Growth Process of Worm-Like Micelles: An Emerging Complex Behaviour. Francesco Aliotta Istituto di Tecniche Spettroscopiche del CNR, 98166 Messina, ITALY

Surfactant molecules in solution can self-assemble into aggregates, whose shapes and sizes depend on the molecular structure of the surfactant, nature of the solvent and concentration. It is now well known that, in a number of systems under appropriate conditions, the micellar aggregates can grow anisotropically, changing their shapes from spheres to rods or highly flexible worm-like aggregates. Such an evidence suggested for some analogies between giant flexible cylindrical micelles and conventional polymeric solutions. The proposed analogy was supported by theoretical models that, in a mean field approximation, suggested for a length distribution function shaped exponential and a mean chain length, <L>, scaling with concentration with a simple power law. When the existence of a gel-like phase was revealed in the systems soybean lecithin/organic solvent/water, it was immediate the interpretation of the observed phenomenon in terms of a polymer-like network of giant flexible worm-like micelles. A number of experimental results were successfully interpreted within this approach. It will be presented a review of recent data from Incoherent Quasi Elastic Neutron Scattering and Small Angle Neutron Scattering experiments that are not interpretable within the proposed analogy with polymer solutions. The observed discrepancies between the theory and the experiment suggest for a critical revision of the currently accepted ideas. In particular, it will be shown how the indication of a monotonous growth process of <L> with no respect of the concentration is wrong in principle, since the process is intrinsically a not linear one. The observed dependence of the mean micellar size by the concentration implies a change in the system polydispersity, driven by inter-micellar interactions. Such a behaviour is an emerging complex property of the system, originated from the competition among all the species (aggregates of different size) existing in solution and resulting, in the semi-dilute regime, in a lower configurational entropy than mean field approach would indicate. From this different point of view, it is possible to formulate new models, able to fit also with the data actually not matching the prediction of the conventional theory.