VOLUME TRANSITIONS, PHASE-SEPARATION, AND ANISOTROPIC SURFACE PHASES IN CHARGED GELS

Thermodynamic analysis of phase transitions in charged gels in poor so lvents is presented. At sufficiently high degree of ionization, the ge l undergoes a discontinuous volume transition tu 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 t he surface phase are calculated and the results are generalize to the case of uniaxially deformed gels. The stability of the surface phase i s studied and it is shown that a dilute surface phase is unstable agai nst 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.