Hydration shell exchange dynamics during ion transfer across the liquid/liquid interface

We examine using molecular dynamics simulations the rate and mechanism of water molecules exchange around the Li+ and Na+ ions during ion transfer across the interface between water and nitrobenzene. As the ions are transferred from the water to the organic phase, they keep their first hydration shell and an incomplete second shell. The rate of water exchange between the first shell and the rest of the interfacial water molecule decreases during the transfer, which is consistent with an increase in the barrier along the ion-water potential of mean force. While in bulk water the exchange of water molecules around the Li+ follows an associative (A) or associative interchange (I-a) type mechanism, the fraction of exchange events of type A increases at the interface. In contrast, while in bulk water the exchange of water molecules around the six coordinated Na+ hydrated species mainly follows a dissociative mechanism, the situation at the interface involves an equilibrium interchange between the four- and five-coordinated hydrated ion. Simulation of the reversed process, in which the hydrated Li+ ion is transferred to the aqueous phase, shows the same general behavior as a function of location from the interface.