Chapter 11:New Insights into the Formation of Wormlike Micelles: Kinetics and Thermodynamics

Wormlike micelles (WLMs) can be formed by adjusting the curvature of surfactant aggregates. This can be achieved by changing the overall geometry of the surfactant molecule, principally the ratio between the polar and non-polar parts. In some cases, for ionic surfactants, this can be achieved by increasing their concentration, or by adding inorganic salts. Specifically, with cationic surfactants, WLMs are promptly formed by the addition of organic molecules, such as 2-hydroxybenzoate. This aspect is interesting for many applications because it enables the adjustment of the viscoelastic characteristics of the solution. However, as shown in this chapter, the formation of WLMs is dependent not only on the molecular structure of the surfactant, but, in many instances, is also highly sensitive to the precise structure of the aromatic co-solute. The spontaneity of WLM formation is discussed from the thermodynamic and kinetics points of view. Recent results involving measurements on changes in enthalpy of WLM formation by using isothermal titration calorimetry are a very promising approach to understanding the interactions between the molecules in this type of micellar assembly. Complementary to these, kinetics aspects are highlighted, by considering recent studies on the mechanism of WLM formation using a stopped-flow set-up coupled with time-resolved small-angle X-ray scattering measurements.