Characterization of Langmuir monolayers of cosmetics surfactants

Abstract

The objective of this research was to evaluate the Langmuir and Langmuir-Blodgett (LB) monomolecular films of surfactants studied in the literature and used in the cosmetics industry: potassium cetyl phosphate, sodium lauryl sulfate (SDS) and sodium dioctyl sulfosuccinate (DSS). Surface pressure versus molar area isotherms were studied under various conditions. The films at the air-water interface were investigated for possible transitions of two-dimensional phases. The study of isotherms showed that, at lower temperatures, 4ºC for example, the films become more rigid and, consequently, mechanically more stable. For the temperatures of 4 ºC, 25 ºC, 45ºC and 55ºC, the isotherm profile obtained implies that the transition between expanded liquid and condensed liquid are second-order. At higher concentrations, the film is more condensed over larger molar areas as seen in 1%. The surface pressure versus molar area isotherm at pH 7 showed the condensed gas-liquid transition. However, the transition was observed for extremely low surface pressure showing that the film has, qualitatively, a more expanded behavior. It was demonstrated that the thermodynamic model based on the PC-SAFT state equation can describe the phase transitions with good correlation of the experimental data.