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The 300 molecular weight of molecules and Steam Distillation.

Dec 22, 2019

    Hi Everyone,

Over the recent past time frame, a very interesting aspect of essential oil quality has come my way. A guy I know that is an EO supplier asked me a question about some insecticides that came up in a special analysis that he had run on some oil that he supplies to a specialized manufacturer. Please keep in mind that a GC/MS DOES NOT typically scan for such items. He had to send it to a specialized lab to have it run. He did not specifically say why he had to run a scan, but I put two and two together and it must have been for a large volume-specific customer that requested it.  Anyway, I am not at liberty to discuss a whole lot about the situation other than some general specifics. He contacted me because he knew that I was involved in agriculture and the chemicals were chemicals that are typically used in agricultural settings, for the most part. He wanted me to help him understand the chemicals and maybe he could understand why they are there and how they got there. He also asked if I knew of some other labs that might be able to run a test to verify if the first test was accurate or not. I suggested that he talk to this one chemist that I know of that is also pretty sharp with essential oils and that I would be interested in what he had to say. I also suggested that based on the scientific propaganda out there on these chemicals that if the EO was applied topically or diffused then the limits of daily use based on the EPA guidelines, that it might be OK. But everything I read said that ingestion of these chemicals is an absolute NO-NO.  The chemist that I suggested he talk to, also told him that he has seen these chemicals show up in a number of essential oils of late.  I guess it is or will become an issue. As I find out more I will discuss it here. I have been going to do a discussion on the one chemical but I haven't figured out how to present it without freaking most people out. The idea of them being in the EO's is nothing compared to the widespread use in food and food preparation. So don't be alarmed with the EO aspect. WE got that covered, lol.

But along with this, I have repeatedly taught that anything that is over 300 Molecular Weight will not come over in the steam distillation process.  I have been taught this from a number of credible sources in the Aromatherapy world.  In some scientific-related essential oil references, it also makes reference to this point. Well, it is now obvious that this idea is wrong. These 2 chemicals are 366 and 422 on their molecular weight. So they came over in the distillation.  The theory is not founded in scientific fact. So I apologize for teaching an incorrect concept. You can rest assured that I won't be making that mistake again. I will have to go through and correct a number of statements in that regard.

I consulted with a number of people with a background in medicine and chemistry for answers. Mikeal McKim gave me a real solid answer and explanation as we attempt to understand the background for this issue.  I asked her to post in this group her findings but to add my explanation to it. So this is the combined post. I appreciate her work in pursuing the details and documentary work.


Below is what I plan on posting to add to what you wrote above, I just wanted to send it to you first and if you like what I wrote I will get it all posted.

I(Mikeal) have been looking for some kind of research that states the 300molecular weight (MW) rule to no avail. I will continue to search and if I find something stating this so-called rule, I will definitely share it with all of you.  But what I suspect is really going on is that the 300 MW ‘rule’ is an oversimplification of what is actually occurring.  I am going to first explain the vapor pressure and boiling point, then explain steam distillation and then I will sum everything up.  So bare with me. 😊

Vapor pressure is the pressure exerted when the molecules leave the surface of a solid or liquid, at the same rate as they return.  The normal boiling point is the temperature in which the vapor pressure of a liquid becomes equal to atmospheric pressure.  In chemistry, there are trends that usually can be followed.  One such trend is that the vapor pressure and boiling point of a molecule are related to the size of the molecule.  Although, there are also intermolecular forces involved.  I'm not going to get into those here, but if these forces are not strong then the vapor pressure of a molecule will be low and the molecule will convert to a gas at a lower temperature than a molecule that is large in size and/or has strong intermolecular forces.

Steam distillation is useful when distilling essential oils because components can be distilled and separated at a temperature lower than their normal boiling point.  This allows for the separation of the oil from water at a lower temperature reducing the chances of decomposition of the components.  The main principle behind steam distillation is that when two immiscible liquids (i.e. water and oil) are heated, each one exerts its own vapor pressure.  When the vapor pressure reaches or exceeds the atmospheric pressure, boiling begins.  The two different vapor pressures are added together and because they must be equal to atmospheric pressure to boil, they both get reduced to lower vapor pressures and consequently to lower boiling points.

Since the vapor pressure of a molecule can be lowered during steam distillation, it is possible that a molecule with a larger MW than 300 could possibly find its way into the oil.  While I do not think this will change most of what we discuss when it relates to what is coming over into the oil, on the whole, it does have some implications when dealing with pesticides and the like.

I hope this helps if anyone has any questions or would like to do some extra reading to understand the vapor pressure as it relates to steam distillation, feel free to ask. Also, check out chemistry LibreTexts.  This is a great source I used when I taught that will give explanations and definitions for as deep into it as you want to go!

End of Discussion.


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