Binding isotherms of Ca 2+ to copolymer gels of acrylamide and acrylate (AA), N-isopropylacrylamide and acrylate (NA), and n-butylacrylate and acrylate (BA) were investigated and were compared with each other for examining the effect of the hydrophobicity of chain on the Ca 2+ binding. The highest binding constant and the largest shrinkage in the volume were observed for the BA gel. The counterion condensation theory developed by Manning could not explain the observed binding isotherms. The analysis using the theory for the cooperative binding developed by McGhee and Hippel indicates that the binding to BA gel is more cooperative than those to the others. The present experimental result indicates that the hydrophobic circumstance promotes the Ca 2+ binding to carboxylate.

A theory to describe the swelling behavior of polyion gels is presented on the basis of de Gennes theory for the conformational entropy of chains under traction and ideal Donnan approximation combined with the counterion condensation of polyelectrolytes for the osmotic expansion force. The Flory exponent is regarded as a parameter in theory to describe the effect of the intrachain interaction on the fractal nature of the chain conformation. The volume of poly͑acrylic acid͒ gel was measured as a function of the ionization degree, the salt concentration, and the polymerization degree of chains between crosslinks. The analysis of the expansion behavior showed that the values varied from about 0.8 to about 0.6 for the gels having long chains when the salt concentration changed from 10 to 100 mM, while for the gels consisting of short chains the exponent was kept to about 0.8, irrespective of the salt concentration change. The results indicate that the Flory exponent of the expanding polyion gel increases with the strength of the electrostatic interaction between ionized groups. The volume of a copolymer gel of fully neutralized maleic acid and styrene consisting of the short chains was also measured as a function of the salt concentration. The analysis of the expansion behavior gave ϭ0.71 in the salt concentration range between 3 and 320 mM in contrast with a value of 0.8 for the poly͑acrylic acid͒ gel under similar conditions. This result suggests that the hydrophobic interaction between the phenyl groups reduces the value.

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