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2.2.3.2.1 Nano-emulsions

Nano-emulsions are highly stable systems that show little coalescence of particles, nor sedimentation or creaming.  They can consist of lipid or polymeric vesicles or particles, in the size range of 20-200 nm.  They can have multiple phases, the simplest being oil-in-water.  They are essentially similar to emulsions in that they require energy input to be made, and differ from micro-emulsions which form spontaneously when surfactants are added to a liquid medium (and are therefore technically not emulsions).  Both nano- and micro-emulsions have particles within the same size range.  The organic phase is non-toxic and can be made from food grade components.  Nano-emulsions of solid particles can be dried, through evaporation of the outer phase, to leave particulates, which can then be suspended in another solution for application.  Through encapsulation, the active ingredient is afforded some protection against atmospheric and environmental conditions, such as oxidation, and is released slowly. 

Nano-emulsions can be produced by both high energy (mechanical process using rotators, ultrasound, or pressure homogenisers) and low energy (either spontaneous emulsification, due to solvent diffusion as a result of mixing or dilution; or by phase inversion temperature, a process which is controlled by specific surfactants, such as polyethoxylated surfactants, in response to temperature change) means[i].  In each process the active elements can be added during the synthesis stage (e.g. agrochemicals, nucleic acids) so that they are encapsulated by the nanoparticle or vesicle.  High energy systems offer more control of size distribution and composition of the resultant nanoscale vesicles or particles, however complex and ‘fragile’ chemicals can be easily degraded during the process and they are not readily scaled up to the large volumes required by the manufacturing industry. In contrast, the phase inversion temperature system in particular, is readily scaled up to industrial levels.  In addition, it affords the opportunity (with certain compositions) of effecting a phase inversion, e.g. oil-in-water to water-in-oil, during a transitional phase by changing the temperature, before the system is stabilised as a nano-emulsion. 

In terms of agricultural applications, nano-emulsions can be used for hydrophilic and hydrophobic pesticides, but are largely being developed for those that are poorly water soluble.  For example, pyrethroids such as γ-cyhalothrin and β-cypermethrin have been successfully formulated as lipid nano-emulsion, obviating the need for the organic solvents such as benzene and toluene normally required for its formulation[ii]^,[iii],[iv], as have more ecologically friendly pesticides such as Artemisia arborescens L essential oil[v].  In one case the effects of such pesticides on aquatic life were studied and found to be reduced compared with conventional formulations, while not affecting efficacy26.

In addition to exhibiting greater stability, nano-emulsions demonstrate increased coating of leaves and uptake through plant cell walls, as a result of low surface tension^27,[vi].

The main advantages to using nano-emulsions are therefore- solubilisation of hydrophobic pesticides (hence no need for toxic organic solvents), no precipitation or creaming (therefore no need for constant mixing), increased stability (protect against oxidation), improved uptake.  However, it should be noted that manufacturing opportunities are limited, as the precise mechanisms by which nano-emulsions form and their properties controlled are still the subject of intense basic research.