A COMPARISON BETWEEN LIPASE-CATALYZED ESTERIFICATION OF OLEIC-ACID WITH GLYCEROL IN MONOLAYER AND MICROEMULSION SYSTEMS

Lipase-catalyzed synthesis-esterification of oleic acid with glycerol - was carried out in L2 microemulsions and in monolayers. The microemulsions were based on isooctane as a nonpolar component and various water-glycerol mixtures as polar component. The substrate, oleic acid/sodium oleate, constituted the microemulsion surfactant. The lipase resides mainly in the water pools. Monolayers of oleic acid/sodium oleate were formed on subsolutions of glycerol and water, and the enzyme solution was injected under the compressed monolayers. Thus, the arrangement of the reactants at the oil-water interface of the microemulsion can be regarded as analogous to that at the air-water interface of the monolayer. The microemulsion structure was characterized by self diffusion nuclear magnetic resonance. It was found that the higher the glycerol-to-water ratio, the lower are the water D-values. The reactions in microemulsions generally gave a low degree of oleic acid conversion. The yield increased with increasing glycerol-to-water ratio. Monoglycerides were the main product, and no triglyceride could be detected. The monolayer experiments gave a somewhat higher degree of conversion, with tri- and diglycerides being the major reaction products. The reason why triglycerides are formed in monolayer experiments but not in microemulsions is believed to be due to an unfavorable partitioning of the diglyceride in the microemulsion systems. Once formed, the diglyceride will partition into the hydrocarbon domain and became inaccessible for reaction with the enzyme-O-acyl intermediate at the oil-water interface. In addition, the interfaces in the two systems are different. The monolayer interface is static, whereas the microemulsion interface is highly dynamic, and this difference may also influence the product pattern.