Single-Phase Microemulsions

WII Winsor II system (W/0 pern in equilibrium with excess water) WIN Winsor III system (pern in equilibrium with excess oil and water) and WIV Winsor IV (single-phase pern). 

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In the Will case, provided that there is enough surfactant but not too much, e.g., 1 wt. %, the system splits into three phases, i.e., a microemulsion in equilibrium with excess water and excess oil. At a higher surfactant concentration than the top vertex of the 3

occurrence generally requires a large amount of surfactant, e.g., 20 wt. %, which is in most practical cases too much for cost reasons. At a very low surfactant concentration, around the CMC, only two phases are in equilibrium, and the tension is not necessarily very low. Hence, the convenient surfactant concentration to carry out a phase behavior study is in the range 0.5-3 wt. % for which three-phase behavior and a very low inter facial tension is exhibited in most Will cases. 

For a given surfactant, the ability to form a single-phase w/o microemulsion is a function of the type of oil, nature of the electrolyte, solution composition, and temperature (54-58). When microemulsions are used as reaction media, the added reactants and the reaction products can also influence the phase stability. Figure 2.2.4 illustrates the effects of temperature and ammonia concentration on the phase behavior of the NP-5/cyclohexane/water system (27). In the absence of ammonia, the central region bounded by the two curves represents the single-phase microemulsion region. Above the upper curve (the solubilization limit), a water-in-oil microemulsion coexists with an aqueous phase, while below the lower curve (the solubility limit), an oil-in-water water microemulsion coexists with an oil phase. It can be seen that introducing ammonia into the system results in a shift of the solubilization... 

Single phase microemulsions are treated in the next section. Two general thermodynamic equations are derived from the condition that the free energy of the system should be a minimum with respect to both the radius r of the globules as well as the volume fraction of the dispersed phase. The first equation can be employed to calculate the radius while the second, a generalized Laplace equation, can be used to explain the instability of the spherical shape of the globules. The two and three phase systems are examined in Sections III and IV of the paper. 

Instability of the Spherical Internal Interface in Single Phase Microemulsions... 

It is of interest to note that, because at equilibrium pi =p, in a single-phase microemulsion the pressure p2 in the globules is expected to be lower than that in the... 

The fractional saturation of water in the oil phase is denoted/Wo- For a single-phase microemulsion,/Wo is less than unity. As the amount of water in the phase increases, /Wo progressively increases and reaches the constant value of unity when an excess aqueous phase coexisting with the microemulsion phase appears, creating a two-phase system. 

When eq 3.1 is used to calculate the size and composition distributions of droplets in a single-phase O/W microemulsion, Xgo and ygo should be replaced by XgVi and ygW and the factor f f0 should be replaced by ffm, where fow denotes the fractional saturation of oil in the water phase. As before, fow is less than unity for a single-phase microemulsion and reaches the constant value of unity when a two-phase system comes into existence with an excess oil phase coexisting with the microemulsion phase. 

In addition to single phase microemulsions, several phase equilibria known as Winsor systems [4] are also shown at low surfactant concentrations. A Winsor I (WI) system consists of an 0/W microemulsion that is in equilibrium with an oil phase, while a Winsor II (WII) system is a W/0 microemulsion in equilibrium with an aqueous phase. A Will system has a middle phase (bicontinuous) microemulsion that coexists with both oil and aqueous phases. 

Figure 4.19 Single phase microemulsion regions within the ternary phase triangles of mixtures of DDAB/water and a range of hydrocarbons. (Adapted from [40]).

Palani Raj, W.R., Sasthav, M. and Cheung, H.M. (1995) Polymerization of single phase microemulsions Dependence of polymer morphology on microemulsion structure. Polymer, 36, 2637-2646. 

The problem of large amounts of amphiphiles (i.e., surfactant and cosurfactant) required to form a single-phase microemulsion can be overcome with multiphase microemulsions. Indeed, two- and three-phase microemulsion systems can be obtained with only about 3-5% surfactant (Scheme 22.5). 

FIGURE 32.18 Schematic representation of the main single-phase microemulsion systems (a) 0/W, (b) W/0, and (c) bicontinuous. 

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