
Figure 3. Swelling degree of PSSA gel in TAABr and ionic surfactant solutions (inset figure) contain hydrophobic ions.
on the gel swelling is completely masked, and ion effects on the hydrogen-bonding hydration and the hydrophobic hydration become significant. Small cations such as Li+ may be more accessible to phenyl ring than other cations through the stabilization of the hydrogen bonding interaction. The interaction between F- and phenyl ring was never reported, and details will be described in our coming paper. Ion-Specific Swelling of PSSA Gel in Solutions with Hydrophobic Ions. PSSA has a structure similar to partially sulfonated polystyrene.37 Electrostatic interactions, hydrophobic interactions, and hydrogen bonding should contribute to the ion-specific effect of PSSA. Furthermore, cation-π interactions should also play important roles in the swelling behavior of this aromatic polymer gel.25,26,32,33 The interactions involved with π electron systems are far from a complete elucidation, although it is very important in biology and life science as well as many other fields. To investigate the π electron concerning interactions, the swelling degrees of PSSA hydrogel in more hydrophobic ambient, such as tetraalkyl ammonium bromide (TMABr, TBABr) and ionic surfactants (C12TAB, SDS), were measured and showed in Figure 3. In a previous work (L. Xu), P4VPh gel was found to show a deswelling, followed by reswelling with increasing concentration of tetrabutyl ammonium chloride (TBACl) and cationic surfactants (e.g., C12TAB), while in anionic surfactant (e.g., SDS) solutions only reswelling was observed. Cation-π interaction and hydrophobic interaction have been nominated as main factors for the binding of hydrophobic ions to phenol group and different swelling behaviors of P4VPh gel in various media.25,26 For PSSA hydrogel, similar deswelling-reswelling with increasing salt concentration was observed in TBABr and C12TAB solution (at the critical association concentration close to cmc), implying that cation-π interaction and hydrophobic interaction also exist in the PSSA-hydrophobic cation system. Similar to the results of P4VPh gel, gel reswelling did not occur in the solutions without enough hydrophobicity, i.e., TMABr. The more significant deswelling-reswelling in more hydrophobic ions (e.g., C12TAB+ > TBA+ > TMA+) imply that hydrophobic interaction plays an important role on the swelling behavior of PSSA gel in solutions containing hydrophobic ions, and the hydrophobicity of the swelling media for gel reswelling had a critical value. However, the reswelling in SDS solution which was observed in P4VPh gel did not occur in the case of PSSA hydrogel. Hydrophobic interaction, which causes the formation of polymersurfactant micelle in P4VPh gel system, seems not sufficient
Ion-Specific Swelling of PSSA Hydrogel to lead to a substantial SDS-PSSA binding, probably due to the much higher hydrophilicity of PSSA than P4VPh. Here, SDS seems to simply act as a salting-out agent, and the swelling degree of PSSA hydrogel in 100 mM SDS is close to that in 100 mM NaCl. The contribution of cation-π interaction can be confirmed by the fact that the reswelling of PSSA gel only occurred in cationic surfactant and tetraalkyl ammonium bromide (TAABr) solutions but not in anionic surfactant ones. These results support the proposal of our previous works, namely, the cation-π interaction, which generally works only in a hydrophobic ambient, can be effective even in an aqueous system if sufficient aid of hydrophobic interaction was available.25,26 The ion-specific swelling behavior of PSSA hydrogel in solutions considering both inorganic and organic ions can be interpreted as: (1) The deswelling of PSSA hydrogel in solutions of low salt concentration is caused by the counterion interactions. When the counterion interaction is saturated, the ionic hydration and hydrophobic hydration should be considered. The higher extent of deswelling in cations surfactant (C12TAB) solution is contributed by the extra cation-π interaction and hydrophobic interaction in the system. (2) After swelling minima (d/d0 ) ca. 0.25 except in TBABr), the water molecules around the gel are restricted so that some special interactions in the systems lead to the binding of ions to polymer chain (especially phenyl ring), resulting in the reswelling of PSSA gel. For SCN-, the slight reswelling can be ascribed to its specific binding to hydrophobic interface; for Li+, the reswelling can be attributed to its strong stabilization of the hydrogen-bonding hydration; for TBABr, the reswelling was caused by the cation-π interaction, which is similar with that for the P4VPh-TBACl case, and the slightly higher value of minimum swelling degree might be caused by the binding of large TBA+ cation to gel matrix; for C12TAB, a polymer-surfactant micelle was formed and the osmotic pressure of the counterion caused the gel to reswell; for F-, the very special reswelling can be ascribed to the specific F- binding ability to phenyl ring. To further confirm our proposed mechanism, the interactions concerning the SO3- group and phenyl ring were investigated by some instrumental methods such as FTIR and NMR. FTIR Analysis. The FTIR spectra of PSSA gel immersed in 2 mol‚L-1 inorganic salt solutions are showed in Figure 4 and the wavelength of the peaks which significantly influenced by the addition of salt are listed in Table 2. The peaks appearing at 1644, 1180, 1127, 1036, and 1008 cm-1 are attributed to the stretching vibration of C-C aromatic skeleton, SO3- group symmetric vibration, in-plane skeleton vibration of phenyl ring, SO3- group antisymmetric vibration, and in-plane bonding vibration of phenyl ring, respectively.34,37 The intensity of the 1644-cm-1 peak in deionized water is much higher than the 1000-1200-cm-1 ones, implying that the 1644-cm-1 peak has been significantly interfered by the presence of water. In 2 mol‚L-1 KSCN, KCl, and KF solutions, the 1000-1200-cm-1 peak has become much stronger than that at 1644 cm-1 due to the dehydration of PSSA hydrogel and the presence of ions near -1 LiCl and NaCl solution, the peaks hydrated SO3 . In 2 mol‚L of both phenyl ring and SO3- are strong, implying the existence of cation-π interaction in these systems. Peak shift has occurred in salt solutions, and the most significant change appeared at 1644 and 1180 cm-1. Compared to H2O, significant blue shift (for the peak near 1644 cm-1) has been observed in KSCN solution, while red shift occurs in KF solution. In a UV-vis study, the phenyl peak of P4VPh shifted to higher wavelength accompanied by aggregation, while
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3395
Figure 4. FTIR spectra of PSSA gel in various solutions with 2 mol‚L-1 (a) anions and (b) cations.
TABLE 2: Wavelength of ν(C-C) and νs(OdSdO) of PSSA Gel in Various Salt Solutions by FTIR H2O 2 mol‚L-1 KSCN 2 mol‚L-1 KCl 2 mol‚L-1 KF 4 mol‚L-1 KSCN 4 mol‚L-1 KF 6 mol‚L-1 KSCN 6 mol‚L-1 KF 2 mol‚L-1 LiCl 2 mol‚L-1 NaCl 100 mM SDS 100 mM C12TAB
ν(C-C) (cm-1)a
νs(OdSdO)b
1644 1627 1652 1656 1626 1660 1630 1660 1646 1643 1651 1654
1180 1189 1190 1192 1190 1194 1191 1189 1189 1185 1220 1216, 1190 split peak
a Represents stretching vibration of C-C aromatic skeleton. b Represents SO3- group symmetric vibration.
blue shift occurred during a dissolution procedure.26 Although the 1644-cm-1 peak might be greatly influenced by water, we can assume that SCN- and F- bind to the phenyl ring in different ways or their distances from π-electron are different; probably F- is capable to bind to the H atom of phenyl ring and SCN- is binding to the hydrophobic part of PSSA near the alkyl chain. Red shift has also been observed in surfactant’s case. The red shift in C12TAB can be ascribed to cation-π interaction and that in SDS solution is unclear. In the peak near 1180 cm-1, red shift occurs in all the solutions and the wavelength increases with decreasing swelling degree. This
3396 J. Phys. Chem. B, Vol. 111, No. 13, 2007
Xu et al. implying its low affinity to phenyl ring. It is binding to phenyl ring through hydrophobic hydration at a position near the β proton and alkyl chain. In the case of cations, the δ values for the chemical shift is complicated, especially for Li+. The δ values for R proton increase in the sequence of Li+ < K+ < Na+, while those for β proton increase in the sequence of K+ < Na+ < Li+. The completely opposite position of Li+ implies that Li+ binds to the π electron above the phenyl skeleton plane, however, in a much closer distance with the R proton, so that the negative charge transfers to Li+ occur and there is a lower interference to the β proton. Conclusion
Figure 5. 1H NMR spectra of PSSS using D2O as solvent.
TABLE 3: Chemical Shifts of Phenyl H Atom of PSSS in D2O Solutions Containing 2 mol‚L-1 Salt by 1H NMR D2O KF KCl KSCN LiCl NaCl
a,a′ (ppm)
δ (ppm)
b,b′ (ppm)
δ (ppm)
7.462 7.460 7.486 7.488 7.478 7.489
0 -0.002 0.024 0.026 0.016 0.027
6.266 6.287 6.310 6.335 6.343 6.332
0 0.021 0.044 0.069 0.077 0.066
result has proved that the deswelling of PSSA hydrogel is mainly attributed to the counterion interaction involved with SO3- and the destabilization of SO3- hydrogen bond by ions. In C12TAB solution, significant red shift has been observed and the peak has been split into two (1216 and 1190 cm-1) due to the strong counterion interaction between TAB+ and SO3-. Although no hint of binding between SDS and PSSA are shown based on the fact that it is deswelling extent is similar to that of NaCl, probably SDS does influence the hydration structure of SO3and phenyl ring, while the mechanism needs further elucidation. NMR Analysis. The 1H NMR spectra of PSSS (Mw 70 000) solution cosoluted with 2 mol‚L-1 Hofmeister ion was measured to simulate the ion effect on the π-hydrogen-bonding hydration of PSSA gel because the interference of SO3- can be effectively masked by this method. The spectra of pure PSSS and the attribution of the peaks are shown in Figure 5.37 The chemical shift and δ (changes of chemical shift compared with those obtained in D2O) of phenyl H atom in each solution are listed in Table 3. The chemical shift of the phenyl H atom in all the cases shifts to a high field except for R proton (a,a′) in KF solution. A clear tendency of decreasing chemical shift with increasing hydration ability of anions was observed, i.e., SCN> Cl- > F-. It is known that π electron movements in the direction of phenyl skeleton plane causes the shift to low field (δ < 0); on the contrary, the movements toward the π electron lead to a high-field shift with increasing distance (δ > 0).38 The shift to a high field is caused by the interaction between cations and π electron above the phenyl skeleton plane; while the lower δ value of R proton than that of β proton (b,b′) can be ascribed to the counterion attraction between cations and SO3-. The lowest δ in KF solution again proves the strong affinity of F- for the phenyl ring, and the binding is in the direction of phenyl skeleton plane. Compared with the very close R-proton chemical shift of SCN- and Cl-, the larger difference between their chemical shifts for β protons implies that the R proton is ready to accept hydrophilic binding and that the β proton favors the hydrophobic binding. The δ values for both kinds of protons in KSCN solution are highest in anions series,
PSSA hydrogel was prepared by radiation crosslinking to study the ion-specific swelling of hydrogels containing phenyl ring. The ion effects of gel swelling in both low and high concentration salt solution are discussed in terms of hydrogen bonding through SO3- and phenyl ring, in addition to other interactions. The deswelling extent of PSSA hydrogel follow Hofmeister series in anions (SCN- < Br- < Cl- < F) in low concentration solutions due to the destabilization of anions to hydrogen bond between SO3- and water. Reswelling was observed in high concentration solutions if some specific interactions exist in the systems. An abnormal reswelling in KF solution is reported for the first time and is ascribed to the specific F- binding to phenyl ring. The reswelling in SCN- is attributed to the SCN- binding to phenyl ring and the hydration forces between the PSSA backbones. On the other hand, the deswelling extent of PSSA hydrogel in cations followed the sequence of Li+ < Na+ < K+ < Ca2+, which is reversed with the Hofmeister series except Ca2+. The significant deswelling in Ca2+ is caused by the physical crosslinking of PSSA hydrogel by divalent ions. The deswelling in cations followed the sequence of counterion interactions between SO3- and cations, which is in reverse sequence with the Hofmeister series. Reswelling was also observed in cations with very strong hydration ability (Li+). Deswelling-reswelling in hydrophobic ions with strong hydrophibicity (TAB+, TBA+) is attributed to cation-π interaction and hydrophobic interaction. As a result, ion-specific swelling of PSSA is mainly contributed by the influence of ions to hydrogen-bonding hydration of SO3- and phenyl ring, cation-π interaction, hydrophobic interaction, and dispersion force. Acknowledgment. This work is financially supported by the China Postdoctoral Science Foundation (Project. No.2006039356). The authors thank Prof. Mitsuru Satoh of the Department of Chemistry and Materials Science, Tokyo Institute of Technology, Japan, for his highly valuable discussions and suggestions on this work. References and Notes (1) Boroudjerdi, H.; Kim, Y. W.; Naji, A.; Netz, R. R.; Schlagberger, X.; Serr, A. Statics and dynamics of strongly charged soft matter. Phys. Rep. 2005, 416 (3-4), 129-199. (2) Hofmeister, F. A. Exp. Pathol. Pharmakol. 1888, 24, 247. (3) Kunz, W.; Nostro, P. L.; Ninham, B. W. The present state of affairs with Hofmeister effects. Curr. Opin. Colloid Interface Sci. 2004, 9, 1-18. (4) Muta, H.; Ishida, K.; Tamaki, E.; Satoh, M. An IR study on ionspecific and solvent-specific swelling of poly(N-vinyl-2-pyrrolidone) gel. Polymer 2002, 43 (1), 103-110. (5) Lopez-Leon, T.; Jodar-Reyes, A. B.; Bastos-Gonzalez, D.; OrtegaVinuesa, J. L. Hofmeister effects in the stability and electrophoretic mobility of polystyrene latex particles. J. Phys. Chem. B 2003, 107 (24), 56965708.
Ion-Specific Swelling of PSSA Hydrogel (6) Lo´pez-Leo´n, T.; Jo´dar-Reyes, A. B.; Ortega-Vinuesa, J. L.; BastosGonzalez, D. Hofmeister effects on the colloidal stability of an IgG-coated polystyrene latex. J. Colloid Interface Sci. 2005, 284, 139-148. (7) Ninham, B. W.; Yaminsky, V. Ion Binding and Ion Specificity: The Hofmeister Effect and Onsager and Lifshitz Theories. Langmuir 1997, 13, 2097-2108. (8) Bostrom, M.; Williams, D. R. M.; Ninham, B. W. Why the properties of proteins in salt solutions follow a Hofmeister series. Curr. Opin. Colloid Interface Sci. 2004, 9, 48-52. (9) Suzuki, A. Phase transition in gels of sub-millimeter size induced by interaction with stimuli. AdV. Polym. Sci. 1993, 110, 199-240. (10) Satoh, M.; Yoda, E.; Komiyama, J. Effects of Hydrophobic Hydration on the Counterion Binding of Polycations. Macromolecules 1991, 24 (5), 1123-1127. (11) Huh, Y. I.; Kotake, N.; Satoh, M.; Komiyama, J. Counterion Specific Swelling Behaviors of Crosslinked Poly(L-glutamic acid) Membranes in Aqueous Alcohols. J. Membr. Sci. 1993, 81 (3), 253261. (12) Okazaki, Y.; Ishizuki, K.; Kawauchi, S.; Satoh, M.; Komiyama, J. Ion-Specific Swelling and Deswelling Behaviors of Ampholytic Polymer Gels. Macromolecules 1996, 29, 8391-8397. (13) Takano, M.; Ogata, K.; Kawauchi, S.; Satoh, M.; Komiyama, J. Ion-specific Swelling Behavior of Poly(N-vinyl-2-pyrrolidone) Gel Correlations with Water Hydrogen Bond and Non-freezable Water. Polymer Gels and Networks 1998, 6, 217-232. (14) Nishiyama, Y.; Satoh, M. Collapse of a charged polymer gel induced by mixing “unfavorable” counterions. Polymer 2001, 42 (8), 39193921. (15) Muta, H.; Miwa, M.; Satoh, M. Ion-specific swelling of hydrophilic polymer gels. Polymer 2001, 42 (14), 6313-6316. (16) Muta, H.; Kojima, R.; Kawauchi, S.; Tachibana, A.; Satoh, M. Ionspecificity for hydrogen-bonding hydration of polymer: an approach by ab initio molecular orbital calculations. THEOCHEM 2001, 536 (2-3), 219-226. (17) Muta, H.; Sin, T.; Yamanaka, A.; Kawauchi, S.; Satoh, M. Ionspecificity, for hydrogen-bonding hydration of polymer: an approach by ab initio molecular orbital calculations II. THEOCHEM 2001, 574, 195211. (18) Muta, H.; Kawauchi, S.; Satoh, M. Ion effects on hydrogen-bonding hydration of polymer an approach by “induced force model”. THEOCHEM 2003, 620 (1), 65-76. (19) Muta, H.; Kawauchi, S.; Satoh, M. Ion-specific swelling behavior of uncharged poly(acrylic acid) gel. Colloid Polym. Sci. 2003, 282 (2), 149155. (20) Minato, T.; Satoh, M. Counterion Binding in Na Poly(acrylate) Gel-A 23Na NMR Study. J. Polym. Sci. Polym. Phys. 2004, 42, 44124420. (21) Yasumoto, N.; Kasahara, N.; Sakaki, A.; Satoh, M. Ion-specific swelling behaviors of partially quaternized poly(4-vinyl pyridine) gel. Colloid Polym. Sci. 2006, 284 (8), 900-908. (22) Daly, E.; Saunders, B. R. A Study of the Effect of Electrolyte on the Swelling and Stability of Poly(N-isopropylacrylamide) Microgel Dispersions. Langmuir 2000, 16, 5546-5552.
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3397 (23) Molina-Bolı´var, J. A.; Galisteo-Gonza´lez, F.; lvarez, R. H.-A. Colloidal stability of protein-polymer systems: A possible explanation by hydration forces. Phys. ReV. E 1997, 55 (4), 4522-4530. (24) Muta, H.; Taniguchi, T.; Watando, H.; Yamanaka, A.; Takeda, S.; Ishida, K.; Kawauchi, S.; Satoh, M. Super-salt-resistive gel prepared from poly(4-vinyl phenol). Langmuir 2002, 18 (25), 9629-9631. (25) Xu, L.; Yokoyama, E.; Watando, H.; Okuda-Fukui, R.; Kawauchi, S.; Satoh, M. Specific swelling behaviors of poly(4-vinyl phenol) gels in tetraalkylammonium chloride solutions. Langmuir 2004, 20 (17), 70647069. (26) Xu, L.; Yokoyama, E.; Satoh, M. Specific interactions of poly(4vinyl phenol) gel with cationic and anionic surfactants. Langmuir 2005, 21 (16), 7153-7160. (27) Zhao, J. X.; Brown, W. Adsorption of Alkyltrimethylammonium Bromides on Negatively Charged Polystyrene Latex-Particles Using Dynamic Light-Scattering and Adsorption-Isotherm Measurements. Langmuir 1995, 11 (8), 2944-2950. (28) Xu, R. L.; Smart, G. Electrophoretic mobility study of dodecyltrimethylammonium bromide in aqueous solution and adsorption on microspheres. Langmuir 1996, 12 (17), 4125-4133. (29) Bhardwaj, Y. K.; Mohan, H.; Sabharwal, S.; Mukherjee, T. Radiation effect on poly (p-sodium styrene sulphonate) of different degrees of polymerization in aqueous solution: pulse radiolysis and steady state study. Radiat. Phys. Chem. 2001, 62 (2-3), 229-242. (30) Bhardwaj, Y. K.; Mohan, H.; Sabharwal, S.; Majali, A. B. Role of radiolytically generated species in radiation induced polymerization of sodium p-styrene sulphonate (SSS) in aqueous solution: Steady state and pulse radiolysis study. Radiat. Phys. Chem. 2000, 58 (4), 373-385. (31) Sjostrom, J.; Piculell, L. Simple gel swelling experiments distinguish between associating and nonassociating polymer-surfactant pairs. Langmuir 2001, 17 (13), 3836-3843. (32) Xu, L.; Watando, H.; Satoh, M. Effects of chemical modifications on the swelling behaviors of poly (4-vinyl phenol) gel. Colloid Polym. Sci. 2006, 284 (8), 862-870. (33) Xu, L.; Yokoyama, E.; Satoh, M. Colloids Surf., A 2007, 296 (13), 277-284. (34) Jamroz, D.; Marechal, Y. Hydration of sulfonated polyimide membranes. II. Water uptake and hydration mechanisms of protonated homopolymer and block copolymers. J. Phys. Chem. B 2005, 109 (42), 19664-19675. (35) Ohrn, A.; Karlstrom, G. A combined quantum chemical statistical mechanical simulation of the hydration of Li+, Na+, F-, and Cl-. J. Phys. Chem. B 2004, 108 (24), 8452-8459. (36) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Infrared Spectroscopy of Size-Selected Benzene-Water Cluster Cations [C6H6-(H2O)n] + (n) 1-23): Hydrogen Bond Network Evolution and Microscopic Hydrophobicity. J. Phys. Chem. A 2004, 108, 10656-10660. (37) Yang, J. C.; Jablonsky, M. J.; Mays, J. W. NMR and FT-IR studies of sulfonated styrene-based homopolymers and copolymers. Polymer 2002, 43 (19), 5125-5132. (38) Liang, X. T. NMR (analyse and application of high resolution 1H NMR); China Academic Press: Beijing, 1976.
