Phase and rheological behavior of salt-free alkyltrimethylammonium bromide/alkanoyl-N- methylethanolamide/water systems

Addition of N-hydroxyethyl-N-methyldodecanamide (NMEA-12) and N-hydroxyethyl-N-methylhexadecanamide (NMEA-16) to dilute solutions of hexadecyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB) in the micellar (W m) phase results in an increase in viscosity. It is found that the CTAB-NMEA-12 or CTAB-NMEA-16 and DTAB-NMEA-16 surfactant systems show viscoelastic behavior typical of systems containing wormlike micelles. The dynamic viscoelastic behavior of the viscoelastic micellar phase follows the Maxwell model at low shear frequency at the composition of maximum viscosity. The mixing fraction of NMEA in total amphiphile for the maximum viscosity increases with decreasing the total amphiphile concentration. Most probably, in a dilute region, as, the effective cross-sectional area per surfactant at the hydrophobic interface in the micelle, increases and more NMEA is needed to decrease the average as. Assuming that one-dimensional growth of micelles takes place upon addition of NMEA and as for the surfactant and NMEA are constant, the rod-micellar length was calculated as a function of the mixing fraction of NMEA in total amphiphile. As a result, the rodlike micellar length is not largely increased up to certain amount of added NMEA, above which the enormous increase in micellar length takes place. The micellar growth can be simply explained by decreasing the effective cross-sectional area per amphiphile upon addition of NMEA.