THERMODYNAMICS OF PHASE-EQUILIBRIUM FOR SYSTEMS CONTAINING GELS

A gel is a three-dimensional cross-linked elastic polymer. When a gel is immersed into a liquid, it can swell, similar to a sponge. If the s urrounding liquid is a solute-containing solution, a solute partitions between the gel and the surrounding liquid. This work derives the fun damental equations of phase equilibrium for such systems. These equati ons take into account the elastic properties of the gel. Criteria are given for the distribution of solutes between a gel and a coexisting l iquid phase. Criteria are also presented for calculating three-phase e quilibria where a swollen gel and a collapsed gel coexist with the sur rounding liquid. Attention is given to a slightly ionized gel (polyele ctrolyte) surrounded by a dilute aqueous electrolyte solution. The equ ations of equilibrium show that, when Donnan equilibria are taken into account, the pH inside the gel is not equal to that of the surroundin g aqueous solution. The discussion presented here may be useful for de sign of gels used in medicine, pharmacy and biotechnology.