Short-range interactions: from simple ions to polyelectrolyte solutions

Although many properties of electrolyte solutions can be successfully described by theories at the McMillan Mayer level of approximation, there are other phenomena that cannot be explained without taking into account the explicit nature of solvent molecules. One of these that have received much attention is the Hofmeister effect that describes the influence of different types of ions on the solubility of hydrophobic molecules in water. In this work we use two simple water models, the ‘fused-spheres’ and the two-dimensional ‘Mercedes-Benz’ models to study ion solvation in water, and test suppositions about their effect on hydrophobicity. Both models give good qualitative agreement with experiment, such as Samoilov ion hydration activation energies, and Setchenow coefficients, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The results suggest that the interactions of ions with water are governed mostly by the ionic charge densities. Water structure is determined by the balance of electrostatic forces and the tendency for hydrogen bond formation. Ions with a high charge density bind water molecules very tightly and therefore exclude the hydrophobe from their first shell, leading to salting-out. The effect decreases with decreasing charge density of the ion.