Azeotrope
One of the oldest of all chemical techniques is distillation. An example of distillation in its simplest form is boiling water. Indeed, boiling water and then condensing it on a cold surface is one of the best methods for water purification.
With water as the solvent, the boiling point of a solution is relatively constant (slight variations can occur due to the presence of impurities at high concentrations). And while this is generally the case for any pure solvent, it is not the case for a mixture of two or more solvents. In a mixture, the boiling point is dependent upon the relative concentrations of all of the species or, more accurately, on their mole fractions.
In the case of mixed solvents, the ability of one component to vaporize can exceed the ability of any other component to vaporize. To understand this, it is necessary to consider what vaporization requires. A molecule must approach the surface of solvent with sufficient velocity of kinetic energy to overcome the inter-molecular interactions that hold it to the surrounding molecules. It is a bit like someone struggling to get clear of a crowd--their ability to escape depends upon their strength and determination.
In the case of molecules, the kinetic energy of the molecule is dependent upon the temperature. But the ability of a molecule to escape is also dependent upon its mass and the type of interactions it has with the other solvent molecules.
All of this is an explanation as to why the vapor composition above a mixture of solvents does not necessarily have the same chemical composition as the solvent itself. But a curious thing happens during a distillation. If the more volatile solvent evaporates more quickly--if the vapor is enriched in one of the components--then the remaining solution is depleted in that component. That is, if a solution consists of a two component mixture, A and B, and A is more volatile, then the mole fraction of A in the vapor will be higher than the mole fraction of A in the liquid, and vice versa for B.
The result is that the composition of the solution changes as the distillation occurs. At some point though, the relative rate of vaporization of the components becomes equal. That is, A may be more volatile but there is less of it in solution. When this occurs, the vapor is no longer enriched in A. Rather, the vapor has the exact same composition as the solution. This is the azeotrope--the point in a distillation curve where the chemical composition of the vapor and the solution are the same.
Azeotropes or azeotropic mixtures occur for a number of important chemical compounds but perhaps the most important is the ethanol-water mixture. Distillation of ethanol mixtures is the basis of the distillery industry which makes such alcoholic beverages as bourbon and gin. However, industrial ethanol is "95%" as this is the azeotropic mixture for the combination of pure ethanol and water. That is, if yeast are cultivated in a sugar water solution, the resulting alcohol that can be distilled is the azeotropic mixture consisting of 95.6% ethanol and 4.4% water.
One of the many industrial uses of ethanol involves blending it with gasoline to make gasohol. There are several reasons for doing this. It produces a cleaner combustion and reduces the consumption of non-renewable resources. However, ordinary ethanol --the 95% ethanol-water azeotrope--is unsuitable for this use as the water interferes with the gasoline combustion and prevents the ethanol from being miscible with the gasoline.
This problem is solved by taking advantage of another azeotrope. The benzene-ethanol-water ternary azeotrope boils at a lower temperature than the ethanol-water azeotrope. By introducing benzene to a water ethanol mixture, the water can be extracted through distillation leaving behind the pure ethanol which contains a minor amount of benzene as an impurity. The result is that almost pure ethanol can be obtained which is suitable for blending with gasoline. (In fact, the benzene in the ethanol is an octane booster. The disadvantage of this fuel additive is that the presence of any water in the gas tank will result in the separation of the ethanol as the water-ethanol mixture will be reestablished.)
The industrial use of azeotropes to distill or purify mixtures of liquids is one of the more important aspects in any distillation process.
This is the complete article, containing 714 words
(approx. 2 pages at 300 words per page).
