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Adsorption and Adsolubilization by Monomeric, Dimeric, or Trimeric Quaternary Ammonium Surfactant at Silica/Water Interface

Adsorption of monomeric, dimeric, or trimeric quaternary ammonium surfactant on silica from its aqueous solution has been investigated by measuring adsorption density, zeta potential, and dispersion stability. The monomeric (dodecyltrimethylammonium bromide, 1RQ), dimeric (1,2-bis(dodecyldimethylammonio)ethane dibromide, 2RenQ), and trimeric (methyldodecylbis[2-dimethyldodecylammonio) ethyl] ammonium tribromide, 3RdienQ) surfactants are used in this study. The amounts adsorbed at saturation decrease with increasing dodecyl chain number of the surfactants from 1RQ to 2RenQ and 3RdienQ. Silica suspensions by adsorption of the surfactants exhibit a process of dispersion-flocculation-redispersion with the surfactant concentration for the three surfactants which can be correlated with the change in zeta potentials. ESR measurements using methyl 12-doxylstearate show that the microviscosities in 2RenQ- and 3RdienQ-adsorbed layers are greater than that in the 1RQ-adsorbed layer. Under a constant feed concentration of 2-naphthol, the adsolubilized amounts of 2-naphthol increase, reach a maximum, and then decrease with the surfactant concentration for the three surfactants. The ratio of maximum amount of 2-naphthol adsolubilized to the adsorbed amount of surfactant on silica increases with an increase in the dodecyl chain number of the surfactants from 1RQ to 2RenQ and 3RdienQ. In addition, from a two-step process of adsorption-adsolubilization, it is suggested that 2RenQ and 3RdienQ adsorb on much stronger than 1RQ, keeping 2-naphthol in their adsorbed layers.