CONTACT POTENTIALS OF SOLUTION INTERFACES - PHASE-EQUILIBRIUM AND INTERFACIAL ELECTRIC-FIELDS

Calculations of the electric fields at the water liquid-vapor interface are used to motivate a general discussion of the electrostatic potential differences between conducting phases in equilibrium with respect to independent transport of charged species. A formula which requires only structural information on the interfacial charge density profile is presented for this contact (or surface) potential. In contrast to the purely thermodynamic basis for introduction of surface potentials, this surface structural perspective suggests that surface potentials are generally measurable quantities. The consistency between thermodynamic and structural definitions of the contact potential is discussed. Although a solvent contact potential can be defined and, in principle, measured, this solvent contact potential generally differs from the infinite dilution electrostatic potential difference involved in the thermodynamics of electrolyte solutions: ionic contributions to thermodynamic contact potentials do not vanish in the infinite dilution limit. In fact, if the solvent conductivity is negligible, then the limiting infinite dilution value of the contact potential depends on the compositional path chosen for the approach to infinite dilution. Further, although the contact potential can be identified with the total dipole moment of the charges in the interfacial regions, the defined solvent contribution to the contact potential is generally not determined only by the interfacial density profile of molecular dipole moments. Finally, it is suggested that electron (and positron) reflectivity experiments would provide the most direct attempt to measure contact potentials of solution interfaces, and some primitive aspects of such experiments are considered.