Ellipsometry Studies of the Self-Assembly of Nonionic Surfactants at the Silica-Water Interface: Equilibrium Aspects

The nature of layers of a series of poly(ethy1ene glycol) monoalkyl ethers (C,,Em) adsorbed on silica surfaces has been systematically investigated by means of null ellipsometry. The results show that adsorption remains low until a well-defined concentration ((=0.6-0.9)cmc) is exceeded. Then, as surfactants in the interfacial region start to self-absemble, it increases abruptly and plateau adsorption is generally observed prior to the cmc. The normal extension of the interfacial aggregates is relatively constant from intermediate to high surface coverage. Increasing the ethylene oxide to hydrocarbon ratio results in a decreased adsorption. The mean optical thickness, on the other hand, is relatively independent of the number of ethylene oxide groups in the surfactant but almost linearly dependent on the length of the hydrocarbon tail. The values obtained for these parameters suggest that the adsorbed layer is built up of discrete surface aggregates, or micelles, with dimensions resembling those observed in bulk solution. A more refined optical model of the adsorbed layer confirms the notion of surface micelles growing with increasing hydrocarbon content. It also points out that the extension of the surface micelles is slightly larger than the measured mean optical thickness. In addition to studies of neat C S , surfactants, we also examine the adsorption of mixed surfactant systems. Changes observed in adsorption on altering the bulk ratio of two surfactants are well correlated to the bulk micellar surfactant ratio calculated by ideal solution theory.