Adsorption of dodecyl pyridinium chloride on monodisperse porous silica.

The adsorption of a cationic surfactant, dodecyl pyridinium chloride, on monodisperse Stober silica particles dispersed in an aqueous solution has been studied as a function of pH and electrolyte concentration. In a previous study it was concluded that the proton charges of these silica particles can be distinguished in charges on the outer surface, charges in the mesopores and charges in a microporous region. From a comparison with the adsorption on non-porous silica it was concluded that dodecyl pyridinium cations can penetrate the mesopores, but are excluded from the (highly charged) microporous region. On the outer surface, adsorption of the cationic surfactant leads to a maximum in the hydrophobicity at about 50% of the maximally adsorbed amount. Charge reversal has already taken place at about 10% of the plateau value. It was concluded that, after charge reversal, head-on adsorption proceeds until monolayer coverage, after which bilayer formation sets in.