W/O microemulsions

The use of surfactants has been an important positive factor for several reasons. They form O/W microemulsions, which must have low viscosity and contain a high oil content later on this oil must be separated fairly easily. 

It is convenient to differentiate between oil-in-water (o/w) microemulsions and water-in-oil (w/o) microemulsions in which water and oil are the respective major components. It is reasonable to regard (o/w) microemulsions as akin to swollen normal micelles and w/o microemulsions as reverse micelles (Section 1). 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Araya H, Tomita M, Hayashi M (2006) The novel formulation design of self-emulsifying drug delivery systems (SEDDS) type O/W microemulsion III The permeation mechanism of a poorly water soluble drug entrapped O/W microemulsion in rat isolated intestinal membrane by the Ussing chamber method. Drug Metab Pharmacokinet 21 45-53. 

Before describing how microemulsion nature and structure are determined by the structure and chain length of surfactant and cosurfactant, it is necessary first to briefly review the theories of microemulsion formation and stability. These theories will highlight the important factors required for microemulsion formation. This constitutes the first part of this review. The second part describes the factors that determine whether a w/o or o/w microemulsion is formed. This is then... 

This means that an o/w microemulsion results in thxs case On... 

These systems were referred to by Clausse t a (21) as Type U systems. On the other hand, with cofurfactants with chain length Cg to Cy (Figur 3 e-g), the Winsor IV domain is split into two disjointed areas that are separated by a composition zone over which viscous turbid and birifringent media are encountered. This second class of systems was referred as Type S systems (24). It can also be seen that the Winsor IV domain reaches its maximum extension at reducing in size below and above C. Moreover, at C, one observes a small monophasic region near the W apex (probably o/w microemulsion of the Schulman s type) which vanishes as the alcohol chain length is increased to Cg. 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

Fig. 15.4 Schematic ternary-phase diagram of an oU-water-surfactant microemulsion system consisting of various associated microstructures. A, normal miceUes or O/W microemulsions B, reverse micelles or W/O microemulsions C, concentrated microemulsion domain D, liquid-crystal or gel phase. Shaded areas represent multiphase regions.

Winsor [15] classified the phase equilibria of microemulsions into four types, now called Winsor I-IV microemulsions, illustrated in Fig. 15.5. Types I and II are two-phase systems where a surfactant rich phase, the microemulsion, is in equilibrium with an excess organic or aqueous phase, respectively. Type III is a three-phase system in which a W/O or an O/W microemulsion is in equilibrium with an excess of both the aqueous and the organic phase. Finally, type IV is a single isotropic phase. In many cases, the properties of the system components require the presence of a surfactant and a cosurfactant in the organic phase in order to achieve the formation of reverse micelles one example is the mixture of sodium dodecylsulfate and pentanol. 

Not all emulsions exhibit the classical milky opaqueness with which they are usually associated. A tremendous range of appearances is possible, depending upon the droplet sizes and the difference in refractive indices between the phases. An emulsion can be transparent if either the refractive index of each phase is the same, or alternatively, if the dispersed phase is made up of droplets that are sufficiently small compared with the wavelength of the illuminating light. Thus an O/W microemulsion of even a crude oil in water may be transparent. If the droplets are of the order of 1 pm diameter a dilute O/W emulsion will take on a somewhat milky-blue cast if the droplets are very much larger then the oil phase will become quite distinguishable and apparent. Physically the nature of the simple emulsion types can be determined by methods such as [95] ... 

Type I O/W microemulsions, in which the oil is solubilized within micelles in an aqueous continuous phase. 

Some more modern semi-synthetic metal-working oils are actually O/W microemulsions [193], Such microemulsions may switch readily to O/W macroemulsions when diluted with water at the time of application. Once applied, the surfactants need to adsorb onto metal surfaces with their hydrophobic groups oriented away from the surfaces in order to reduce friction and ensure wetting of the metal by hydrocarbons present in the metal-working liquid or emulsion. Rosen and Daha-nayake [193] list the commonly used surfactants for this application. 

Some agrochemicals are formulated as emulsifiable concentrates. Here active ingredients that are not very soluble in water are dissolved in a solvent that is, in turn, emulsified into the aqueous phase, either in the concentrate itself (an emulsion concentrate) or else upon dilution in the spray tank [865]. Some emulsion concentrates are designed so that when water is added to them they spontaneously emulsify to form an O/W microemulsion [225],... 

When both oil (o) and water (w) are present, o/w microemulsions will be formed when v/a0lc < 1 w/o microemulsions when v/ac c>U and lamellar phases when v/a0lc 1 (Israelachvili etal., 1980 Mitchell and Ninham, 1981). The v/a ratio depends on the surfactant chemical structure (lc and v) and on surface repulsions between headgroups (aD), (Mitchell and Ninham, 1981). When repulsions increase, the surfactant parameter (v/aGlc) increases and micelles get smaller. As a consequence, size and CMC are related surfactants with low CMC aggregate into large molecules, while the higher the CMC, the smaller the micelles. 

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

At low temperatures an O/W microemulsion (0/Wm) is formed which is in equilibrium with an excess oil phase. This condition is termed a Winsor I system. At high temperatures the headgroup requires less space on the interface and, thus, a negative curvature can result. A phase inversion o ccurs and a W/O microemulsion (W/Om) is formed which is in equilibrium with an excess water phase. This situation is termed a Winsor II system. At intermediate temperatures three phases - a water phase, a microemulsion D and an oil phase - are in equilibrium. This is called a Winsor III system. Here the curvature of the interfaces is more or less zero. Hence, the interfacial tension is minimum as depicted in Figure 3.24 (right) for the system C12E5, tetradecane and water. 

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. 

Figure 8. Volume fractions of droplets as a function of the volume ratios of alcohol to surfactant in microemulsions. All the system characteristics are identical to those described for Figure 3. Filled symbols are for O/W microemulsions, and open symbols refer to W/O microemulsions.

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