Free thin liquid films (foam films) from rhamnolipids: type of the film and stability

Abstract

For the first time the foam film model has been used to study the interaction behavior of microbial type biosurfactants adsorbed at the air/solution interface. The studies were carried out with microscopic foam films formed from solutions of anionic type rhamnolipid biosurfactants at a constant concentration of 10−4 M. Measurements of the equilibrium film thickness (h) as a function of the electrolyte (NaCl) concentration (Cel) show that with the increase of Cel from 5×10−4 to 1 M, h gradually decreases from about 100 to 5 nm. Formation of common films (h>30 nm), common black films (CBF) with h∼6–20 nm and 5 nm thick Newton black foam films (NBF) was found. The experimentally determined critical electrolyte concentration of CBF–NBF transition (Cel,cr) is 0.8 M NaCl. The direct measurements of disjoining pressure isotherms at different Cel corroborate the type of the common films and demonstrate the existence of an aqueous core in the common and CBF, and the bilayer structure of the NBF. The experimental studies show the role of the Derjaguin–Landau–Verwey–Overbeek (DLVO) surface forces for the stability of the common films. A possible action of additional structural (hydration) and eventual short-range steric forces and their effect for the stability of the thin CBF and NBF are discussed. For thick common films the surface electric parameters—potential of the diffuse electric layer (ϕ0) and surface charge density (σ0) are assessed by the equilibrium film method using the equations of the DLVO theory. The comparison of the obtained ϕ0=71 mV with the previous results obtained for films stabilized with the salt of a fatty acid—Na oleate, demonstrates the role of the carboxylic group of the rhamnolipid molecule for the surface charge of the studied foam films. The investigations can contribute to gaining new knowledge not only for the different type of interactions in thin films stabilized with microbial type biosurfactants, but also for the interactions between two surfaces through a thin film.