Microstructure considerations of new five-component winsor IV food-grade microemulsions studied by pulsed gradient spin-echo NMR, conductivity, and viscosity

The microstructure of an unusual Winsor IV isotropic region of five-component microemulsions based on nonionic surfactants was studied. The microemulsions are composed of R(+)-limonene, water, propylene glycol (PG), ethanol (EtOH), and polyoxyethylene sorbitan monostearate (Tween 60) with a 1:1:3 R(+)limonene/ethanol/surfactant weight ratio. The phase diagrams of the system are characterized by an extended single continuous isotropic region starting from an oil-rich solution containing no aqueous phase (reverse micelles) to the water/propylene glycol (1/1) corner (swollen direct micelles). The microemulsions seem to be attractive for food applications. The microstructure changes gradually, smoothly, and continuously upon increasing the aqueous phase content. The microemulsion transforms from a water-in-oil (W/O) microemulsion to a bicontinuous phase and to an oil-in-water (O/W) microemulsion. The microstructure of the microemulsion along a selected dilution line, is probed using pulsed gradient spin-echo NMR, conductivity, and viscosity. The results are compared with a corresponding non-food-grade system based on C18:1E10 (Brij 96v). The hydrodynamic radius (R-H) of O/W microemulsion samples based on Brij 96v and Tween 60, at 90% aqueous phase, was determined to be 5.4 +/- 0.2 and 5.2 +/- 0.2 nm, respectively, and the calculated values of the area of the polar headgroup (a) are 87 for Brij 96v and 160 Angstrom(2) for Tween 60-based microemulsions. The ratio of the water self-diffusion coefficient, D-W, to the PG self-diffusion coefficient (D-PG) (termed the L ratio)was found to be sensitive to the aqueous phase content. In the O/W microemulsions, the water/PG mixture diffuses as hydrogen bonded entity (molecular diffusion controlled), while in the W/O microemulsions, the PG partitions, in part, at the interface and the dominate diffusion process is neither aggregate nor molecular controlled.