
Miniemulsion

A miniemulsion (also known as nanoemulsion) is a special case of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases (for example, oil and water), one or more surfactants and, possibly, one or more co-surfactants (typical examples are hexadecane or cetyl alcohol).
There are two general types of methods for preparing miniemulsions: high-energy methods and low-energy methods. For the high-energy methods, the shearing proceeds usually via exposure to high power ultrasound of the mixture or with a high-pressure homogenizer, which are high-shearing processes. For the low-energy methods, the water-in-oil emulsion is usually prepared and then transformed into an oil-in-water miniemulsion by changing either composition or temperature. The water-in-oil emulsion is diluted dropwise with water to an inversion point or gradually cooled to a phase inversion temperature. The emulsion inversion point and phase inversion temperature cause a significant decrease in the interfacial tension between two liquids, thereby generating very tiny oil droplets dispersed in the water. 
Miniemulsions are kinetically stable but thermodynamically unstable. Oil and water are incompatible in nature, and the interface between them is not favored. Therefore, given a sufficient amount of time, the oil and water in miniemulsions separate again. Various mechanisms such as gravitational separation, flocculation, coalescence, and Ostwald ripening result in instability. In an ideal miniemulsion system, coalescence and Ostwald ripening are suppressed thanks to the presence of the surfactant and co-surfactant. With the addition of surfactants, stable droplets are then obtained, which have typically a size between 50 and 500 nm. 
Miniemulsions have wide application in the synthesis of nanomaterials and in the pharmaceutical and food industries. For example, miniemulsion-based processes are, therefore, particularly adapted for the generation of nanomaterials. There is a fundamental difference between traditional emulsion polymerisation and a miniemulsion polymerisation. Particle formation in the former is a mixture of micellar and homogeneous nucleation, particles formed via miniemulsion however are mainly formed by droplet nucleation. In the pharmaceutical industry, oil droplets act as tiny containers that carry water-insoluble drugs, and the water provides a mild environment that is compatible with the human body. Moreover, miniemulsions that carry drugs allow the drugs to crystallize in a controlled size with a good dissolution rate. Finally, in the food industry, miniemulsions can not only be loaded with water-insoluble nutrients, such as beta-carotene and curcumin, but also improve the nutrients' digestibility.