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

Abstract

International audienceThe hydration entropy of the monovalent cations and 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.: Cond. Matter, 2010, 22, 284108]. 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 implies fewer hydrogen-bond arrangements because the cation substantially inhibits 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 of 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 help explain the ordering in Hofmeister series of ions whereby anions increase this number but cations decrease it

Instantaneous, parameter-free methods to define a solute’s hydration shell

“…Its n values agree well with those from other simulations that use either the SAHB method but a different ion force field, 39 for which F − to I − are 6.1, 6.9, 7.5, and 7.6, or that use the GC method based on g(r XH ) 60,68 whose n values are 6.2-6.5 for F − and 7.0-7.4 for Cl − . GC n values obtained from g(r XH ) by neutron diffraction 8,45,54,69 are also close at 6.…”

“…Hydrogen bonds are determined using the strongest-acceptor definition. 38,39 1. g(r) cutoff (GC): All water molecules within a distancecutoff are in the solute's hydration shell. The radial distribution function g(r XO ) is constructed from all trajectory frames, where r XO is the solute-oxygen distance.…”

“…A more general form of this definition considers electrostatic force in place of distance. 39 This takes into account both distance and charge and is useful if there are multiple types of acceptor. These two definitions had previously been referred to as "topological" because they are based on the idea of resolving hydrogen-bond arrangements using the separating transition states at which a donor experiences opposing, equal forces from two acceptors.…”

“…For the more specialized case of hydrogen-bond coordination, there is a parameter-free hydrogen-bond definition 38,39 whereby a hydrogen donates to the nearest acceptor as long as that is the shortest donor-acceptor distance between the two molecules involved. If there is a shorter distance, then the donor hydrogen is said to have a broken hydrogen bond.…”

The Solvation of Anions in Propylene Carbonate

“…In the case of structured solvents there is also a solvent-solvent term which is positive, especially in the case of kosmotropic electrolytes, because the ions disrupt the solvent structure [34].…”