Characteristics of K+ and Rb+ as “structure-breaking” ions in dilute aqueous solution: Insights from ONIOM-XS MD simulations

- ONIOM-XS MD simulations of aqueous K+ and Rb+ solutions have been performed.
- The first hydration shell of both K+ and Rb+ shows a high degree of flexibility.
- The average coordination numbers of 6.4 and 6.6 were observed for K+ and Rb+, respectively.
- The results clearly characterize K+ and Rb+ as “structure-breaking” ions in water.

The structural and dynamical properties of the K+ and Rb+ hydrates have been investigated by means of the ONIOM-XS MD approach. For both the K+ and Rb+ hydrates, the ONIOM-XS MD results clearly reveal a high degree of flexibility within the first shell, with the average coordination numbers of 6.4 and 6.6, respectively. In this respect, it is observed that water molecules in the vicinity of K+ and Rb+ are rather labile, i.e., they are arranged with respect to the influence of the ion as well as to the hydrogen bond (HB) interactions among the first-shell waters and between the first-shell waters and the bulk. These observed phenomena clearly confirm the very loose first hydration shell of both the K+ and Rb+ ions, i.e., the weak ion-water interactions which correspond to the “structure-breaking” effects of K+ and Rb+ in aqueous solution.

(a) M-O and (b) M-H radial distribution functions, where M = K+ and Rb+, and their corresponding integration numbers, as obtained by the ONIOM-XS MD simulations.111