PHYSICAL CHARACTERIZATION OF NONIONIC SURFACTANT LAYERS ADSORBED AT HYDROPHILIC AND HYDROPHOBIC SOLID-SURFACES BY TIME-RESOLVED ELLIPSOMETRY

Non-ionic surfactant adsorption at hydrophobic and hydrophilic surface s has been studied by time-resolved ellipsometry. Adsorption isotherms obtained for polyethylene glycol monoalkyl ethers (C(n)E(m)) at these surfaces are presented together with data on the adsorbed layer thick ness and refractive index, giving information about the concomitant ev olution of the molecular organisation within the adsorbed layers. It i s shown that, whereas the surfactants are adsorbed as sub-monolayers o r monolayers at hydrophobic surfaces, they form surface micelles or bi layer-type aggregates at hydrophilic silica. The adsorption/desorption kinetics of the surfactants at the different surfaces have also been investigated. A kinetic model based on the assumption of diffusive tra nsport of surfactants through a stagnant layer, coupled with the idea of a local equilibrium existing between the adsorbed layer and the sol ution in its immediate vicinity, is used to interpret the experimental results. This was found to reproduce quantitatively the main features of the time dependence observed during adsorption and desorption at h ydrophilic as well as hydrophobic silica.