AGGREGATIVE ADSORPTION OF NONIONIC SURFACTANTS ONTO HYDROPHILIC SOLID WATER INTERFACE - RELATION WITH BULK MICELLIZATION

The adsorption isotherms of nonionic surfactants such as polyethylene glycol alkyl ether or alkylphenol ether onto a hydrophilic surface have a typical sigmoidal shape for short or medium polar chain lengths. The rising part of the isotherm is always located below but close to the critical micellar concentration (cmc). Equilibrium as well as kinetic data show that the adsorption layer is built from isolated molecules coming from aqueous solution and not by direct adsorption of micelles. The plateau in the adsorption isotherms stems from stabilization of the surfactant chemical potential slightly above the cmc. Several experimental arguments support the existence of extended but finite molecular aggregates at the solid/water interface below the cmc. Furthermore the process leading to this aggregative adsorption appears to have some similarities with bulk micellization taking place above the cmc. A 2-fold theoretical analysis of the aggregative adsorption is proposed. First a thermodynamic model of the micellization is developed and its predictions are compared with available experimental results. Second, the thermodynamic properties of an aggregative adsorption layer in equilibrium with the bulk solution are discussed, then by use of a grand partition function, theoretical isotherms are computed and compared with experiments.