The K+ hydration shell structure in non-polar and low-polar environments

Knowledge of the structure of hydrated ions in hydrophobic media is crucial for understanding the mechanism of many biological processes and is relevant to a variety of technological applications. We focus here on the hydrated shell of the K+ cation, which has been widely investigated, both experimentally and theoretically, in gas phase and in aqueous solution. Recent experimental work has also been devoted to investigate Na+ and K+ halides transfer through a water/organic interface suggesting that the traditional view of hydrated ions entering the hydrophobic phase is not valid. We report a quantum mechanical study of K+(H2O)n clusters (n = 1, 8) in gas phase and in dielectric media with dielectric constants ε = 2, 10 corresponding to non-polar and low-polar environments, respectively. We show that dielectric effects significantly modify the stability and size of the hydrates. In particular, our computations suggest that in non-polar media, mono- and di-hydrates (n = 1, 2) are the most stable species and that hexahydrates (n = 6) have two water molecules in the second solvation shell.