Solute-Solvent Contact by Intermolecular Cross Relaxation: II. The Water-Micelle Interface and the Micellar Interior

The intermolecular dipole-dipole cross-relaxation is measured between F-19 nuclei of sodium perfluorooctanoate in micelles and H-1 nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furo, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced F-19 chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.