Fluorescence and Light-Scattering Studies of the Aggregation of Cationic Surfactants in Aqueous Solution: Effects of Headgroup Structure

Abstract

Aggregation of the following cationic surfactants has been studied in aqueous solution: cetyltrimethylammonium chloride, CMe3ACl; cetyldimethylphenylammonium chloride, CMe2PhACl; cetyldimethylbenzylammonium chloride, CMe2BzACl; cetyldimethyl-2-phenylethylammonium chloride, CMe2PhEtACl; and cetyldimethyl-3-phenylpropylammonium chloride, CMe2PhPrACl. Critical micelle concentrations, cmc's, were obtained from surface tension and conductance measurements; data of the latter were also employed to determine the degree of the surfactant counterion dissociation, α. Static and quasi-elastic light-scattering measurements were employed to obtain micellar weight-average molecular weights, aggregation numbers, Nagg, micellar hydrodynamic radii, Rh, and interfacial area/surfactant headgroup. The latter area was also obtained from surface tension measurements. Finally, time-resolved fluorescence decay measurements with pyrene as probe were employed to obtain Nagg. The structure of micelles of CMe2PhACl is different from that of the other phenyl-group-bearing surfactants because its aromatic ring cannot fold back on the micellar interface. Increasing the number of the methylene segments in the headgroup results in an increase in α and interfacial surface area/headgroup and a decrease in the cmc, Nagg, and Rh. There is good agreement between micellar properties obtained by the different techniques employed.