Abstract
This work reports a new approach for in situ studies of thin adsorbed films (less than 100 Å) on layered substrates by null ellipsometry. The values of the absorbed film properties are obtained by first characterizing the substrate in different ambient media, having different refractive indices. These data are then used to apply the appropriate optical model for interpreting the adsorption data. We show that it is possible by this approach to make rather accurate and time-resolved measurements of the thickness and refractive index of thin adsorbed layers, in addition to the ordinary output data on the adsorbed amount. Besides the pure methodological part of the work, we also investigate the general features of the adsorption of polyethylene glycol monoalkyl ethers (CnEm) at the silica-water interface. The results featuring a series of surfactants with varying compositions confirm the cooperative nature of the adsorption process at the silica-water interface. This involves the formation of interfacial aggregates, with well-defined extensions normal to the surface, at a critical concentration.
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