Microemulsion terminology

This ambiguity in the microemulsion terminology remains today (41). The microemulsions are defined as the clear thermodynamically stable dispersions of two immiscible liquids containing appropriate amounts of surfactants or surfactants and cosurfactants. The dispersed phase consists of small droplets with diameter in the range of 100-1000A0. Because of these properties, such systems have several advantages over macroemulsions for industrial applications. 

McClements DJ. Nanoemulsions versus microemulsions Terminology, differences, and similarities. Soft Matter. 2012 8(6) 1719-1729. 

Fig. 2. The phase diagrams and terminology of a microemulsion system close to its two critical end points, where the middle phase and one of the binodals...

Several categories of microemulsions that refer to equilibrium phase behaviours and that distinguish, for example, the number of phases that can be in equilibrium and the nature of the continuous phase. They are denoted as Winsor Type I (oil-in-water), Type II (water-in-oil), Type III (most of the surfactant is in a middle phase with oil and water), and Type IV (water, oil, and surfactant are all present in a single phase). The Winsor Type III system is sometimes referred to as a middle-phase microemulsion , and the Type IV system is often referred to simply as a microemulsion . An advantage of the Winsor category system is that it is independent of the density of the oil phase and can lead to less ambiguity than do the lower-phase or upper-phase microemulsion type terminology. Nelson type emulsions are similarly identified, but with different type numbers. 

Macroemulsion In enhanced oil recovery terminology, the term macroemulsion is sometimes employed to identify emulsions having droplet sizes greater than some specified value, and sometimes simply to distinguish an emulsion from the microemulsion or micellar emulsion types. See also Emulsion. 

The system has two phases an excess oil phase and a water-external microemulsion phase. Because microemulsion is the aqueous phase and is denser than the oil phase, it resides below the oil phase and is called a lower-phase microemulsion. At a high salinity, the system separates into an oil-external microemulsion and an excess water phase. In this case, the microemulsion is called an upper-phase microemulsion. At some intermediate range of salinities, the system could have three phases excess oil, microemulsion, and excess water. In this case, the microemulsion phase resides in the middle and is called a middle-phase microemulsion (Healy et al., 1976). Such terminology is consistent with their relative positions in a test tube (pipette) with the water being the dense liquid. In the environmental sciences and engineering, however, a dense nonaqueous phase liquid (DNAPL) could be denser than water (UTCHEM-9.0, 2000). Fleming et al. (1978) used y, P, and a to name the lower-phase, middle-phase, and upper-phase microemulsions, respectively. 

Grossiord et al. (21) diseuss an additional method termed, by them, the oily isotropic dispersion process. We prefer a more accurate terminology of emulsified microemulsions. The idea is to disperse an oil phase within water by smfactant and to form an L2 phase. This phase is... 

FIGURE 1.8 Part of the phase diagram water (W), SDS, and pentanol (C5OH). The areas named microemulsions in this Figure 1.4 were called micellar solutions in Ekwall s terminology (From Ekwall, P., m Advances in liquid Crystals, Brown, G.H. (Ed.), Academic Press, New York, 1975, pp. 1-139. With permission.)... 

For compositions outside the monophasic microemulsion domains of the phase diagrams, polyphasic systems are obtained for which the Winsor terminology is widely adopted (see Chapter 1, Section In this terminology a homoge-... 

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