Study of the effects of added salts on micellization of cetyltrimethylammonium bromide surfactant

Sodium bromide (NaBr) or sodium salicylate (NaSal) can be added to aqueous solutions of cetyltrimethylammonium bromide (CTAB) to convert spherical micelles to rod-like (cylindrical) micelles that are believed to be necessary for the surfactant to be an effective drag-reducing agent. The effects of the added salt (inorganic or aromatic) on the micellization behavior have been tested by several available molecular-thermodynamic models. Models proposed both by Blankschtein and coworkers and by Nagarajan and coworkers show ability to predict the effects of nonadsorbing salts such as NaBr on the micellization of CTAB solutions. However, their application is limited when the additive is an organic salt (e.g., NaSal) providing that ions penetrate into the surfactant (e.g., CTAB) micelles. A model that combines the molecular thermodynamic approach with energetic and structural data obtained from atomistic scale Monte Carlo simulations of the micelle shell, has been developed (27) that predicts the micellization behavior of pure CTAB aqueous solutions and also performs well for the system with penetrating organic ions. The Monte Carlo simulation data from the complementary model show that intramicellar molecular ordering is crucial to the estimation of free energies and that ordering is affected by the curvature of the micelles. Finally, it was shown that the molecular ordering of the salicylate ions causes the deviation from the ideal mixing of the CTA(+) and Sal(-) ions, especially for cylindrical micelles. (C) 2003 American Chemical Society.