Despite its first appearance in the literature over 100 years ago, the Hofmeister effect has evoked modern-day interest in understanding the structural, dynamical, and energetic effects that an ion imparts on other molecules in solution. While notable scientific progress has been made, an understanding of the interplay of ions and water, especially the dependence on the identity of the ion, is not yet complete. The many-body expansion (MBE) was used to obtain a quantitative account of the interplay between the ion-water and water-water interactions in aqueous ionic clusters containing ions in the Hofmeister series. The many-body terms were further divided into contributions from the ion-containing fragments and the pure water fragments of the expansion to better understand the influence of the ion in the expansion and how its presence alters the water-water interactions. Through this analysis, noteworthy differences between kosmotropes and chaotropes were identified. The MBE of kosmotropes is dominated by ion-water interactions (ranging from 89% - 106% of the cluster binding energy for the ions considered). The disruption due to the dominant ion results in weak, unfavorable water-water interactions. Conversely, the MBE for chaotropes was found to converge more quickly as it more closely resembles that of pure water clusters. Chaotropes exhibit weaker overall binding energies and ion-water interactions (55% - 95% of the total cluster binding energy) with more favorable water-water interactions, somewhat recovering the pattern of the 2-4 body terms exemplified by pure water clusters. In addition, both kosmotropic and chaotropic ions exhibit a remarkable anti-correlation between the 2-B ion-water (I-W) and water-water (W-W) interactions as well as between the 3-B (I-W-W) and (I-W) interactions. These results have important implications for the development of many-body ion-water interaction potentials used to understand the solvation of ions and the different behavior of kosmotropes and chaotropes.

Using the many-body expansion (MBE) to dissect the interplay between ion-water and water-water interactions in aqueous ionic clusters of the Hofmeister series

ACS Spring 2022

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