**1.3 Current trends in nanoemulsions**

As already mentioned, nanoemulsions are being used in a wide range of fields. There is a lot of research and development work done in the field of nanoemulsions. Many bioactive substances are present in natural available substances, emulsification of these bioactive components is a trending research topic. In September 2018, water compatible form of coconut oil through nano-emulsification was developed [12]. The nanoemulsion was made successfully using PHC as a surfactant at a concentration of 5% {w/w}. Nanoemulsions have also found an important space in field of pharmaceuticals. Many of the oral drugs synthesized do not have aqueous stability almost insoluble) and have low bioavailability. A low energy method to make composite hydrogel beads encapsulated with single and multiple hydrophobic drugs was developed [13]. This makes nanoemulsions a promising carrier of hydrophobic drugs. It was shown that nanoemulsions were used to enhance the antileishmanial activity of *Copaifera* spp. oleoresins against both *Leishmania amazonensis* and *Leishmania infantum* strains [14].

Recently a new technique for making Pickering nanoemulsions using Silica nanoparticles was developed which is highly scalable and energy efficient. Nanoemulsions are usually stabilized using surfactants. The use of surfactants has some disadvantages which include surfactant desorption and Ostwald ripening. Hence a new interest of making nanoparticle stabilized nanoemulsion (Pickering nanoemulsions) has evolved. Nanoparticles have higher desorption energy barrier. However, the limitation of nanoparticles as stabilizing agent was obtaining the size in nano-range. In the traditional method to make Pickering emulsions (high energy

*Nanoemulsions - Properties, Fabrications and Applications*

photosensitive side of a photographic film [3].

nanoemulsion, and miniemulsion (**Table 1**).

**1.2 Nanoemulsions**

systems, the two phases are dispersed and continuous. One liquid is dispersed (the disperse phase) in the other (the continuous). The different types of emulsion systems can include: oil-in-water (o/w), water-in-oil (w/o), and oil-in-oil (o/o) [1]. The oil-in-oil phase can be polar oil dispersed in non-polar oil, or vice versa. An emulsifier is usually used to disperse immiscible liquids. The emulsifier also plays an important role in the formation and long-term stability of the emulsions. Emulsions can also be classified on the type of emulsifier or the structure of the system. Emulsions being liquids do not have any static internal structure. The droplets are assumed to be statistically distributed in the liquid matrix. According to IUPAC, in emulsions the droplets can be amorphous, liquid-crystalline, or any mixture. The droplet diameters in the dispersed phase range between 10 nm and 100 μm (which may exceed the size limits for colloidal particles) [2]. Some common example of emulsion systems are homogenized milk, some cutting fluids for metal working, egg yolk is an emulsion with the emulsifying agent lecithin, butter is an emulsion of water in fat, and an emulsion of silver halide in gelatin is used as a coating in the

Emulsion system can be classified based on their droplet size as macroemulsion,

Emulsion systems find a wide range of applications in the field of food, cosmet-

ics, agriculture, pharmaceuticals (preparation of drugs and drug delivery).

since moderate surfactant concentration is sufficient for their making [5]. Nanoemulsions are said to be kinetically stable and thermodynamically unstable. Their stability can be altered by their preparation methods like adding specific co-surfactants. They usually use high energy methods for their preparation but low energy based methods can also be used with the help of some special conditions using certain chemical potential of the component [6]. Nanoemulsions are said to be transparent, biodegradable, and biocompatible. Normal emulsions usually undergo sedimentation by gravity, which is overcome by nanoemulsions. Nanoemulsions exhibit Ostwald ripening phenomenon. Due to this, small

**Emulsion type Droplet size (μm)** Macroemulsion 1–100 Microemulsion 10–100 Nanoemulsion 20–500

As the name suggests, the size of the droplets in this type of emulsion is in nanometer ranges. They not only differ in size but also in the many properties and method of preparation. The main difference between nanoemulsion and conventional emulsion (macroemulsion) is the size and shape of the droplets in the continuous phase. In macroemulsion, the shape is usually spherical but in nanoemulsions a variety of shapes can be seen like swollen micelles and bicontinuous structures. Though micro and nanoemulsions are similar in their sizes the method of preparation differs between them. Both of them require energy inputs, but nanoemulsions mostly use mechanical shear while micro emulsions make use of spontaneous emulsification methods. Microemulsions also need a high surfactant concentration compared to nanoemulsions. The application of nanoemulsion in pharmaceutical, food, cosmetic, and chemical industry is comparatively more than microemulsion

**66**

**Table 1.**

*Emulsion type and its droplet size [4].*

and low energy methods, described in the next section) many steps were involved, a single step method was developed using vapor condensation. Moreover the traditional methods used in the preparation of Pickering nanoemulsions had some disadvantages. High energy methods reduced the adsorption of the particles on the droplets while, low energy methods were unable to produce Pickering nanoemulsions and clogging of nanoparticles was seen. The concentration of nanoparticles required in the new methods was also less compared to that required in traditional methods. In this process, oil was taken and cooled below the dew point, during which the water condenses on the oil. If the oil has the right properties and sufficient concentration of nanoparticles, then water drops self-disperse within the oil. The nanoparticles then will self-assemble around them to form nanoemulsions [15]. Nanoemulsions are studied in great detail due to their potential applications. Improvements in their preparation methods and the fields in which they can be used are the ongoing trends in nanoemulsions.
