Azeotropes in Rotary Evaporation

Azeotropes are mixtures of two or more liquids whose ratios cannot be changed through simple distillation. When an azeotropic mixture is boiled, the resulting vapor has the same composition as the unboiled liquid mixture. Azeotropes are often referred to as constant boiling point mixtures.

When using a rotary evaporator, it’s important to be aware of azeotropes and how they can affect your process. Azeotropes can even be used in some cases to help facilitate a process.

In this post, we discuss in more detail what azeotropes are and how they impact the rotary evaporation process.

What Is an Azeotrope?

If a mixture of two or more liquids results in a vapor of the same composition as the liquid when boiled, it is said to be an azeotropic mixture. An azeotrope forms at a certain ratio of the two liquids . For example, 1-propanol and water form an azeotrope at 71.7% 1-propanol. At that point, no further separation of the two components can occur via simple distillation. 

The boiling point of an azeotrope is different to that of either of its constituents. The boiling point of the 1-propanol/water azeotrope mentioned above is 87.7°C, whereas the boiling point of 1-propanol is 97°C and that of water is 100°C.

Azeotropes can be classed by a variety of means:

• Positive and negative azeotropes: A positive (or minimum-boiling) azeotrope has a lower boiling point than any of its constituents, whereas a negative (or maximum-boiling) azeotrope has a higher boiling point than any of its constituents.
• Homogeneous and heterogeneous azeotropes: In a homogeneous azeotrope, the constituents of the mixture are miscible in any proportion. Heterogeneous azeotropes occur inside the miscibility gap of constituents that are not miscible.
• Binary and ternary azeotropes: These azeotropes consist of two or three constituents, respectively.

Although the azeotrope itself is formed at a certain composition, the behavior of mixtures of liquids that form azeotropes is different to that of non-azeotropic mixtures at other proportions as well. Take an ethanol and water mixture, for example. At 95.5% ethanol, these two liquids form an azeotrope that has a boiling point of 78.1°C. At this composition, the resulting vapor when the mixture is boiled has the same composition as the liquid (95.5% ethanol).

So what about lower levels of ethanol? At lower concentrations of ethanol, you will still see some water in the resulting vapor when the mixture is boiled. And the boiling point will change depending on the composition. A full table showing the boiling points of aqueous ethanol solutions can be found here, but the below table shows some select values.

Considerations When Evaporating Azeotropes with a Rotovap

Depending on your specific application, azeotropic behavior may be a help or a hindrance. Here are some factors to consider:

1. Desired compounds can form azeotropes with solvents

First and foremost, when designing a process, you’ll want to ensure that your desired compound does not form an azeotrope with your solvent. Otherwise, you’ll automatically lose your precious sample upon evaporation. If forming an azeotropic mixture is unavoidable, you’ll need to consider methods of separation, such as using an entrainer (more on that below).

2. Azeotropes may be responsible for unexpected observations

It’s important to understand when you have an azeotropic mixture so you know what to expect during evaporation. Azeotropes can mess with expected results. For example, when distilling a solution of ethanol mixed with water, you might expect that the ethanol, with a boiling point of 78°C, would quickly evaporate, leaving almost pure water. However, based on the information above, you’ll find that the proportions of water and ethanol in the vapor will alter according to the mixture's changing composition. 

3. Entrainers can sometimes be used to break azeotropes

In some cases, it may be desirable to manipulate azeotropes to avoid issues such as lengthy distillation times or to separate their components. Depending on the azeotrope, it may be possible to use what’s known as an “entrainer.” An entrainer is a separating agent that can be used to break azeotropes in azeotropic distillation and other separation methods.

An example of an azeotropic distillation setup.

For example, benzene or toluene can be added as an entrainer to an ethanol/water mixture. The added compound changes the molecular interactions of the mixture, eliminating the azeotrope. 

4. Azeotropes can be used to separate zeotropic mixtures

A zeotropic mixture is a combination of solvents that don’t form an azeotrope when combined in any proportion. For example, acetic acid and water never form an azeotrope.

Due to the similar boiling points of these two liquids (acetic acid has a boiling point of 118.1°C), it is difficult to separate them using distillation alone. We discussed entrainers above, and they can actually be used in this case too. An entrainer can act as a separating agent that enhances the separation of two liquids with similar boiling points.

In the case of an acetic acid and water mixture, an appropriate entrainer would be ethyl acetate. A 69.2% aqueous solution of ethyl acetate forms an azeotrope with a boiling point of 70.4°C. When you add the correct amount of ethyl acetate to the water/acetic acid mixture, you can easily distill off the resulting azeotrope to leave pure acetic acid.