THEORETICAL-STUDY OF VAPOR-PRESSURE OF PURE LIQUIDS IN POROUS-MEDIA

There are reports in the literature describing vapor pressure reductio n ill 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 fo r pores of radii less than 0.1 mu m. In fact, substantial vapor pressu re reduction has been observed in larger pores. This is explained here on the basis of the effect of long-range surface forces exerted by th e solid on the liquid, while the application of the Kelvin equation to capillary-held liquids tacitly assumes the solid to exert no influenc e. A generalized Clausius-Clapeyron equation in the presence of extern al force fields is first obtained. The resulting excess enthalpy of va porization is next semi-quantitatively related to the molar polarizati on of the liquid and the porous solid based on a molecular approach. ( C) 1998 Elsevier Science B.V. All rights reserved.