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

Abstract

The phase and rheological behaviour of mixed nonionic surfactant system – polyoxyethylene cholesteryl ether (ChEO₁₅)–alkanoyl-N-methylethanolamide (NMEA-n)–water was studied at 25 °C. With addition of NMEA to the ChEO₁₅–water binary system, a micellar (W_m)–lamellar (L_α) phase transformation takes place at low ChEO₁₅ concentration. Within the W_m phase region of the ChEO₁₅–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.