Theoretical study of vapor pressure of pure liquids in porous media

There are reports in the literature describing vapor pressure reduction in capillaries or porous materials that is substantially in excess of that predicted by the classical Kelvin equation, which relates the vapor pressure over the curved vapor–liquid interface inside a pore to its radius of curvature. However, this effect is negligible except for pores of radii less than 0.1 μm. In fact, substantial vapor pressure reduction has been observed in larger pores. This is explained here on the basis of the effect of long-range surface forces exerted by the solid on the liquid, while the application of the Kelvin equation to capillary-held liquids tacitly assumes the solid to exert no influence. A generalized Clausius–Clapeyron equation in the presence of external force fields is first obtained. The resulting excess enthalpy of vaporization is next semi-quantitatively related to the molar polarization of the liquid and the porous solid based on a molecular approach.