ORIENTED DIPOLES AT INTERFACES - CALCULATION OF SURFACE-POTENTIAL AND SURFACE-TENSION

A simple method is introduced that estimates the surface free energy of pure, polar liquids. For long-chain amphiphiles, the calculation is almost intuitive. In addition to the refractive index spectrum and dielectric permittivites used to calculate the van der Waals component from the Lifshitz theory, knowledge of the partial charge distribution in the dipolar molecule, the molecular conformation, and a model of the molecular packing at the liquid/vapor interface is required. For smaller, highly polar molecules, the molecular packing at the interface is not as easily predicted intuitively so that knowledge of the surface potential jump, chi, is also required. This parameter is not experimentally accessible and the models used presently to calculate it from the surface potential, psi, are inadequate. In response to this need, a new model has been developed which predicts the surface potential jump from experimental data. Finally, the surface free energies of water, methanol, and several other polar liquids are calculated as the sum of a van der Waals contribution arising from the interactions between randomly oriented molecules and an electrostatic, orientational term calculated from the partial charge distribution in the molecules.