For situations in which capillary condensation of water occurs (e.g., within porous media), the thermodynamics can usually be studied by means of the Laplace and Young equations and a compressibility equation. We show here that the compressibility equation for the liquid does not reflect the criteria of equal gas and liquid phase potential changes at equilibrium, for departures from a specified reference state.

Carman (1953) concluded that the capillary condensate can exist in a state of tension for large vapor-liquid pressure differentials and may exhibit physical properties that differ substantially from normal values. The familiar Kelvin equation is used to calculate the capillary curvature and is derived from the general Laplace and Young relationship. Melrose (1966) has obtained this relationship by matching the coefficients of the internal free energy and hydrostatic balances of the system shown in Figure 1.