Complementary use of simulations and free energy models for CTAB/NaSal systems

Sodium salicylate (NaSal) is added to aqueous solutions of cetyl trimethyla mmonium bromide (CTAB) to convert spherical micelles to wormlike micelles, thus producing an effective drag-reducing agent. A theoretical understandin g of why NaSal is so effective at causing this transformation has not been developed at this time. Using a model that combines aspects of free energy models with simulations, we show change in ordering of the amphiphiles with in the micelles of different curvatures. The way NaSal transforms the spher ical micelles to wormlike micelles is by insertion of the salicylate ion in to the surfactant shell, which reduces the preferred mean curvature of the surfactant shell and thus causes the transformation. This mechanism is diff erent from that of a counterion shell formed in the presence of electrolyte s. These models help us to understand the interactions between amphiphiles and additives in the micellar headgroups. The results of this model agree w ith some experimentally observed trends and help to account for others. Com parisons of our models with predictions based only on free energy models hi ghlight the significance of accounting for intramicellar ordering in calcul ating micelle free energies. In general, such a model can predict the effec t of the inclusion of other organic additives into micellar structures.