Capillary Condensation

the condensation of vapor in the capillaries and microcracks of porous bodies or in the interstices between closely packed particles.

A necessary condition for capillary condensation is the wetting of the surface of the material (particles) by the liquid. Capillary condensation starts with the adsorption of vapor molecules by the condensation surface and the formation of liquid menisci. The pressure of the saturated vapor over concave menisci is, according to Kelvin’s equation, lower than the saturated vapor pressure p₀ over a flat surface. Capillary condensation proceeds, therefore, at lower vapor pressures than the saturation pressure P₀. The volume of liquid condensed in pores attains its limiting value at external vapor pressure p = p₀, where the liquid-gas interface has zero curvature (plane, catenoid).

The complex capillary structure of a porous body may be the cause of capillary hysteresis, that is, the dependence of the quantity of liquid condensed in the pores not only on vapor pressure but also on the prehistory of the process, that is, how the given state was attained—by the condensation or by the evaporation of the liquid.

Capillary condensation increases the absorption (sorption) of vapors by porous bodies, particularly near the point of vapor saturation. The phenomenon of capillary condensation is used industrially in trapping liquids by finely porous materials (sorbents). Capillary condensation also has an important role in the processes of drying and moisture retention in soils, construction materials, and other porous materials.