Physical characterization of non-ionic surfactant layers adsorbed at hydrophilic and hydrophobic solid surfaces by time-resolved ellipsometry

Non-ionic surfactant adsorption at hydrophobic and hydrophilic surfaces 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 thickness and refractive index, giving information about the concomitant evolution of the molecular organisation within the adsorbed layers. It is shown that, whereas the surfactants are adsorbed as sub-monolayers or monolayers at hydrophobic surfaces, they form surface micelles or bilayer-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 transport of surfactants through a stagnant layer, coupled with the idea of a local equilibrium existing between the adsorbed layer and the solution 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 hydrophilic as well as hydrophobic silica.