ELLIPSOMETRY STUDIES OF THE SELF-ASSEMBLY OF NONIONIC SURFACTANTS AT THE SILICA-WATER INTERFACE - KINETIC ASPECTS

We have investigated the adsorption and desorption kinetics of polyethylene glycol alkyl ethers at hydrophilic silica surfaces by in situ ellipsometry. The adsorbed amount and the mean optical thickness are by this technique monitored independently with time. Five different kinetic regimes, each displaying a characteristic time dependence of the surface excess, are identified during a typical adsorption-desorption cycle. A strong coupling is observed between the kinetics in these regimes and the self-assembly properties of the surfactants at the interface as well as in the solution. A kinetic model that incorporates these phenomena is developed to explain and predict the rates of mass transfer during adsorption and desorption. It is shown that adsorption is controlled by the diffusion of monomers and micelles, if present, from solution to the interface through a stagnant layer. The concentration of surfactant monomers in the vicinity of the adsorbed layer is determined by a local equilibrium between adsorbed surfactant aggregates and surrounding monomers. We can from adsorption and desorption data calculate monomer and micellar diffusion coefficients as well as critical surface aggregation concentrations for the surfactants studied. Moreover, from desorption data in the exponential regime, observed toward the end of the desorption process, the dissociation rate constants and the half-lifes of the adsorbed aggregates (micelles) are estimated. Finally, it is shown that the adsorbed layer thickness is constant during the major part of the adsorption-desorption cycle, indicating that surface aggregates (micelles) with well-defined thicknesses normal to the surface are present throughout most of these processes.