The aim of this study was to investigate the effect of the state of the silica surface and of the surfactant molecular structure on the adsorption of cationic surfactants onto silica. Thus, the adsorption of DTAB (dodecyltrimethylammonium bromide) and of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and on HCl-washed silica (SiH) has been investigated under "free" system conditions. The amount of surfactant adsorbed (adsorption isotherm), the sodium ion and bromide ion concentrations and the pH in the equilibrated supernatant, and the silica particle electrophoretic mobility have been measured along the isotherms. The adsorption mechanisms of the two surfactants on the raw and washed silica are qualitatively similar. Nevertheless, important quantitative differences are observed which are all due to (i) the larger number of surface sites present at the surface of SiNa with respect to SiH and (ii) the larger ionic strength of the supernatant in SiNa/surfactant systems with respect to SiH/surfactant systems, due to the much larger amount of sodium ions released by SiNa upon surfactant binding. Thus, the amounts of surfactant adsorbed at the point of zero charge and at saturation of the silica particles, of sodium ions released by the surface and the decrease of critical micelle concentration (cmc) in the supernatant with respect to pure water are all larger for the raw silica than for the treated silica. For the four silica/surfactant systems investigated, the first adsorption step corresponds to the adsorption of individual surfactant ions on the negative sites of the silica surface. It is driven by electrostatic interactions and strongly dependent on the number of surface sites and ionic strength associated to the released ions. At the end of the first adsorption step, which is clearly seen with SiH/surfactant systems, the second adsorption step starts. This step is driven by hydrophobic interaction between surfactant alkyl chains and results in the formation of surface aggregates. The surfactant adsorption on the surface is shown to continue even after the cmc in the supernatant is reached. Copyright 1999 Academic Press.