Disjoining Pressure

a thermodynamic parameter that characterizes the state of a thin layer, or film, of liquid or gas in the gap between the surfaces of bodies. When the system is in equilibrium, the disjoining pressure is P = P₂ – P₁, where P₂ is the normal pressure on the film exerted by the bodies, and P₁, is the pressure in the volume of liquid or gas from which the film was formed (Figure 1). If the disjoining pressure has a positive value (P > 0), then the film is stable; if it has a negative value (P < 0), then the film spontaneously thins until it ruptures.

Figure 1

Disjoining pressure was first detected by the Soviet scientists B. V. Deriagin and E. V. Obukhov in 1934. It arises when two surface layers mutually overlap and is due to the total effect of forces that are different by nature. Thus, electrostatic forces, the forces of “elastic” resistance of solvated, or adsorbed solvated, films, and the forces of molecular interaction can all act as components of the disjoining pressure. The pressure depends on the thickness of the film, the composition and properties of the interacting phases (bodies), and the temperature. The study of disjoining pressure served as the basis for the Deriagin-Landau-Verwey-Overbeck (DLVO) theory of the stability of hydrophobic colloids and explains many surface phenomena. Overcoming a positive disjoining pressure, which prevents the thinning of the film under external forces, leads to adhesion or fusion as the bodies come into contact. In the case of colloidal systems, this means the coagulation or coalescence of particles of the dispersed phase. The disjoining pressure has a decisive influence on the effectiveness of such important practical processes as the swelling and peptization of clayey minerals, the stabilization of foams, and flotation, impregnation, and gluing.