However, dehydration required for conventional sample preparation can severely affect the native structure [ 3 , 7 , 8 ].
Apart from the existence of the unitary phase, the size of droplets of the dispersed phase of a microemulsion is a primary characteristic, since it determines much of the physical behavior and functionality. For macroemulsions, both sound acoustics, electroacoustics and visible light turbidimetry, nephelometry, laser diffraction can be used to characterize the droplet size.
This leads to a useful property that microemulsions are not only transparent but also requires different techniques to characterize them. Here, the measurement is the fluctuation of scattered laser light intensity, which is related to the Brownian motion of particles droplets in the medium.
Fitting the autocorrelation of the scattered light intensity to models of particles in specific media can lead to determining the diffusion constant, particle size distribution, and in some cases, shape anisotropy [ 9 — 11 ]. SANS data give access to both the droplet size and dynamic properties; however, the relative cost and data collection time limit the applicability of this technique [ 3 ].
Apart from scattering techniques, nuclear magnetic resonance NMR and related techniques are widely used in the characterization of microemulsions [ 3 , 13 ]. It follows that most techniques which give a measure of the droplet size of microemulsions will also have access to the size distribution or polydispersity. However, due to the confluence of particle size variation and shape anisotropy, analyzing, and deconvoluting, these data may not always be straightforward.
Similarly, because most techniques use the motion of aggregates droplets to elucidate the sizes of dispersed droplets, light scattering techniques have access to the diffusion rates as well.
However, it is important to note that fluorescence correlation spectroscopy FCS [ 3 ] can be used to measure the diffusion constants in addition to particle size and size distribution [ 18 ] , particularly useful in dilute solutions and other conditions where light scattering techniques fail [ 3 ]. However, when the droplet sizes of the emulsions become comparable to the incident beam focus size, the FCS is not reliable leading to the technique being used as a complementary method to DLS [ 3 ].
NMR relaxation is very sensitive to the droplet size but is insensitive to interactions allowing accurate droplet size measurements [ 13 ]. Although all of the techniques discussed so far give information about the morphology, electron microscopy provides a robust method to directly visualize the nanoscale structure and morphology of microemulsions.
In each of these cases, the sample preparation requires dehydration which, in the case of microemulsions, can severely affect their native structure [ 3 ].
To alleviate this problem, cryo-electron microscopy cryo-EM techniques have been utilized where emulsions are frozen in their native hydrated states.
The rheological properties of microemulsions are often crucial in their application because they will affect the processability, and kinetics, and stability under various conditions [ 19 ]. Microemulsions show varying rheology depending on the phase point. Although the effect of the molecular structure of emulsions has a large impact on the behavior of microemulsions [ 20 ], the characterization techniques are generally the same as their macroscopic counterparts [ 21 , 22 ].
The applications of microemulsions are plenteous and span in areas including drug delivery, cosmetics, food, fuel, lubricants and coatings, detergents, agrochemicals, analytical chemistry, nanoparticle synthesis, biotechnology, and chemical reactors [ 23 ]. Further, ultralow interfacial tension, the presence of nanosized droplets of dispersed phase, slow release and protection of encapsulated material, and the ability to penetrate through biological membranes are some attributes that make the microemulsions find significant applications in various sectors.
A brief description of the applications of microemulsions in selected fields is given below. Emulsions are opaque gels or creams in which a drug is dispersed for topical application. The effect of the drug released from the gel depends on the permeability of the drug through the skin barrier.
Microemulsions considering the small size of the droplets can serve as better delivery vehicles thereby improving the drug solubility, penetrability, and shelf life [ 24 ]. It has been shown that penetrability of hydrophobic drugs is improved by encasing the drug in a lipid vesicle [ 26 ].
Therefore, it is apparent that the smaller the size of the droplet, the better the delivery of a hydrophobic drug. Drug diffusion was shown to follow kinetics related to models such as Higuchi model resulting in the slow release of the drug [ 27 ]. In this case, the permeation was shown to increase when glycolipids were incorporated into the microemulsion indicating that they could outperform macroemulsions in topical drug delivery. Poorly soluble drugs such as cyclosporine and paclitaxel have shown improved oral bioavailability in microemulsion systems and have been patented along with other drugs such as ritonavir and saquinavir [ 28 ].
Cosmetics and cosmeceutics currently utilize microemulsion systems and demonstrate the enormous potential of using these systems for various products. Skin care products, hair care products, and perfumes are the main types of microemulsion products available in the market. The surfactants are either ionic or nonionic [ 23 ].
Bioactive agents, including antioxidants and skin whitening agents, have been incorporated in and delivered to the skin via microemulsion cosmetic products [ 29 , 30 ].
Interestingly, antioxidant and moisturizing effects of olive oil, which can be utilized as the main ingredient in microemulsions, increase upon incorporation in microemulsions thus making such systems apt for cosmetic applications [ 31 ].
Numerous attributes of microemulsions render these systems excellent to be used in the food sector. Among such attributes, their ability to protect, slowly release, and enhance the activity of the encapsulated material, and the possibility of formulating microemulsions using edible substances, stand out.
Further, the bioactive compounds—crocin, safranal, and picrocrocin—of saffron encapsulated in multiple emulsions have shown enhanced slow release properties and greater stability in gastric conditions [ 34 ]. Moreover, microemulsions encapsulating steppogenin have shown to be effective in reducing enzymatic browning of apple juice.
The number of studies on applications of microemulsions in food is plenteous and is still growing. The high interfacial tension between the crude oil and reservoir brine keeps the residual oil in the reservoir.
The interfacial tension can be lowered via the preparation or introduction of microemulsions, and thus, this area is actively investigated. The surfactants stimulate the formation of a microemulsion in the porous reservoir between reservoir brine and crude oil, which reduces the interfacial tension between the two.
Hence, the oil recovery is enhanced [ 36 ]. Also, numerous studies have been conducted to evaluate the use of ionic liquids as green chemicals in place of surfactants in microemulsions in enhancing oil recovery [ 39 ]. Microemulsions have been used as fuels with many attractive properties. These fuels are used to decrease the emission rates of gases such as nitrogen oxides and carbon monoxide, and particles soot [ 40 ].
Although alcohols frequently used in microemulsion biofuels decrease the cetane number. The incorporation of cetane improvers has significantly increased the cetane number thus improving the properties of microemulsion fuels [ 41 ]. Microemulsion systems are frequently used as lubricants. Microemulsions prepared using ionic liquids and copper nanoparticles are some recent advances in this field [ 43 , 44 ].
As cutting oils, microemulsions serve as lubricants and absorbers of the heat of friction [ 45 ]. As corrosion inhibitors, microemulsions may show many mechanisms of action. The corrosion causing factors may be soluble in the microemulsion so that those factors may be unavailable for the metal surface.
Also, the hydrophobic coating on the metal may prevent corrosion [ 46 ]. Further, microemulsion coatings are better than emulsion coatings with respect to scrub resistance, stain resistance and color intensity [ 23 , 47 , 48 ]. The suitability of microemulsions in textile finishing has also been demonstrated by many researchers.
In fact, microemulsions have shown better properties than both conventional textiles finishing aids and normal emulsions [ 23 , 49 ]. The structure hinders the unfavourable contact area between non-polar groups and water, which favour the thermodynamics of the colloidal dispersion system. 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.
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. Nanoemulsions are considered to be a classic liquid emulsion formation from two immiscible liquids that is thermodynamically unstable. 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 mentioned above.
The only distinctive difference that separates a nanoemulsion from a microemulsion is their thermodynamic stability, i. 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:. Schematic illustration between microemulsions and nanoemulsions, with its separated phase state, respectively.
Microemulsions have a relative lower free Gibbs energy than the phase-separated state; therefore, it is unlikely to break down even after a long storage period provided the storage condition remains unchanged. Meanwhile, nanoemulsions have a higher free Gibbs energy, leading to breakdown and revert back to its original separated state despite the assistance of surfactants. 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 period.
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 is not surprising due to its unstable thermodynamic as discussed previously. Therefore, if an emulsion system has multiple peaks formation in its size distribution, it can possibly be considered as a system that has both microemulsions and nanoemulsions.
However, the noticeable approach to be used in this scenario is to undergo proper physical characterization such as Zetasizer or using scanning electron microscope for proper size measurement, as well as storing it for a period of time, i. These approaches might provide other significant evidence to validate the type of emulsion formation. This chapter is a timely content of nanoemulsion development as this research field has shown significant publication output for the past decade due to growing interest as a result of nanotechnology development as seen in Figure 3 using Web of Knowledge Thomson Reuters online search engine [ 15 ].
Presently, microemulsion-related publication is dominating in terms of publications output per year. This is in line with the history of microemulsion for the past seven decades which covers a comprehensive research range from science i. On the contrary, nanoemulsions received much lesser focus until the mids.
It has relative incomplete fundamental knowledge understanding in terms of properties, characterizations and fabrication methods, and it also has limited applications presently.
Therefore, this book is important in attempting to mend the knowledge gap and form the trend. As nanotechnology matures gradually, the outlook of nanoemulsion looks bright and it aims to stretch its capability to apply across different fields that will benefit mankind.
Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications.
Edited by Kai Seng Koh.
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