Some plants produce essential oils as part of their metabolic processes. It is estimated that only about 1% of all flowering plants will in fact produce essential oils in any significant amounts. These plants store their essential oils in different parts of their structure, and depending on the plant in question, can be found in its petals , its leaves, twigs, its fruit or rind of fruit, in its wood, bark or roots.
While science has not been able to establish with certainty, as to why some plants produce essential oils there are a number of theories as to what their function may be, including:
When essential oils are analyzed, it can be seen that each essential oil has its own distinct chemical composition. The proportions may fluctuate depending on many factors such as: where the plant is grown, climate, harvesting, distillation process etc but the essential oil of lavender will have a specific range of components, different to the specific range of components found say in eucalyptus.
In the world of Chemistry, essential oils are classified under the category of Organic Chemistry as their individual components are mostly made up of Carbon, Hydrogen and Oxygen. The Hydrocarbons – Monoterpens and Sesquiterpens and the Functional Groups, which are hydrocarbons with the addition of Oxygen.
David G Williams in his book The Chemistry of Essential Oils has some interesting definitions for atomic and molecular weight:
For the purposes of this book, the atomic weight of an element can be defined as the number of times that one atom of the element is heavier than one atom of hydrogen taken as 1.
The molecular weight of a compound is the number of times that one molecule of the compound is heavier than one atom of hydrogen taken as 1.
When a typical essential oil is allowed to evaporate from a smelling strip, the odour of the oil is observed to change. This is readily shown using Geranium Oil or one of the citrus oils, by taking a half cm dip of the oil, allowing it to evaporate for, say, 20 minutes, then comparing the odour with that of a fresh dip. The difference in odour is caused by the evaporation of constituents of the oil of lower molecular weight, leaving behind others of higher molecular weight. Since different constituents have different odours, the odour of the sample changes.
We don’t need to know the molecular weight of specific essential oils in order to work with them, however if for any reason one wanted to know it, first figure out the molecular weight of each of the components and then add them together. The one significance of knowing the molecular weight would be to help one understand the possibility of an essential oil or essential oil component penetrating the skin.
Shirley and Len Price, in their book, Aromatherapy for Health Professionals tell us that:
The molecular size and shape will also have a bearing on the speed of penetration of the skin. The smaller the molecule the faster it will penetrate. Molecules having a molecular weight over 500 will be unlikely to pass through the skin. Essential oils, because they are the product of distillation are usually limited to a maximum molecular weight of 225.
Because of the mechanics of the steam distillation process, not all of the essential oil components found in the plant will make their way into the final essential oil produced through steam distillation. Essential oils produced through cold expression (like many of the citrus) can have larger molecular weights as all the essential oil components found in the plant will find their way into the final product.