Middle-phase microemulsion, definition

Fig. 1. Phase diagram of an amphiphile—oil—water system that forms a middle-phase microemulsion, definition of microemulsion, and illustration of the...

Nevertheless, possibilities for confusion abound. Until recently (50), it was thought that all nonmultiple emulsions were either oil-in-water (O/W) or water-in-oil (W/O). However, from the definitions of microemulsions and macroemulsions and from Fig. 16.2, it follows that in many macroemulsions one of the two or three phases is a microemulsion. The phase diagram of Fig. 16.2 makes clear that there are six possible nonmultiple, two-phase morphologies, of which four contain a microemulsion phase. These six two-phase morphologies are oleic-in-aqueous (017AQ, or O/W) and aqueous-in-oleic (AQ/OL, or W/O), but also, oleic-in-microemulsion (OL/Ml), microemulsion-in-oleic (MI/OL), aqueous-in-microemulsion (AQ/MI), and microemulsion-in-aqueous (MI/AQ) (50). [Although they have not yet aU been reported, theoretically there are 12 three-phase emulsion morphologies formed by the top, microemulsion (i.e., middle), and bottom phases (51,52) of three-phase microemulsion systems.]... 

The polyphasic region contains a three-phase zone surrounded by three two-phases zones. Systems whose composition lies in the three-phase zone separate into an amphi-phUe-rich phase (m), which is in the middle of the diagram at the boundary of the single-phase region, and two excess phases, which are essentially pmre aqueous phase and pure oil. This amphiphile-rich phase, which is found to obey, in most cases, the definition proposed for a bicontinuous microemulsion or a liquid crystal, has been called a middle phase because its intermediate density makes it appear in between the oil and water phases in a test tube. Because the middle phase is at equilibrium with both excess phases, it cannot be diluted either by water or oil, and it is thus neither water nor oil-continuous. This is another hint of the bicontinuous nature of this phase, as conductivity and viscosity measurements and other experimental evidences have shown [20-22,37-40]. 

Another important phase classification that can often be found in literature has been introduced by Winsor (Winsor 1948), who found four general types of phase equilibria. A Winsor type IV phase corresponds to classical single phase microemulsions consistent with Hoar s Schulman s definition (Hoar Schulman 1943). A Winsor type I system denotes two phases in equilibrium, an o/w structure and an almost pure oil upper phase. On the contrary, in a Winsor type II system an aqueous phase containing surfactant is in equilibrium with an w/ o microemulsion in the upper phase. Finally, Winsor type III structures equal a three-phase system consisting of a surfactant poor water phase, a bicontinuous middle phase, and an upper almost pure oil phase. 

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