Vapour Pressure of Immiscible Liquids

2 liquids are said to be immiscible if they're completely insoluble in each other. Such a system actually consists of 2 phases, though it is usually referred to as a mixture. Examples include benzene and water, kerosene and water, etc.

Though classified as immiscible, in actual fact, there is still some degree of mutual solubility. For example, again using the example of benzene and water, a small amount of benzene will be dissolved in the water phase, and vice versa.

Since by definition, immiscible liquids do not interact with each other in any way whatsoever, they will evaporate completely independent of each other. In other words, each liquid will contribute its own characteristic equilibrium vapour pressure regardless of the presence of the other liquid. Thus, the total pressure exerted by the mixture is the sum of the equilibrium vapour pressures.

For example, the 2 liquids A and B are immiscible. At equilibrium, they each exerts equilibrium vapour pressure PVP,A and PVP,B respectively. Thus the total pressure exerted is

P = PVP,A + PVP,B

The equilibrium vapour pressures of water and benzene at 100 oF (37.8 oC) are 0.95 and 3.30 psia respectively. The total pressure exerted is therefore (0.95 + 3.30) = 4.25 psia. Note here that in the calculations of total pressure, the actual amount of each is immaterial.

An important outcome is that the boiling point of an immiscible mixture must be lower than that of either of its components. Again, using water-benzene mixture, we note that the mixture will boil when the total pressure exerted by the mixture equals the atmospheric pressure, 14.7 psia.

This temperature is 156 oF (68.9 oC), when the equilibrium vapour pressures of water and benzene are 4.45 and 10.25 psia respectively (thus, total 14.7 psia). Note that the mixture boiling point is lower than that for pure water 212 oF (100 oC) and pure benzene 176 oF (80.2 oC).

[ The phenomenon illustrated above forms the basis of an operation known as steam distillation, where steam is added to an organic mixture to lower the boiling point of the mixture. Water forms immiscible mixture with most organic substances, thus these mixtures will boil at a temperature below that of either water or the other material.]

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