A mixture of chemicals is said to be zeotropic if the composition of the vapor and the liquid phase at the vapor-liquid equilibrium state are never the same. Dew point and boiling point curve do not touch each other over the entire composition range with the exception of the pure components (curve end points). Mixtures where the both curves are touching each other in at least a single point - indicating the same composition in the vapor and liquid phase - are azeotropic mixtures.
Vapor pressures and boiling temperatures of a zeotropic mixture are always between the pure component boiling points resp. vapor pressures. Azeotropic mixtures show a pressure maximum/temperature minimum or pressure minimum/temperature maximum that are above/below the values of the pure components.
Stating compositions of zeotropic mixtures is common for some commercially available blends but the term zeotropic is valid only for a given temperature and pressure but not for specific compositions.
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Zeotropic mixtures can be separated by normal distillation. The separation factor never become exactly 1 but might be very close to 1 making a separation by distillation very difficult or even technically impossible.
Some specific zeotropic mixtures are used as refrigerants (Code names are R-4xxx). Zeotropic mixtures have the disadvantage that their composition is changing during the repeated evaporation and condensation processes. This causes a temperature glide – a change in the boiling temperature at constant pressure. Additionally, this leads to changing thermodynamic properties, notably, heat of vaporization and heat capacity which are the most important in the refrigeration process. Azeotropic mixtures on the other hand have constant properties at their azeotropic composition and behave like pure components. If zeotropic blends are used it is common to select mixtures with small separation factors (so called near-azeotropic mixtures).[2]
Zeotropy occurs when the boiling difference is large or when the chemicals in a blend are similar, e. g. two ketones or two amines. Azeotropy occurs if the boiling point difference is smaller and the chemicals contain different functional groups (e. g. Chloroform and Methanol).
A mixture can be zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure. A good example is the mixture of Ethanol and Water which is azeotropic above approximately T=305 K and P=12 kPa. Below that temperature and pressure the mixture is zeotropic. The separation factor at the limiting temperature is still very small and thus the differences between liquid and vapor composition is too small to be of use for distillation.