Generalised line tension theory revisited

A generalised thermodynamic theory of the line tension in thin liquid films and layers [1,2] is reviewed in the light of numerical computation of line tension values and sign in function of the shape of the disjoining pressure isotherm and physico-chemical parameters. The line tension depends on the parameters characterising the shape of the disjoining pressure isotherm II(H) in the liquid films, as well as on the contact angle, capillary pressure P-c and interaction free energy (the grand thermodynamic potential) Delta Omega H-f in the film of thickness H-f The most significant effect on the line tension and its sign is produced by the shape of the II(H)-isotherm rather than by the absolute value of the interaction free energy Delta Omega H-f i.e. by the contact angle theta(f) II(H)-isotherms with only one coagulation minimum or one coagulation barrier lead to line tensions with negative or positive signs, respectively. It is a general rule that the sign of the line tension coincides with the sign of the interaction free energy Delta Omega H-f, for these simple shape II(H)-isotherms. As soon as either a coagulation barrier is added to the II(H)-isotherms with a near coagulation minimum, or a coagulation minimum appears besides the coagulation barrier, the sign of the line tension tends to change. The values and signs of both, the interaction free energy Delta Omega H-f and the line tension tau, characterise the shape of the II(H)-isotherm. This justifies the efforts directed towards an experimental determination of the line tension, since a knowledge of value and sign of this parameter should provide new information on the shape of the disjoining pressure isotherm. (C) 1999 Elsevier Science B.V. All rights reserved.