One of the best definitions of microemulsions is from Danielsson and
Lindman [1] “a microemulsion is a system of water, oil and an amphiphile
which is a single optically isotropic and thermodynamically stable liquid
solution”. In some respects, microemulsions can be considered as small-scale
versions of emulsions, i.e., droplet type dispersions either of oil-in-water (o/w)
or of water-in-oil (w/o), with a size range in the order of 5−50 nm in drop
radius. Such a description, however, lacks precision since there are significant
differences between microemulsions and ordinary emulsions (or
macroemulsions). In particular, in emulsions the average drop size grows
continuously with time so that phase separation ultimately occurs under
gravitational force, i.e., they are thermodynamically unstable and their
formation requires input of work.