Spatial Organization of Organic Solvents at Interfaces and Its Consequences for Local Electric Potential

The development of modern membranes for energy-storage devices such as supercapacitors and membranes for ionic separations depends on the description of ions at solid interfaces, often provided by the electrical double layer (EDL) model. The classical EDL model however, does not consider important factors such as the influence of solvents on electrochemical potential distributions. First, we show in a model system of acetonitrile at a silica interface, that the solvent organization dictates the ionic distributions and interfacial potential. Specifically, ion transport measurements in nanopores, nonlinear spectroscopy, and molecular dynamics simulations reveal that the distribution of ions is determined by the long-range, bilayer-like organization that the interface imposes upon the liquid. The solvent organization enables the effective surface potential to switch from negative in low salt concentrations to positive in high salt concentrations.

To support the claim that the interfacial potential is extremely sensitive to molecular organization of the solvent, we also performed experiments with propylene carbonate that exists in racemic and enantiomerically pure forms. Our experiments revealed that the effective surface potential is indeed dependent on the chiral form of the solvent. Electrochemical and electrokinetic measurements of ionic transport through glass pipettes and polymer pores revealed that the effective surface potential is significantly lower in solutions prepared using enantiomerically pure propylene carbonate than in solutions prepared using the racemic form. Our results emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces and the relatively unexplored role that chirality can play in electrokinetic phenomena.