Competitive Interactions Within Cm(III) Solvation in Binary Water/Methanol Solutions

Abstract

Competitive forces exist in multicomponent solutions, and within electrolytes they consist of both ion-solvent and solvent-solvent interactions. These can influence a myriad of processes, including ligand complexation. In the case of water/alcohol solutions, recent work revealed an interesting dilemma regarding the overall solution dynamics and organization as compared to solute-solvent interactions. This is particularly true for highly charged ions in solution, whose ion-solvent interactions were demonstrated to be highly sensitive to the composition of the immediate solvation environment. Faster solvent exchange should be observed about the ion, considering that second-order Møller-Plesset perturbation theory predicts an average decrease in ion-solvent dissociation energy when methanol enters the first solvation shell of Cm3+(aq). Yet the addition of methanol to water causes the dynamic features of the hydrogen-bond network of the entire solution to slow. The apparent competition between these contrary forces was examined using a combination of electronic structure calculations with both ab initio and classical molecular dynamics simulations, using binary water/methanol solutions and Cm3+ as a representative solute. This combination of theoretical methods predicts that, among the competitive effects of the solvent-solvent and ion-solvent interactions, the solution-phase dynamics imparted by the addition of methanol to water kinetically restricts the solvation exchange rates about Cm3+ in these binary solutions.