Heterogeneous and Homogeneous Azeotropes

Homogeneous and heterogeneous are the two types of azeotropes. Homogeneous azeotropes are miscible liquids. Heterogeneous azeotropes are liquids that are immiscible. Heterogeneous azeotropes are formed through two liquid phases after the process of condensation. Thus, these azeotropes have a high boiling point. 

For homogeneous azeotropes, there are two methods used for the separation of these azeotropes: pressure swing and extractive distillation. The difference between azeotropes and other chemical compounds/mixtures is that azeotropes maintain their chemical composition during and after distillation. This is in contrast to other substances called zoetropes. This entails that the liquid used in the distillation process and the vapour are of the same chemical properties. For this reason, a simple distillation process will not help in separating the components and thus, other methods have to be used. 

Different types of azeotropes

There are many ways in which azeotropes can be classified, for example based on composition – homogeneous or heterogeneous. Homogeneous azeotropes are those substances that are miscible while the latter are immiscible and divide into different layers.

 Another way to classify azeotropes is into positive and negative azeotropes. 

The ones whose boiling points are lower than the boiling points of their constituents are called positive azeotropes. These azeotropes also show a huge positive deviation from Raoult’s Law. 

On the other hand, negative azeotropes are those whose boiling points are higher than those of their constituents. These show a huge negative deviation from Raoult’s Law. 

Raoult’s Law states that in an ideal solution, the intermolecular forces acting between dissimilar molecules is the same as that between similar molecules. This means that if vapour pressure is applied on any solvent part of a solution, it will be equal to the pressure applied on a pure solvent at similar temperatures scaled by the solvent’s mole fraction.

We can also classify azeotropes as binary or ternary. Binary azeotropes are those mixtures with only two components, like ethanol and water. Ternary azeotropes are those azeotropes that have three components in them, such as a mixture of chloroform, methanol and acetone. 

Examples

There are many examples of azeotropes that are used in the industry today and also some that are naturally occurring. One of them is a solution of ethanol and water. If we boil a mixture of 60% ethanol with water, we will get a vapour of 60% ethanol with water. This azeotrope will also be homogeneous as alcohols and water are miscible. 

Another kind of azeotrope is that of chloroform and water. However, this kind of mixture is a heterogeneous azeotrope. This can be observed when we mix these two liquids;, they form two layers, the top layer consisting mainly of water and a small amount of chloroform, and vice versa in the next layer. The boiling point of this mixture is observed to be lower than that of the boiling point of either water or chloroform. 

Difference between heterogeneous and homogeneous azeotropes

The main difference between homogeneous and heterogeneous azeotropes is that a homogeneous azeotrope has only one phase throughout the solution while a heterogeneous azeotrope has more than one phase and each is distinct.  

For homogeneous azeotropes, the composition is constant throughout the mixture and so there is only one such region. This is because it is uniform throughout its composition. 

On the other hand, in a heterogeneous azeotrope, different parts have different compositions, which is why there is more than one phase in it.

In a mixture of salt and water, the upper regions have the same proportions of salt and water composition as the lowermost portions – example of a homogeneous azeotrope.

On the other hand in a sand and water mixture, the proportion of sand is much more at the bottom than at the top and vice versa for water – example of a heterogeneous azeotrope.

Conclusion

Azeotropes are mixtures of two or more liquids that have a constant boiling point and that share the same constituents as their vapour form. For this reason, azeotropes cannot be separated by a simple distillation method and need pressure changes in multiple distillation chambers or the addition of extra components in a mixture before separation. 

The above article has talked in detail about azeotropes, and has also mentioned how we can classify azeotropes into various categories and some of the examples of different azeotropes such as the mixtures of ethyl alcohol and water or chloroform and water.