Thermodynamics

In this chapter we examine the thermodynamics of the liquid-gas systems in porous media. The treatment centers around the examination of two effects, namely, the effect of the liquid-gas interfacial tension and curvature (when the radius of curvature becomes very small), and the effect of the solid-fluid interfacial forces which results in a significant surface adsorption (when specific interfacial area becomes very large for small pores). First the classical treatments, which are centered around the effect of the meniscuscurvature on the thermodynamic state, are discussed. Both single- and multicomponent systems are considered. Then we examine the thermodynamics of thin liquid-film extensions of perfectly wetting liquids and the role of the van der Waals forces on the equilibrium state. Next, the capillary condensation (adsorption) and evaporation (desorption) in small pores is discussed. The hysteresis in the adsorption isotherm, as well as the other features of adsorption and desorption are discussed. The curvature arguments made for the existence of the hysteresis are discussed before the introduction of the more modern descriptions based on the molecular interaction theories. Then two of these modern theories and their predictions of the phase change and the stability of the thin liquid films in small pores are discussed. Some thermodynamic aspects of solid-liquid phase change are discussed in Section 12.6.