THERMODYNAMIC (INCLUDING MICROCALORIMETRY) STUDY OF THE ADSORPTION OF NONIONIC AND ANIONIC SURFACTANTS ONTO SILICA, KAOLIN, AND ALUMINA

Adsorption from aqueous solutions (of three alkylbenzene polyoxyethylenic surfactants and a sodium octylbenzene sulfonate) is followed as a function of chain length, temperature, and salinity on various samples of silica (4 to 200 m2g-1), α-alumina (9 m2g-1), and kaolin (20 m2g-1). A close examination of the adsorption isotherms and of the corresponding differential enthalpics of displacement supports the view that micelle-like surface aggregates of limited size are eventually formed when the density or energy of the adsorbing sites is low (case here of the nonionic surfactants but also of the anionic one at a pH where the surface charge is low). In the other case, two-dimensional hemimicelles may form even at low coverage, whereas bilayers are possible at saturation. © 1991.