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

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 Moller Plesset perturbation theory predicts an average decrease in ion-solvent dissociation energy when methanol enters the first solvation shell of Cm-(aq)(3+). 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.