Why is steam distillation useful for collecting volatile monoterpenes?

Steam distillation is a highly effective method for collecting volatile monoterpenes because it allows for the distillation of substances at temperatures significantly lower than their normal boiling points.1

Monoterpenes typically have high boiling points, ranging from 150 to 185 °C (300 to 365 °F). Heating these compounds to such high temperatures can cause them to decompose or degrade. Steam distillation is applicable because monoterpenes are generally immiscible (incapable of mixing) and chemically nonreactive with water. By passing steam into the plant material, the process supplies the necessary heat to vaporize the volatile oils at a reduced temperature, preventing thermal damage while separating the desired compounds from nonvolatile plant impurities.213