Confocal micro-Raman spectroscopy of black soap films

Abstract

Black soap films from aqueous solutions of sodium dodecyl sulphate are studied by micro-Raman confocal spectroscopy. At the end of the draining process films of different thicknesses are obtained depending on the experimental conditions: Working in a closed humidified chamber leads to common black films while, under evaporation or in the presence of electrolyte, Newton black films are observed. From the Raman spectra of these films, quantitative information is deduced about the conformational and lateral order of the aliphatic surfactant chains, as well as the thickness of the residual water layer. More accurate measurements of the thickness of these ultimate films have been carried out by transmission ellipsometry and their effective refractive index measured by Brewster angle reflectivity. The thinner films present higher molecular organization and their aqueous core exhibits unusual spectral features.