Adsolubilization of 2-Naphthol by Cationic Multi-chained Surfactants at Silica/Water Interface

Adsolubilization of 2-naphthol by cationic multi-chained surfactants such as gemini (m-2-m, m=alkyl chain length ; 10-2-10, 12-2-12) and trimeric surfactants (m-2-m-2-m ; 10-2-10-2-10, 12-2-12-2-12) on silica has been investigated. For comparison, a similar experiment has been performed using a cationic single-chained surfactant, dodecyltrimethylammonium bromide (DTAB). Adsorption of the surfactants increases with an increase of the concentration and reaches a saturation, while adsolubilization of 2-naphthol increases and shows a maximum and then decreases with the surfactant concentration for all the surfactant systems. The ratio of maximum amount of 2 -naphthol adsolubilized to the adsorbed amount of surfactant on silica is decreased as in the order, 10-2-10-2-10> 12-2-12-2-12>12-2-12=DTAB>10-2-10. In addition, the order of the ratio per one-hydrocarbon chain is 10-2-10-2-10 =12-2-12>12-2-12-2-12 =10-2-10, where the ratio of DTAB is considerably large compared to those of the multi -chained surfactants. These results can be interpreted by the compactness of surfactant adsorbed layer. The effect of the surfactant structure on the adsolubilization is also discussed using admicellar partitioning coefficients of 2-naphthol.