Manifestations of Micelle Formation

FIGURE 15.5. A number of important and accessible physical manifestations of micelle formation have been exploited in studies of surfactant aggregation. The most classic techniques include surface tension and conductivity titration, and turbidity (or light scattering), all of which show relatively sharp changes at the onset of micelle formation. 

For light scattering (bottom curve), the change in solution turbidity indicates the appearance of a scattering species of significantly greater size and optical properties than the monomeric solute. The effect on surface tension (middle curve) has already been introduced, but will be discussed in more detail below. These and many other types of measurement serve as evidence for the formation of aggregates or micelles in solutions of surfactants at relatively well-defined concentrations. 

The results of studies of surfactant solution properties were classically interpreted in terms of a spherical association of surfactant molecules the micelle. The structure was assumed to be an aggregate of from 50-100 molecules with a radius approximately equal to the length of the hydrocarbon chain of the surfactant Fig. 15.6). The interior of the micelle was described as being essentially hydrocarbon in nature, while the surface consisted of a layer or shell of the head groups and associated counterions, solvent molecules, and similar items. 

Modern studies using techniques unavailable just a few years ago have produced more detailed information about the microscopic nature of the association structures. We now know, for example, that micelles are not static species. They are very dynamic in that there is a constant, rapid interchange 

FIGURE 15.6. Micelle formation is a rapid and dynamic process involving continuous movement of surfactant molecules into and out of the micelle and, perhaps, submicellar aggregates. The residence time of a given molecule in a micelle is estimated to be between 10 and 10 s. 

---