Elasticity of the Solution Surface

Surfactants on an interface. It is well-known [480] that pure liquids cannot form stable foams. Liquids gain the capacity to form a foam if they contain dissolved or mixed surfactants. 

Specific adsorption and surface pressure. If we introduce the notions of specific adsorption T (the mass of surfactants per unit area of the surface or interface) and of the saturation adsorption Too (specific adsorption of surfactants for the limit filling of the adsorption layer), then it is suggested to consider the collection of surfactant molecules in the adsorption layer as a peculiar two-dimensional gas governed by the Van Laar equation [45, 240, 250] 

Here Act is the surface pressure, cto is the surface tension of the pure solvent, a is the surface tension of the solution, R is the gas constant, and T is the absolute temperature. 

Elastic properties of interface. The surface tension of the solution interface is less than the surface tension of the pure solvent interface. The difference is equal to the surface pressure of surfactant molecules [9, 109, 414], This does not contradict the fact that the films forming the skeleton of the foam possess increased strength and elasticity. The equilibrium surface layer of a pure liquid is ideally inelastic. Under the action of external forces, the free surface increases not because of extension (an increase in the distance between the molecules in the near-surface layer) but because new molecules are coming from the bulk. A decrease in the equilibrium tension as some amount of surfactant is added does not mean that the elasticity of the surface decreases, since this surface does not possess elastic properties under slow external actions. Nevertheless, we point out that even surfaces of pure liquids possess elastic properties [465] (dynamic surface tension [232]) under very rapid external actions whose characteristic time is less than the time of self-adsorption relaxation of the surface layer. This property must not depend on the existence of an adsorption layer of surfactant. At the same time, surfactants impart additional elastic properties to the surface both at low and high strain rates. 

The first term on the right-hand side determines the surface (equilibrium or dynamic) tension a, and the second term is the modulus of elasticity of the adsorption layer 

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