Influence of ion size and charge in ion transfer processes across a liquidvertical bar liquid interface

In this work, molecular dynamics simulations have been used to study the tr ansfer of some alkaline ions (Na+, K+, and Rb+), an alkaline-earth ion (Sr2 +), and an organic ion (N(CH3)(4)(+)) across the water/2-heptanone liquid/l iquid interface. Potentials of mean force were calculated and the ion trans fer mechanisms were analyzed. The computed free energies of transfer exhibi t a clear dependence on the ionic size and charge. In clear agreement with the experimental results obtained for several liquid/liquid biphasic system s, the free energies of transfer increase with the ionic charge and decreas e with the ionic size. In all cases investigated, the potential of mean for ce 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 tra nsfer 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 conne cts 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). Fo r the organic and the alkaline ions, the water finger may be as long as 10 Angstrom and, for the alkaline-earth ion, as long as 14 Angstrom. In additi on, it has been found that all the ions drag a part of their hydration shel l into the organic phase, a phenomenon well documented experimentally. For similar ions, the number of water molecules and the fraction of the hydrati on shell dragged into the organic phase increased with the robustness of th eir shell. The N(CH3)(4)(+) ion drags slightly more water molecules than th e 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 qua ntitative dependence on the ionic size and charge.