DLVO AND NON-DLVO FORCES IN THIN LIQUID FILMS FROM RHAMNOLIPIDS

Microscopic foam films (r = 100 μm) stabilized with a single rhamnolipid with a well-known structure (α-L-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (R1)) are investigated, and the obtained results are compared with results obtained from studies of foam films formed from solutions of rhamnolipid mixtures. The studies are carried out employing the Scheludko-Exerowa microinterferometric method. The dependence of foam film thickness on the electrolyte concentration (C el ) of the solution is monitored, and formation of common films (CF), common black films (CBF) and Newton black films (NBF) is found. The continuous CBF-to-NBF transition is considered as evidence of the action of repulsive forces that are not described by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloid stability. These non-DLVO repulsive forces lead to an additional positive component of the disjoining pressure. To understand better the surface forces operating in the rhamnolipid foam films, direct measurements of the disjoining pressure/film thickness (Π(h)) isotherms are carried out employing the thin liquid film–pressure balance technique. The comparison of the obtained experimental Π(h) isotherm for CF (C el = 10−3 mol dm−3 NaCl) to the DLVO theoretical predictions yields a diffuse electric layer potential of ∼ 5 mV and surface charge density of ∼ 50 mC m−2 at the film solution–air interfaces. The deviation of the experimental curve from the theoretical one found for films thinner than about 40 nm evidences the action of non-DLVO surface forces. The experimental steplike Π(h) isotherms obtained for the CBF (C el = 0.15 mol dm−3 NaCl) are considered to result from an aggregation process, leading to the formation of lamellar structures in the foam film. The obtained results show that the surface forces operative in rhamnolipid foam films are determined not only by the structure and organization of the adsorbed layers but also by the molecular state of the bulk solution.