Investigations in the field of the thermodynamics of capillary condensation

Abstract

1. The capillary condensation of benzene vapors on wide-pored and fine-pored silica gels within the temperature interval ∼ −50 to +50° was investigated by the method of direct measurement of the adsorption isosteres. 2. A thermodynamic analysis was made of the phenomena of capillary condensation. The thermodynamic criteria of the Kelvin equation are the negative values of the differential entropy and thermal coefficient of the logarithm of maximum work in the case of a constant filled volume. Equations were derived permitting the calculation of the differential heats and entropies of capillary condensation as a function of the filling according to one adsorption (desorption) isotherm, well confirmed experimentally. 3. The experimental data are in full agreement with the thermodynamic criteria obtained. The Kelvin equation in the system studied begins to be fulfilled at radii of the meniscus greater than ∼ 16 A. 4. It was shown that at the end of the hysteresis loop, the difference of the differential isosteric heats of desorption and adsorption is always greater than zero, while at the beginning of the hysteresis, this difference may be either positive or negative, depending on the shape of the hysteresis loop. These conclusions are confirmed experimentally. 5. The beginning of the sharp rise in the differential heats of capillary condensation at various values of the adsorption occurs at various temperatures, but at the same degrees of volume filling; hence, the corresponding “breaks” on the various isosteres lie on one isochore. 6. The capillary condensation isochores in plots of log p versus T−1 are expressed with good approximation by a bundle of straight lines, intersecting at one point when T=Tcrit.