Azeotrope Definition and Examples

What Is an Azeotrope?

An azeotropic solution retains its composition when distilled.
An azeotropic solution retains its composition when distilled. Rafe Swan / Getty Images

An azeotrope is a mixture of liquids that maintains its composition and boiling point during distillation. It is also known as an azeotropic mixture or constant boiling point mixture. Azeotropy occurs when a mixture is boiled to produce vapor that has the same composition as the liquid. The term is derived by combining the prefix "a", meaning "no," and the Greek words for boiling and turning. The word was coined by John Wade and Richard William Merriman in 1911.

In contrast, mixtures of liquids that do not form an azeotrope under any conditions are called zeotropic.

Types of Azeotropes

Azeotropes may be categorized according to their number of constituents, miscibility, or boiling points.

  • Number of Constituents: If an azeotrope consists of two liquids, it is known as a binary azeotrope. An azeotrope consisting of three liquids is a ternary azeotropes. There are also azeotropes made of more than three constituents.
  • Heterogeneous or Homogeneous: Homogeneous azeotropes consist of liquids that are miscible. They form a solution. Heterogeneous azeotropes are incompletely miscible and form two liquid phases.
  • Positive or Negative: A positive azeotrope or minimum-boiling azeotrope forms when the boiling point of the mixture is lower than that of any of its constituents. A negative azeotrope or maximum-boiling azeotrope forms when the boiling point of the mixture is higher than that of any of its constituents.

    Azeotrope Examples

    Boiling a 95% (w/w) ethanol solution in water would produce a vapor that is 95% ethanol. Distillation cannot be used to obtain higher percentages of ethanol. Alcohol and water are miscible, so any quantity of ethanol can be mixed with any quantity to prepare a homogeneous solution that behaves as an azeotrope.

    Chloroform and water, on the other hand, forms a heteroazeotrope. A mixture of these two liquids will separate, forming a top layer consisting mostly of water with a small amount of dissolved chloroform and a bottom layer consisting mostly of chloroform with a small amount of dissolved water. If the two layers are boiled together, the liquid boils at a lower temperature than either the boiling point of water or of chloroform. The resulting vapor will consist of 97% chloroform and 3% water, regardless of the ratio in the liquids. Condensing this vapor results in layers that exhibit a fixed composition. The top layer of the condensate will account for 4.4% of the volume, while the bottom layer will account for 95.6% of the mixture.

    Azeotrope Separation

    Since fractional distillation cannot be used to separate components of an azeotrope, other methods must be employed.

    • Pressure swing distillation applies pressure changes to change the composition of a mixture to enrich the distillate with the desired component.
    • Another technique is the addition of an entrainer, which is a substance that alters the volatility of one of the azeotrope components. In some cases, the entrainer reacts with a component to form a nonvolatile compound. Distillation using an entrainer is called azeotropic distillation.
    • Pervaporation separates components using a membrane that is more permeable to one constituent than to the other. Vapor permeation is a related technique, using a membrane more permeable to the vapor phase of one component than another.