Alcohol and Water

Questions and Answers

Alcohol (ethanol) and Water Molecule
(pictures with friendly permission of Dr. Bernd Meynhardt/Universitaet Kiel)

Whisky consists primarily of water and alcohol. Other constituents (usually < 1%) in the whisky are taste materials and flavours, called "congeners". When refering to alcohol, we mean ethyl alcohol, also known as ethanol. The chemical formula is C2H5OH.

The process of whisky production ensures that no methyl alcohol (methanol, CH3OH) is present in the finished product. The yeast employed in the conversion of sugars to alcohol have been selectively developed in such a way as to minimize or eliminate the production of methanol. During distillation, the removal of the foreshots and feints, and the collection of only the "middle cut" provides further assurance.

The molecular model in the upper left diagram represents the ethanol molecule. The gray atoms are carbon atoms (C), the red atom is the oxygen (O) and the white-gray atoms represent hydrogen (H). A model of the water molecule (H2O) is shown to the right.

A closer examination of the spatial properties of the ethanol molecule shows that the molecule without the oxygen atom is symmetrical with respect to the perpendicular plane through the carbon atoms. To form ethanol, only a hydrogen atom has been replaced by an oxygen-hydrogen group (the OH group). The ethanol molecule is electrically neutral in total but has local separation of electrical charge (so-called dipoles d+, d-). This is due to the differing affinity for electrons by oxygen and the hydrogen atom attached to it. The oxygen attracts electrons, thus developing a slightly negative charge, whilst the hydrogen atom suffers an equivalent deficiency of charge, and is therefore slightly positive. Water molecules also exhibit these dipoles, since they also are non-linearly arranged (angle 109 deg). Because opposite charges attract, dipoles on the ethanol and water molecules allow the formation of weak hydrogen bonds between negative and positive charges on the different molecules.

 
Hydrogen Bonds between Water and Ethanol

The OH group on the ethanol is said to be hydrophilic (water-loving).The remainder of ethanol, the C2H5-group, is said to be hydrophobic(water-hating) or lipophilic (fat-loving). Naturally the formation ofthe hydrogen bonds is global, so that all dipoles of all the molecules,as illustrated on the drawing above, establish such bonds. Inconsequence, ethanol and water are able to mix in all proportions (theyare completely miscible).

Questions and Answers

Why does high proof whisky become warm when diluted with water?
The OH groups of ethanol form hydrogen bonds with the water molecules. The bonded molecules are in a lower energy state than the separate molecules. The excess energy released by the bonding is converted into additional molecular vibrations and rotations, which is perceived as warming. Add enough water, and enough bonding will take place so that the energy released as heat can readily be felt on the hand.

Why do some whiskys show cloudiness when diluted with water?
In whiskies of high ethanol concentration, there are congeners (other molecules, usually quite large ones, and often carrying much of the taste of the whisky) which are less soluble in water than in ethanol. As more water is added, the solubility of these in the mixture thus decreases. They precipitate out of solution, usually as very fine particles, too small to settle by gravity. We see this as the haze or cloudiness.

Why are some whiskys cooled down and then filtered prior to bottling (chill-filtered)?
Most (non-ethanol) constituents of whisky are more soluble at higher temperatures than at lower temperatures. The whisky may be stored after sale at a lower temperature and some haze may develop. The marketing people have concluded that customers don't like to drink slightly cloudy whisky. So they chill-filter, to make the whisky look clearer and "brighter" to the public. But they remove some of the taste in doing it.

Why do streaks form if whisky is mixed with water?
This is a physical, rather than a chemical effect. Light is bent (refracted) as it passes through a liquid. Different densities cause different amounts of refraction. When we add water, we are changing the density locally. We see the refracted light as the streaks, where different densities exist locally. Once the whisky has been well mixed, the density is the same throughout the liquid, so no refraction, so the streaks disappear.

Why is it not possible to produce 100% ethanol by simple distillation?
A liquid boils because it's vapor pressure equals atmospheric pressure. At low ethanol concentrations, the vapor pressure of the ethanol is higher than that of water, so it distills off first. As the concentration of ethanol rises, eventually a mixture is produced (95.6% ethanol, 4.4% water) whose vapor pressure is constant. This is called an azeotropic mixture. Water and ethanol then distill off at the same concentration in the vapor form as in the bulk liquid. The azeotrope has it's boiling point somewhat below the boiling point of pure ethanol (78.15°C at 1 bar (760mm)). When simmering an ethanol water mixture, the temperature of the azeotrope can therefore never rise to the boiling point of 100% ethanol, since the 96/4 mixture boils first.

This can be overcome by altering the azeotropic mixture so that it's boiling point is higher than that of pure alcohol. This then allows the ethanol to be distilled off. Industrially, many different chemicals have been used to form an azeotrope with water, including benzene (C6H6), which is both poisonous and carcinogenic. Modern techniques, such as the use of molecular sieves are healthier, and permit the production of almost 100% pure ethanol.

Some physically data

¹⁾ Pure water
²⁾ Pure alcohol

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