Confined self-diffusion of lecithin in the lecithin-water system

The self-diffusion of lecithin molecules in a disperse system obtained by t he mechanical mixing of lecithin with deuterated water was investigated by the pulsed magnetic-field gradient H-1 NMR technique at 310 K. The experime ntal conditions were the following: the maximal gradient amplitude was equa l to 50 T m(-1) the diffusion time varied from 4.5 ms to 1 s, the lecithin concentration varied from 10 to 40%, and the residual diethyl ether content in the system was 3-6%. At all concentrations and diffusion times, the dif fusion decay of spin-echo signals had a complex shape and was represented a s a sum of two components corresponding to the "phases" characterized by co mpletely confined and unconfined self-diffusion. The phase characterized by unconfined self-diffusion, on the basis of the character of its independen ce of the diffusion time and population, was assigned to protons of water a nd ether molecules. The behavior of the component whose shape depended on t he diffusion time was quantitatively analyzed in terms of the model of self -diffusion of molecules in a thin spherical layer. To reach the best agreem ent between calculated and experimental diffusion decays, a bimodal distrib ution of spheres was introduced as the sum of two lognormal distributions. Parameters of these distributions were determined for all the investigated concentrations. The average radius of spheres for the mode characterized by larger radii is close to published values of the radius of vesicles formed in lecithin-water systems obtained by mechanical mixing.