CONCENTRATION-DEPENDENCE OF MICELLAR SIZE AND COMPOSITION IN MIXED ANIONIC CATIONIC SURFACTANT SOLUTIONS STUDIED BY LIGHT-SCATTERING AND PULSED-GRADIENT FT-NMR SPECTROSCOPY

Light scattering and pulsed-gradient FT-NMR spectroscopy (PGNMR) have been measured for aqueous solutions of sodium decanesulfonate (C10SO3)/octyltrimethylammonium bromide (OTAB) and sodium octanesulfonate/OTAB as a function of total surfactant concentration, c, at different mixing ratios (the mole fraction of OTAB in the total mixed solute, x2, is 0.1-0.5). From the analysis of self-diffusion coefficients of surfactants obtained by PGNMR, the mole fraction of OTAB in the mixed micelle, x2m, is determined. At concentrations much higher than the critical micelle concentration (cmc), x2m is close to x2. As c is decreased toward the cmc, x2m is increased toward equimolar composition. These results suggest that micelles grow with decreasing concentration. Light-scattering results are consistent with this prediction. It is also shown that in the C10SO3/OTAB system, the mixed micelles grow rapidly when x2m exceeds about 0.4.