Rotary evaporation is often favored over standard distillation, with one of the main reasons being that you can achieve a faster evaporation rate with a rotovap setup. You might be wondering why this is the case.
For an at-a-glance look, the factors that can help speed up evaporation are:
- Decreased pressure within the system
- Increased temperature of the solvent
- Rotation of the flask
While the first two factors can be changed with both rotary evaporation and standard distillation, only rotary evaporation has the added benefit of rotating the sample. In this post, we’ll take a closer look at the similarities and differences between the two methods and why rotary evaporation is more efficient.
Decreased Pressure Within the System
The presence of a vacuum decreases the pressure within the system. This decreases the vapor pressure required for boiling to occur, thus lowering the boiling point of the solvent. This means less heat energy is needed for it to evaporate.
Aside from lowering the boiling point, a lower pressure can speed up the rate of evaporation. That is, a solvent will evaporate quicker when it is close to its boiling point. Pressure pushing down on a solvent makes it difficult for molecules to escape as vapor. A lower pressure means those molecules can enter the atmosphere at a faster rate.
Many standard distillation setups include the use of a vacuum, so pressure can be controlled in these systems. As such, rotary evaporation and standard distillation could be considered comparable in this manner.
Increased Temperature of the Solvent
An increase in temperature will increase the vapor pressure, leading to evaporation. However, whether you’re using increased temperature in a rotary evaporation or standard distillation setup, it takes time and energy to heat a bath. This reduces the efficiency of using increased temperature to evaporate the solvent.
Water takes a lot of energy to heat, and heating a bath from room temperature (usually 20 - 25°C) to, for example, 80°C, will take a lot more time than heating it from room temperature to 30°C (a common target vapor temperature for rotary evaporation). Oil baths may take even longer to heat, as they can go as high as 180°C.
What’s more, at higher temperatures, more heat is lost to the ambient environment and less heat is added to the bath, as the difference in temperature between the bath liquid and internal heating elements decreases. This makes operating at higher temperatures slower and less energy efficient.
Both standard distillation and rotary evaporation setups use increased temperature, so this factor does not necessarily make rotary evaporation more efficient than standard distillation. The key is in the rotation.
Rotation of the Flask
The rotation of the flask plays an important role in a rotary evaporation setup for two major reasons: increased surface area of the solvent and agitation of the water bath liquid.
In a static setup, as is seen in standard distillation, the surface area of the solvent is limited. In a rotating flask, the solvent forms a thin film around the sides of the flask, greatly increasing the surface area and speeding up evaporation.
The arrows in this figure represent heat transfer.
What’s more, as the flask rotates, it agitates the liquid surrounding the flask in the water bath. This aids heat transfer to the flask and the solvent.