Liquid—Vapor Interface Potential for Water

Abstract

The water molecule has been idealized as a point dipole plus point quadrupole, encased in a spherical exclusion envelope. Classical electrostatics has been applied to the determination of the potential field surrounding such a molecule in the wide interfacial region between the liquid and vapor phases, just below the critical temperature. From this result, the mean torque on the molecule in this inhomogeneous region follows, and produces a spontaneous interfacial polarization. The consequent potential difference across the interface has thus been evaluated at several temperatures, and demonstrates the tendency for surface water molecules to orient their protons away from the vapor and into the liquid. A related optical experiment is suggested, involving electric‐field dependence of reflected‐light ellipsometry.