Ion-Specific Swelling and Deswelling Behaviors of Ampholytic Polymer Gels

Ampholytic polymer gels were prepared by γ-ray irradiation to aqueous solutions of two kinds of poly(vinyl alcohol)s containing sulfonate and quaternary ammonium groups, respectively. By adjusting the mixing ratio of the two polymers, three kinds of ampholytic gels were obtained: anion-rich, cation-rich, and 1:1 ampholytic ones. These gel samples were immersed in many kinds of salt solutions and buffers to observe the swelling behavior. The swelling degrees of the cation-rich and the anion-rich gels decreased with the increase in the salt concentration (from 0 to 10 -3 −10 -2 M), followed by reswelling with a further increase in the ionic strength of the immersing solutions. The initial deswelling is common to ionic gels and the latter swelling may be ascribed to scission of preformed ionic cross-linking. A similar swelling was also observed for the 1:1 gel, which did not show any decrease in the swelling degree in the lower salt concentration region. A remarkable ion-specificity was noted at the highest salt concentration region above 0.1 M; the three kinds of gels significantly deswelled in solutions containing strongly hydrated anions, such as F -, SO 4 2-, HPO 4 2-, and citrate anion. The ion-specific swelling behavior is discussed in terms of the compatibility of ionic hydration with the polymer hydroxyl group through the electron pair donation and acceptance abilities of the water molecules.