Hydration of monovalent and divalent cations near a cathode surface

Hydration of monovalent (Li⁺, Na⁺, K⁺, and Cs⁺) and divalent (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) cations on a cathode surface was studied by a classical molecular dynamics simulation. The potential of mean force (PMF) for each cation species was calculated as a function of the distance from the cathode surface, and the potential barriers for dehydrating the first and second hydration shells near the cathode surface were estimated. The positions of the minimum of the PMF closest to the cathode surface were found to be in the order Li⁺ < Na⁺ < Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺ < K⁺ < Cs⁺. It was found that Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions are most likely doubly hydrated when they are adsorbed on the cathode surface without an applied voltage, whereas Na⁺, K⁺, and Cs⁺ ions are most likely singly hydrated at room temperature. On the other hand, when a voltage of 1 V was applied to the electrodes, all the cation species that we studied appeared most likely to be singly hydrated on the cathode surface. The depths of the potential well closest to the cathode surface under an applied voltage of 1 V were found to be in the order Ba²⁺ < Sr²⁺ < Ca²⁺ < Mg²⁺ for the divalent cations and Li⁺ < Na⁺ < K⁺ < Cs⁺ for the monovalent cations in the set of models that we used. These orders coincide with the Hofmeister series from the kosmotropic to the chaotropic.