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Water core Polar solute
Polar head group
Nonpolar fluid (e.g. ethane, propane or octane)
Figure 1. Structure of a water-in-oil microemulsion droplet showing partitioning of polar and nonpolar solutes into the water and oil microphases.
The shape, size, and structure of these dispersed droplets depend upon a multitude of variables including the surfactant type, ionic strength, the presence of cosurfactant(s), and the amount of water. Commonly used surfactants are of the five general categories; anionic, cationic, nonionic, amphoteric and zwitterionic. The nature of amphoteric surfactants, i.e., whether or not they behave as anionic or cationic species, is dependent on the pH or ionic strength of the aqueous phase.
The amphiphilic nature of surfactant molecules having both highly polar or charged hydrophilic head groups and nonpolar, lipophilic tails, promotes aggregation. Their selective absorption into the interface reduces the surface tension of the medium. Hence, the origin of the name surfactant is derived from the expression, surface-active agent. Cosurfactants are small amounts of somewhat amphiphilic substances, typically low molecular weight alcohols, having some solubility in both oil and water, that greatly enhance the water- or oil-solubilizing capacity of the microemulsion. The shape of the surfactant molecule is a very important factor in predicting the types of aggregates formed. For reverse micelles and water-in-oil microemulsions, the surfactant must have a “wedge” shape where the hydrocarbon tails have a much higher volume than that of the polar head groups. This allows for efficient packing to form a stable inverted structure. Typical reverse micelle surfactants have twin alkyl tails in order to satisfy this packing requirement. Another option is to use cosurfactants such as low molecular weight alcohols in combination with single-tail surfactants to obtain the proper surface curvature required for reverse or inverted structures.
Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (TR = T/Tc> 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes.
The amount of water added to a water-in-oil (w/o) microemulsion is defined by the molar water-to-surfactant ratio, W. Generally, the greater the W value, the larger the size of the nanometer-sized
droplets in the microemulsion phase. When the volume of the dispersed water phase exceeds approximately 26% of the total microemulsion volume, a transition may occur from an inverted structure to what is postulated as a bicontinuous structure1251 and finally to normal structures at water volumes greater than 74%J261
The ability to dissolve highly polar or ionic species in the polar microdomains of the microemulsion is, to a large part, dependent upon the amount of water incorporated into the microemulsion. The molar water-to-surfactant ratio (W) is a convenient measure of the polar solvent strength of the microemulsion. For ionic surfactants, the microemulsion will have a solvent power for polar species approaching that of bulk water when W is above 10. At lower W values, the water acts to hydrate the head groups and counter-ions of the surfactant yielding a microsolvent environment that has a dielectric constant appreciably lower than that of bulk water and, hence, a much reduced capacity for the dissolution of ionic or highly polar species. Manipulation of the W value of the microemulsion can be viewed as a means to adjust the polarity of the microemulsion for cleaning operations. For supercritical microemulsions, the W value can be adjusted through controlled addition of water to the solution or through the manipulation of the pressure or temperature of the solution.
It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species.