Micelles shape change

One of our goals in applying FTTR spectroscopy to the study of micelle shape changes is to obtain data which addresses simultaneously methylene chain ordering (conformation), and headgroup spacing, for a comparison with these models. 

A combination of these two effects is responsible for separation of the Li phase into micelle-rich and micelle-lean phases at the cloud point temperature. It is well known that entropic effects cause phase separation to occur in relatively dilute solutions of long, cylindrical particles, even in the absence of interparticle attraction. Hence phase separation is facilitated by micelle shape changes caused by higher values of v lal and, of course, also by a shift from a repulsive to an... 

The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144,145]. For example, the d>,rans to Cis for stilbene derivatives show the following trend solvent system methylcyclohexane > SDS micelle multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. 

As micelles become larger, they have been shown to become more asymmetric, with their shape changing from spherical to cylindrical and lamellar. In lamellar mesophases (Figure 12.2c), the surfactant molecules are arranged in parallel sheets with the hydrocarbon tails forming the inner layers. The water is stratiLed between the sheets, hydrating the external polar head groups (Schot 1995). 

The self-diffusion of the individual components is strongly affected by the formation of micelles in the solution. This applies to the surfactant, the counterion, the water, and to solubilized molecules. As illustrated in Fig. 2.11 for sodium dodecyl sulfate, surfactant and counterion diffusion are very weakly dependent on concentration below the CMC while a marked decrease in the micellar region is observed for the surfactant and a less marked one for the counterion37. Water diffusion shows a stronger concentration dependence below the CMC than above it. Self-diffusion studies using radioactive tracers have been performed to obtain information on CMC, on counterion binding, on hydration and on intermicellar interactions and shape changes. 

Although a number of infrared bands can be used to establish that a micellar shape change has occurred, it is difficult to determine the actual shape unambiguously from the spectroscopic data alone. We therefore make use of micelle aggregation numbers and solution rheological properties, which depend on micelle size and shape, for correlation with the structural information (packing) provided by the FTIR spectra. 

It is also known that additives may change the size and shape of micelles.At a certain point, as the surfactant or additive concentrations change, ionic micelles may change shape from spherical or nearly spherical to rodlike or other elongated forms. This may also affect the solubilization of the additive. It appears that alkane solubilization increases as the micelles become large, rodlike aggregates, whereas for polar additives like alcohols the solubilization decreases. ... 

The shape of the micelle may change when material is solubilized by the micelle (Chapter 4, Section IIIA) and by change in molecular environmental factors (Figure 4-4). 

As surfactant concentration in solutions with c0 > CMC increases, not only the concentration of spherical micelles becomes higher, but also their shape changes. Spherical micelles turn into anisometric ellipsoidal and cylindrical micelles and further to rod-like, thread-like and lamellar micelles... 

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