COLLIGATIVE PROPERTIES OF POLYELECTROLYTE SOLUTIONS

The solution properties of polyelectrolytes are not well understood de spite increasing theoretical and experimental effort, particularly dur ing the last 10 years. Some of the current models for polyelectrolyte solutions are valid only at infinite dilution because they account onl y for long-range interactions. Nagvekar and Danner [M. Nagvekar, R.P. Danner, An Excess Gibbs Free Energy Model for Polyelectrolyte Solution s, Fluid Phase Equilibria 53 (1989) 219.] have developed an excess Gib bs free energy model for polyelectrolyte solutions. The key feature of the model is to express the excess Gibbs free energy as the sum of th e contributions from long-range and short-range interactions. The 'lim iting laws' of Manning [G.S. Manning, Limiting Laws and Counterion Con densation in Polyelectrolyte Solutions. I. Colligative Properties, J. Chem. Phys. 51 (1969a) 924; G.S. Manning, Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions. Ii. Self-diffusion of Smal l Ions, J. Chem. Phys. 51 (1969b) 934.] were used to account for the l ong-range interactions while a local composition model of the Non-Rand om Two Liquid (NRTL) type was used for short-range interactions. In th is work, we present the expressions for counterion activity coefficien ts and the motic coefficients in polyelectrolyte solutions with and wi thout added salts at finite concentrations. Data for a number of polye lectrolyte systems with univalent and divalent counterions are analyze d using Nagvekar's proposed model. In the low concentration regime, th e osmotic coefficient data are practically independent of concentratio n, and are in reasonable agreement with the Manning model. With increa sing polyelectrolyte concentration, the osmotic coefficient data are a strong function of concentration, and the local composition model is shown to successfully represent the data. (C) 1998 Elsevier Science B. V.