Internal structure of water around cations

In the present paper, we use statistical mechanics to probe into the changes induced by cations (Al3+, Mg2+, Ca2+, Li+, Na+, K+, Rb+, Cs+) on the structure of water. The theory aims to find the minimum free energy state, taking into account the hydrogen bonding interactions between water molecules, electrostatic interactions between water and the ion, and thermal energy. Water molecules in the first shell of Na+ ions are found to largely retain their structure; the average number of H-bonds () and the average dipolar alignment () of a water molecule are only marginally different from the corresponding values of bulk water. This is made possible by the "caging" of the Na+ ions by water molecules. The magnitudes of and are, however, found to decrease for ions on either side of Na+ ions in the Hofmeister Series. Water molecules around small ions with high charge density (e.g. Al3+) are found to strongly align their dipoles in the direction of field, despite the reduction in the number of H-bonds per molecule. Those around large ions with low charge density (e.g. Cs+) are oriented such that one of their H-bond axes involving a lone pair of electrons is directly facing the ion, thereby maximizing their H-bond interactions at the other three bonding sites. Beyond the first shell of all the ions studied, the degree of hydrogen bonding is similar to that of bulk water molecules. Changes in the molecular orientation and non-linear polarization effects, however, persist up to similar to 3-4 hydration shells in the case of salting-in ions, and similar to 7-9 shells in the case of salting-out ions. (C) 2012 Elsevier B.V. All rights reserved