Onsager's Real Cavity model near solid interfaces

TL;DR

This paper extends Onsager's real cavity model to accurately describe Casimir-Polder interactions of molecules near interfaces, accounting for local-field effects and cavity geometry, with implications for dispersion forces in liquids.

Contribution

The authors develop a closed-form analytical extension of Onsager's model for molecule-surface interactions, incorporating cavity geometry and dielectric properties.

Findings

Interaction magnitude depends on local-field screening, molecular polarizability, and liquid permittivity.

Transition from open to closed cavity regimes affects the interaction profile.

Limitations include point-dipole approximation and neglect of repulsive forces.

Abstract

We develop an extended Onsager real-cavity framework to describe the Casimir-Polder interaction of small molecules dissolved in dielectric liquids near planar interfaces. By analytically resolving the geometry of the cavity opening, we derive a closed expression that arises when the molecule approaches a surface and connects them smoothly to the asymptotic medium-assisted interaction. Using experimentally established dielectric functions for water, propanol, and PTFE together with accurate molecular polarisabilities for O2 and N2, we compute the full distance-dependent potential for four molecule (O2 and N2)-liquid (water and propanol)-surface (PTFE) combinations. The results reveal how local-field screening inside the cavity, molecular polarisability, and liquid permittivity jointly determine the magnitude and shape of the interaction, including the characteristic transition from the open cavity (small separations) and closed cavity (large separations). The framework provides a transparent baseline for dispersion forces in liquids, while highlighting limitations associated with the point-dipole description, the absence of repulsive contributions, and the breakdown of the dipole approximation at ultrashort separations.