with water and ice First a strongly
hydrophobic essential oil such as
trans-anethole is dissolved in a water-
miscible solvent, such as
ethanol, and the ethanol itself forms a
solution (a
homogeneous mixture) with water. If then the concentration of ethanol is lowered by addition of more water the hydrophobic substance precipitates from the solution and forms an
emulsion with the remaining ethanol-water-mixture. The tiny droplets of the substance in the emulsion scatter light and thus make the mixture appear white. Oil-in-water
emulsions are not normally stable. Oil droplets
coalesce until complete
phase separation is achieved at
macroscopic levels. Addition of a small amount of
surfactant or the application of high
shear rates (strong stirring) can stabilize the oil droplets. In a water-rich ouzo mixture the droplet coalescence is dramatically slowed without mechanical agitation, dispersing agents, or surfactants. It forms a stable homogeneous fluid
dispersion by liquid–liquid
nucleation. The size of the droplets when measured by
small-angle neutron scattering was found to be on the order of a
micron. Using
dynamic light scattering, Sitnikova
et al. showed that the droplets of oil in the emulsion grow by
Ostwald ripening, and that droplets do not coalesce. The Ostwald ripening rate is observed to diminish with increasing ethanol concentrations until the droplets stabilize in size with an average diameter of . Based on thermodynamic considerations of the multi-component mixture, the emulsion derives its stability from trapping between the
binodal and
spinodal curves in the
phase diagram. However, the microscopic mechanisms responsible for the observed slowing of Ostwald ripening rates at increasing ethanol concentrations appear not fully understood. ==Applications==