Pressure-induced volume phase transition of polyacrylamide gels in acetone–water mixtures

Abstract

Equilibrium swelling curves of ionized polyacrylamide gels immersed in acetone–water mixtures were measured as a function of pressure up to pressures of 300 MPa. The gels, which shrank at atmospheric pressure, underwent an abrupt volume change (pressure-induced volume phase transition) from a shrunken state to a swollen state at the transition pressure. The transition pressure increased with an increase of acetone concentration. The pressure-induced volume phase transition can be interpreted by taking account of the free-energy change ΔV⋅P between swollen (hydrated) and shrunken (dehydrated) states. The ΔV represents the difference between the molar volume of water structured around hydrophilic groups of polyacrylamide chains and that of free water in the bulk mixtures. The estimated value of ΔV is −3.3 mL/mol, which qualitatively agrees with that obtained from the experiments of denaturation of proteins. The pressure-induced volume phase transition is generally expected in many hydrogels.