Premicellar aggregation of fatty acid N-methylethanolamides in aqueous solutions

The aggregation behaviors of an excellent nonionic foam booster, namely, fatty acid N-methylethanolamide (NMEA-X; X indicates the carbon number of the acyl group), in aqueous solutions have been studied by equilibrium surface tension (gamma), solubilization of oil-soluble dye, and steady-state fluorescence techniques. NMEA, having a longer alkyl chain than NMEA-08, clearly had two break points on the gamma versus log C (where C is concentration) curves. The solubilization of the oil-soluble dye for NMEA aqueous solutions began at the break point of higher concentration in the gamma versus log C curves, so this concentration was confirmed to be the critical micellization concentration (cmc). Above the cmc, however, a separate oil phase of NMEA was observed instead of micelles of limited size. Another break point at lower concentration was also observed in plots of the fluorescence intensity ratio of pyrene, I1/I3, versus log C of NMEA. The gradual decrease of I1/I3 and the appearance of excimer emission of pyrene in the concentration region between the two break points suggest the existence and growth of premicellar aggregates and the solubilization ability of pyrene. Consequently, this break point at lower concentration was assumed to be the critical premicellization concentration (cac). The surface tension reduction in the premicellar region decreased with increasing alkyl chain length of NMEA.