An experimental and theoretical investigation on the swelling of N-isopropyl acrylamide based ionic hydrogels in aqueous solutions of (sodium chloride or di-sodium hydrogen phosphate)

Hydrogels are three-dimensional polymeric networks of hydrophilic monomers. When at least some of the monomers are electrolytes, such networks are called polyelectrolyte hydrogels. This paper reports new experimental results for the swelling equilibrium of polyelectrolyte hydrogels consisting of the neutral monomer N-isopropyl acrylamide and the ionic comonomer sodium methacrylate in aqueous solutions of a single salt (either sodium chloride or di-sodium hydrogen phosphate) at 298 K. The effect of the salt on the swelling equilibrium of such hydrogels is described by a thermodynamic model which combines an expression for the Gibbs energy of a liquid phase with an equation for the Helmholtz energy of an elastic network.