Formation of viscoelastic wormlike micellar solutions in mixed nonionic surfactant systems

The addition of short-EO-chain polyoxyethylene dodecyl ether (C12EOn) surfactants to a dilute solution of polyoxyethylene cholesteryl ether (ChEO(m), m = 10 and 15) induced unidimensional micellar growth leading to the formation of viscoelastic solutions. The viscoelastic systems show Maxwellian behavior over a wide range of shear frequency and are considered to consist of a transient network of wormlike micelles. With increasing concentration Of C12EOn (n = 1-4) at fixed ChEO(10) concentration in a micellar solution phase, at first a gradual and then a steep increase in zero-shear viscosity (eta(o)) was observed. The mixing fraction of C12EOn to increase eta(o) or induce the micellar growth increases in the following order: C12EO1 approximate to C12EO2 < C12EO3 < C12EO4. On increasing m of ChEO(m) from 10 to 15 in the ChEO(m)-C12EO3 system, a sharp increase in eta(o) is shifted to relatively higher mixing fraction Of C12EO3. These results indicate that the average section area per surfactant in a micelle decreases upon the addition of short-EO-chain C12EOn and that micelles grow to form long rod micelles. Assuming that the cross-sectional area of each of the amphiphiles at the hydrophobic interface, a, is constant (ideal surface mixing), the rod micellar length was calculated as a function of the mixing fraction Of C12EOn in the total amphiphile, X, using the data obtained from experimental results. The calculated results on micellar growth well explained the results of rheological measurements.