**2. Terminology**

In this section, some fundamental concepts on similarity as well as difference for both microemulsions and nanoemulsions will be provided, respectively, in terms of its physical and chemical properties, stability and thermodynamic difference as well as the current fabrication methods. These terminologies are meant to clarify the common confusions found in most literatures.

#### **2.1 Physical and chemical properties**

### *2.1.1 Microemulsions*

The definition of microemulsions is commonly described as a stable liquid droplet in microscale (r < 100 μm) formed by mixing of two immiscible phases, i.e., oil and water in the presence of surfactant. The emulsion's structure, size, composition and surface behaviour can be manipulated by different fabrication methods. They may form one or multiple phases (with different shapes) that are in equilibrium with one another [14]. As for analysis, common analytical equipment such as light-inverted microscopy, scanning electron microscopy, X-ray powder diffraction, electrical conductivity and rheology are usually chosen as tools for the study focus. Therefore, it is advisably to classify microemulsion (O/W) as a thermodynamically stable colloidal dispersion. The structure hinders the unfavourable contact area between non-polar groups and water, which favour the thermodynamics of the colloidal dispersion system. For instance, the surfactant molecules in an oil-in water (O/W) microemulsions has the setup of non-polar tails that associate with each other forming a hydrophobic core. The hydrophilic head groups extrude onto the surrounding aqueous phase of the microemulsion. Meanwhile, the hydrophobic oil molecules may assimilate into the interior of a micelle as a separate core or serves as a barrier between the surfactant tails as shown in **Figure 1**. If oil molecules have the same polar groups, they may then be integrated into the micelle in such a way that it creates a visible distance into the water. This behaviour is particularly important in pharmaceutical industry as it serves as a fundamental core structure for self-microemulsifying drug delivery system. It extends the knowledge for drug delivery system design in terms of the optimum composition of the initial system and the optimum method to dilute the surface of the microemulsion.

#### *2.1.1 Nanoemulsions*

Nanoemulsions are considered to be a classic liquid emulsion formation from two immiscible liquids that is thermodynamically unstable. Theoretically, this small spherical droplet (r < 100 nm) could be formed using oil (as continuous phase) and water (as dispersed phase) without addition of a surfactant. However, this system will be highly unstable; thus, most nanoemulsions would require the assistance of surfactant (often it is more than one type of surfactants used) to facilitate its droplet formation. The fundamental component formation of a nanoemulsion is very similar to those found in a microemulsion as

**5**

period.

*2.1.1.2 Particle size*

*Introductory Chapter: From Microemulsions to Nanoemulsions*

mentioned above. The only distinctive difference that separates a nanoemulsion from a microemulsion is their thermodynamic stability, i.e., a microemulsion is thermodynamically stable while a nanoemulsion is not. As nanoemulsion only has a short development history of ~25 years [15], there are much area of interest that are waiting to be explored with most of the current understanding for Nanaoemulsions is adapted based on Microemulsions understanding. The summary of similarities and differences between microemulsions and nanoemulsions

*Oil-in-water microemulsions: (a) oil molecules assimilate between the surfactant tails; and (b) oil molecules* 

The chapter so far outlines the similarity and differences between nanoemulsions and microemulsions based on their general physical and chemical properties, as well as other characteristics. In this section, an attempt to propose practical methods to distinguish nanoemulsions from microemulsions will be made. The key concept to make this comparison is based on thermodynamic point of view as shown in **Figure 2** below. As the terms used for microemulsions and nanoemulsions remain confusing in most literatures, the following two factors can be used as a

Nanoemulsions are not thermodynamically stable, whereas microemulsions are. This simply means microemulsions will not undergo deformation at an infinite period so long the storage condition remains constant. For nanoemulsions, degradation of structure is noticeable due to Ostwald ripening, flocculation, coalescence and gravitational separation. These result in particle size distribution change, physical properties change as well as chemical properties change. Practically, it can be difficult to just make a justification to distinguish nanoemulsions from microemulsion merely based on long-term storage, since microemulsions commonly suffer from chemical degradation and microbial contamination during the storage

As mentioned above, microemulsions have tendency to form single narrow size distribution, ought to its maturity in terms of fabrication methods. Adding to its thermodynamic stability, the size of a microemulsion will not undergo changes once it is formed by a specific approach. This is different from nanoemulsions where they tend to have multiple peaks in its size distribution. This

guideline to distinguish nanoemulsions from microemulsions:

*incorporated as a hydrophobic core. Reprinted with permission from Ref. [3].*

*DOI: http://dx.doi.org/10.5772/intechopen.87104*

can be seen in **Table 1**.

**Figure 1.**

*2.1.1.1 Long-term storage*

**Figure 1.**

*Nanoemulsions - Properties, Fabrications and Applications*

the common confusions found in most literatures.

**2.1 Physical and chemical properties**

whole.

**2. Terminology**

*2.1.1 Microemulsions*

range of different industries as well as many literatures, this chapter will outline the similarities and differences between microemulsions and nanoemulsions, to articulate the scientific terminology used to describe each of the terms and also to look into research progress of both microemulsions and nanoemulsions as a

In this section, some fundamental concepts on similarity as well as difference for both microemulsions and nanoemulsions will be provided, respectively, in terms of its physical and chemical properties, stability and thermodynamic difference as well as the current fabrication methods. These terminologies are meant to clarify

The definition of microemulsions is commonly described as a stable liquid droplet in microscale (r < 100 μm) formed by mixing of two immiscible phases, i.e., oil and water in the presence of surfactant. The emulsion's structure, size, composition and surface behaviour can be manipulated by different fabrication methods. They may form one or multiple phases (with different shapes) that are in equilibrium with one another [14]. As for analysis, common analytical equipment such as light-inverted microscopy, scanning electron microscopy, X-ray powder diffraction, electrical conductivity and rheology are usually chosen as tools for the study focus. Therefore, it is advisably to classify microemulsion (O/W) as a thermodynamically stable colloidal dispersion. The structure hinders the unfavourable contact area between non-polar groups and water, which favour the thermodynamics of the colloidal dispersion system. For instance, the surfactant molecules in an oil-in water (O/W) microemulsions has the setup of non-polar tails that associate with each other forming a hydrophobic core. The hydrophilic head groups extrude onto the surrounding aqueous phase of the microemulsion. Meanwhile, the hydrophobic oil molecules may assimilate into the interior of a micelle as a separate core or serves as a barrier between the surfactant tails as shown in **Figure 1**. If oil molecules have the same polar groups, they may then be integrated into the micelle in such a way that it creates a visible distance into the water. This behaviour is particularly important in pharmaceutical industry as it serves as a fundamental core structure for self-microemulsifying drug delivery system. It extends the knowledge for drug delivery system design in terms of the optimum composition of the initial system and the optimum method to dilute the surface of the

Nanoemulsions are considered to be a classic liquid emulsion formation from two immiscible liquids that is thermodynamically unstable. Theoretically, this small spherical droplet (r < 100 nm) could be formed using oil (as continuous phase) and water (as dispersed phase) without addition of a surfactant. However, this system will be highly unstable; thus, most nanoemulsions would require the assistance of surfactant (often it is more than one type of surfactants used) to facilitate its droplet formation. The fundamental component formation of a nanoemulsion is very similar to those found in a microemulsion as

**4**

microemulsion.

*2.1.1 Nanoemulsions*

*Oil-in-water microemulsions: (a) oil molecules assimilate between the surfactant tails; and (b) oil molecules incorporated as a hydrophobic core. Reprinted with permission from Ref. [3].*

mentioned above. The only distinctive difference that separates a nanoemulsion from a microemulsion is their thermodynamic stability, i.e., a microemulsion is thermodynamically stable while a nanoemulsion is not. As nanoemulsion only has a short development history of ~25 years [15], there are much area of interest that are waiting to be explored with most of the current understanding for Nanaoemulsions is adapted based on Microemulsions understanding. The summary of similarities and differences between microemulsions and nanoemulsions can be seen in **Table 1**.

The chapter so far outlines the similarity and differences between nanoemulsions and microemulsions based on their general physical and chemical properties, as well as other characteristics. In this section, an attempt to propose practical methods to distinguish nanoemulsions from microemulsions will be made. The key concept to make this comparison is based on thermodynamic point of view as shown in **Figure 2** below. As the terms used for microemulsions and nanoemulsions remain confusing in most literatures, the following two factors can be used as a guideline to distinguish nanoemulsions from microemulsions:
