Aggregation of sodium dodecylsulfate in aqueous nitric acid medium

Abstract

Nitric acid medium is invariably used for nitration of organic molecules. Although surfactants are known to influence reaction rates, little is known about the aggregation behavior of surfactants in nitric acid medium. Micellization characteristics of sodium dodecylsulfate (SDS) in aqueous nitric acid are investigated in this work by using the conductance method. The critical micelle concentration (cmc) and the aggregation number were also determined by the surface tension and the steady-state fluorescence methods, respectively. This study reveals that in acidic medium SDS exhibits both normal and unusual conductivity behaviors. Equations developed on the basis of the mixed electrolyte model, Debye–Hückel–Onsager approach, and the pseudophase ion-exchange model successfully simulate the conductivity data. The exchange of sodium and hydrogen counterions at the micellar surface has no significant effect on the cmc of SDS. Acid concentration, surfactant concentration, and cmc control the competitive binding of sodium and hydrogen counterions. Analysis of conductivity data revealed hydrolysis of about 12% SDS when [HNO3]⩾0.02moldm−3. Hydrolysis of SDS has been confirmed by nitrating some of the substituted phenols. It has been predicted that SDS+aqueous HNO3 medium with [HNO3]⩾0.02moldm−3 may be used as a green medium for nitration.