Aqueous microemulsions phase diagram

The locations of the tietriangle and biaodal curves ia the phase diagram depead oa the molecular stmctures of the amphiphile and oil, on the concentration of cosurfactant and/or electrolyte if either of these components is added, and on the temperature (and, especially for compressible oils such as propane or carbon dioxide, on the pressure (29,30)). Unfortunately for the laboratory worker, only by measuriag (or correcdy estimatiag) the compositions of T, Af, and B can one be certain whether a certain pair of Hquid layers are a microemulsion and conjugate aqueous phase, a microemulsion and oleic phase, or simply a pair of aqueous and oleic phases. 

Nevertheless, possibiUties for confusion abound. From the definitions of microemulsions and macroemulsions and from Figure 1, it immediately follows that in many macroemulsions one of the two or three phases is a microemulsion. Until recentiy (49), it was thought that all nonmultiple emulsions were either oil-in-water (O/W) or water-in-oil (W/O). However, the phase diagram of Figure 1 makes clear that there are six nonmultiple, two-phase morphologies, of which four contain a microemulsion phase. These six two-phase morphologies are oleic-in-aqueous (OL/AQ, or O/W) and aqueous-in-oleic (AQ/OL, or W/O), but also, oleic-in-microemulsion (OL/MI), microemulsion-in-oleic (MI/OL), aqueous-in-microemulsion (AQ/MI), and microemulsion-in-aqueous (MI/AQ) (49). 

Figure 4.11 Scheme of a phase diagram of the system water-isooctane-AOT-PEO with the polymer content Cp as a function of the aqueous phase water content Xwp- L2 is the w/o-droplet phase in the region 2 a microemulsion coexists with an aqueous phase al specifies a solid polymer coexistence with a microemulsion in the region t2 solid polymer, aqueous polymer solution and a microemulsion are found. The real phase diagram with more phase regions is found in [52]. (From Ref. [52], reprinted with permission of the American Chemical Society.)... 

Figure 4.12 Phase diagram of the system water-isooctane-AOT-PEO as a function of the temperature T and the surfactant concentration y. The isooctane content was fixed at 40wt.% and the polymer content in the aqueous phase was either Cp = 5 or 20 wt.%. The phase regions are 2 for microemulsion coexisting with excess water, 2 for microemulsion coexisting with excess oil, 3 for three-phase coexistence of a microemulsion with two excess phases. Inside the fish-tail there are one-phase regions L Li for o/w-droplet microemulsions, L2 for w/o-droplet microemulsions, La for the lamellar phase and 2a for a coexistence of a lamellar with a microemulsion phase. Note that the whole fish-tail is shifted to higher surfactant concentrations upon polymer addition. (From Ref. [53], reprinted with permission of the American Chemical Society.)...

When an aqueous system containing a surfactant, cosurfactant and of intermediate salinities is allowed to equilibrate with crude oil, the mixture sometimes separates into three phases. One of these phases is the aqueous phase which contains very little surfactant. This is called the lower phase. The second phase is called the middle phase. This phase is a microemulsion which contains large amounts of both oil and water and nearly all the surfactant. The third (upper) phase contains the oil. Systems of oil and aqueous phases which show this phase behavior are said to exist in the "beta" region of the phase diagram. The "beta" type systems have been shown to form the least stable emulsions and thereby result in enhanced oil recovery (30). 

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