Viscoelastic worm-like micelles

Viscoelastic Worm-Like Micelles in Nonionic Fluorinated Surfactant Systems... 

In this chapter, a brief theoretical background on the rheological behavior of viscoelashc worm-like micelles is given. It is followed by a discussion on the temperature-induced viscosity growth in a water-surfactant binary system of a nonionic fluorinated surfactant at various concentrations. Finally, some recent results on the formation of viscoelastic worm-like micelles in mixed nonionic fluorinated surfactants in an aqueous system are presented. 

A remarkable contribution in recent years was to have shown for the first time the formation of highly viscoelastic worm-like micelles (Figure 12) in mixed nonionic surfactant systems [110]. This finding allowed to clarify the relation between packing constraints of hydrophobic chains and micellar growth because the complex interactions between counterions (present in ionic surfactant systems) and headgroups had not to be taken into consideration. 

He showed the formation of viscoelastic worm-like micelles in various nonionic and ionic surfactant systems and described the evolution of micellar growth namely by rheology and small-angle X ray scattering [110, 112, 118, 133, 137, 144—155], Zero-shear viscosities 3 x 10 times that of water was reported for certain systems [118],... 

Because of the interaction of the two complicated and not well-understood fields, turbulent flow and non-Newtonian fluids, understanding of DR mechanism(s) is still quite limited. Cates and coworkers (for example, Refs. " ) and a number of other investigators have done theoretical studies of the dynamics of self-assemblies of worm-like micelles. Because these so-called living polymers are subject to reversible scission and recombination, their relaxation behavior differs from reptating polymer chains. An additional form of stress relaxation is provided by continuous breaking and repair of the micellar chains. Thus, stress relaxation in micellar networks occurs through a combination of reptation and breaking. For rapid scission kinetics, linear viscoelastic (Maxwell) behavior is predicted and is observed for some surfactant systems at low frequencies. In many cationic surfactant systems, however, the observed behavior in Cole-Cole plots does not fit the Maxwell model. 

The viscoelasticity of the worm-like micelles arises because of the entanglement of very long and flexible worm-like micelles to form a transient network, similar to a solution of flexible polymers. Unlike polymers, however, worm-like micelles break and re-form dynamically. When the network of worm-like micelles is deformed or the equilibrium conditions are suddenly changed, the relaxation occurs within a definite time, and the equilibrium condition is restored again. For a deformation with a hme period shorter than the relaxation time, Tr, the system... 

As said before, when entangled worm-like micelles are formed, viscoelastic behavior only follows Cates model at low and intermediate frequencies, indicating that other fast relaxation processes exist (see Figure 12.5). Moreover, at cosurfactant-surfactant ratios above the viscosity maximum, further addition of cosurfactant decreases viscosity and viscoelastic functions up to phase separation, where lamellar liquid crystal appears. As said above, this has been related to the fact that, after the maximum in viscosity, the decrease in spontaneous curvature produces branching of micelles before phase separation [9, 10). 

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