azeotropes

An azeotrope is a mixture of two or more liquids that, at a given pressure, boils at a constant temperature and has the same composition in the liquid and vapor phases. In other words, at the azeotropic composition the vapor produced by distillation has the same composition as the liquid, so simple distillation cannot separate the components beyond that point.

Azeotropes are categorized by their boiling behavior. Minimum-boiling, or positive, azeotropes have a boiling point lower than either of the pure components, while maximum-boiling, or negative, azeotropes have a boiling point higher than either component. The occurrence depends on non-ideal interactions between molecules in the liquid mixture, which cause deviations from Raoult’s law. When unlike interactions are weaker than like interactions, a minimum-boiling azeotrope can form; when unlike interactions are stronger, a maximum-boiling azeotrope can form. The defining feature remains that at the azeotropic composition, the liquid and vapor have identical compositions.

A well-known example is the ethanol–water system, which forms a minimum-boiling azeotrope at about 95% ethanol by volume at 1 atmosphere. Other systems also exhibit azeotropes, including various organic–water mixtures, illustrating how molecular interactions govern the appearance and type of azeotrope.

Practically, azeotropes constrain separations by ordinary distillation. To overcome them, processes such as azeotropic or extractive distillation using a third component, or pressure-swing/distillation techniques, may be employed to break the azeotropic behavior and achieve further purification.