Abstract
If one can prepare small droplets in the form of emulsion or microemulsion without addition of surfactant, the droplets would have a clear, discontinuous interface as well as highly hydrophobic interior and show properties different from those of droplets in ordinary microemulsions. From an energetic viewpoint, such surfactant-free droplets should be thermodynamically unstable and can be prepared only transiently in solution, either by nucleation of supersaturated molecules (solute) or by enforced dispersion of organic liquid (solute) in water. Surfactant-free emulsion is the simplest system of liquid/liquid dispersions, which can help us clarify the evolution and growth processes, stability mechanisms, and determination of droplet sizes in emulsion systems. In this review, the discrete size distribution of oil droplets, the relationship between formation of fine droplets (and/or dispersion stability) and viscosity (and/or vapor pressure) of oils, difference of the growth processes between S class droplets (below 100 nm in diameter) and M class droplets (above 100 nm in diameter), influence of structure of oil molecules on stability, and dispersion stability of oil droplets by addition of another oil are discussed.
