Self-desorption of mixtures of anionic and nonionic surfactants from a silica/water interface

The adsorption of a mixture of two surfactants, the anionic sodium dodecyl sulfate and the nonionic nonylethylene glycol n-dodecyl ether, from water t o a hydrophilic silica dispersion has been studied at pH = 5.0 and 25 degre esC in a large concentration range at several surfactant compositions by a classical depletion method. The adsorption of the latter compound onto sili ca induces the incorporation of the former by the formation of mixed surfac tant aggregates. However, at high total surfactant concentration, about 100 times the mixed critical micelle concentration, both surfactants are parti ally desorbed from the silica surface. This behavior is qualitatively inter preted as the consequence of two phenomena: on one hand, the strong interac tion between the anionic and the nonionic surfactant in the bulk above tile critical micelle concentration and on the other hand the repulsion which o ccurs between the negatively charged dissociated silanol groups and the ani onic surfactant in the mixed aggregates at the silica/water interface. At h igh total surfactant concentration and above a specific anionic/nonionic su rfactant ratio tile former. phenomenon is favored over the latter, hence th e desorption of both surfactants.