Motivation for surfactant systems as reaction media

Surfactants by definition self-organise in water giving rise to micelles of varying size and shape. The core of micelles is non-polar and can solubilise reactants that are insoluble in water. Thus, a simple surfactant-water system at a surfactant concentration well above the critical micelle concentration can be used to overcome the problem of reactant incompatibility the polar reagent will be situated in the bulk aqueous domain, the non-polar reagent will be present in the micelles, and the reaction will occur at the micelle boundary. Organic reactions in micellar systems have been reported more than 40 years ago [1,2]. 

micellar solutions consist of three regions of distinctly different solvation properties, a continuous polar aqueous domain, non-polar cores and interfacial regions of intermediate polarity. They are all present in a single homogeneous, thermodynamically stable solution. The totality of the three regions can be treated as separate reaction regions 

True micellar systems have low capacity for dissolving non-polar reactants, however. They are therefore of limited preparative value. Microemulsions, which contain not only surfactant and water but also an oil component, can dissolve appreciable amounts of both a polar and a non-polar reactant and are therefore much more practically useful as media for organic synthesis. There has been considerable interest in the use of microemulsions as media for organic reactions in recent years [7—11]. Not only can such a formulation be a way to overcome compatibility problems, the capability of microemulsions to compartmentalise and concentrate reactants can also lead to considerable rate enhancement compared to one-phase systems. A third aspect of interest for preparative organic synthesis is that the large oil-water interface of the system can be used as a template to induce regioselectivity. These aspects will be dealt with in this chapter. 

Sn2 type performed in a microemulsion and in two liquid crystalline phases of different geometry. The higher overall rate obtained in a liquid crystalline system as compared to a microemulsion is most probably an effect of the higher interfacial area between the polar and the non-polar domains of the former systems. Almost all added surfactant will be located at the interface in both systems, which means that the volume of the reaction zone will be proportional to the amount of surfactant in the system. As will be discussed later in this chapter, the reaction rate for a typical bimolecular substitution reaction is proportional to the interfacial area, provided one of the reactants is only soluble in the polar domain and the other reactant is only soluble in the non-polar domain of the organised surfactant system. 

However, for practical use as reaction media the liquid crystalline systems are hardly realistic. They need very high surfactant concentrations, which make the work-up procedure complex, and these systems are highly viscous, which make mixing and heat removal a problem. Surfactant liquid crystals are of more interest as templates for making meso-porous oxides and such materials, in the form of suspensions of small particles, are also of interest for overcoming compatibility problems in organic synthesis [20]. The profiles for reactions in such systems are also included in Fig. 5.1. 

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