Abstract: A molecular thermodynamic model for polyelectrolyte solutions without the added salt has been developed in this work. It includes three contributions. The first term is the contribution from the mixture of charged hard spheres which is determined by using Blum′s MSA model. The second term is that from bonding of polyion segments which is calculated through two-particle cavity correlation function accounting for the correlation in the nearest-neighbour segments of polyion. The third term accounts for the stickiness between the polyion and its counterions beyond the territorial binding. Thermodynamic properties of polyelectrolyte solution can be described satisfactorily.

Key words: Polyelectrolyte, Molecular thermodynamics, Sticky, Osmotic coefficient, Activity coefficient