Type of organised surfactant system

Surfactants in solutions show a broad variety of microstructures caused by molecular selforganisation. The observed structures depend essentially on the physical interactions of the involved components and the composition of the mixtures. For the selection of a suitable type of reaction medium the required composition of the reaction mixture is more important than the question of whether a micellar solution, a bicontinuous microemulsion, a w/o- or an o/w-microemulsion is formed. For synthetic purposes high concentrations of reactants are indispensable in order to avoid high-energy cost for work-up procedures. Therefore, a reaction system that allows high-reactant concentrations needs to be chosen. For stoichiometric reactions involving reactant incompatibility, like nucleophilic substitution reactions with an inorganic nucleophile, often an aqueous solution of this reactant has 

The small droplets act as microreactors when they contain the water-soluble catalysts. For hydroformylation reactions with water-soluble Rh/TPPTS in the droplets, the alkene, carbon monoxide and hydrogen approach the micelle surface where the reaction occurs, as is illustrated in Fig. 5.13. After the reaction is completed, phase separation can be achieved by changing the temperature of the reaction mixture. When the mixture is cooled down an aqueous bottom phase, containing most of the surfactant and the water-soluble catalyst separates from the organic upper phase, which contains the hydrophobic products and unconverted reactants. In case of incomplete catalyst recovery the micelle remaining in the product phase can be separated by means of ultrafiltration. 

A first attempt at hydroformylation in a micellar system using a water-soluble rhodium catalyst (Rh-TPPTS) was made by Tinicci and Platone from Eniricerche in 1994 [ 59 ]. They converted olefins with carbon numbers up to 12 using a mixture of an anionic surfactant (SDS) and butanol (as co-surfactant). It has been shown that microemulsions made with non-ionic surfactants of the alcohol ethoxylate type are advantageous compared to ionic 

As was mentioned earlier in this chapter, it is not necessary to transfer every reaction mixture into a thermodynamically stable one-phase system. Often the presence of one organised surfactant phase in equilibrium with one or two excess phases is sufficient to give an appropriate reaction rate. In such two- or three-phase systems the reaction occurs in the surfactants phase while the coexisting phases act as reservoir for the reactants. This approach has been demonstrated for alkylation of phenol [28] and for rhodium catalysed hydroformylation of dodecene [50]. A major practical advantage with the multi-phase systems is that substantially less surfactant is needed. This reduces costs and simplifies the work-up. 

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