4. Conclusions
Possible applications of nano-emulsions are strongly
limited by the stability of nano-emulsions, except for nanoparticle
preparation when the process of physical or chemical
solidification takes place within the period of stability of nanoemulsions.
Nano-emulsions are proposed for numerous applications
in pharmacy as drug delivery systems because of their capacity
of solubilizing non polar active compounds. Due to the
stability problems, most of proposed formulations are selfemulsifying
systems and the nano-emulsions are produced
just before their application. Although there have not been
reported too many applications in other fields, there is a great
potential for nano-emulsion applications if Oswald-ripening
destabilization mechanism is limited by using very insoluble
oils.
Concerning optimization in preparation of nano-emulsions
by shear, an optimum shear or time shearing can exist if breaking
and coalescence are competing phenomena during the process.
Concerning optimization in the preparation of nano-emulsions
by low-energy methods, recent literature confirms that
crossing bicontinuous or aqueous continuous phases during
emulsification allows obtaining O/W nano-emulsions of small
droplet size and low polydispersity.
Optimizations by selective variation of parameters or experimental
designs allow to conclude that, with respect to
composition variables, generally there is an optimum surfactant
mixture composition, or HLB, and that the higher the oil
surfactant ratio the greater the droplet size. The preparation
variables, as addition, agitation or cooling rate, generally do
not have a significant influence if the system is optimized with
respect to composition.
This last conclusion has a very important derivation: if
preparation variables do not have influence, the system can be
scaled-up, from lab to industrial, and similar results can be
expected.
As a final comment, judging from the most recent literature,
the interest in nano-emulsion preparation and application is
growing, but few of the numerous ideas reported become commercial
final applications