Phase behavior of cationic microemulsions near the tricritical point

The three-phase behavior of a microemulsion was investigated near the tricr itical point in a NaBr/water/hexadecyl-trimethylammonium bromide (CTAB)buta nol/decane system at 25 degrees C. The three-phase body, consisting of wate r-rich, microemulsion (surfactant), and oil-rich phases, exists between two critical endpoints where the water (or oil) phase becomes identical with t he microemulsion phase. It is found in the phase diagram that the three-pha se body shrinks with the change of NaBr concentration in water, and eventua lly disappears at the optimum concentration 0.5 wt % of NaBr. Not only the salinity but also the butanol/decane ratio effects the reduction of the thr ee-phase body. This system was characterized by means of density, interfaci al tension, and static lightscattering measurements. The change of the dens ity values of two phases near the tricritical point where the three-phase b ody is not observed, apparently showed that the oil droplets disperse in wa ter (O/W type) in the lower phase, while in the upper phase the water dropl ets disperse in oil (W/O type). Under a fixed decane/butanol ratio, the tri critical point tends to be reached with increasing CTAB concentration. As t he tricritical point was approached, at which two phases look bluish, the l ight scattering experiment showed that the microemulsion droplets in the up per and the lower phases increased in size and their sizes become of the sa me order. At the tricritical point, the microemulsions with the maximum sol ubilization of water or oil merge with each other due to efficient droplet size and strengthened interaction between the particles. Consequently, the bicontinuous structure was formed.