Polyelectrolyte Hydration: Experiment, Theory, and Simulation

Properties of polyelectrolytes differ from those of low–molecular electrolytes as also from neutral polymers. In addition to the long–range Coulomb interaction, the water mediated ion–specific forces can be important. These effects are crucial for understanding the processes involving biological polyelectrolytes as also industrial applications. Examples showing importance of the ion–specific interaction are given in Refs. [1,2]; the enthalpies of dilution ?H of polyelectrolyte solutions were found to be either negative or positive, depending on the nature of counterion, temperature, and the charged group on a polyion. The classical model1?3, treating solvent as dielectric continuum, polyion as charged hard cylinder and ions as charged hard spheres, predicts for ?H to be always negative. In the present study a theory based on an extension of the product reactant Ornstein–Zernike theory4,5 was applied to model polyelectrolyte solution. Solvent molecules were modeled with four square–well sites so they can coordinate with each other and charges in the solution. Flexible polyions with 120 monomer units and equivalent number of oppositely charged counterions were present in the system6. The strength of the water–counterion interaction was varied, while the water–polyion and water–water interactions were constant. Note that in this model the “water” molecules had no charges; dielectric constant of the solution was assumed to be equal to that of pure water under these conditions. Thus, the pair potential for the present model was written as a sum of the hard?sphere term, Coulomb term, and the term describing association. Despite of its simplicity the model was able to explain basic experimental results of real polyelectrolytes6. For strongly hydrated counterions we obtained ?H