Determination of genuine diffusivities in heterogeneous media using stimulated echo pulsed field gradient NMR

Pulsed field gradient (PFG) NMR diffusion measurements in heterogeneous med ia may lead to erroneous results due to the disturbing influence of interna l magnetic held gradients. Here, we present a simple theoretical model whic h allows one to interpret data obtained by stimulated spin echo PFG NMR in the presence of spatially varying internal held gradients. Using the result s of this theory, the genuine self-diffusion coefficients in heterogeneous media may be extrapolated from the dependence of the apparent diffusivities on the dephasing time of the simulated echo PFG NMR sequence. Experimental evidence that such extrapolation yields satisfactory results for self-diff usion of hexadecane in natural sediments (sand) and of n-octanol in doped M gO pastes is provided. (C) 2001 Academic Press.