Characterization of Adsorption of Quaternary Ammonium Cationic Surfactants and Their Adsolubilization Behaviors on Silica

Adsorption of quaternary ammonium cationic surfactants with one, two, or three alkyl chains on silica has been studied by measuring adsorption density, ζ potential, and dispersion stability. The adsorbed amounts at saturation decrease with increasing chain number of the surfactants. Silica suspensions exhibit a process of dispersion−flocculation−redispersion with the surfactant concentration for three surfactants which can be well correlated with the change in ζ potentials. Fluorescence and ESR measurements using probes show that the microproperties in the adsorbed layers are considerably affected by their chain numbers of the surfactants. Under a constant feed concentration of 2-naphthol, the adsolubilized amounts of 2-naphthol increase, reach a maximum, and then decrease with single-chain or double-chain surfactant concentration, whereas only a slight decrease in the adsolubilized amount of 2-naphthol is observed for the triple-chain surfactant. The ratios of amount of 2-naphthol adsolubilized to the adsorbed amount of surfactant on silica for the double-chain and triple-chain surfactants are not so different and are quite large compared to that for the single-chain surfactant. In addition, from a two-step adsorption−adsolubilization procedure, it is found that the double-chain or triple-chain surfactant adsorbs strongly on the silica surface, keeping 2-naphthol molecules in the adsorbed layer in comparison with the single-chain surfactant.