Nanoemulsions, a class of emulsions with a very small and uniform droplet size, typically in the range of 20 nm–500 nm, are becoming increasingly popular as vehicles for the controlled delivery and optimized dispersion of active ingredients. They often are referred to as mini-emulsions, submicrometer-sized emulsions, ultrafine emulsions, etc.1
Their small droplet size makes some nanoemulsions transparent or translucent, resembling microemulsions; but if they are inadequately prepared to control droplet size distribution and stabilized against Ostwald ripening that occurs when the oil has some finite solubility in the continuous medium, nanoemulsions may lose their transparency with time as a result of an increase in droplet size.
Unlike microemulsions that are thermodynamically stable, nanoemulsions are only kinetically stable. However, the long-term physical stability of nanoemulsions with no apparent flocculation or coalescence makes them unique and they are sometimes referred to as approaching thermodynamic stability. The inherently high colloid stability of nanoemulsions can be well understood by considering their steric stabilization (when using non-ionic surfactants and/or polymers) and how this is affected by the ratio of the adsorbed layer thickness to droplet radius, as will be discussed.