Nanostructured fluids based on propylene carbonate/water mixtures

The enzymatic activity of Lipase VII from Candida rugosa was studied in the quaternary water-in-oil microemulsion cetyltrimethylammonium bromide (CTAB)/water/pentanol (POH)/hexane (C6). The enzyme-catalyzed hydrolysis of p-nitrophenylbutirate (p-NPB) was found essentially independent from the water/surfactant ratio. On the other hand, the enzyme kinetics was strongly affected by the cosurfactant/ surfactant ratio. The interfacial composition and the reverse micelles size, as well as the substrate partition between organic bulk and surfactant aggregates, have been determined through pulsed gradient spin-echo NMR measurements. The correlation of the microemulsion structure with enzyme activity has therefore been possible. The lipase affinity for p-nitrophenylbutirate is lower in microemulsion compared to water because the hydrophobic effect is weaker in the former system. The turnover number is unaffected from the reverse micelles size and from the interfacial bromide concentration, while is strongly dependent on the interfacial composition. The data presented indicate a marked influence of the extent of interfacial surface on the catalytic efficiency of lipase.

The microstructure of aggregates formed by sodium dodecyl sulfate (SDS) and 1-pentanol in mixtures of water and a polar aprotic solvent (propylene carbonate, PC) was investigated by means of pulsed gradient spin-echo NMR, dynamic light scattering, viscosity, and conductivity measurements. PC partitions itself between micelles and aqueous bulk. The fraction of micellized propylene carbonate remains constant along PC-dilution, and the phase separation takes place when the composition of continuous phase attains the PC/water miscibility gap. The micellized PC is present mainly in the micelle's palisade and strongly increases the total interfacial area, thus acting as a cosurfactant. At high PC content, the system is composed by very small aggregates (around 10 A in radius) made by few SDS molecules (10-6) and PC and pentanol. The resulting system can be described as a nanostructured fluid with a huge interfacial area and a small dispersed phase. © 2005 American Chemical Society.