About Thermodynamic Theory of Water Retention Capacity and Dispersity of Soils

On the basis of thermodynamic concept on competitive interphase interactions, the idea is forwarded about the ionic-electrostatic mechanism of water retention capacity and aggregative stability of fine solid particles in soil physical systems. The author applies modern data on the nanostructure of colloidal disperse complex of soil physical systems in the gel form, i.e., in the form of noncapillary two-phase system of resistant to aggregation particles separated by solvate layers. Disputable questions are discussed about the role of surface energy and hydration of cations in water retention as well as about the relationship between the matrix (disjoining) and the osmotic water pressures. The fundamental ionic-electrostatic model of water retention capacity is analyzed for the colloidal-disperse bodies in the areas of sorptive and film water on the water retention curve (WRC) in the form of interaction between thermodynamic potential (disjoining pressure), water content, and dispersity (effective specific surface area) of the solid phase. On the basis of this model, the potential dependence is shown between the dispersity and the mobile thermodynamic factors (i.e., temperature, charge, and concentration of ions in the electrical double layer), and the method of estimating the effective specific surface area from the WRC is proposed as an alternative to the commonly adopted BET method. For standard conditions, under which the dispersed system with monomolecular liquid layer maintains its aggregative stability, the specific surface area is calculated according to the WRC slope (the angle coefficient in semi-logarithmic coordinates) and the value of the effective size of water molecules. Simultaneously with the dispersity assessment from the WRC, the energy indices of interphase interactions— the generalized Hamaker constant and the surface tension at the solid–liquid interphase—are determined.