Phase and rheological behaviour of viscoelastic wormlike micellar solutions formed in mixed nonionic surfactant systems

The phase and rheological behaviour of mixed nonionic surfactant system - polyoxyethylene cholesteryl ether (ChEO(15))-alkanoyl-N-methylethanolamide (NMEA-n)-water was studied at 25 degreesC. With addition of NMEA to the ChEO(15)-water binary system, a micellar (W-m)-lamellar (L-alpha) phase transformation takes place at low ChEO(15) concentration. Within the W-m phase region of the ChEO(15)-NMEA-12 water system, there exists a high-viscosity region consisting of a viscoelastic micellar solution of entangled wormlike micelles, as suggested by rheological measurements. These solutions obey the Maxwell model of a viscoelastic fluid. Substitution of NMEA-12 by its analogue with a longer hydrophobe (NMEA-16) in the mixed surfactant system increases the extent of micellar growth, however, a phase separation occurs before a highly viscoelastic micellar system is formed. This may be attributed to the increase in packing constraints in the lipophilic core of the micelle that favours unidimensional growth.