Pulsed gradient NMR study of anisotropic surfactant diffusion in the caesium perfluoro octanoate/D₂O system

Abstract

We use pulsed field gradient ¹⁹F NMR to measure the diffusion coefficients of surfactant molecules in the isotropic and lamellar phases of the caesium perfluoro octanoate (CsPFO)/D₂O system. An aligned lamellar sample is created by cooling through the nematic phase in the presence of a 1·4 T magnetic field. The director in the lamellar phase does not respond to ordinary field strengths, thus the aligned sample can be rotated clockwise or counterclockwise to place the director at a magic angle, where measurement of diffusion coefficients becomes possible. From a pair of so-obtained coefficients, we derive the principal values of the diffusion tensor corresponding to the directions parallel and perpendicular to the director (D_⊥ and D _⊥). We found D _∥ to be at least 20 times D _∥ a much larger anisotropy than is seen in electrical conductivity and water diffusion in similar systems. These results are compared to electrical conductivity, water and dye diffusion measurements.