Rheological behavior of worm-like micelles in a mixed nonionic surfactant system of a polyoxyethylene phytosterol and a glycerin fatty acid monoester.

In the present study, we examine a worm-like micelle consisting of a nonionic surfactant system of polyoxyethylene phytosterol (PhyEO(m))/glycerin fatty acid monoester (GFA-C(n))/Water (m=10, 20, 30; n=8, 10, 12) using rheological measurements looking towards potential application of the system as a gel base. Phase diagrams in the dilute region of the PhyEO(m)/GFA-C(n)/Water systems show the formation of worm-like micelles in some of the surfactant combinations. It is thought that the worm-like micelles form with the GFA-C(n) solubilized in the palisade layer of a spherical or rod-like micelle consisting of PhyEO(m), resulting in a decrease in the interfacial curvature of the molecular assembly. The rheological properties of micellar solutions were examined while changing the ratio of GFA-C(n) (R) with the total concentration of the surfactants (wt%) fixed. Steady-flow viscosity measurements in the region of worm-like micelle formation showed Newtonian flow in the low shear rate region and non-Newtonian flow at higher shear rates. This result shows that the network structure of worm-like micelles does not break in the low shear rate region, but does break in the high shear rate region. The zero-shear viscosity (eta(0)) was calculated from the steady-flow viscosity curve and was found to change dramatically with changing R value, increasing to a value 10000 times that of other values at the maximum. Thus, there is an optimal composition of surfactants which leads to the greatest entanglement of the worm-like micelles. To consider the change in eta(0) in detail, dynamic viscoelasticity measurements were carried out. Consequently, the viscoelastic behavior of the worm-like micelles was found to be similar to the Maxwell model, which represents the most basic model for a viscoelastic body, and it was shown that this worm-like micelle had a single relaxation time. Moreover, it was found that the change in eta(0) of a worm-like micelle was influenced by its mechanical strength.