Free-Energy Analysis for Facilitated Ion Transfer of Multivalent Ions at the Liquid-Liquid Interface

This work investigated the microscopic mechanisms of facilitated ion transfer of the divalent Ca²⁺ ion through a water-oil interface assisted by two tetrakis[4-chlorophenyl]borate (TPBCl^-) anions by calculating multidimensional free-energy surfaces with molecular simulation. The landscape of three-dimensional free energy indicated a reduced barrier for the ion transfer induced by 1:2 complex formation. Two TPBCl^- ions form the 1:2 complex with Ca²⁺ almost simultaneously at the interface near the barrier point, and the dynamics of barrier passage is associated with the break of the water finger. On the other hand, the dissociation of two TPBCl^- ions from the complex occurs stepwise as it moves into the oil phase. The influence of transient complex formation is on the order of nanometers for the interface, and it defines the effective thickness of the interface for the ion transfer. The free-energy profile elucidated the catalytic role of TPBCl^- at the interface, known as the shuttling mechanism.