Thermodynamically stable colloidal system interfacial tension

Microemulsions, like micelles, are considered to be lyophilic, optically isotropic and thermodynamically stable colloidal dispersions (the first microemulsions were termed oleopathic hydromicelles [128]). In some systems, the addition of a fourth component, a cosurfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to zero or near-zero values, easily on the order of 10 -10 mN m or lower, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about lO-lOOnm [129]. > The droplets can be so small that they scatter little light, so the emulsions appear transparent. Unlike coarse emulsions, micro emulsions are thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence and possibly transient negative interfacial tensions, but this remains an area of continued research [130-134]. It is sometimes helpful to think of microemulsions as systems of very highly swollen micelles. 

As emphasized earlier, similarity in the composition of contacting phases in the vicinity of the critical temperature leads to a decrease in the interfacial tension to very small values, at which the spontaneous formation of thermodynamically stable liophilic colloidal systems is possible (see Chapter VI). 

The ease of emulsion formation increases and the droplet size achievable decreases as the interfacial tension falls. Systems in which the interfacial tension falls to near zero j<10-3 mNm (dyne cm-1)] may emulsify spontaneously under the influence of thermal energy and produce droplets so small (<10 nm diameter) that they scatter little light and give rise to clear dispersions. The micro emulsions so formed occupy a place between coarse emulsions and micelles. They are usually effectively monodispersc and unlike coarse emulsions are thermodynamically stable. Microemulsion droplets have sometimes been classified as swollen micelles. In fact, there probably exists an essentially continuous sequence of states from association colloids to coarse emulsions,... 

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