Monitoring the different micelle species and the slow kinetics of tetraethylammonium perfluorooctane-sulfonate by 19F NMR spectroscopy

Abstract

Since we lack effective tools that can monitor the structures of surfactant micelles in situ, the different equilibrium species and the slow kinetics of micelles are still not well understood. Herein, by using 19F NMR, we simultaneously monitored that micelles of tetraethylammonium perfluorooctanesulfonate (TPFOS, C8F17SO3N(C2H5)4) in water grow more complex in virtue of hydrophobic counterions and the slow kinetic exchange process exists in the system. Apart from the monomeric signals, three sets of micelle signals which correspond to spherical micelles, wormlike/wormlike micelles with rings in end caps and toroidal micelles were successfully detected on the NMR time scale because of the slow exchange rate for surfactant molecules between the monomer and the micelle states. By comparison, other fluoro- and hydrocarbon surfactants with different tail lengths and counterions (+N(CH3)4, +N(C3H7)4, Li+ and Na+) have been studied, and the coexistence of different micelles could also been observed for the aqueous solution of C9F19COON(CH3)4. However, only one set of averaged NMR signals could be observed for these surfactants. The micellization of TPFOS in water is demonstrated to be a predominantly entropy-driven process. Molecular dynamics (MD) simulation revealed an unusual distribution of counterions, providing further understanding of the mechanism of the micelle formation process.