Paramagnetic nuclear relaxation study of the structure and dynamics of lyotropic lamellar phases

Abstract

Lamellar phases of double tailed sodium alkyl-phosphates/water systems have been investigated by ³¹ P and ¹³ C relaxations enhanced by paramagnetic divalent ions exchanging rapidly among the polar heads of the surfactant molecules. In order to extract both structural and dynamic information from these experiments, the paramagnetic probes have been chosen such that their electron spin-lattice relaxation times are significantly larger (Mn²⁺, VO²⁺) or smaller (Ni²⁺) than the time scales of molecular motions generally found in the 0.1-2.0 ns range. The paramagnetic relaxation of surfactant nuclei results from intra- and intermolecular contributions, the latter corresponding to dipolar interactions in a two dimensional lattice. The sum of these contributions has been computed as an average over all accessible conformers resulting from the trans-gauche isomerizations of alkyl chains and fitted to the experimental relaxation rates at several magnetic field strengths. This procedure allows the determination of the populations of the main conformers, of the reorientational correlation times and of the lateral diffusion coefficient of the surfactant, which are sometimes difficult to obtain by other N.M.R. methods.