Hydrodistillation involves the use of water or steam to recover volatile principles from plant materials. The fundamental feature of hydrodistillation is that it enables a compound or mixture of compounds to be distilled and subsequently recovered at a temperature substantially below that of the boiling point of the individual constituents.
During hydrodistillation, water and essential oil form a heterogeneous system of immiscible liquids. By the principle of distillation of mutually immiscible liquids, the total vapor pressure of the mixture at its boiling point will be equal to the sum of their partial vapor pressures. Hence, the vapor pressure exerted by each component is less than its vapor pressure if present alone at its boiling point. Therefore, the boiling temperature for any two-phase liquid will always be lower than the boiling point of either of the pure liquids at the same total pressure. Thus, in the case of an essential oil, the constituent compounds distill at temperatures below 100°C when boiled with water at atmospheric pressure. From the vapor pressure data of individual compounds, it is possible to calculate their boiling points on steam distillation and the relative ratio of oil to water in the distillate at different pressures. The composition of the vapor formed from a two-phase liquid mixture depends on the partial pressures of the pure liquids. The ratio of the weights of the two components in the vapor (and the distillate) is expressed as the ratio of their partial vapor pressures multiplied by the ratio of their molecular weights (Guenther, 1972). Thus, for oil-water system,
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