MOLECULAR-DYNAMICS STUDY OF THE FREE-ENERGY FUNCTIONS FOR ELECTRON-TRANSFER REACTIONS AT THE LIQUID LIQUID INTERFACE

Free energy functions for electron-transfer reactions at the interface between immiscible polar and nonpolar solvents are investigated by using molecular dynamics simulation. The full free energy curves are calculated by using an umbrella sampling procedure for an electron transfer between two centers located either parallel or perpendicular to the interface, and these are compared with the free energy curves for the same process in the bulk polar solvent. In most cases, the parabolic representation of the free energy curve (obtained from its curvature near the minimum) overestimates the high free energy values, with the largest deviation observed for charged solute in the bulk. For the systems at the interface, the parabolic representation is somewhat better, and the curvature of the free energy curve near the minimum is shown to depend on the orientation with respect to the interface of the line joining the charge-transfer centers. The role of solute charge and solvation structure in determining the free energy curves for the different systems is discussed.