Microemulsions are isotropic, clear, or translucent, thermodynamically stable dispersions containing water, oil, surfactant, and a cosurfactant, usually alcohol. Because of their unique properties, including an ultralow interfacial tension, large interface, and the ability to solubilize otherwise immiscible liquids, microemulsions are widely used in various fields, including tertiary oil recovery, separation, pharmaceuticals, nanoparticle synthesis, and chemical engineering. They are either “water in oil” (W/O) or “oil in water” (O/W) dispersions, stabilized with pure or mixed amphiphiles. Amphiphiles are required to significantly reduce the oil–water interfacial tension by their adsorption to the interface, which minimizes the positive free energy change of dispersion associated with surface formation. Microemulsions differ physicochemically from macroemulsions, which are normally called “emulsions.” In macroemulsions, the particle size is much larger; they are nontransparent, have only short-term stability, and allow only restricted handling. Occasional homogenization with agitation is required to prevent macroemulsions from breaking or phase separation. Therefore, they are kinetically stable, whereas microemulsions can form with the expenditure of very little energy (which can be provided by the thermal energy of the system) and are thus thermodynamically stable. A mixture with the right composition of water, amphiphile, and oil may spontaneously homogenize, forming a microemulsion. A water–oil microemulsion is topologically similar to reverse micelles (in which the polar heads of the amphiphile molecules are oriented inward and the nonpolar tails are oriented toward the oil continuum), and their distinction rests on the availability of free water in the core, called the “micro-pool.” Compositions yielding a rigid interior or core are called “reverse micelles,” in which the water present is immobilized by the hydration of the polar or ionic heads of the amphiphile and the counterions (when present). Compositions with mobile or free water in the core after the hydration requirements of the amphiphile head groups and counterions are satisfied are called “microemulsions.” The size of the reverse micelles is normally restricted to 5 nm because a grown size <5 nm changes the status of the mixture to a microemulsion. This dimension-dependent change in status may not apply to oil–water dispersions where the solvent-dependent bound and free oil in the pool is less physicochemically significant, as in reverse micelles. Of course, normal micelles can consume oil and grow in size, forming a microemulsion.
- Starch Microemulsion Preparation
- Starch Microemulsion Properties
- Applications of Starch Microemulsions
- Application of Starch Microemulsions to Foods
Starch, an inexpensive, renewable, biodegradable natural polymer, is widely used in food and industrial fields as a thickener, gelling agent, bulking agent, and water-retention agent. However, native starch has several limitations, including its poor processability and solubility, which limit its industrial applications. However, starch can be modified with physical, chemical, or enzymatic treatments to improve its properties.