Ion-specific swelling behavior of uncharged poly(acrylic acid) gel

The ion-specific swelling behavior of poly(acrylic acid) (PAA) gel prepared by γ-ray irradiation was investigated as a function of salt concentration in the presence of 0.01 M HCl. The anion specificity for the swelling ratio was similar to that for many kinds of hydrogels, i.e., Cl−<Br−<NO3−<I−, while the cation specificity proved to be rather unusual, i.e., Mg2+<Ca2+<Li+<Na+<K+<Cs+. In order to find any differences in the hydration of uncharged PAA from that of other polymers having typical polar groups, the hydrogen-bonding hydrations on the relevant polar groups were compared for small molecule analogues with an ab initio molecular orbital calculation. According to the results, the marked deswelling of PAA gel in the presence of strongly hydrated cations was ascribed to the unfavorable hydration to the acidic proton of PAA due to the reduced availability of water oxygen as well as to the destabilization of hydrophobic hydration developing around the uncharged PAA.