Structural comparison between non-aqueous and aqueous microemulsions from multicomponent self-diffusion measurements

Abstract

Microemulsions are known to be formed also with non-aqueous solvents like glycerol, formamide and its derivatives. The isotropic region is fairly large and is often lar- ger than for the corresponding aqueous system. Although some structural information of aqueous microemulsions is available in literature, there is currently still no information on the structure of these non-aqueous microemulsions. From a comparison of phase be- haviour of these non-aqueous systems with that of the aqueous ones, the microstructures in these optically isotropic solutions are often believed to be the same. But this approach of gaining information about the structure is too indirect. We have carried out multicom- ponent serf-diffusion measurements in these systems by Fourier Transform NMR spin- echo technique and compared the results with those of aqueous ones. Our results reveal considereable difference in micro-structural behaviour in these systems. The self-diffu- sion data in the aqueous system show that these systems are quite structured when a cosurfactant like hexanol or octanol is used. The serf-diffusion coefficients of all the com- ponents in the non-aqueous system are quite high, so the idea of segregation into distinct domains does not seem to hold good. The non-aqueous microemulsions thus seem to be less structured than the aqueous ones and are probably doser to the structureless simple solutions. Abstract: Microemulsions based on aliphatic hydrocarbon, surfactant and aqueous buffer have been used as reaction medium for the lipase catalyzed transesterification of a trigly- ceride and a fatty acid. Both AOT (sodium bis (2-ethylhexyl)sulfosuccinate) and certain alcohol ethoxylates could be used as surfactant to produce a triglyceride having a fatty acid composition similar to that of natural cocoa butter from a palm oil distillation frac- tion. The nonionic surfactant gives a higher reaction rate than AOT, presumably due to a more favourable association of water in the microemulsion. Recovery of the enzyme is easier with the former surfactant. However, the ethoxylate is found to participate in an unwanted side reaction, viz, formation of esters with free fatty acids in the solution.