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 (Wm) 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, a s, the effective cross-sectional area per surfactant at the hydrophobic interface in the micelle, increases and more NMEA is needed to decrease the average a s. Assuming that one-dimensional growth of micelles takes place upon addition of NMEA and a s 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.

Phase diagrams of water; sodium dodecyl sulfate (SDS); and a new foam booster, alkanoyl-Nmethylethanolamide (C8, NMEA-8; C12, NMEA-12; C16, NMEA-16), were constructed at 25 °C. NMEA is hardly soluble in water: liquid-liquid phase separation occurs with NMEA-8, lamellar-phase formation occurs with NMEA-12, and solid precipitation occurs with NMEA-16. In the presence of a small amount of NMEA, the surfactant hexagonal phase (H1) is extended to the dilute region. In the aqueous micellar solution beyond the H1 phase, the viscosity dramatically increases and a viscoelastic solution is formed in NMEA-12 and NMEA-16 systems. The viscoelastic micellar solution formed in SDS-NMEA-12 systems follows the Maxwell model typical of wormlike micellar systems at low shear frequencies. In the SDS-NMEA-16 system, a gel-like highly viscoelastic solution is formed in the maximum-viscosity region. Rheological measurements show that the ability of NMEA to induce micellar growth increases in the following order: NMEA-8 , NMEA-12 < NMEA-16. In agreement with this result, dynamic light scattering measurements show that with an increasing mixing fraction of NMEA-12 or NMEA-16 in SDS-NMEA systems the micellar size increases, leading to the formation of wormlike micelles.