Surface forces and properties of foam films from rhamnolipid biosurfactants

Foam films are considered as a convenient model to study the interaction behaviour and surface properties of microbial rhamnolipid type biosurfactants. The Scheludko–Exerowa microinterferometric methodology of film thickness measurements is employed for experimental studies of microscopic foam films formed from aqueous solutions of a single rhamnolipid Rh1 (with one rhamnosyl head group) and of mixtures of rhamnolipid surfactants Rh1 and Rh2 (with two rhamnosyl head groups) at ratios Rh2/Rh1 = 1.2 and Rh2/Rh1 = 0.69. The measurements of the equilibrium thickness (h) of the obtained films as a function of surfactant concentration (Cs) and electrolyte (NaCl) concentration (Cel) determine the conditions for obtaining common, common black and Newton black films. The saturation values of the diffuse electric layer potential φ0 ∼ 60 mV for the Rh1.2 and φ0 ∼ 94 mV for the Rh0.69 common films conform the ionic character of the rhamnolipids. The h(Cel) curves of the rhamnolipid foam films and the directly measured disjoining pressure (Π(h)) isotherms indicate the ranges of action of the DLVO and non-DLVO surface forces. The obtained foam film parameters allow their practical use in ecology and in various technological processes where rhamnolipid surfactants are used. Experiments with model lung surfactant (Infasurf) foam films with rhamnolipid added outline a perspective for the potential application of the foam film for investigating the effect of rhamnolipids on human alveoli.