An excess Gibbs free energy model for polyelectrolyte solutions

Abstract

The current models for polyelectrolyte solutions are valid only at infinite dilution because they account exclusively for long-range interactions and completely ignore the discrete nature of the charged groups. In this work, a local composition model of the Non Random Two-Liquid (NRTL) type is introduced to describe cylindrical polyelectrolytes and their interactions with ions in solution. The excess Gibbs free energy is expressed as the sum of the contributions from long-range and short-range interactions. The ‘limiting laws’ of Manning are used to account for the long-range interactions while the local composition model is used for the short-range interactions. From the free energy model, expressions can be derived for colligative properties like the activity and osmotic coefficients. Correlations using the proposed model are shown for polyelectrolyte systems, both with and without added salts.