Effect of electrolyte species on the adsorption of a cationic surfactant to silica: the common intersection point

Adsorption isotherms for cetyltrimethylammonium bromide (CTAB) on silica have been measured using optical reflectometry (OR) in the presence of 1 mM and 10 mM NaBr, KBr and CsBr in order to elucidate the role of specific adsorption of the electrolyte to silica. In this regard the surface excess at concentrations up to the common intersection point (CIP) is most revealing as Koopal and co-workers have demonstrated that in the presence of an indifferent electrolyte the CIP corresponds to the isoelectric point and the charge compensation point. CsBr exhibited a CIP at 0.035 mM considerably higher than the CIP for NaBr and KBr (0.025 mM), indicating specific adsorption of Cs⁺ ions to silica.