Fluorescence and light-scattering studies of the aggregation of cationic surfactants in aqueous solution: Effects of headgroup structure

Aggregation of the following cationic surfactants has been studied in aqueous solution: cetyltrimethylammonium chloride, CMe(3)ACl; cetyldimethylphenylammonium chloride, CMe(2)PhACl; cetyldimethylbenzylammonium chloride, CMe(2)BzACl; cetyldimethyl-2-phenylethylammonium chloride, CMe(2)PhEtACl; and cetyldimethyl-3-phenylpropylammonium chloride, CMe(2)PhPrACl. 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, alpha. Static and quasi-elastic light-scattering measurements were employed to obtain micellar weight-average molecular weights, aggregation numbers, N-agg, micellar hydrodynamic radii, R-h, 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 N-agg. The structure of micelles of CMe(2)PhACl 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 a and interfacial surface area/headgroup and a decrease in the cmc, N-agg and R-h. There is good agreement between micellar properties obtained by the different techniques employed.