DIFFUSION OF THE DISPERSED PHASE IN A HIGHLY CONCENTRATED EMULSION - EMULSION STRUCTURE AND FILM PERMEATION

A highly concentrated emulsion, made up of lecithin, C(12)EO(4), n-hep tane, and brine (1 wt% NaCl solution), has been investigated by means of the NMR PFG method. The emulsion is of the W/O type and contains 96 wt% brine. The self-diffusion of the water has been investigated at t hree different diffusion times, and it is found that the water diffusi on is Gaussian over the range of the relevant experimental parameters covered, with a value for the self-diffusion coefficient that is reduc ed by a factor of roughly 10, as compared with the bulk water diffusio n coefficient. The reduction is caused by the presence of a thin liqui d film which separates the emulsion droplets and which exerts a diffus ion barrier for the water molecules within the droplets. The data are interpreted by means of a simulation procedure, in which particles per form a random walk in droplets, the surface of which is partially perm eable to the particles. With independent structural information the si mulation approach yields a permeability coefficient for the water cros sing the film. Finally, it is argued that concentrated emulsions are e xcellent model systems for developing PFG methods in studying the gene ral class of porous systems. (C) 1996 Academic Press, Inc.