Volume Transitions, Phase Separation, and Anisotropic Surface Phases in Charged Gels

Thermodynamic analysis of phase transitions in charged gels in poor solvents is presented. At sufficiently high degree of ionization, the gel undergoes a discontinuous volume transition to a new homogeneous and isotropic phase. When the two-phase coexistence line is reached under fixed volume conditions, the new phase forms a thin anisotropic layer on the surface of the gel. The concentration and the deformation of the surface phase are calculated and the results are generalized to the case of uniaxially deformed gels. The stability of the surface phase is studied and it is shown that a dilute surface phase is unstable against long wavelength fluctuations with wave vectors oriented along the surface of the gel. The connection between the theory and experiments on phase transitions in gels is discussed.