Small-angle neutron scattering study of a world-wide known emulsion: Le Pastis

‘Pastis’ is a famous aniseed based beverage, symbolic of hot summer days in the south of France. But who really knows the origin of the milky color when adding water? Each time you drink a glass of ‘Pastis’, you observe a complex phenomenon in physical chemistry! Pastis is mainly composed of anethol, an aromatic molecule, water, ethanol and additive compounds that improve the taste. Anethol has a very low solubility in water and further addition of water to the ‘dry’ preparation induces the formation of an emulsion made of large anethol droplets in a water-plus-ethanol solvent that scatter light. The origin of the spontaneous formation of the emulsion is a well-known phenomenon, which appears in liquid systems with three or more compounds, when the sample composition is trapped between the binodal and spinodal curves on the phase diagram. The radius of the droplets has been determined using the small-angle neutron scattering technique (D22, Institut Laue-Langevin, Grenoble). Of the order of a micron, the droplet size depends on the anethol/ethanol volume ratio and grows with time and temperature.