The purpose of rotary evaporator
The purpose of distillation is to separate a given mixture into its components based on their respective volatilities, through the process of evaporation and condensation (liquid-gas-liquid).
We use the rotovap for two main purposes: 1) to concentrate non-volatile components in a mixture (for example, concentrating the purest and freshest flavors from a blood orange by removing the water), and 2) to extract the volatile aroma and flavor molecules from mixtures gently and at low temperatures (for example, extracting the desired flavors from a blend of alcohol, herbs, and fruit without heating the mixture up).
The key to understanding any distillation is to remember that it is a separation. Sugars, acids, colors, and most bitter compounds are separated from aromas, alcohols, water, and small flavor molecules, etc. What is phenomenal about rotovap distillation, as opposed to standard distillation, is that it can separate food compounds from one another without altering them.
Two key components of the rotovap allow for a gentle, precise, and efficient distillation not found in conventional distilling apparatuses: 1) the solvent is removed under vacuum, which lowers its boiling point, eliminating the need for high-temperature distillation, and 2) the rotation of the evaporating flask, immersed in a heated water bath, increases the surface area of the product, greatly speeding distillation and also, through forced convection, keeping the mixture evenly mixed and heated to promote stable, even evaporation.
An additional benefit of laboratory-type stills is that laboratory condensers, being sealed and made of glass, are easy to visually monitor and adjust. We can, therefore, recover almost 100% of the solvent.
Basic Structure of rotary evaporator
The main components of a rotary evaporator are:
1.A motor unit that rotates the evaporation flask or vial containing the user’s sample.
2.A vapor duct that is the axis for sample rotation, and is a vacuum-tight conduit for the vapor being drawn off the sample.
3.A vacuum system, to substantially reduce the pressure within the evaporator system.
4.A heated fluid bath (generally water) to heat the sample.
5.A condenser with either a coil passing coolant, or a “cold finger” into which coolant mixtures such as dry ice and acetone are placed.
6.A condensate-collecting flask at the bottom of the condenser, to catch the distilling solvent after it re-condenses.
7.A mechanical or motorized mechanism to quickly lift the evaporation flask from the heating bath.
Generally, the component liquids of interest in applications of rotary evaporation are research solvents that one desires to remove from a sample after an extraction, such as following a natural product isolation or a step in an organic synthesis. Liquid solvents can be removed without excessive heating of what are often complex and sensitive solvent-solute combinations.
Rotary evaporation is most often and conveniently applied to separate “low boiling” solvents such a n-hexane or ethyl acetate from compounds which are solid at room temperature and pressure. However, careful application also allows removal of a solvent from a sample containing a liquid compound if there is minimal co-evaporation (azeotropic behavior), and a sufficient difference in boiling points at the chosen temperature and reduced pressure.
The advantages of a rotary evaporator
1.that the centrifugal force and the frictional force between the wall of the rotating flask and the liquid sample result in the formation of a thin film of warm solvent being spread over a large surface.
2.the forces created by the rotation suppress bumping. The combination of these characteristics and the conveniences built into modern rotary evaporators allow for quick, gentle evaporation of solvents from most samples, even in the hands of relatively inexperienced users. Solvent remaining after rotary evaporation can be removed by exposing the sample to even deeper vacuum, on a more tightly sealed vacuum system, at ambient or higher temperature (e.g., on a Schlenk line or in a vacuum oven).
The disadvantage of a rotary evaporator
A key disadvantage in rotary evaporations, besides its single sample nature, is the potential of some sample types to bump, e.g. ethanol and water, which can result in loss of a portion of the material intended to be retained. Even professionals experience periodic mishaps during evaporation, especially bumping, though experienced users become aware of the propensity of some mixtures to bump or foam, and apply precautions that help to avoid most such events. In particular, bumping can often be prevented by taking homogeneous phases into the evaporation, by carefully regulating the strength of the vacuum (or the bath temperature) to provide for an even rate of evaporation, or, in rare cases, through use of added agents such as boiling chips (to make the nucleation step of evaporation more uniform). Rotary evaporators can also be equipped with further special traps and condenser arrays that are best suited to particular difficult sample types, including those with the tendency to foam or bump.