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-heptane, 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 three different diffusion times, and it is found that the water diffusion is Gaussian over the range of the relevant experimental parameters covered, with a value for the self-diffusion coefficient that is reduced by a factor of roughly 10, as compared with the bulk water diffusion coefficient. The reduction is caused by the presence of a thin liquid film which separates the emulsion droplets and which exerts a diffusion barrier for the water molecules within the droplets. The data are interpreted by means of a simulation procedure, in which particles perform a random walk in droplets, the surface of which is partially permeable to the particles. With independent structural information the simulation approach yields a permeability coefficient for the water crossing the film. Finally, it is argued that concentrated emulsions are excellent model systems for developing PFG methods in studying the general class of porous systems. (C) 1996 Academic Press, Inc.