Thermodynamics of adsorption of surfactant mixtures on minerals

Abstract The adsorption of sodium dodecylsulfate and a polyethoxylated nonylphenol and well-defined mixtures thereof was measured on gamma alumina. The thermodynamics of mixing upon formation of the surface aggregates (admicelles) was studied as well as that associated with mixed micelle formation for the system. Regular mixing theory was obeyed upon formation of mixed micelles, but significant deviations from the predicted behavior were observed upon formation of mixed admicelles. This result is not surprising since it has been shown that the physical assumptions necessary to derive the regular mixing relations are not valid even for mixed micelle formation. It is concluded that whereas it is coincidental that regular mixing theory describes the formation of mixed micelles for this sytem, regular mixing theory is completely unable to describe the formation of mixed admicelles for the same mixture of surfactants.