Micelles relaxation behaviour

The relaxation behaviour of water can be quite different from bulk water if it is enclosed in reversed micelles (Wong et al., 1977), or bound to frozen erythrocytes (Zipp et al., 1976), or bound to red-cell membranes (Finch and Schneider, 1975). 

In other cases, several discrete relaxation times or distributions of relaxation times can be found [39]. This is typically the case if the stress relaxation is dominated by reptation processes [42 ]. The stress relaxation model can explain why surfactant solutions with wormlike micelles never show a yield stress Even the smallest applied stress can relax either by reptation or by breakage of micelles. For higher shear rates those solutions typically show shear thinning behaviour and this can be understood by the disentanglement and the orientation of the rod-like micelles in the shear field. 

The two fast relaxation times increase almost linearly with total concentration, figure 3a, which is similar to the behaviour predicted for the one relaxation time in one-component micelles. It is noteworthy that the values at the cmc are surprisingly close to those predicted by equations 12a and b. 

In addition, the zwitterionic alkyldimethylaminoxide surfactants (C DMAO) that are known to form rod-like micelles already at low concentrations, show a linear behaviour of log rjo log c in an extended concentration region, as can be seen in Figure 10.7. Some curves show a break, which indicates that at different concentrations a different relaxation mechanism for an applied stress operates. At the lowest concentrations above c, the slope is the highest, being close to 4.5, as observed for polymers. This result is somewhat surprising because... 

There are two specific examples of liquids that often show simple MaxweU-like behaviour within the normal measuring range (lO - 10 Hz), with the expected shapes of the G and G" curves and a single relaxation time, viz. associative-thickener-type polymers (see chapter 16 for details) and worm-like surfactant micellar systems, otherwise known as threadlike or rod-like micelles (linear or branched). (The latter are also called living polymers because if they break imder large stresses, they can reform under conditions of rest or low stress (see chapter 18).) The typical response of such a system is shown in figure 14, where a representative relaxation time would be around 1 s, see [10]. 

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