Hydration of simple ions. Effect of the charge density.

Ions are often classified as 'kosmotropes' or 'chaotropes' depending on whether they order or disorder water, as determined by how much they change the viscosity of water. Collins has proposed that water ordering by an ion depends on its charge density, using a simple fused-sphere model (K. D. Collins, Biophys. J., 1997, 72, 65-76). To make his idea more quantitative, we have applied the site-site integral equation of Chandler, Silbey and Ladanyi to obtain the potential of the mean force between ions and water molecules. From the potential of mean force curves we obtain the activation energy for transferring a water molecule from near the ion to the bulk. The results for this quantity are in semi-quantitative agreement with experimental data for alkali halides and with theoretical results based on the TIP3P model of water.