What are Azeotropes?

What are Azeotropes?

An azeotrope is a liquid mixture with a unique feature: its vapor phase possesses the same composition as the liquid phase at its boiling point. This means that boiling an azeotrope does not alter the relative concentrations of its components. Unlike typical mixtures where the more volatile component evaporates first, azeotropes boil as a single entity, maintaining their equilibrium. 

 

Types of Azeotropes 

Azeotropes can be classified by a variety of means:

Homogenous and Heterogenous Azeotropes:

  1. Homogeneous Azeotropes: These mixtures, like ethanol-water, are miscible and for a singly, uniform phase. 
  2. Heterogeneous Azeotropes: These mixtures, like oil-water, are immiscible and exist as distinct layers within the flask. 

Positive and Negative Azeotropes:

  1. Positive Azeotropes: Also known as “minimum-boiling” azeotropes. They have lower boiling points than their components.
  2. Negative Azeotropes: Also known as “maximum-boiling” azeotropes. They have higher boiling points than their components.

Temperature-Minimum and Pressure-Maximum Azeotrope Mixtures of Chloroform and Methanol[1]

Binary and Ternary Azeotropes:

As their names suggest, binary azeotropes are made up of two constituents, whereas ternary azeotropes are made up of three constituents. 

 

Significance in Rotary Evaporation

Azeotropes can both hinder and aid the separation process. On one hand, if your desired compound forms an azeotrope with your solvent, complete purification through simple evaporation becomes impossible. This presents a hurdle in isolation, requiring alternative techniques like extraction or azeotrope breaking with entrainer molecules. However, you can harness azeotropes to your advantage:  If there is a contaminant in your solvent, and this contaminant forms an azeotrope with the solvent at a lower boiling point than the desired product, you can exploit this property to selectively remove the contaminant. Here’s how it works:

  1. Identify the Azeotrope Formation: Determine if there is an azeotrope formation between the solvent and the contaminant.
  2. Boiling Point Selection: If the contaminant forms an azeotrope with the solvent at a lower boiling point, you can choose a specific temperature for evaporation.
  3. Selective Evaporation: By evaporating the mixture at this specific temperature, you will selectively remove the azeotrope-forming contaminant, leaving behind the remaining desired product.

 

Entrainer Molecules

Entrainer agents, also called azeotropic agents, are molecules that can be added to a liquid mixture to help in the separation of components using distillation, specifically when your mixture forms an azeotrope. In rotary evaporation, entrainer agents are used to break down azeotropes and assist in separating its components. Here’s how they work:

1) Breaking Azeotropes: Entraining agents work by disrupting the vapor-liquid equilibrium, efficiently altering the azeotropic behavior. By using an entrainer, the azeotropic composition is affected, leading to separation based on the altered boiling points.

2) Improved Separation Efficiency: Entrainers can upregulate the volatility of one component compared to the other, contributing to its preferential evaporation. This selective vaporization leads to an improved separation efficiency, specifically when working with components that have similar boiling points or form azeotropes.

3) Enhancing Distillation Performance: Azeotropic agents can also alter the relative volatility of components, decreasing the temperature needed for separation. This contributes to energy savings and enhanced efficiency in rotary evaporation.

Selecting Suitable Entrainers

Choosing an entrainer depends on the following factors: the type of components that need to be dissociated, their boiling points, solubility, and compatibility with the system. Examples of entrainer molecules include ethanol, benzene, toluene, and different chlorinated solvents, among others. You will need to look up what entrainers, if any, are available for your particular azeotrope, as there are no universal entrainers.

 

Azeotropes in Practical Applications 

Controlled Evaporation:

  • Paint thinners and degreasers: The acetone-chloroform azeotrope (68% acetone) exhibits a controlled evaporation rate crucial for these applications. The azeotrope ensures consistent cleaning performance without flash evaporation or rapid drying, making it ideal for precise removal of grease and paint residues.
  • Drying heat-sensitive materials: Certain azeotropes can be tailored to evaporate at lower temperatures, making them suitable for drying heat-sensitive materials.

Separation and Purification:

  • Recovery of high-purity ethanol: While the ethanol-water azeotrope limits ethanol concentration in homebrewing (specially with a recent rise in the use of rotary evaporators by professional mixologists in craft bars), techniques like azeotropic distillation using benzene can break the azeotrope and yield purer ethanol (around 99.5%). This process, though not recommended for home use due to safety concerns, demonstrates the potential of azeotropic distillation in achieving high purity. It is also necessary to mention that benzene cannot be used in any applications intended for personal consumption, as it is highly carcinogenic. 
  • Extraction of essential oils: Some essential oils form azeotropes with specific solvents, allowing for their selective extraction from plant materials. 

 

References:

WilfriedC at English Wikipedia, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons