Mixtures and solutions

All substances are made up of atoms and molecules that contain electrical charges. Depending on the type of charge and the way it is distributed, substances can attract or repel one another very much, very little, or not at all.

When substances are mixed together, different things can happen depending upon the nature of their interaction. If the molecules actively repel one another, like oil and water, they will separate from one another as completely as possible and are said to be immiscible. These substances may be easily separated by mechanical means - filtering in the case of a solid and a liquid (sand and water, for example), or siphoning off in the case of two liquids (the previously mentioned oil and water). There is an exception to this rule: when the immiscible components are made into extremely small particles, the motion caused by thermal energy keeps re-mixing them so they can't separate themselves. This special case is a process called homogenization, but it does not affect the process of distillation.

If the molecules are strongly attracted to one another, they can chemically react to form a wholly new and different compound, consuming the original substances in the process. The metabolism of living things is a series of controlled chemical reactions, and a good example of this process is the conversion of sugar to carbon dioxide and ethanol. There is no way to get the original sugar back, because it no longer exists.

If the attraction between molecules is moderate, the substances can go into solution. In a solution, the molecules of the two substances are intimately mixed together, and there is no way to separate them by mechanical means like filtering or centrifuging. However completely mixed they are, they still exist and maintain their unique properties, like vapor pressure.

Water is a good solvent for many different things, and the complex composition of seawater or blood is a testament to this. Salts and most (but not all!) solids have a vapor pressure so low that we can consider it to be zero. If you dissolve a teaspoon of salt in a cup of water, and then allow the water to evaporate, you will end up with a cup containing a teaspoon of salt. The water evaporates, but the salt doesn't. Boiling the water speeds things up, but the end result is the same.

The same is true of oil dissolved in gasoline. Oil and gasoline are chemically similar molecules, but the oil molecules are larger and have a much lower vapor pressure than gasoline. If you want to put a very thin film of oil evenly over a component, dissolve the oil in gasoline and dip the component in it. When the gasoline evaporates, you're left with a very thin, even coating of oil all over the component (this is an old watchmaker's trick!)

It is relatively easy to separate the components of a solution when their vapor pressures are quite different. What happens when you try to separate components with similar vapor pressures?

Pure water is a liquid that boils at 100°C (212°F) at normal atmospheric pressure (760 mm mercury) and has a surface tension of 54.9 dynes/cm2 (a dyne is a unit for force, defined in Appendix 1). Pure ethanol is a liquid that boils at 78.5°C (173.3°F) and has a surface tension of 21.38 dynes/cm2. You've probably guessed that the ethanol has a higher vapor pressure than the water, because it has a lower surface tension. This is true, but they are much closer than salt and water or oil and gasoline.

Now mix water and ethanol together, like you did with the water and salt, and try to separate them by evaporation. What do you get? An empty cup! If you leave the mixture for a limited time, the more volatile ethanol does evaporate faster than water. You would get a cup containing a little water, but if you collected the vapor coming from it, you would not have pure ethanol. You would have a mixture of water and ethanol. Ethanol and water are much more difficult to separate than water and salt or oil and gasoline.

You can try to hurry things along by heating the mixture to a temperature above the boiling point of pure ethanol but below that of water, on the assumption that the ethanol will boil off but the water won't. Right?

Wrong! Many people have tried this, they've all been beaten by the laws of physics. As you heat the mixture up, ethanol molecules will escape faster than water molecules, but both will still escape. You may end up with a little pure water in the bottom of the cup, but the vapor will be a mixture of ethanol and water. However, this idea is getting closer to the solution.

What happens if you boil the mixture as rapidly as possible? Exactly the same as before, but faster! After all, boiling is really just fast evaporation.

To make sense of this, we need to examine the behavior of mixtures. In general, the boiling point of a solution is found somewhere between the boiling points of its components. What allows the distillation process to work is the fact that a mixture boils at a temperature that depends upon the relative concentrations of the components of the mixture, and produces a vapor that is a mixture of the two substances. The vapor produced is not just any mixture, but a predictable one. At any given temperature, the substance with the higher vapor pressure produce more vapor than the less volatile substance.

For ethanol and water, this means that you end up with a vapor containing a higher proportion of ethanol molecules than the starting mixture. Excellent! You can now condense this vapor and enjoy a good drink, which is exactly what they did in the Good Old Days. It shouldn't take long to figure out that repeating the process again provides something even richer in ethanol, and so on...

What we've just described with water and ethanol applies to many other substances, including the congeners mentioned in chapter 1. Some of them have higher vapor pressure than ethanol, and some lower. Some of them provide flavor to the spirits, and some create headaches. Repeated distillation increases the concentration of ethanol, but also alters the concentration of other substances in the mixture. The end result is more ethanol with less flavor and fewer hangovers.

Two basic approaches evolved to deal with this limitation. Spirits produced in bulk were distilled two, three, or even four times, and then either treated or matured (more on this later) before being consumed. This was fine for an ongoing commercial operation, because production from previous years was available while the fresh spirits aged.

On a smaller scale, and often for the production of medicines and tonics, the ethanol was repeatedly distilled to make it as pure as possible. At the same time, botanicals were steeped or distilled for their flavors and added to the purified spirits. These extracts were then blended with other substances to produce the exact flavor, color and palate desired. In general, these products required less maturation than their bulk cousins, but they often benefited from it, because some subtle flavors can't be produced in any other way.

These herbal tonics and medicines were often produced as a holy calling by monastic orders. The brothers had the combination of herbal, medicinal and process knowledge, the space to cultivate and protect the rare herbs, and the time to perform the difficult processes of extraction and purification.

To complete the discussion of concentrating ethanol by repeated distillation, we must consider azeotropism. By repeatedly boiling and condensing a mixture of water and ethanol, we get distillates that contain higher and higher proportions of ethanol. This works until the solution contains about 96% ethanol and 4% water. At this point, the proportions of ethanol and water molecules entering the vapor phase remain the same as the proportions in the solution, 96% and 4%. This is caused by the extra attraction of the molecules in solution, and defines the upper limit of concentration by distillation. Another result of azeotropism is that the boiling temperature of the 96% ethanol solution is lower than that of pure ethanol!

To obtain 100% pure ethanol (something only chemists and alternative fuel enthusiasts need), you have to "break the azeotrope", which requires adding a third substance to disrupt the molecular attraction between water and alcohol. Due to the principles of distillation, this substance will also appear in the mixture of vapors. The most effective breaker of the ethanol/water azeotrope is benzene, which is highly toxic

Fortunately, you don't need to bother. 96% is an excellent solvent for making extracts and essences, and if you're going to produce a 40% vodka, why take all the effort to remove the water only to put it back?

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Responses

  • xavier
    Does ethanol repel water?
    6 years ago

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