Onsager's Real Cavity model near solid interfaces

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.