Influence of Ion Size and Charge in Ion Transfer Processes Across a Liquid Liquid Interface

摘要

In this work, molecular dynamics simulations have been used to study the transfer of some alkaline ions (Na~+, K~+, and Rb~+), an alkaline-earth ion (Sr~(2+)), and an organic ion (N(CH_3)_4~+) across the water/2-heptanone liquid/liquid interface. Potentials of mean force were calculated and the ion transfer mechanisms were analyzed. The computed free energies of transfer exhibit a clear dependence on the ionic size and charge. In clear agreement with the experimental results obtained for several liquid/liquid biphasic systems, the free energies of transfer increase with the ionic charge and decrease with the ionic size. In all cases investigated, the potential of mean force for the transfer shows a monotonic increase in the Gibbs free energy as the ion progresses into the organic liquid. The major increase of the free energy occurs when the ion is on the organic side of the interface. The transfer seems to be an activationless process because there is no free energy barrier, this is true even in the case of the transfer of the organic ion. The transfer mechanism involves the formation of a water finger that connects the ions in the organic phase to the water phase during the transfer in both directions (i.e., from water to the organic phase and vice versa). For the organic and the alkaline ions, the water finger may be as long as 10 A and, for the alkaline-earth ion, as long as 14 A. In addition, it has been found that all the ions drag a part of their hydration shell into the organic phase, a phenomenon well documented experimentally. For similar ions, the number of water molecules and the fraction of the hydration shell dragged into the organic phase increased with the robustness of their shell. The N(CH_3)_4~+ ion drags slightly more water molecules than the alkaline ions, although the fraction of its hydration shell that remains in the organic solvent is much smaller. The mechanisms of the ion transfer processes studied here are all qualitatively similar, showing however a quantitative dependence on the ionic size and charge.