Abstract

The aim of this work is to develop and implement efficient theoretical approaches based on first-principles to investigate key features of ion hydration in clusters and solutions. For this purpose, the parameter set of the self-consistent-charge density-functional tight-binding (SCC-DFTB) model, an approximate version of (first-principles) density-functional theory, has been extended to include halogen atoms and used to describe the interatomic interactions in molecular dynamics (MD) simulations performed to investigate the hydration features of halides. The results of these first-principles-based simulations unambiguously demonstrate the higher affinity of the larger halides for the cluster surface, hence resolving a long-standing controversy. Given the accuracy and computational efficiency of SCC-DFTB in describing halide cluster hydration, the model was further validated against high-level quantum-chemistry data and employed to investigate ionic clusters containing the polyatomic anions of the Hofmeister series: different hydration extents were found for the anions investigated, consistent with their order in the series. For instance, kosmotropic ions, i.e. those favoring water structure, tend to adopt fully hydrated structures, while chaotropic ions, i.e. those that disrupt hydrogen-bonded water networks, tend to be expelled from the water droplet and adopt surface hydration structures. Turning our attention to cations, the hydration behavior of alkyl diammonium of varying alkyl chain length was also investigated in clusters by MD simulations with empirical force fields validated against SCC-DFTB. In light of the increased surface propensity of longer dications found in water clusters, the relationship between the hydration extent of these ions and their effect on the salting out of organic molecules in aqueous solvent mixtures was also investigated: while fully hydrated shorter dications promote phase separation, partially hydrated longer dications have a stabilizing effect on organic aggregates in the mixture. These findings helped rationalize previous experimental data on environmentally-friendly “switchable” solvents, the design of which could be greatly assisted by further such simulations. The methodology developed, based on first-principles, not only allowed studies that helped unveil a possible relationship between the hydration extent of ions and their specific effect in solutions, but should also find a broad range of applications.