Ion-water clusters, bulk medium effects, and ion hydration

TL;DR

This paper investigates how the bulk medium influences ion-water cluster configurations and hydration free energies, highlighting differences between gas-phase calculations and aqueous ion thermodynamics, especially for soft ions.

Contribution

It evaluates the primitive quasichemical approximation and clarifies the bulk medium's role in stabilizing cluster configurations and affecting hydration thermodynamics.

Findings

Bulk medium stabilizes configurations not seen in gas phase.

Bulk medium lowers the excess chemical potential of ions.

Differences in optimal cluster size between gas-phase and aqueous conditions.

Abstract

Thermochemistry of gas-phase ion-water clusters together with estimates of the hydration free energy of the clusters and the water ligands are used to calculate the hydration free energy of the ion. Often the hydration calculations use a continuum model of the solvent. The primitive quasichemical approximation to the quasichemical theory provides a transparent framework to anchor such efforts. Here we evaluate the approximations inherent in the primitive quasichemical approach and elucidate the different roles of the bulk medium. We find that the bulk medium can stabilize configurations of the cluster that are usually not observed in the gas phase, while also simultaneously lowering the excess chemical potential of the ion. This effect is more pronounced for soft ions. Since the coordination number that minimizes the excess chemical potential of the ion is identified as the optimal or most probable coordination number, for such soft ions, the optimum cluster size and the hydration thermodynamics obtained without account of the bulk medium on the ion-water clustering reaction can be different from those observed in simulations of the aqueous ion. The ideas presented in this work are expected to be relevant to experimental studies that translate thermochemistry of ion-water clusters to the thermodynamics of the hydrated ion and to evolving theoretical approaches that combine high-level calculations on clusters with coarse-grained models of the medium.