The relation between stability of asymmetric films of the liquid/liquid/gas type, spreading coefficient and surface pressure

Abstract

We consider the possibility of using spreading coefficients S and entry coefficients E as criteria for the stability of an asymmetric film. Dependencies of the spreading coefficients on the surface and interface tensions and as a function of adsorption and pressure of saturated vapour are measured. It is shown that the entry coefficient corresponds to the surface energy change not at the drop entry from the solution to the interface surface but at the plane parallel asymmetric film breaking. Both coefficients E and S do not consider the existence of a disjoining pressure in the thin film. The typical disjoining pressure isotherms and the dependencies of the surface tension on the inverse adsorption (1/ Γ) for the asymmetric films are analysed. In the generalised case the spreading coefficient is neither a sufficient nor a necessary criterion for stable/unstable asymmetric films. The so-called generalised spreading coefficient is less informative than the Π( h) isotherm. The use of the spreading and entry coefficients is discussed in respect to foam breaking in heterogeneous systems containing antifoam drops as a criterion for asymmetric film stability and as a condition for effective foam breaking.