Phase Behavior of Cationic Microemulsions near the Tricritical Point

Abstract

The three-phase behavior of a microemulsion was investigated near the tricritical point in a NaBr/ water/hexadecyl-trimethylammonium bromide (CTAB)/butanol/decane system at 25°C. The three-phase body, consisting of water-rich, microemulsion (surfactant), and oil-rich phases, exists between two critical endpoints where the water (or oil) phase becomes identical with the microemulsion phase. It is found in the phase diagram that the three-phase body shrinks with the change of NaBr concentration in water, and eventually 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 three-phase body. This system was characterized by means of density, interfacial tension, and static light scattering measurements. The change of the density values of two phases near the tricritical point where the three-phase body is not observed, apparently showed that the oil droplets disperse in water (O/W type) in the lower phase, while in the upper phase the water droplets disperse in oil (W/O type). Under a fixed decane/butanol ratio, the tricritical point tends to be reached with increasing CTAB concentration. As the tricritical point was approached, at which two phases look bluish, the light scattering experiment showed that the microemulsion droplets in the upper and the lower phases increased in size and their sizes become of the same order. At the tricritical point, the microemulsions with the maximum solubilization 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.

Preferential solubilization of dodecanol from dodecanol–limonene binary oil mixture in sodium dihexyl sulfosuccinate microemulsions: Effect on optimum salinity and oil solubilization capacity

“…Variations of the lower critical salinity (where the Winsor type I-III transition occurs) and the optimum salinity with dodecanol concentration could also be a good indication of approaching tricritical behavior [28,29]. It is seen in Fig.…”

“…For example, increasing the overall oil polarity may result in approaching a tricritical point [27][28][29][30] or appearance of liquid crystalline or surfactant-rich sponge phases [31,32]. These changes result in the disappearance of the microemulsion behavior.…”

“…The tricritical point is defined as the point at which three conjugate phases become simultaneously identical [27], resulting in a single-phase system. The single-phase system is brought about by merging the lower and upper critical endpoints at sufficiently high surfactant concentrations [28,29]. Tricritical systems are associated with the weakening of the structure [29,30].…”

Formation of Microemulsions with Gemini-Type Surfactant.

“…It means that the surfactant aggregate changes from W m (discrete normal micelle) phase to O m (reverse micelle) phase without phase separation. This is a typical phase behavior beyond the tricritical point, at which two critical endpoints of W-D and O-D are connected (13,14). Figure 7(a) and (b) show the phase diagrams of the 3 wt% NaCl aq./GS/ MEH/octane and tetradecane systems at 25 , respectively.…”

Modeling solubilization of oil mixtures in anionic microemulsions

“…But by analyzing the variation of the interfacial tension between the coexisting phases it might be possible to identify the non-wetting/wetting transition point [41]. Beyond the wetting regime, when the tricritical point is reached along this path, the bicontinuous microemulsion structure is lost [42]. To predict this point for ionic surfactant systems, the observation that ionic microemulsions can only be formed above a certain electrolyte concentration [43] might be a starting point.…”