Abstract
In the present study, we examine a worm-like micelle consisting of a nonionic surfactant system of polyoxyethylene phytosterol (PhyEOm)/glycerin fatty acid monoester (GFA-Cn)/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 PhyEOm/GFA-Cn/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-Cn solubilized in the palisade layer of a spherical or rod-like micelle consisting of PhyEOm, 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-Cn (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 (η0) 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 η0 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 η0 of a worm-like micelle was influenced by its mechanical strength.
