Micellar properties of octyldimethylbenzylammonium bromide in water

The micellar properties of octyldimethylbenzylammonium bromide (C8BBr) in aqueous solution were studied by carrying out conductivity measurements at 20, 25, 30, and 37 °C, density measurements at 25 and 35 °C, and vapor pressure osmometry (v.p.o.) at 37 °C. For comparative purposes the latter technique was also applied for solutions of octyltrimethylammonium bromide. The three techniques yielded identical, within the experimental error, values of the critical micelle concentration (cmc) for C8BBr. The aggregation number, n, of the micelles at cmc was estimated from conductivity data by applying the Kimizuka and Satake approach. Compared to the data estimated earlier for the decyl and dodecyl homologues the cmc value for C8BBr appears to be unexpectedly high. At the same time the degree of ionization, β, of the micelles of this surfactant was found to be low. An explanation of these discrepancies is offered by suggesting that the micelles of C8BBr assume a compact arrangement of the amphiphilic cation with the benzyl group oriented towards the aqueous phase. The volumetric data in the micellar range seem to support this view.