Swelling and shrinking processes of sodium polyacrylate-type super-absorbent gel in electrolyte solutions.

Some effects of electrolytes on the swelling and shrinking processes of sodium polyacrylate-type super-absorbent gel were examined systematically, using several kinds of uni-univalent and di-univalent electrolyte solutions. A gel of this type goes through a maximum degree of swelling and then shrinks in electrolyte solutions. The reason was found to be that the sodium ions dissociated from the functional groups in the gel, –COONa, are exchanged easily with the other cations in the solution. The swelling and shrinking mechanism can be reasonably explained in terms of the electric force acting on the charged network of the gel. This is different from the previously proposed mechanism, which involves the osmotic pressure. The final degree of swelling is closely related to the selectivity of –COO– ion for the cation in the solution and to the cation concentration in the solution.