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Steam distillation is a separation technique that harnesses the low boiling point property of immiscible mixtures. It is predominately used to separate temperature-sensitive organic molecules from a non-volatile contaminant. The organic molecule must be immiscible in water.
In steam distillation, the immiscible mixture is heated to boiling, causing the distillation of both water and the volatile organic compounds. This means that the gaseous mixture travels upwards to a condenser, which then condenses the vapor to liquid so that it can be collected. In contrast to simple distillation, steam distillation uses a water reservoir to replenish water in the heated mixture throughout the process. The immiscible organic component is slowly distilled along with the water, while the non-volatile component remains in the heated mixture. Once the organic component is distilled, it can then be separated from the water using liquid-liquid extraction.
For a miscible mixture that forms a homogeneous solution, the vapor pressure of each component is dependent on the vapor pressure of the pure component and its mole fraction in the liquid mixture according to Raoult’s law.
pA = pA*xA
where pA is the vapor pressure of one liquid component in a miscible liquid mixture, pA* is the vapor pressure of the pure liquid, and xA is the mole fraction of that liquid in the mixture, which is equal to nA/nt. nA is the number of moles of the individual liquid in the mixture, and nt is the total number of moles of all the liquids in the mixture.
The total vapor pressure above the miscible liquid mixture is equal to the sum of the partial vapor pressure of each component in it, which is known as Dalton’s law. The vapor pressure of a liquid increases with temperature as more molecules gain kinetic energy to escape the liquid phase to the gas phase. In a miscible mixture containing two liquids, the total pressure can be described as:
P = pA + pB
where pA and pB are the vapor pressures of liquid A liquid B, respectively, above the mixture. P is the total vapor pressure of both liquids above the mixture. Combining the equations describes the relationship between the total vapor pressure of the solution and the mole fraction of the individual components:
P = pA*xA + pB*xB
In an immiscible mixture, where the components form a heterogeneous mixture, the vapor pressures of each component contribute independently to the total vapor pressure. Thus, the total vapor pressure is equal to the sum of the individual pure vapor pressures. In an immiscible mixture composed of two liquids, the total pressure is defined as the vapor pressure of the first liquid plus the vapor pressure of the second liquid.
P = pA* + pB*
As a liquid is heated, the vapor pressure increases. Each component in a mixture has its own boiling point. In a mixture of miscible liquids, boiling occurs at a temperature between the boiling points of the constituent liquids.
For an immiscible mixture, boiling occurs at a much lower temperature than the boiling points of the individual components. As each individual component contributes independently, less heat is required to increase the total vapor pressure to the atmospheric pressure.
For example, consider the immiscible mixture of benzene and water. The boiling point of benzene at normal atmospheric pressure is 80.1 °C, and the boiling point of water at normal atmospheric pressure is 100 °C. The solution boils when the total vapor pressure reaches 760 mm Hg (normal atmospheric pressure). At 69.3 °C, the vapor pressure of water is 227 mm Hg and the benzene vapor pressure is 533 mm Hg, which in total equals the necessary 760 mm Hg required to boil. This is well below the boiling point of either individual component.