As a summary of this point, nano-emulsions are emulsions
(non-equilibrium systems, defined according to [1•]) with a
remarkable small droplet size (in the nanometer range, e.g. 20–200 nm), regardless of the preparation method. A photography
of an oil-in-water (O/W) nano-emulsion with a schematic example
of the structure is presented in Fig. 1.
Evidently the size range may vary depending on the authors.
Some authors consider 500 nm as the upper limit [1•,6]. In any
case, the size limit is not a key issue because no qualitative
differences are established by droplet size. The formation,
properties and stability of nano-emulsions are well established in
numerous papers which are reviewed [1•,5•]. Regarding applications,
nano-emulsions were firstly developed, and used for
a long time, to obtain nanoparticles by polymerization [7•], the
so-called miniemulsion polymerization method, and more
recently to obtain solid lipid nanoparticles [8–10], and ceramic
particles [11]. At present, new applications are being developed
to use nano-emulsions as consumer products.
In this review, recent literature on the new applications of
nano-emulsions as consumer products is reviewed and classified
according to the field of application. This direct application of
nano-emulsions requires the optimization with respect to formulation
and preparation variables in order to obtain the desired
characteristics. Recent literature on optimization of nano-emulsion
preparation is also reviewed and classified according to three
approaches: considerations on phase behavior, selective variation
of parameters and experimental designs.