Lateral intermolecular forces in the physisorbed state: Surface field polarization of benzene and n-hexane at the water/and mercury/vapor interfaces

The available experimental data on the dependence of the surface tensions of water and mercury on the adsorption of benzene and hexane from the vapor phase are critically analyzed and interpreted to obtain the two-dimensional second virial coefficients [B ₂(T)] for these adsorbed nonpolar molecules. Calculations based on the unperturbed Lennard-Jones (L-J) 12-6 formalism for benzene and the related 12-5 Salem formalism for long chains in two dimensions for hexane require that B ₂(T) for both molecules is less negative for both adsorbates. On water, the experimental data indicate that B ₂(T) for both molecules is less negative than expected from the unperturbed L-J and Salem estimates, and on mercury the B ₂(T) values from experiment are positive. These findings are analyzed first in terms of a possible reduction in the attractive component of the potential of mean force between physisorbed molecules arising from their frequency-dependent interaction with their electrostatic images in the bulk phases, as described by McLachlan. It is concluded that the McLachlan corrections are small for these molecules and surfaces. A second analysis considers the effect of an extra repulsion between the adsorbed molecules arising from the induction of dipoles normal to the interface by the surface electric field. Surface field polarization (SFP) accounts reasonably well for the experimental results, leading to estimates of the surface fields at the mercury and water surfaces which are consistent with estimates from contact potentials for mercury and computation from modeling the water surface. SFP may have a wide impact in determining the form of physisorption isotherms.