INTERFACIAL BEHAVIOR OF NONIONIC SURFACTANTS AT THE SILICA WATER INTERFACE REVEALED BY ELLIPSOMETRY

This work reports a new approach for in situ studies of thin adsorbed films (less than 100 angstrom) on layered substrates by null ellipsome try. The values of the adsorbed film properties are obtained by first characterizing the substrate in different ambient media, having differ ent refractive indices. These data are then used to apply the appropri ate optical model for interpreting the adsorption data. We show that i t is possible by this approach to make rather accurate and time-resolv ed measurements of the thickness and refractive index of thin adsorbed layers, in addition to the ordinary output data on the adsorbed amoun t. Besides the pure methodological part of the work, we also investiga te the general features of the adsorption of polyethylene glycol monoa lkyl ethers (C(n)E(m)) at the silica-water interface. The results feat uring a series of surfactants with varying compositions confirm the co operative nature of the adsorption process at the silica-water interfa ce. This involves the formation of interfacial aggregates, with well-d efined extensions normal to the surface, at a critical concentration.