NONEQUILIBRIUM STRUCTURE OF WATER-IN-OIL GEL EMULSIONS

Nonequilibrium structure of water in oil (W/0) type gel emulsions (or highly concentrated emulsions) was investigated by means of the ESR spin probe method. The change in apparent order parameter ''S'' and the isotropic hyperfine splitting constant 'aN'' of spin probes (5- and 16-doxylstearic acids) in gel emulsions, ordinary W/0 (middle- or low-internal-phase-ratio) emulsions, and single oil phases were determined along a tie line in which the oil/surfactant ratio is kept constant and only water content is changed in a water/triethylene glycol dodecyl ether (or tetraethylene glycol hexadecyl ether)/cyclohexane system. The apparent order parameter ''S'' and the isotropic hyperfine splitting constant ''a(N)'' of 5-doxylstearic acid increase slowly but continuously in an ordinary emulsion region beyond the solubilization limit and then abruptly increases in the gel emulsion. Whereas in case of 16-doxylstearic acid, both S and a(N) remain unchanged over a wide range of water contents except in the gel emulsions. Using a simple model, we have calculated the distribution of surfactant molecules between the reverse micelle and the water-oil interface in emulsions. The result predicts that there is no reverse micelle in the continuous oil phase and the spin probe exists at the water-oil interface in gel emulsions, and the water droplet size increases in the gel emulsion region with increasing water content. These predictions are in good agreement with the ESR data and the microscopic observations.