Entry and spreading of alkane drops at the air/surfactant solution interface in relation to foam and soap film stability

Abstract

For droplets of hydrocarbon oil dispersed in surfactant solution to be effective in reducing foam stability it is necessary for the droplets to be capble of entering the air/solution interfaces of the lamellae within the foam. If entry is possible, then soap film rupture (hence foam destruction) can, in principle, occur by one of several mechanisms. These include macroscopic or molecular spreading along lamellae surfaces (potentially resulting in film thinning by a Marangoni process), and ‘bridging’ in which the oil drop enters both surfaces of a lamella producing an unstable bridge across the film. In the present work we have focused on the entry of drops into surfaces by obtaining (a) entry coefficients (from appropriate surface and interfacial tensions) as a measure of the feasibility of drop entry and (b) half-lives of single drops placed beneath the air/solution interface to assess the rate of drop entry. The findings are used to gain an insight into the way in which dispersed oil drops reduce both foam and single film stability. We have used a homologous series of alkanes as oils, chosen such that some homologues can enter the surface of the surfactant solution (3.8 mmol l^–1 AOT in 0.03 mol l^–1 NaCl) and some cannot. It is found that (fortuitously) all those oils which are capable of entry also spread as multi-molecular films, and reduce foam and film stability. It is believed that the observed effects of alkane chain length on single film stability result from the variation of single drop half-life with chain length rather than with any variation of spreading coefficients. Oils which are unable to enter the air/solution interface are found to enhance foam and film stability, possibly by a reduction in the rate of film thinning resulting from the pressence of oil droplets in the Plateau border regions.