MODELING EFFECTS OF PH AND COUNTERIONS ON SURFACTANT ADSORPTION AT THE OXIDE WATER INTERFACE/

A model is presented that describes the effect of pH on the adsorption of an isomerically pure anionic surfactant species at a mineral oxide /water interface. A site-binding model, to account for effects of pH, surface heterogeneities, and counterions, is incorporated into a patch wise, phase-separation modeling approach, making it possible to predic t both the surface charge and the counterion association beneath an ad sorbed surfactant aggregate. Parameters for the site binding model on or-alumina are obtained from experimental surface charge measurements. The formation of both local monolayers (hemimicelles) and bilayers (a dmicelles) is allowed, although the isotherms studied in this paper ar e fit by parameter values that predict admicelle formation only, The m odel is able to predict experimental measurements of the adsorption of an isomerically pure, anionic surfactant species on alpha-alumina as a function of pH. It reproduces several previously unexplained experim ental observations; in particular, it offers an explanation for the ob servation of significant adsorption of anionic surfactant above the po int of zero charge (pzc) of a mineral oxide surface.