Solvation Forces in Soap Films

Abstract

THE stability of soap films and colloidal dispersions involve the same forces, thus leading to the same type of curves for potential energy plotted against distance. Such curves frequently show two minima¹ (Fig. 1). Stabilization in the secondary minimum at relatively large inter-particle separation distances (10²–10³ Å) is controlled by electrostatic double layer repulsion forces and this region of the curve has been investigated in some detail² for aqueous soap films in air. Stabilization in the primary minimum at small separation distances (10–20 Å) has been considered to be governed by a short range repulsive force as a result of interaction between solvation layers a few molecules thick which are associated with the two flanking surfactant monolayers³. Any closer approach of the two interfaces entails desorption which manifests itself as a rapidly increasing repulsive force as successive solvation layers are removed⁴. Because of the lack of specific data, this short range repulsive force is conventionally represented in the potential energy diagram by a very steeply rising curve at a certain critical separation distance¹, implying that primary minimum, or Perrin, films must be subjected to very large pressure differences before they can be further thinned. One method for investigating this region of the potential energy curve is to measure film thickness as a function of relative water vapour pressure.