Microemulsions phase diagram with

Fig. 5 shows a hypothetical phase diagram with representation of microemulsion structures. At high water concentrations, microemulsions consist of small oil droplets dispersed in water (o/w microemulsion), while at lower water concentrations the situation is reversed and the system consists of water droplets dispersed in oil (w/o microemulsions). In each phase, the oil and water droplets are separated by a surfactant-rich film. In systems containing comparable amounts of oil and water, equilibrium bicontinuous structures in which the oil and the water domains interpenetrate in a more complicated manner are formed. In this region, infinite curved channels of both the oil and the water domains extend over macroscopic distances and the surfactant forms an interface of rapidly... 

The phase behavior observed in the quaternary systems A and B is also evidenced in ternary systems. Figure 4 shows the phase diagrams for systems made of AOT-water and two different oils. The phase diagram with decane was established by Assih (14) and that with isooctane has been established in our laboratory. At 25°C the isooctane system does not present a critical point and the inverse micellar phase is bounded by a two-phase domain where the inverse micellar phase is in equilibrium with a liquid crystalline phase, as for system B or system A when the W/S ratio is below 1.1. In the case of decane, a critical point has been evidenced by light scattering (15). Assih and al. have observed around the critical point a two-phase region where two microemulsions are in equilibrium. A three-phase equilibrium connects the liquid crystalline phase and this last region. 

Figure 4.9 Phase diagram of the system water-decane-CioE4 at equal volume fractions of water and decane as a function of the temperature T and the surfactant concentration cf>7. At low (f>7 there is a three-phase coexistence, while at moderate cf>7 the one-phase bicontinuous microemulsion appears. At even higher cf>7 the lamellar phase appears. At high and low temperatures a microemulsion phase coexists with either excess water or oil. The polymer fraction cf>p is raised symmetrically for the water- and oil-soluble polymers, and the one-phase microemulsion window closes continuously. The 2 K temperature shift is due to the use of heavy water. (From Ref. [40], reprinted with permission of the American Chemical Society.)...

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 17 Illustration of the fact that microemulsion structure is not simply a function of composition. Shown are partial ternary phase diagrams with nonionic and cationic surfactants at room temperature. For a similar composition (approximately 15% surfactant, 65 wt% water, and 20 wt% oil), the microstructures of the two systems are widely different, as shown by the ratio of the water and oil diffusion coefficients, Dn /Dhc where he here denotes oil (hydrocarbon). The nonionic system has an oil-in-water structure (D //)hc = 200), while the cationic system has a water-in-oil structure (D,/Z)h. = 1/200).

This study supports the assumption that the alcohol influences the phase behavior of oil-brine-surfactant system and its effect is related to the saturation concentration in the water-phase. On the other hand, the ternary phase diagram with a constant amount of the cosolvent seems to be more valuable for selecting microemulsion formulations than the pseudo-ternary representation in which both alcohol and surfactant are varied simultaneously. 

Lattice models have been studied in mean field approximation, by transfer matrix methods and Monte Carlo simulations. Much interest has focused on the occurrence of a microemulsion. Its location in the phase diagram between the oil-rich and the water-rich phases, its structure and its wetting properties have been explored [76]. Lattice models reproduce the reduction of the surface tension upon adsorption of the amphiphiles and the progression of phase equilibria upon increasmg the amphiphile concentration. Spatially periodic (lamellar) phases are also describable by lattice models. Flowever, the structure of the lattice can interfere with the properties of the periodic structures. 

FIG. 2 Example media (a) Surfactant-water phase diagram. (Reprinted from Ref. 206, Copyright 1991, with permission from Elsevier Science.) (b) Ordered periodic and bicontinuous structures. (Reprinted from Ref. 178 with permission from Academic Press, Ltd.) (c) Nonordered membrane structures from ternary microemulsions. (Reprinted with permission from Ref. 177, Copyright 1989, American Chemical Society.)... 

Winsor reported that the phase behavior of SOW systems at equilibrium could exhibit essentially three types, so called Wl, Wll and Will, illustrated by the phase diagrams indicated in Fig. 1. In the Wl (respectively, Wll) case, the surfactant bears a stronger affinity for the water (respectively, oil) phase and most of it partitions into water (respectively, oil). As a consequence, the system exhibits a two-phase behavior in which a microemulsion is in equihb-rium with excess oil (respectively, water). 

The aim of the experiment is to determine the microemulsion (i.e. clear, single-phase) region for the three components already discussed and map out the results on a triangular phase diagram. The microemulsion region is determined by making up a series of mixtures in lOcm stoppered Erlenmeyer flasks with compositions that span the anticipated range. The procedure is to start with a volume of about 2cm of oil... 

The review by Attwood and Florence (1998) is valuable because it provides a comprehensive survey of the earlier pharmaceutical literature. From this we can learn that microemulsions are formed spontaneously from fixed compositions of surfactant(s), oil, and water but these compositions change with temperature. As a stable system is diluted progressively with, for example, water, the microemulsion phase will spontaneously revert to other phases on the diagram, including unstable emulsions or a solution. This becomes relevant when considering what happens to the system when it is administered. No matter how it is administered the microemulsion phase will be diluted, most likely with water in some form or another, and revert to some other composition in the body. 

A totally different way of looking at microemulsions —and one that connects this topic with previous sections of the chapter —is to view them as complicated examples of micellar solubilization. From this perspective, there is no problem with spontaneous formation or stability with respect to separation. Furthermore, ordinary and reverse micelles provide the basis for both O/W and W/O microemulsions. From the micellar point of view, it is the phase diagram for the four-component system rather than y that holds the key to understanding microemulsions. 

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