Azeotropia

Azeotropy is a phenomenon in liquid mixtures where the components reach a fixed overall composition that boils at a constant temperature, such that the vapor produced has the same composition as the liquid. At the azeotropic composition, distillation of the mixture by ordinary means cannot change the ratio of components in the liquid or the distillate.

Azeotropes arise from non-ideal vapor–liquid equilibrium and deviations from Raoult’s law. Depending on the interactions between molecules, a mixture can form a minimum-boiling azeotrope, where the boiling point is lower than that of the pure components, or a maximum-boiling azeotrope, where it is higher. At the azeotropic point, the relative volatility of the components equals unity, so simple fractional distillation cannot alter the composition.

Separation of azeotropic mixtures typically requires special techniques. One approach is adding an entrainer or forming a new azeotrope that shifts the composition, allowing further separation. Another method is pressure-swing distillation, since the azeotropic composition can change with pressure for some systems. Extractive distillation, where a solvent is used to modify activity coefficients, and certain membrane separations can also be employed to break or bypass azeotropes.

Common examples include the ethanol–water system, which forms an azeotrope around 95.6% ethanol by volume at atmospheric pressure with a boiling point near 78.1°C. Other organic–water systems, such as acetone–water, also exhibit azeotropy. Understanding azeotropy is important in chemical engineering for designing methods to purify liquids and to optimize industrial distillation processes.