Microemulsions as reaction medium for surfactant synthesis

Various microemulsion formulations were evaluated as reaction media for synthesis of the surface active compound decyl sulfonate from decyl bromide and sodium sulfite. The reaction rate was fast both in water-in-oil and in bicontinuous microemulsions based on nonionic surfactants. Two-phase systems with added phase transfer agent (quaternary ammonium salt or crown ether) were much less efficient. It is postulated that the low efficiency of the phase transfer agents in catalyzing the reaction is caused by strong ion pair formation between the product formed, decyl sulfonate and the phase transfer agent. To prove this point, decyl bromide was reacted with two other nucleophiles, sodium cyanide and sodium azide. Neither of these gave a reaction product that could form an ion pair with the phase transfer agent. With these reagents, phase transfer catalysis was almost as efficient as synthesis in a microemulsion. It was also demonstrated that the rate of decyl sulfonate formation in microemulsion could be increased further by the addition of a small amount of cationic surfactant. However, the choice of surfactant counterion is decisive concerning the effect on reaction rate. Whereas a small nonpolarizable ion such as acetate gives a considerable reaction rate increase, a large polarizable ion such as bromide slows down the reaction. Bromide is believed to interact so strongly with the interface that it prevents the reacting ion, sodium hydrogen sulfite, from reaching into the interfacial zone. (C) 1997 Elsevier Science B.V.