A hydrogel was prepared by cross-linking poly(4-vinyl phenol) with ethylene glycol diglycidyl ether in
order to examine the ion-specific swelling behavior. The swelling ratios to various kinds of aqueous salt
solutions except for N(CH3)4Cl were almost constant, or the gels remained in a swollen state, up to their
saturated concentrations. This unexpected result was ascribed to stabilization of hydrogen-bonding
hydrations to the phenol OH proton and to the phenol ring (π electrons) by anions and cations, respectively.
The supposed mechanism was supported by ab initio calculations for the corresponding small model systems.
Swelling behaviors of poly(4-vinyl phenol) (P4VPh) gel in aqueous tetraalkylammonium chloride (TAACl) solutions were investigated to find a very specific swelling behavior. Especially for the tetrabutylammonium chloride (TBACl) system, P4VPh gel remarkably deswelled with increasing salt concentration (=2.0 M) and then sharply reswelled in a higher concentration region (>/= ca. 2.1 M). A similar swelling profile was also observed in the swelling time-course; upon immersion of a water-swollen P4VPh gel into 2.5 M TBACl solution, the gel first deswelled in an early stage (approximately 0.1 h) and then remarkably reswelled with time. Relative amounts (mol/mol) of TBA cation and water per monomer residue were estimated as ca. 1 and 0 for the deswollen state and 5 and 50 for the reswollen state, respectively. This result, together with those of attenuated total reflection Fourier transform infrared measurements performed for the gels swollen in various kinds of TAACl and inorganic salt solutions, suggested that in the highly deswollen and almost dehydrated state, phenol rings aggregated with intervening TBA cations, while the aggregation reswelled upon further binding with TBA cations.
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