No abstract

No abstract

Micellar solutions of cetyltrimethylammonium bromide, with D20 or formamide (FA) as solvent, have been studied through longitudinal NMR relaxation (170 and 12 34H for D20; 13C, 170, 14N, and 15N for FA) and self-diffusion measurements. Both determinations concern only the solvent molecules. The data are analyzed on the basis of a fast exchange between two states; free solvent and solvent molecules bound to micelles. From these measurements, we derive structural and dynamic information about (i) the relaxation rate of the solvent (D20, FA) in the bound state, (ii) the number n of solvent molecules per surfactant polar head, (iii) the possible variation of micelle size in water and formamide, and (iv) reorientational features of the solvent (FA, D20) in the bound state and in the free state.

The heteronuclear Overhauser effect between protons of formamide (FA) and carbon-1 3 of cetyltrimethylammonium bromide has been investigated in temperature and surfactant concentration conditions where the surfactant forms micelles in this nonaqueous solvent. Unexpectedly, responses from carbons of the surfactant tail are observed. These responses are of sign opposite to the conventional ones. These experimental observations are qualitatively explained and rationalized on the basis of spin diffusion amid the surfactant proton spin system, by taking into account the signs of the various cross-relaxation terms involved in the transmission of nuclear Overhauser processes. This is a clear demonstration of solvent penetration into micelles formed in a nonaqueous medium.

No abstract