CHARACTERIZATION OF THIN NONIONIC SURFACTANT FILMS AT THE SILICA WATER INTERFACE BY MEANS OF ELLIPSOMETRY

This paper describes a new approach for in situ characterization of thin films adsorbed on multilayer substrates by means of null ellipsometry. The values of the adsorbed film properties are obtained by first measuring the substrate in different ambient media, having different refractive indexes, and then using these data to apply the appropriate optical model for fitting the adsorption data. The investigation is focused on the adsorption properties of penta- and hexaethylene glycol monododecyl ether, C-12E5 and C-12E6, on silica wafers, but the methodology can be used for adsorption studies in more general terms. Results obtained in this study on the thickness of adsorbed surfactant films, as well as on adsorbed amounts, are in good agreement with those obtained by other techniques. Moreover, since the measurements are rapid, ellipsometry also provides us with crucial information on the evolution of these properties with time. Thus, we show that ellipsometry can be used for time-resolved studies of thicknesses of adsorbed films that are very thin (<50 angstrom) and have low optical contrast with respect to the ambient media.