Prediction and interpretation of the hydration entropies of monovalent cations and anions

The hydration entropy of the alkali metal cations and halide anions are calculated directly from molecular dynamics simulations of the hydrated ion and bulk water using theory previously applied to the hydration of noble gases [Irudayam and Henchman, J. Phys.: Condens. Matter 22, 284108 ( 2010)]. Extensions are included to account for differential hydrogen-bonding of first-shell waters with themselves, the ion, and bulk water. The entropies, enthalpies and Gibbs energies agree reasonably with simulation and experiment when the effect of force field is taken into account. The anions' entropy losses are mostly vibrational and librational, consistent with their stronger hydration. The cations' entropy losses are mostly orientational which imply fewer hydrogen-bond arrangements because the cations substantially inhibit the ability of surrounding water molecules to accept hydrogen bonds. Owing to the many entropy terms and different decompositions, it is shown that the terms, kosmotropes and chaotropes must be appropriately applied so as not to lead to contradictions. It is also proposed that the number of hydrogen-bond arrangements helps explain the ordering in Hofmeister series of ions whereby anions increase this number but cations decrease it.