Abstract:

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 wt. ratio. The phase diagrams of the system are characterized by an extended single continuous isotropic region starting from an oil-rich soln. contg. no aq. 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 aq. 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 diln. line, is probed using pulsed gradient spin-echo NMR, cond., and viscosity. The results are compared with a corresponding non-food-grade system based on C18:1E10 (Brij 96v). The hydrodynamic radius (RH) of O/W microemulsion samples based on Brij 96v and Tween 60, at 90% aq. phase, was detd. to be 5.4 ± 0.2 and 5.2 ± 0.2 nm, resp., and the calcd. values of the area of the polar headgroup (a) are 87 for Brij 96v and 160 \AA2 for Tween 60-based microemulsions. The ratio of the water self-diffusion coeff., DW, to the PG self-diffusion coeff. (DPG) (termed the L ratio) was found to be sensitive to the aq. phase content. In the O/W microemulsions, the water/PG mixt. diffuses as hydrogen bonded entity (mol. diffusion controlled), while in the W/O microemulsions, the PG partitions, in part, at the interface and the dominant diffusion process is neither aggregate nor mol. controlled. [on SciFinder(R)]