Kinetics of Nonionic Surfactant Adsorption and Desorption at the Silica−Water Interface: Binary Systems

This paper is the second of two dealing with the adsorption and desorption kinetics of nonionic surfactants at a solid−liquid interface. The first paper described a model of the kinetics of single nonionic surfactant adsorption. 1 This work is now being completed by extending the theoretical model to cover binary surfactant systems. The evolution of the total surface excess during the adsorption and desorption has been modeled and compared with experimental results obtained by in situ null ellipsometry. In this comparison, the surface behavior of the two nonionic surfactant pairs C 14 E 6 −C 10 E 6 and C 12 E 5 −C 12 E 8 at a planar silica−water interface was studied. These binary systems represent two different types of polydispersity: different lengths of the hydrocarbon chains and unequal numbers of ethylene oxide groups in the hydrophilic headgroups. The critical micelle concentrations (cmcs) of the surfactants in the former pair therefore differ a great deal, whereas those of the surfactants in the latter pair are similar. A comparison between experiments and simulations showed good agreement. In an attempt to further analyze the experimental results, individual amounts adsorbed and concentration profiles were calculated. The results of these simulations showed that each surfactant in a given pair has a characteristic adsorption and desorption path. According to the model, this path is determined mainly by the mutual relationship between their cmcs.