Mass Transfer: Azeotrope

The properties of a species in its pure state are different from that in solution. The difference in the property value represents deviation from ideal solution. The deviations are of two types positive  deviation and negative deviation.

When the intermolecular forces between the unlike molecules (molecules of A and B) in the solution are stronger than that between like species in pure state, the molecules have more tendency to stay in liquid phase and exert lower pressure than that of ideal state. This is negative deviation and when the intermolecular forces between the unlike molecules (A-B) in the mixture are weaker than that between like molecules (A-A and B-B) in pure state, the molecules have the more tendencies to be in vapor phase and exert higher pressure than that of ideal state, positive deviation.

When the two components have close vapor pressures, and have large deviations from ideality, they are likely to form azeotrope. An azeotrope is a mixture of constant boiling i.e. mixture has same composition in vapor phase and liquid phase.

A system having large positive deviation that means it will exert high pressure at the same temperature compared to that for ideal state, so the boiling point at the azeotrope composition will be lower than the boiling points of pure components. This type of azeotrope is called the minimum boiling azeotrope. 

A system having large negative deviation that means it will exert less pressure at the same temperature compared to that for ideal state, so the boiling point at the azeotrope composition will be higher than the boiling points of pure components. This type of azeotrope is called the maximum boiling azeotrope.