Peanuts, Rods, Worms and Trees : How Do Giant Micelles Evolve ?

Orateur : Alan PARKER (Rational Formulation, Annecy, France)

Surfactant micelles can take a wide variety of shapes, depending on a subtle balance between packing constraints and intermolecular forces. Since shape controls functionality, understanding these shape changes is of practical, as well as fundamental, interest. Theories based on molecular packing have led to a lot of progress and intuition. However, it is a big stretch from the molecular level to the macroscopic, so mesoscopic theories, which ignore molecules, also have a role to play. First, I discuss our study of the effects of ionic strength and perfume molecules on giant ionic micelles. Increasing the ionic strength pushes the viscosity over an approximately Gaussian curve as the micelle shape is driven through the sequence : sphere → rod → wormlike chain → branched chain → lamella. Adding perfume molecules changes the height and position of the salt curve, but not its shape. I argue that this similarity is due to the key roles of micellar persistence length and spontaneous curvature. In the second part, I concentrate on the sphere → rod transition (SRT). It seems reasonable that the SRT should reflect the similarities found in the full salt curve. It turns out that this transition has been little studied experimentally, although a nice molecular-thermodynamic theory exists. I discuss the information that can be gathered from rheological and light scattering measurements and the existing theory. Finally, I discuss how a mesoscopic theory could be extended to cover the sphere → rod transition.

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