Self-Desorption of Mixtures of Anionic and Nonionic Surfactants from a Silica/Water Interface

Abstract

The adsorption of a mixture of two surfactants, the anionic sodium dodecyl sulfate and the nonionic nonylethylene glycol n-dodecyl ether, from water to a hydrophilic silica dispersion has been studied at pH = 5.0 and 25 °C in a large concentration range at several surfactant compositions by a classical depletion method. The adsorption of the latter compound onto silica induces the incorporation of the former by the formation of mixed surfactant aggregates. However, at high total surfactant concentration, about 100 times the mixed critical micelle concentration, both surfactants are partially desorbed from the silica surface. This behavior is qualitatively interpreted as the consequence of two phenomena: on one hand, the strong interaction between the anionic and the nonionic surfactant in the bulk above the critical micelle concentration and on the other hand the repulsion which occurs between the negatively charged dissociated silanol groups and the anionic surfactant in the mixed aggregates at the silica/water interface. At high total surfactant concentration and above a specific anionic/nonionic surfactant ratio the former phenomenon is favored over the latter, hence the desorption of both surfactants.