Thin wetting films from aqueous solutions of surfactants and phospholipid dispersions

Abstract

The microscopic thin wetting film method was used to study the stability of wetting films from aqueous solution of surfactants and phospholipid dispersions on a solid surface. In the case of tetradecyltrimethylammonium bromide (C 14TAB) films the experimental data for the receding contact angle, film lifetime, surface potential at the vapor/solution and solution/silica interface were used to analyze the stability of the studied films. It is shown that with increasing C 14TAB concentration charge reversal occurs at both (vapor/solution and solution/silica) interfaces, which affects the thin-film stability. The spontaneous rupture of the thin aqueous film was interpreted in terms of the earlier proposed heterocoagulation mechanism. The presence of the mixed cationic/anionic surfactants was found to lower contact angles and suppresses the thin aqueous film rupture, thus inducing longer film lifetime, as compared to the pure amine system. In the case of mixed surfactants hetero-coagulation could arise through the formation of ionic surfactant complexes. The influence of the melting phase-transition temperature T c of the dimyristoylphosphatiddylcholine (DMPC) on the stability of thin films from dispersions of DMPC small unilamellar vesicles on a silica surface was studied by measuring the film lifetime and the TPC expansion rate. The stability of thin wetting films formed from dispersions of DMPC small unilamellar vesicles was investigated by the microinterferometric method. The formation of wetting films from diluted dispersions of DMPC multilamellar vesicles was studied in the temperature range 25–32 °C. The stability of thin film of lipid vesicles was explained on the basis of hydrophobic interactions. The results obtained show that the stability of wetting films from aqueous solutions of single cationic and mixed cationic–anionic surfactants has electrostatic origin, whereas the stability of the phospholipid film is due to hydrophobic interaction.