Molecular dynamics study of the calcium ion transfer across the water/nitrobenzene interface

A study on the calcium ion transfer across the water/nitrobenzene interface is presented. The potential of mean force was calculated and a good agreement was found between the experimental and the calculated free energy of transfer. This is a monotonically increasing function of the distance to the interface, and the process was found to be non-activated. The evolution of the first and second hydration shells was analysed as a function of the distance to the interface; the first hydration shell remains intact whereas the second hydration shell suffers a severe water loss. Water finger formation was also found, with behaviour similar to that already described for other ions in different interfaces. As far as we know, a direct comparison between the calculated number of water molecules dragged with an ion into the organic phase and the experimental results is presented for the first time and a very good agreement was found.