Swelling and shrinking of a polyelectrolyte gel induced by a salt solution

Abstract

The stability of a polyelectrolyte gel in solution results from a delicate balance between several competing thermodynamic forces, viz.

• (i) osmotic pressure of free ions in the gel,

• (ii) molecular interaction of solvent and polymer molecules,

• (iii) network elasticity,

• (iv) Debye-Htickel interaction of ions.

That balance may be upset by a decrease of temperature and by the addition of salt to the solvent. This results in a decrease of osmotic pressure and collapse of the gel to a small fraction of the initial volume. The effect can be reversed by increasing temperature and by removing salt from the solution. This paper presents an attempt to describe swelling and shrinking quantitatively and to understand the nature of the opposing forces. The volume of a particular polyacrylamide gel in a water acetone solution is represented as a function of the salt content and of temperature.