Bursting of soap films. Part 4.—The behaviour of ions on a crowded surface

Abstract

The growing hole of a bursting soap film is preceded by an aureole of accelerating, contracting, and thickening film. Measurements of the cross-sectional profile of such aureoles are reported and provide the basis for an interpretation in terms of surface tension changes accompanying the contraction of the film which is complete in less than a millisecond. The surface tension of sodium dodecyl sulphate solution decreases to below 15 mN/m (dyn/cm) under these conditions. As desorption is negligible, the data can be interpreted in terms of an extension of surface pressure-area per molecule curves towards high pressures and low areas. This extension shows an unexpected decrease of slope at low areas and indicates a rapid increase in intermolecular attractions as the surfactant ions become crowded on the surface.