Formation of Viscoelastic Wormlike Micellar Solutions in Mixed Nonionic Surfactant Systems

The addition of short-EO-chain polyoxyethylene dodecyl ether (C 12 EO n ) 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 C 12 EO n ( 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 (η o ) was observed. The mixing fraction of C 12 EO n to increase η o or induce the micellar growth increases in the following order: C 12 EO 1 ≈ C 12 EO 2 < C 12 EO 3 < C 12 EO 4 . On increasing m of ChEO m from 10 to 15 in the ChEO m −C 12 EO 3 system, a sharp increase in η o is shifted to relatively higher mixing fraction of C 12 EO 3 . These results indicate that the average section area per surfactant in a micelle decreases upon the addition of short-EO-chain C 12 EO n 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 s, is constant (ideal surface mixing), the rod micellar length was calculated as a function of the mixing fraction of C 12 EO n 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.