Micellization and surface activity of long-chain monoquaternary and diquaternary ammonium salts

Abstract

Six monoquaternary ammonium bromide amphiphiles (CH/N⊕, − 20 1 and nine diquaternary ammonium species (bola amphiphiles) (CH/N⊕, 11 1 and 16 1 ) have been synthesized. The critical micelle concentration of each has been determined using equilibrium surface tension and equivalent conductivity data. Water-air interface parameters have also been derived from the surface tension data using the Gibbs adsorption isotherm. The monoquaternary ammonium salts behave more or less like conventional surfactants. However, each contains an ester group in its long chain and this appears to be located via a “loop-like” conformation at both the micelle and the water—air interfaces. The diquaternary ammonium salts also display aggregation phenomena, with apparent CMC values below that expected for their CH/N ratio, but consistent with the overall large hydrocarbon structure of each. The terminal positions of the cationic groups seem likely to make packing into spherical micelles problematical, and the aggregates formed in this case might be lamellar in nature. The dye solubilization behavior is also consistent with such a structure. Both N⊕ centers in each bola species are located at each interface, again via a “loop-like” structure. Each also shows evidence for discrete ion-pair formation ⊝Br ⊕N fx245-1N ⊕ in the bulk solution below the CMC, and a second discontinuity in the surface tension data at concentrations far below the CMC. The latter appears to be due to the binding of ester groups in the hydrocarbon skeleton at the interface, via “multiple loops.” Salts bearing a methacryloxyethyl substituent directly on N ⊕ show evidence for this being intramolecularly associated with N ⊕ in the micellar interface.