Microemulsions emulsification failure

The phase stability of organic extractant phases and classical reverse microemulsions are thus governed by the same rule. This general conclusion would not have been valid for extractant systems in the case of an emulsification failure mechanism, namely rejection of the internal phase, but this was not observed in liquid/liquid extraction systems when salt is extracted. 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

Figure 1.19 Micrographs of microemulsion droplets of the o/w-type in the system II2O- n-octane-CnEs prepared near the emulsification failure boundary at ya = 0.022, wb = 0.040 and T = 26.1 °C. (a) Freeze-fracture direct imaging (FFDI) picture showing dark spherical oil droplets of a mean diameter = 24 9 nm in front of a grey aqueous background. Note that each oil droplet contains a bright domain of elliptic shape which is interpreted as voids, (b) The freeze-fracture electron microscopy (FFEM) picture supports the FFDI result. Each fracture across droplets which contain bubbles shows a rough fractured surface. (From Ref. [26], reprinted with permission of Elsevier.)...

Olsson, U. and Schurtenberger, P. (1993) Structure, interactions, and diffusion in a ternary nonionic microemulsion near emulsification failure. Langmuir, 9, 3389-3394. 

An example of such a simple O/W system was investigated with the nonionic surfactant pentaethylene glycol dodecyl ether (C12E5) together with hydrocarbon and water, i.e., the simplest possibility to form a microemulsion (surfactant, oil, water).This system had been studied in some detail before [47-49], and it is well established that it contains spherical aggregates close to the emulsification failure, i.e., when it is saturated with oil. 

The second chapter, by D. Vollmer (Germany), brings a quantitative comparison of experimental data and theoretical predictions on thermodynamic and kinetic properties of microemulsions based on nonionic surfactants. Phase transitions between a lamellar and a droplet-phase microemulsion are discussed. The work is based on evaluation of the latent heat and the specific heat accompanying the transitions. The author focuses on the kinetics of phase separation when inducing emulsification failure by constant heating. The chapter is a comprehensive, detailed study of all the aspects related to the phase separation phenomenon in microemulsions. 

The scheme outlined here has turned out to make up a convenient approach, first, to treating Winsor I and Winsor II microemulsion systems where an excess phase is present [38], in which case it is sufficient to consider merely the first interfacial term of Eq. (191), and, second, to the corresponding one-phase microemulsions elose to the two-phase region (i.e., on the border to what occasionally is referred to as emulsification failure) [47]. 

The minimization of the total free energy with the volume fraction of the dispersed phase gives information on the stability of the microemuisions droplets. For small attractive interactions, the microemulsion will demix with an excess of water because of the curvature effect, with this mechanism being called the emulsification failure . On the other hand, for a sufficiently strong attractive droplet interaction, the droplets are destabilized, thus leading to a phase separation of a microemulsion with a smaller droplet volume fraction. 

Figure 15.6. Self-diffusion measurements of the oil component in a microemulsion containing didodecyldimethylammonium sulfate (DDAS)/dodecane/water (D2O) at different ratios of surfactant to oil. The X-axis represents the total volume of oil plus surfactant, i.e. the volume fraction of aggregates/micelles according to equation (15.7) represents oil diffusion when the system solubilizes a maximum amount of oil (the emulsification failure line), with the continuous line being a fit of equation (15.7) to the data represents the oil diffusion at lower oil-to-surfactant ratios when the system forms prolate structures the corresponding line is simply a guide for the eye (Nyden, unpublished data)...

The Winsor I and Winsor II equilibria (emulsification failure) results from a finite swelling of droplets, and correspond to saturated solutions in equilibrium with excess solubilizate. In the Winsor III equilibrium, the (middle-phase) microemulsion has a bicontinuous structure. Similar to the Winsor I and II equilibria, the three-phase equilibrium can be considered as a finite swelling of the bicontinuous microstructure. The three-phase triangle forms from critical end-points, on the water-rich and one on the oil-rich side. The onset of three phase equilibria appears to be correlated with a micellar to bicontinuous structural transition in the microemulsion phase. 

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