Phase diagrams, emulsions microemulsions

Scientifically described for the first time in 1943 by Hoar and Schulman (2), the latter author coined the term microemulsion in 1959 to describe these optically isotropic transparent oil and water dispersions (3). Since this early work, many experimental and theoretical efforts have shown that these dispersions are actually solutions, namely thermodynamically stable equilibrium phases (4). Consequently, the most widely, but still not universally accepted definition of a microemulsion is that of a thermodynamically stable mixture of oil and water. Occasionally, the term microemulsion (5) or miniemulsion (6) is used to describe long-lived emulsions with ultra-small droplet sizes (30-100 nm). Sometimes, stable emulsions may be created by agitation of systems while passing through regions of the phase diagram where microemulsion phases form however, the final state is in the emulsion region (7, 8). In this present chapter, we use the most widely accepted definition of microemulsions, namely equilibrium phases of oil and water (9). 

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). 

The interfacial tension y at the planar interface has a minimum near the temperature Indeed, at the latter temperature r is small, A/jt0 = 0 and because d ij w/d J and dfi /dT have opposite signs and s is also small (because T is small), dy/d I 0. The temperature T0 is provided by Eq. (25) and is independent of the concentration of surfactant. In other words, the two minima of Fig. 4 which correspond to the phase inversion temperatures of a macroemulsion (the curve with a positive minimum) and microemulsion (the curve with a negative minimum) are the same. When emulsions are generated from a microemulsion and its excess phase, the emulsion is of the same kind as the microemulsion, the phase inversion temperature is obviously located in the middle of the middle phase microemulsion range and the above conclusion remains valid. The above results explain the observation of Shinoda and Saito [6,7] that the phase inversion temperature (PIT) of emulsions can be provided by the ternary equilibrium phase diagram. 

FIGURE 4.25 Three-component phase diagram for the solubilization. Cre, Cremophor RH 40 Gly, glyceride Pol, poloxamer 124 L, isotropic microemulsion G, gel E, crude OAV emulsion E2, W/O emulsion. [Graph reconstructed from data by Kim et al. Pharm. Res., 18, 454 (2002).]... 

The term microemulsion to describe such systems is not well chosen it conveys the idea of an actual emulsion characterized by submicrometer (below 0.1 tun) droplets. As is well known, an emulsion is not thermodynamically stable and cannot be represented by a single-phase domain in a thermodynamic phase diagram. The so-called microemulsions must be considered as real micellar solutions containing oil in addition to water and surfactants. These solutions, although very far from ideal in the thermodynamic sense, are nevertheless always real in the thermodynamic sense. Another important difference between microemulsions and emulsions is that, in general, a microemulsion requires significantly more surfactant than an emulsion. 

Microemulsions are thermodynamically stable phases, which can be represented by clear areas in equilibrium phase diagrams. Nanoemulsions are really small emulsions, with the main characteristics of emulsions they are not thermodynamically stable and the way they are prepared has a great impact on their physical stability. The only difference with common emulsions is their very small droplet size, which ranges from 10 to 500 nm. Accordingly, nanoemulsions may look bluish, due to light diffusion (brown/yellow by transmission), just like microemulsions close to a critical point. 

Pioneering work by Bates, Lodge and co-workers [5-12] demonstrated that the addition of diblock copolymer (A-B) into the mixture of its corresponding homopolymer A and B drives self-assembly into varied structures including droplet-type microemulsions [8], bicontinuous micro emulsions [5, 6, 8, 10, 12], hexagonal phases [10, 11] and lamellar phases [5, 8, 12]. Figure 7.1 shows a typical temperature-composition phase diagram of symmetric polyethylene (PE)/polyethylenepropylene (PEP)/PE-PEP mixtures [5]. To... 

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). 

These AOT microemulsions are characterized by a high surfactant to monomer ratio, (2.5-3). If the amount of AOT is too low, the optically transparent microemulsions evolve towards turbid and unstable latexes during polymerization, due to a shift in the emulsion region of the phase diagram [55,56]. However, it should be noted that AM plays the role of a cosurfactant in these systems, owing to its surface-active properties, thus leading to an increase in the micellar stabilization capacities [48]. 

Microemulsions are prepared by the spontaneous anulsilication method (phase titration method) and can be depicted with the help of phase diagrams. Construction of phase diagram is a useful approach to study the complex series of interactions that can occur when different components are mixed. Microemulsions are formed along with varions association structures (including emulsion, micelles, lamellar, hexagonal, cubic, and varions gels and oily dispersion), depending on the chemical composition and concentration of each component. 

The viscosity of bicontinuous microemulsions is less straightforward to describe theoretically than systems of isolated aggregates. In general, bicontinuous emulsions also exhibit low viscosity and simple Newtonian flow behavior, but it should be instructive to compare their rheological properties to that of O/W or W/O microemulsions, which in the phase diagram border on the bicontinuous microemulsion. 

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