Free energy of transfer of hydrated ion clusters from water to an immiscible organic solvent.

The free energies of transfer of hydrated alkali and halide ion clusters from bulk water to bulk 1,2-dichloroethane (DCE) are calculated using molecular dynamics computer simulations. Cluster sizes that include one to four water molecules are considered, as well as the bare ions. For each ion, the free energy of transfer decreases as the number of water molecules in the cluster increases. This dependence is stronger for small ions than for larger ions. The simulation results can be used to determine the most likely cluster sizes in DCE. The free energies of transfer are in reasonable agreement with the experimental free energies of ion transfer. An examination of the different energy terms involved in the solvation of the hydrated ion clusters in DCE and in water shows that several energy terms and, in particular, specific interactions between the cluster and the rest of the solvent, vary with ion size and charge and must be accurately calculated to determine the free energy of transfer.