**4. Microemulsions**

*Microemulsion - A Chemical Nanoreactor*

Emulsion is usually formed by the action of mechanical mixing with the existence of a surfactant molecule. The mechanical mixing action can generate a turbulence effect and break down the two immiscible phases, while surfactant is contributing in building an adhesion force on the surface between the droplets of the two phases (see **Figure 4**). This process of the formation of an emulsion is called emulsification. There are two types of emulsification, spontaneous emulsification and self-emulsification. The spontaneous emulsification is the emulsification process where there is no involvement of external heat or physical action or energy that can affect the emulsification of the two immiscible liquids. In the spontaneous emulsification, the two phases usually take a significant amount of time to completely become an emulsion. In the self-emulsification, the complete emulsification occurs when appropriate surfactants are used [14]. Emulsification can be observed in many activities and processes ranging from simple events people frequently do in daily life to more complicated and sophisticated processes performed in industry. Producing mayonnaise is an example of the emulsification processes where egg yolks and oil are mixed and emulsified by stirring action to form a thick emulsion in which the egg yolk acts as a continuous phase while the oil is the dispersed phase. Similarly, mixing water in oil can generate a water-in-oil or oil-in-water emulsion, depending on which phase is predominate. In this case, however, an emulsified material such as asphaltenes is needed to stabilize the emulsion. Fingas and Fieldhouse [6] mentioned that asphaltenes are the main factor that cause (W/O) emulsion stabilization as it has the ability to build a rigid cross-linked and elastic films. Besides asphaltenes, resins can also act as stabilizing agents. As outlined by Fingas [7], although resins normally assist the asphaltenes by acting as a solvent that can stabilize as the asphaltenes migrates. This shows that asphaltenes and resins can be considered as the main emulsifiers that exist naturally in crude oils. In addition, to asphaltenes and resins, other organic and inorganic stabilizers frequently found in crude oils include waxes and clays [15]. Crude oil does not always appear as a stable emulsion as there is certain phenomenon that continuously destabilizes it. Examples of such phenomenon are flocculation, creaming, and coalescence. Flocculation is the process of the combination of small emulsion droplets each other due to excess surfactants in the continuous phase. When the emulsion is shaken, however, these particles can disperse again in the medium.

*Main basic types of microemulsions; the three basic types of microemulsions are direct (oil dispersed in water, o/w),* 

**3. Emulsification**

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**Figure 4.**

*reversed (water dispersed in oil, w/o) and bi-continuous [13].*
