Azeotropes: why are they difficult to separate?

Those who work in industries having intensive use solvent — such as the pharmaceutical, chemical, printing or painting industries — know that not all liquid mixtures can be easily separated. Some seem to ‘resist’ separation, as in the case of azeotropes: combinations of liquids that behave abnormally and render traditional distillation ineffective.

An azeotrope is a mixture of two or more liquids that, once a certain proportion is reached, behaves as a “single liquid” during distillation. In practice, the vapor that is formed has the same composition as the liquid mixture: this makes simple distillation ineffective, which instead is based on the different volatility of the components.

Where does the problem arise?

The problem with azeotropes arises from the fact that, during distillation, the behaviour of the mixture does not follow the usual rules based on individual boiling points. Although one of the liquids in the mixture is much more volatile, the intermolecular forces between the components cause them to evaporate simultaneously. This happens because the equilibrium between the liquid phase and the vapor phase is achieved in such a way that the composition does not change, even after repeated cycles of evaporation and condensation. In practice, it is as if the mixture behaves as a single substance and stays together, making it impossible to obtain a pure component with simple distillation alone.

A concrete example: ethanol and water.
By distilling a mixture of water and ethanol, it is possible to increase the concentration of ethanol up to 95.6%. At this point the azeotrope is reached: the mixture boils at about 78.1°C, and the vapor has exactly the same composition as the liquid. We cannot go any further without alternative methods.

Types of azeotropes

• Minimally boiling: The azeotropic mixture has a lower boiling point than that of the individual components. Ex: ethanol-water.
• Of maximum boiling: the opposite, with a higher boiling point. E.g.: hydrochloric acid-water.

Some well-known examples

• Acetone/Methanol: azeotrope at 56.5°C
• Chloroform/Methanol: azeotrope at 56.5°C
• Acetic acid/Water: azeotrope at 100.1°C
• Isopropanol/Water: azeotrope at 80.3°C

When classic distillation is not enough, there are more advanced solutions:

• Azeotropic distillation: adding a third component that “breaks” the balance.
• Pressure variation: changing the operating conditions changes the balance.
• Membrane technologies: selectively separate components.

Conclusion
If you find yourself dealing with mixtures that are difficult to separate, it is very likely that an azeotrope is involved. Knowing how they behave is the first step to dealing with the problem effectively. If you want to learn more and understand how we can help you manage azeotropic solvents, contact us: let’s find the best solution together.