Polyelectrolyte gels in salt solutions

TLDR

The study builds on the electrostatic free energy framework of polyelectrolyte solutions. The authors aimed to develop a general theory that accurately describes equilibrium swelling and salt accumulation in polymethacrylic acid gels across the full ionization range in salt solutions up to 0.5 M. They investigated equilibrium swelling and salt uptake in highly swollen polymethacrylic acid gels exposed to 0.8 × 10⁻²–0.5 M LiCl, NaCl, and KCl at varying polymer ionization degrees, and formulated a theory to capture these results. All three cations produced identical equilibrium swelling, indicating negligible specific ion–polyacid interactions; the uncorrected Donnan model fails for highly charged gels, and the gel-phase salt activity is governed mainly by electrostatics, closely matching that in polyelectrolyte solutions.

Abstract

Abstract Equilibrium swelling and salt accumulation were investigated for highly swollen polymethacrylic acid gels in aqueous salt solutions of 0.8 × 10 −2 to 0.5 M LiCl, NaCl, and KCl, at varying degrees of ionization of the polymer. The same equilibrium results were obtained with all three cations, thus showing that in the range investigated, specific interactions between the cations and the polyacid network may be disregarded. It was shown that the Donnan distribution, uncorrected for activity factors, cannot be applied to highly charged, highly swollen gels, even as a rough approximation. A general theory was developed which satisfactorily describes these equilibrium results in the entire ionization range of the gel in salt solutions up to 0.5 M . for the electrostatic free energy of polyelectrolyte solutions. It was found that the activity factor of the salt in the gel‐phase is chiefly determined by the electrostatic factor, and closely approximates the activity factor in polyelectrolyte solutions.