Destabilization of Nanoemulsion System
All the previously mentioned components are mostly considered to provide stability to a nanoemulsion system; however, being a metastable system, it may break down over an extended period through several physicochemical mechanisms like gravitational separation which specifically includes creaming and sedimentation, coalescence, flocculation, chemical degradation, and Ostwald ripening (McClements and Rao, 2011). All mentioned destabilization mechanisms are discussed serially in the next section.
The most common form for the destabilization of nanoemulsions is gravitational separation, either in the form of creaming or in sedimentation that varies based on relative densities of dispersed and the continuous phases. Droplets having a lower density starts to move upwards due to a relatively lower density than its liquid surrounding them, which is termed as creaming. Conversely, when the droplets due to a higher density than the surrounding liquid start moving downwards are termed as sedimentation. This is the major reason that conventional O/W emulsions are more prone to creaming as oil has a lower density than surrounding liquid water therefore creaming is more prominent in liquid edible oil, and due to similar reasons, the W/O emulsion is more prone to sedimentation. In some cases, due to the presence of crystalline lipids the density of lipids increases in an O/W emulsion, the increased density due to lipid crystallization. Decreasing the droplet size, adding weighting agents, and adding thickening agents, for increasing the viscosity of the aqueous phase, are some possible ways to reduce the gravitational separation (McClements and Jafari, 2018).
Flocculation and Coalescence
These are other issues with droplet formation that occurs due to colloidal interactions among the droplets. In flocculation, mostly the clusters are formed as two or more droplets interact together due to the presence of attractive forces that acts between the droplets. However, coalescence is the formation of a large droplet which is a result of the merging of multiple small droplets that aggregate together. The main reason for coalescence is the attractive interactions, caused due to van der Waals forces, hydrophobic forces, and depletion forces, between the smaller droplets that bring them close enough to form a larger aggregate. These attractive interactions can be negated by factors termed as repulsive interactions that include electrostatic and steric interactions. To avoid coalescence the repulsive interactions must be kept greater than the attractive interactions (McClements and Rao, 2011).
It is a process where the formation of larger droplets starts from shrinking of smaller droplets. The process where the mean size of existing droplets present in the dispersed phase starts increasing with time which is frequently termed as Ostwald ripening (Kabalnov, 2001). The solubility of the dispersed phase is more in smaller droplets when compared to bigger droplets as smaller droplets that possess higher curvature whereas bigger droplets possess smaller curvature. This difference of curvature leads to gradual diffusion of oil molecules resulting in the formation gradient which leads to the growth in the size of droplets between the intervening fluid (McClements and Jafari, 2018).
Nanoemulsions Preparation Methods
Preparation of nanoemulsions often requires a huge amount of energy as being nonequilibrium systems; therefore, their preparation either requires a large amount of energy or the use of surfactants and most of the time both. Therefore, in this section, a major focus is on the most utilized energetic methods for nanoemulsion preparation. Briefly, these are categorized as high-energy emulsification and low-energy emulsification methods. Both methods hold equal weightage in generating small droplets; however, high-energy methods are preferred in the food industry as lower levels of an emulsifier is required and the use of a natural emulsifier is also feasible using this approach. Additionally, it can also be easily customized with huge mechanical devices for large-scale production (Salvia-trujillo et al., 2017). In the next section, a detailed description of high- and low-energy methods is specified; we initially elaborate on various high-energy methods followed by low-energy methods.
High-energy methods require the application of high disruptive forces with a mechanical device that is capable of causing the breakdown of oil droplets and dispersing them into the water phase. High-energy methods involve high-pressure homogenization (HPH), ultrasonic homogenization (USH). microfluidization, high- pressure microfluidic homogenization (HPMH), and colloid mills (Salvia-trujillo et al., 2017; McClements and Jafari, 2018; Espitia et al., 2019).
High-Pressure Valve Homogenizers
HPH is the most common and widely used method for generating small-sized droplets in the food and pharmaceutical industries due to its versatility and feasibility. Rather than generating emulsions, this method is more effective in reducing the size of droplets already generated during coarse emulsion from the two distinct liquids. This type of homogenizer uses a piston pump that pulls the coarse emulsion, generated using a high-shear mixer; pushes that into a chamber; and then propels it through a narrow valve located at the end of the chamber. This constant flow through the valve is maintained by uninterrupted forward and backward strokes of the piston that ultimately generate intense turbulence and hydraulic shear force through which macroscale droplets get broken down into relatively smaller droplets. A conventional HPH system operates in a pressure range not exceeding 150 MPa however, few high- capacity ultra-HPH may function beyond maximum operating pressures, which could go up to 350 to 400 MPa, resulting in the generation of ultra-small droplets. Droplet diameters ranging as small as 1 nm are reported to be produced using this method (Donsi et al., 2012; Qadir et al., 2016; Horison et al., 2019). Further reduction in the droplet size by increased homogenization is not recommended for this type of method as a further increase in pressure may lead to coalescence and ultimately leads to increased-sized droplets. The major disadvantage of this technique is the consumption of high energy, which causes increased emulsion temperature during the process. Nevertheless, this technique may further be utilized as hot and cold HPH types, in which the cold HPH technique is highly favored and mostly utilized, especially for temperature-sensitive compounds. A melted lipid phase is used to disperse the compound in both techniques, but in the hot HPH technique, a hot surfactant is used to disperse the mixture above its boiling point using a high-speed stirrer. In contrast, in the cold HPH technique, prior grounding and cooling of active compound and lipid phase are required into a cold surfactant solution which results in the formation of a cold pre-suspension (Azmi et al., 2019).
Ultrasonic Homogenization Nanoemulsions
USH employs making use of sonotrode also termed as a sonicator probe which generates mechanical ultrasound vibrations when it comes into contact with the liquid causing cavitation to occur. Cavitation is defined as the formation and collapse of vapour cavities present in the liquid. This probe, when inserted into the coarse emulsion, results in subsiding and breaking down the microdroplets near the inserted probe with the help of ultrasound energy generated from the sonotrode. USH requires a two-step mechanism; initially, the interfacial waves are generated under the acoustic field which breaks the dispersed phase resulting in a continuous phase. Next, acoustic cavitation aids in collapses these droplets into smaller droplets via pressure fluctuations. Finally, the formation of nanoemulsion droplets occurs which is the overall result of interaction among droplet breakup and droplet coalescence. Factors influencing the efficiency of USH are the duration of treatment and the power, frequency, and amplitude of the ultrasound waves. Mostly medium-scale homogenizers are utilized in those laboratories where the droplet size requirement of nanoemulsion is approximately 0.2 pm. These ultrasonic homogenizers have very simple operational protocols and are energy-efficient and affordable. Additionally, this process requires low emulsifier content and displays excellent dispersion stability. Being aseptic, these can be safely introduced into the coarse emulsion, and therefore, the risk of microbial contaminants entering the processing stage is further reduced. Sonication can be performed either in batch mode or in continuous mode, which solely depends on the design of the operating chamber. Flowever, reports exist that highlight continuous mode to generate broader particle size distribution as compared to batch mode; the reason attributed for this is the non-homogeneous treatment of the fluids in the flow chamber. However, nanoemulsions generation through the sonication method holds several potential drawbacks, one of them being a high shear rate which may instigate an increase in temperature of the emulsion, that may go up to 80°C. This may induce abnormal detrimental effects on the heat-sensitive compounds mainly lipids. Apart from this, the sonication probe being metal derivatives may result in metal leaching into the emulsion because of cavitational abrasion. This could be attributed to additional limitations for limiting its vast commercial application (Salvia-trujillo et al., 2017).
A microfluidization technique utilizes a device called a “microfluidizer” which looks similar in design to a HPH. However, compared to HPH the designing of channels for making emulsion flow within the homogenizer is extremely complicated. This method uses an interaction chamber that uses high pressure to force the product through a narrow orifice, resulting in droplet disruption. The interaction chamber from the inner side consists of small channels, termed as microchannels, that utilize a positive displacement pump having a high pressure of about 500 to 20,000 psi. As pre-emulsification is not required in this technique because the dispersed phase is directly injected into the continuous phase, by making use of microchannels, therefore, this technique is also known as the “direct” emulsification technique. Initially, the course emulsion is prepared by mixing aqueous phase and oily phase which is then further processed in the microfluidizer channels w'hich works on the principle of bifurcating the flow of emulsion into two separate streams. This is followed by merging these separate channels in the interacting chamber which results in generating of intense disruptive forces as the fast-moving streams superimpose on each other leading to an efficient droplet disruption that further generates nanoemulsion of the submicron range (Salvia-trujillo et al., 2017; Pathania et al., 2018).
18.104.22.168.1 High-Pressure Microfluidic Homogenization
This method is a step ahead of the microfluidization technique. Both HPH and HPMH use a positive displacement pump that generates high pressure between
30 MPa and 120 MPa to create nanoemulsions, but still, the specific design for both differ considerably. However, HPMH is extremely useful in generating customizable-size, tailored nanoemulsions at a large scale. Several reports highlight the usefulness of HPMH to be more effective in the formation of nanoemulsions when compared to HPH. In similar research using the HPMH method, authors had claimed to generate stable nanoemulsions having a particle size ranging to 275.5 nm, a zeta- potential of -36.2 mV, and a viscosity of 446 cP (Liu et al., 2019).
This method makes use of a machine suitable for homogenizing mixtures to reduce the droplet sizes for preparing coarse emulsion. Colloid mills are also termed stirred mills and bear a high-speed rotor that can generate hydraulic shear by running up to 18,000 rpm twists, thereby disrupting the dispersed phase. The high-speed rotor generates high hydraulic shear to reduce the droplet size up to 1 pm; however, it can never achieve further size reduction and reach nanoscale size. Therefore, this process can be used as pre-preparative process for generation of micro-nanoemulsions. However, under certain circumstances, droplets as small as 10 nm can be generated by this method. Reports emphasize that lowering the processing temperature beyond the solidification temperature of the dispersed phase augments the process of droplet reduction, which can be justified as grinding rather than fluid disruption (Espitia et al., 2019).