A THEORY OF AQUEOUS EMULSIONS .2. MECHANISM OF FILM CURVATURE AT OIL/WATER INTERFACE

The scope of previous theories has been broadened to account for curvature of monolayers of finely divided powders as well as of molecularly dispersed species whether stabilizing macro- or microemulsions. It is submitted that when work is done on these systems, the stress provided by different tensions at the film/water and film/oil boundaries causes the adsorbed tenants to occupy different areas at each side of of the film, the difference in these areas depending upon the amount of work expended. Without this stress there can be no curvature. In monomolecular films, the film/water and film/oil surfaces are each characterized by their own individual pressure-area curves. These suggest that only the flat film is duplex in nature and that curvature dissipates the initial pressure gradient. The direction of curvature thus depends upon the relative magnitudes of the original pressures and the degree of curvature depends upon σe σi at the external and internal sides of the curved film. Whether the system is macro or micro depends upon the magnitude of the pressure gradient. It is this which determines the different initial tensions. In powder films, different tensions at the solid/water and solid/oil boundaries give rise to an equilibrium contact angle θ. When the powders are spheres, the ratio of the area of solid in contact with each liquid is (1 + cosθ)/(1 - cos θ). Here the direction of curvature depends upon whether cos θ is positive or negative and the degree of curvature upon the magnitude of cos θ. Droplet size in both cases is determined by the degree of curvature of the film and its thickness. © 1969.