Specific Cation Effects on Structure and Dynamics of Carbonyl with Water in N-Methylacetamide Aqueous Salt Solutions

Specific ion effects are typically manifested as differences in the behavior of salts in solution; these effects have largely been focused on preferential binding interactions at specific sites. However, the direct interactions between metal cations and functional groups were confirmed to be far weaker, most cations were not observed to exhibit any binding. It has therefore been proposed that the indirect role of water is equally important in understanding the specific cation effects, yet this significant contribution of the solvent has received less attention. Herein, we performed molecular dynamics simulations to investigate how cations affect the structural and dynamical properties of the solvation shell at the carbonyl-water interface in N-methylacetamide aqueous chloride salt solutions. Our simulations reveal that the distinct solvation behaviors of the carbonyl group exhibit a pronounced dependence on the specific cation present. Moreover, different cations produce specific effects on the structure and dynamics of the solvation of the carbonyl group, which well correlate with both the size and valence of the cation. It has been also observed that salt ions present the considerable effects on the structure and dynamics of the solvation of the carbonyl group via the salt concentration. These findings combined with established theories of the rigid solvation shell induced by cation could provide a molecular basis for understanding the specific cation effects.