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Let me boil this down for you: Azeotropes

30 Jan 2011

Chromatographic techniques are not the only means of separating the components of a mixture. I wrote previously about mechanical separation, which turned out to be an enabling process for making columns. In today's post I'll try to draw a similar connection between chromatography and yet another process that allows us to pull apart things that have been mixed together.

a couple glass jars with liquid in them
has been employed for centuries to increase alcohol content beyond what fermentation can yield. (Come to think of it, I've written previously about strong drink too...) The distillation concept is straightforward: application of heat drives off the more volatile fractions of the mixture first, and their vapors can be recondensed and captured in a second vessel. The common misconception is that the fractions "wait their turns" to boil off, so that if you did it right you could end up with pure water in the pot, and a distillate of pure ethanol. But distillers have known for a long time that no matter how long you let it run, or how many cycles you put it through, the limit of ethanol purity achievable via ordinary distillation is about 96%.

Which brings us to our word of the day: azeotrope. Certain mixtures of solvents, such as water/ethanol, present this challenge. (Also water/acetonitrile, making this a topic of interest to chemists running gradient HPLC analyses and wishing to recycle solvents.) As distillation progresses, at some point the ratio of the components in the vapor phase is identical to that in the remaining liquid. From there, no amount of boiling will alter the proportions further.

It is, however, possible to "break" an azeotrope by changing the parameters of the distillation process.

One of the most common methods is to introduce another component to the mix as an entrainer. In the case of water/ethanol, an effective entrainer is benzene. The water/benzene fraction can then be distilled off, leaving behind the alcohol. Two major drawbacks: the ethanol "bottoms" are contaminated with ppm levels of benzene, and the water is enmeshed with benzene in yet another azeotrope. A related but distinct approach is the use of a drying agent such as lime (calcium oxide). This produces no toxic contaminants in the alcohol, and the hydration of the lime is reversible to reclaim the water.

There is another widely used technique which requires no additional chemical ingredients. Called pressure-swing distillation, it exploits the fact that azeotropic ratios are not fixed, but a function of pressure. Thus, when distillation has reached a given stage the system pressure can be altered to "jump" the mix over the curve and allow distillation to run to completion.

For further reading, you can start at Wikipedia: