Surface Activity ofn-Alkyltrimethylammonium Bromides in Concentrated Orthophosphoric Acid

Abstract

The aggregation and interface behavior of alkyltrimethylammonium bromides (CnH2n+1(CH3)3N+Br−: CnTMA with n = 14, 16, 18) have been investigated in concentrated orthophosphoric acid by means of surface tension measurements. Adsorption of these cationic surfactants at the interface between air and aqueous H3PO4solutions has been determined at 298 K using the Wilehlmy plate method. Measurements in the most concentrated H3PO4-H2O mixture (95% by weight of acid or 17.5 mol/L) were done at then-decane/H3PO4interface using the ring method to avoid any contamination with atmospheric moisture. The critical micelle concentration (CMC), the maximum surface concentration, and surface pressure of C16TMA in water-H3PO4mixtures and of the homologous series of surfactants, CnTMA, in 95%-H3PO4, were determined at 323 K. Benzene has been used as a test solute for solubilization measurements in 95%-H3PO4containing C16TMA micelles. From this experimental work, it can be concluded that micelles are forming even in the most concentrated solutions in acid and that the CMC displays a large depression between 4 and 10 mol/L of H3PO4. The maximum surface concentration increases by steps in the lowest (0 to 1 mol/L) and highest range of concentration (8 to 10 mol/L) in acid. The Klevens equation relating the CMC in relation to the chain length of the surfactants holds in 95%-H3PO4. Values obtained for the parameters of the Klevens equation and for the free energy of micellization in 95%-H3PO4have been compared to other strongly acidic media like concentrated sulfuric or methanesulfonic acids.