Other Lyophilic Colloids Microemulsions

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be 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 [224,225], 

The Winsor microemulsion classification system distinguishes among three different types based on their phase behaviour  

Much work has also gone into understanding the chemical composition requirements for microemulsification [229]. For example, surfactants having extended hydrocarbon tails (linkers) have been developed to induce additional orienting of oleic phase molecules at depth from the interfaces [230,231]. Table 11.3 gives some idea of the range of components that may be formulated into a microemulsion. 

Microemulsion system applications span many areas including enhanced oil recovery, soil and aquifer decontamination and remediation, foods, pharmaceuticals (drug delivery systems), cosmetics, and pesticides [2,5,33,37,232,233]. Some of these are listed in Table 3.6. The widespread interest in microemulsions and use in these different industrial applications are based mainly on their high solubilization capac- 

In using microemulsions to enhance oil recovery from petroleum reservoirs (see Section 11.2.2) the concept of optimal salinity has evolved. By optimal salinity is meant the salinity for which O/W interfacial tension is lowest and oil recovery is 

For a given chemical system, phase-type diagrams can be constructed that show the regimes in which each type of microemulsion will exist [136, 137]. These can be used to understand and predict the effects of, for example, increasing salinity or decreasing HLB, which tend to shift the emulsion type directionally from type I to type III to type II. The middle-phase (in type III) microemulsions can be thought of as bicontinuous, as opposed to drop-like or lamellar. It is thought that phase inversion from a W/O microemulsion to an O/W microemulsion takes 

4) Note that the size range for microemulsions spans the size ranges for nanoemulsions and macroemulsions. 

The surfactant affinity difference (SAD) is used to model emulsion phase behaviour based on the chemical potentials of surfactant in aqueous and oil phases. SAD is the negative of the free energy of transfer of a surfactant molecule from an oil to a water phase (see the papers by Salager et al. [132, 142, 143]). 

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