Azeotrope

An azeotrope is a mixture of two or more liquids whose composition cannot be altered by ordinary distillation because the vapor has the same composition as the liquid at the boiling point. At an azeotropic composition, the liquid and vapor are in equilibrium with identical proportions of the components, so fractional distillation cannot change the relative amounts. Azeotropic mixtures are sometimes called constant-boiling because their boiling point remains essentially fixed during distillation.

Azeotropes arise from non-ideal behavior in liquid solutions, often due to specific inter-molecular interactions that cause deviations from Raoult’s law. They are classified by the temperature at which they boil relative to the pure components: minimum-boiling azeotropes boil at a lower temperature than either component (positive deviation), while maximum-boiling azeotropes boil at a higher temperature (negative deviation).

In binary systems, the azeotropic composition corresponds to a local extremum on the boiling-point versus composition curve. Outside this point, the mixture may be separable by distillation; at the azeotropic composition, simple distillation cannot increase the purity of either component.

Common examples include the ethanol–water system, which forms a well-known minimum-boiling azeotrope around 95–96% ethanol by volume at 1 atm. Other systems, such as water with certain acids or organic solvents, also form azeotropes under specific conditions. Temperature and pressure can influence azeotropy, with some azeotropes disappearing at different pressures. To separate components beyond an azeotrope, techniques such as azeotropic distillation with an entrainer, extractive distillation, or pressure-swing distillation may be employed.