ABSTRACT
Microemulsions are thermodynamically stable mixtures of oil and water. The stability is due to the presence of fairly large amounts (several percent) of surfactants. Microemulsions are often transparent, but scattering of light, X-rays, etc. indicate that oil and water are not molecularly dispersed but are rather more coarsely mixed. By “coarse” in this case we mean that oil and water are present in domains of a few to over a hundred nanometers in size. Consequently, microemulsions contain huge oil/water interfacial areas, and to allow stability the interfacial tension must be quite low, usually ≪ 1 mN/m. In that case the entropy of mixing, although small on account of the coarseness of the mixture, may be large enough to compensate for the positive interfacial free energy and to give the microemulsion a free energy lower than that of the unmixed components.
