Acid–base equilibria in polyelectrolyte systems

Abstract

Acid–base equilibria in carboxylic polyelectrolyte solutions and gels have been investigated by studying ion-exchange equilibria involving hydrogen and alkali ions. The investigations were carried out with solutions of poly(acrylic acid), poly(methacrylic acid), carboxymethyl cellulose and a carboxymethyl cellulose gel. The acid–base dissociation constants of the polyacids were determined, and acid strength was found to increase in the order poly(methacrylic acid), poly(acrylic acid), carboxymethyl cellulose. In the latter case a marked decrease of acid strength with increasing degree of substitution was observed, which was attributed to increased fluctuations in the concentration of carboxylic groups at high degrees of substitution. In the presence of simple salt the ionisation of the polyacids increased with increasing salt concentration. The effect is analogous to the increased ionisation of low-molar-mass weak acids, caused by the salt-induced depression of the ionic activity coefficients. However, the effect is more pronounced in polyelectrolytes and depends on the nature of the alkali-metal counterion, the ionisation being higher for K⁺ and Cs⁺ ions than for Na⁺ and Li⁺ ions.