CHARACTERIZATION OF EMULSIONS BY NMR METHODS

Pulsed-gradient spin-echo NMR experiments are performed on emulsions stabilized by nonionic, cationic, and anionic surfactants. The mean displacements during the measuring time of the liquid inside the emulsion droplets are of the same order of magnitude or larger than the diameter of the droplet, and as a consequence the molecules undergo restricted diffusion. Murday and Cotts' equation for restricted diffusion inside spherical cavities can be fitted to the experimental data using the bulk diffusion coefficient value for the liquid. The continuous medium gives normal diffusion coefficients, slightly reduced due to obstruction effects of the emulsion droplets. For some cases the stimulated spin-echo technique must be used to evaluate the diffusion coefficients as shortT2's make it difficult to perform experiments with the ordinary PGSE sequence. The approach can be used in dilute and nondilute systems to infer information about the droplet size and size distribution and whether a particular emulsion is of the O/W or the W/O type. © 1991 Academic Press, Inc. All rights reserved.