azeotropism

Azeotropism, or the formation of azeotropes, is a phenomenon in liquid mixtures where the liquid and vapor phases become identical in composition at a specific composition and pressure. At this azeotropic point, the mixture boils at a constant temperature and the vapor has the same composition as the liquid, meaning that simple distillation cannot alter the overall composition beyond that point.

Azeotropes arise from non-ideal interactions between components. Deviations from Raoult’s law can cause the total vapor pressure of the mixture to show a maximum or a minimum at a particular composition. When a minimum occurs, the mixture forms a minimum-boiling azeotrope; when a maximum occurs, a maximum-boiling azeotrope results. Azeotropic composition and behavior depend on pressure, so changing the pressure can shift or remove the azeotrope, giving rise to different azeotropic points on phase diagrams.

Commonly cited examples include the water–ethanol system, which forms a minimum-boiling azeotrope near 95.6% ethanol (by volume) that boils about 78.2 C at 1 atm. Other systems in which azeotropes occur include certain organic–water mixtures and some hydrocarbon–water combinations; the exact compositions and boiling temperatures vary with pressure. In industrial contexts, azeotropes present a challenge for purification because they limit the effectiveness of conventional distillation.

Strategies to overcome azeotropism include the use of entrainers (azeotropic or extractive distillation), pressure-swing distillation, or membrane-based separations such as pervaporation. These methods exploit changes in azeotropic behavior to achieve separation of components that cannot be drastically enriched by ordinary distillation alone. See also: Raoult’s law, phase diagrams, distillation.