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A new model for the excess Gibbs energy of aqueous solutions of polyelectrolytes is presented and applied for the correlation of the activity of water in aqueous solutions of polyelectrolytes without as well as with an added (single) salt. The model considers the phenomenon of counterion condensation, i.e., the partial dissociation of highly charged polyelectrolytes in water. Three parameters (a binary interaction parameter between polymer segments, the equilibrium constant of the dissociation reaction and a parameter which accounts for the polymer configuration) were fitted to the experimental results. The model allows for a reliable correlation of experimental results for the osmotic coefficient of aqueous solutions of a single polyelectrolyte (without as well as with an added salt).

Experimental results for the activity of water in aqueous solutions of 10 single polyelectrolytes (two polysodium acrylates, two polysodium methacrylates, three polyammonium acrylates, two polysodium ethylene sulfonates, and one polysodium styrene sulfonate) at (298.2 and 323.2) K are reported. The isopiestic method was employed in these experiments with aqueous solutions of sodium chloride as references. The polyelectrolytes were characterized by three averaged molecular masses determined by gel permeation chromatography. Furthermore, the density and the refractive index increments of the aqueous polyelectrolyte solutions are reported. Although a similar pattern for the activity of water was observed for all systems (i.e., the osmotic coefficient increases with rising polyelectrolyte concentration), the experimental results show that this property depends on the monomer type as well as on the size of the polymer chain. The temperature (varied from (298.2 to 323.2) K) has only a small influence on the act...