**4.1 Type and structure of microemulsion**

Microemulsion structure has a key role in the different physicochemical properties of the applied fields. The specific structures of the microemulsions have been extensively studied by many scientist researchers [16–22]. The three basic types of microemulsions are direct (oil dispersed in water, o/w), reversed (water dispersed in oil, w/o) and bi-continuous. Like multiple emulsion, sometimes multiple microemulsion is also possible. In this type, another layer is formed outside the o/w or w/o microemulsions. The schematic diagram of the basic three types of microemulsions is revealed in **Figure 5**. Microemulsion structure depends on salinity, water content, co-surfactant concentration and surfactant concentration. At higher water content, the microemulsion would be a water-external system, with oil solubilized in the cores of the micelles. Although the mixtures remain single phase and thermodynamically stable, the microemulsion structure changes through a series of intermediate states [23]. The structures of these intermediate states are not well known. However, the solutions are thermodynamically stable and isotropic. Salinity also can reverse the structure of the microemulsion. As salinity increases, the direct microemulsion changes to reverse microemulsions. At low salinity, the system remains in water-external phase, but with increasing salinity the system separates into an oil-external microemulsion.

**Figure 5.**

*Main basic types of microemulsions; the three basic types of microemulsions are (A) reversed (water dispersed in oil, w/o), (B) direct (oil dispersed in water, o/w), and (C) bi-continuous [16].*

#### *Microemulsion - A Chemical Nanoreactor*

The term "microemulsions" has been introduced for the first time by Schulman et al. [24]. The term was used to describe what is called transparent solutions in a model four component system consisting of water, hydrocarbon, surfactant, and co-surfactant. Microemulsions can be experimentally investigated and described using different characterization apparatus such as low-angle X-ray diffraction [25] and viscosity measurements [26]. Microemulsions [27–29] can also be identified and described using phase mapping which is a tetrahedron shape showing the components of the microemulsions (i.e., water, hydrocarbon, ionic surfactant, and co-surfactant) in the four corners of the tetrahedron, as shown in **Figure 6**.

**Figure 6.**

*The four component of microemulsion system water (W), hydrocarbon (H), ionic surfactant (S) and co-surfactant (CoS) [30].*

#### **Figure 7.**

*Typical water-surfactant-oil microemulsions as depicted by the ternary phase diagram [31, 32].*

Similar to emulsion classifications, microemulsions are classified into three types as follow:


**121**

which is the temperature and the quantity of asphaltenes [32, 37].

*Application of Emulsions and Microemulsions in Enhanced Oil Recovery and Well Stimulation*

• Bi-continuous microemulsion: also referred to as "sponge-like" microemulsions. In this type the volume fractions of oil and water re almost similar and both can be considered as continuous, while the surfactant molecules is forming rapidly

The phase diagram for a typical there-component surfactant-water-oil is shown in

Demulsification play an important key in separating water and salts from crude oil which is vital during industrial procedure [33]. Since crude oil consists of impurities and if it is not removed, thus it will cause fouling and major corrosion to the equipment in the industry. As a matter of fact, this process can only be executed when demulsifiers are utilized. This chemical component is capable to inflict the coalescence of water droplets by forming film drainage and elevate the surface activity as the gradient is inversed. Thus, it means that demulsifier can undeniably change the interface physical characteristics. When demulsifiers are added to a dilute emulsion of low concentration, a specific process call adsorption takes place and it will adsorb the emulsion particles and situate it on the surface in forms of droplets. Furthermore, the demulsifier contain organic particles and it will locate and adsorb the dispersed phase in such a way that the non-polar part will be in the crude oil while the polar part will maintain in water [34]. Demulsifiers are also known as a non-ionic that consists of two separate parts of hydrophobic and hydrophilic. The hydrophobic group contains oxypropylenes, alkyls or alkyl phenols whereas the hydrophilic group contains amine groups, carboxyl, hydroxyl or oxyethylene. There are also several methods in separating the emulsions which involves separation in terms of chemical, electrical and mechanical [34]. Usually, it is observed in the industry, the use of chemical method is the most common. The coalescence of (W/O) emulsion is maximized and the protective film breaks with the use of chemical demulsifier [35]. As a result, researchers are looking for a way to accelerate the demulsification process effectively. Several methods were applied, for example, the use of microwave energy, adding Janus magnetic microparticle in demulsifier and others. Based on the research conducted by Martínez-Palou et al. [3]. It was found that the rate of separation of (W/O) emulsion were highly efficient when microwave power was applied compared to the use of chemical non-ionic surfactants and also the solubility of surfactants in water decreases with the aid of a saline solution like seawater. Conversely, the application of the Janus magnetic submicronic particles or P(MMAAA-DVB)/Fe3O4 was very promising as it exhibits a high rate of coalescence whereby the water droplets are settled magnetically through the attraction of an outer magnetic field and it is also recyclable, hence lowering the cost in the petroleum industry [36]. To enhance the demulsification efficiency, the factors that can affect it are the molecular weight, hydrophile lipophile balance (HLB), concentration, water content, temperature and asphaltene content. Concerning the molecular weight, the lower the molecular weight of the demulsifier, the higher the rate of partitioning the emulsion as an elevation in the molecular weight causes a difficulty for the demulsifier to spread in the highly thick or viscous crude oil. Beside, a rise in the amount of HLB, concentration and water content shows an increasing rate in the efficiency of this process. However, the asphaltenes which acts as stabilizing agents can prevent the demulsification of crude oil because when the content becomes higher, it will result a dense film that protects the emulsion. To summarize, the complete separation of (O/W) emulsion can be attain based on two prime factors

*DOI: http://dx.doi.org/10.5772/intechopen.84538*

fluctuating interfaces [27, 28].

**Figure 7** [31, 32].

**5. Demulsification**

*Application of Emulsions and Microemulsions in Enhanced Oil Recovery and Well Stimulation DOI: http://dx.doi.org/10.5772/intechopen.84538*

• Bi-continuous microemulsion: also referred to as "sponge-like" microemulsions. In this type the volume fractions of oil and water re almost similar and both can be considered as continuous, while the surfactant molecules is forming rapidly fluctuating interfaces [27, 28].

The phase diagram for a typical there-component surfactant-water-oil is shown in **Figure 7** [31, 32].
