Contact angles in thin liquid films : III. Interaction forces in Newton black soap films

The interaction parameters of Newton black soap films stabilized by NaDS, as derived from contact angle experiments, have been interpretated in terms of the structure and the interaction forces in the films. From the film thickness and the difference between the surface excess of the salt in the film and at the bulk surface it is concluded that (a) the diffuse double-layer overlap in the film is practically complete; (b) the film only contains absorbed DS⁻ ions and an equal amount of Na⁺ counterions, but no salt; and (c) the double layer at the bulk surface is still partly diffuse. A model for the structure of the NB films is proposed according to which the adsorbed DS⁻ ions with their counterions form a two-dimensional square lattice at each film surface. It is found that the interaction free energy of the NB films can be explained by taking into account the electrostatic interactions between the discrete ions in the two opposing surface lattices. The model of the NB film is qualitatively in agreement with the experimental results of other workers.