The size rang is smaller than 1 micron in the range (100 nm-1000nm).

Its not just the size. Microemulsions by definiton are thermodynamically stable. Nanoemulsions -- not necessarily.

Dimiter hit the nail on the head. We do our best to stabilize nanoemulsions by suppressing Ostwald ripening and coalescence, but in general they will coarsen over time. Microemulsions, are thermodynamically stable, but often have a narrow composition range over which they exist.

Just to be clear compare to Dimiter's comment: nanoemulsions (or equivalently miniemulsions) are NOT thermodynamically stable, only kinetically (to various extent).

Also, the size range of microemulsions starts sooner than 10nm. You can have 1nm radius microemulsions. What is not clear is whether nanoemulsions might contain a microemulsion (i.e. small stable droplets coexisting with large unstable ones). This coexistence might explain the long term stability of some nanoemulsions, as it would be a living system with permanent exchange of components between the small stable droplets and the large (in principle unstable) ones, due to Ostwald ripening, coalescence and fission.

As far as I know (but I could be wrong), typically Ostwald ripening is not suppressed, instead only a part of the oil is based on strongly hydrophobic compounds with ultra-low solubility in water so that droplets cannot disappear through Ostwald ripening (they cannot become empty, and osmotic pressure probably does the rest of the job).

I am not sure that Sylvain's statement about Ostwald ripening is strictly correct. The driving force for Ostwald ripening to occur is the difference in chemical potential between small and large droplets. As molecular diffusion occurs from the small drops, the driving force actually increases, as the curvature continues to increase. According to Lifshitz-Slezov-Wagner theory, what stays constant is the size distribution function for the normalized droplet radius. Droplets less than the critical radius will in fact disappear... Perhaps what Sylvain was thinking was that Ostwald ripening cannot lead to phase separation of an emulsion, as can occur via droplet coalescence. At some point the driving force for ripening will no longer drive continued droplet coarsening. The point at which this occurs depends primarily on the solubility of the dispersed phase in the continuous phase, and other factors accounted for in LSW theory.

I do not think that we are in disagreement. My point was, using apolar compounds with no measurable solubility in water prevent small droplets from disappearing because these compounds cannot escape the micellar core via diffusion through water.