What Is The Mechanism Of Rotary Evaporation

Rotary evaporation is a commonly used separation and concentration technology. By placing the solution in a rotary bottle and reducing the pressure while heating, the solvent quickly evaporates, thus realizing the separation and concentration of the sample. Its mechanism mainly includes the following aspects:

1. Evaporation under vacuum conditions

In the process of rotary evaporator, vacuum conditions are usually created inside the evaporation bottle.

Under vacuum condition, the pressure inside the chemical rotary evaporator is reduced, which will lead to the decrease of the boiling point of the solvent. Because the boiling point refers to the temperature at which a liquid turns into a gas under a certain pressure, and lowering the pressure is equivalent to lowering the energy threshold for the liquid to turn into a gas, which makes the solvent more prone to evaporation. In this way, even at a low temperature, the solvent can evaporate quickly, thus realizing the concentration and separation of the sample.

Under vacuum condition, the evaporation process of solvent is more limited by intermolecular collision and space in gas phase than by ambient atmospheric pressure. This can reduce the formation of gas-liquid interface and make it more difficult for volatile substances to escape into the air, thus avoiding the loss of sample volatilization. This is especially important for samples with strong volatility, which can ensure that the samples will not be lost due to volatilization during concentration.

2. The role of centrifugal force

In the lab rotary evaporator, centrifugal force is an inertial force generated by rotating the bottle. When the solution is heated and evaporated in a rotating bottle, due to the high-speed rotation of the bottle, the components in the solution are subjected to the outward centrifugal force, which will cause the relatively light components to be pushed to the container wall, while the relatively heavy components are closer to the center of the container. This separation effect is beneficial to the separation and collection of target substances during evaporation.

The role of centrifugal force in the rotary evaporation can be understood as follows:

A. Separation effect: Due to the centrifugal force, the components in the solution will show different distributions, and the relatively light components will be more easily pushed to the container wall, while the relatively heavy components will be closer to the center of the container. This separation effect is beneficial to the separation and concentration of different components in the evaporation process, so that the target substance can be collected more easily.

B. Accelerated evaporation: Under the action of centrifugal force, the components in the solution are forced to disperse on the wall of the container, so that more surface area is exposed to vacuum under the condition of heating, thus accelerating the evaporation speed of the solvent and improving the evaporation efficiency.

C. Improve the yield: Centrifugal force can help concentrate the target substances on the container wall, making them easier to be collected and extracted, thus improving the yield of the target substances.

3. Heating action

In the process of evaporation, the solution is heated by an external heating source, so that the solvent evaporates quickly, thus realizing the concentration and separation of the sample.

A. Low temperature evaporation: For some samples sensitive to high temperature, low temperature evaporation is a common choice. Evaporation is usually carried out between room temperature and 60℃, such as the extraction of some olefin compounds, and water bath heating or external heating circulation system can be used.

B. Medium temperature evaporation: For most conventional samples, the medium temperature evaporation is carried out between 40℃ and 80℃. The evaporation of general organic solvents, such as ethyl acetate, dimethylformamide, etc., can choose water bath heating or external heating circulation system.

C. High-temperature evaporation: Some organic solvents that are difficult to volatilize may need to be evaporated at a higher temperature, such as high-boiling solvents or viscous samples, usually between 80℃ and 120℃, and heated by oil bath or electret heating.

Normal operation process of rotary evaporation

1. Preparation: First, put the solution or mixture to be treated in the evaporation bottle of the rotary evaporator, and carry out pretreatment as required, such as filtering or stirring.

2. Start the rotation: turn on the power of the rotary evaporator and start the rotation of the turntable. The rotating speed of the rotating disk can be adjusted according to the actual demand.

3. Heating: the solution in the rotary evaporator is heated to the evaporation temperature by a heater or a heated water bath. The heating temperature depends on the nature and requirements of the solution to be treated.

4. Evaporation: As the solution is heated, the solvent in it begins to evaporate to form steam. Steam enters the condenser through the outlet at the top of the evaporation bottle.

5. Condensation: in the condenser, steam is cooled and converted into liquid, that is, condensed into recovered solvent. Usually, the condenser uses external cooling medium (such as cooling water) to reduce the temperature of the tube wall and promote steam condensation.

6. Recovery: the condensed solvent liquid is collected at the bottom of the condenser and can be recovered and separated through the corresponding outlet. The recovered liquid can be further used in other processes or experiments.

7. End of operation: when the rotary evaporation process is completed, stop heating and rotation, and clean and maintain the equipment after the system cools down.

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