Transfer of a tetramethylammonium ion across the water-nitrobenzene interface: Potential of mean force and nonequilibrium dynamics

The transfer of a tetramethylammonium ion across the water/nitrobenzene liq uid/liquid interface is studied at 300 K using molecular dynamics computer simulations. The potential of mean force, calculated using two different te chniques, shows that the transfer is accompanied by a relatively small chan ge in the free energy, in agreement with experiments. A comparison of the p otential of mean force with a continuum electrostatic model shows reasonabl e agreement. The mechanism of ion transfer across the interface under condi tions of an external electric field is investigated by nonequilibrium molec ular dynamics calculations. Despite the fact that this process has only a s mall free energy driving force, several of the structural and dynamical cha racteristics of the system during the transfer are similar to those observe d in the study of the transfer of small ions with a large free energy of tr ansfer.