Understanding the Chemistry of Cannabis Oil – Part 1: Terms & Key Concepts

POSTED Sept. 14, 2020

Covering purity, potency, polarity, solubility, and extraction.

Not everyone needs a degree in chemistry to work in the cannabis industry, but a working knowledge of some basic chemistry terms will help you navigate through the murky waters of sales pitch claims plus the ambiguous and vague terminology thrown around without any scientific meaning. Asking vendors and equipment sellers the right questions so both parties are clear on what the language means can help avoid unrealistic expectations. In the end, you will be a smarter buyer and more confident consumer.

Purity

Often when people describe a desirable extract or oil concentrate, it’s referred to as ‘clear,’ or ‘pure.’ Unfortunately, using such simplified terms doesn’t properly define the anticipated end-product. The word ‘pure’ in chemical terms is talking about a single chemical substance. All extracted or distilled oils are going to contain a mixture of chemical substances. More specifically, a homogeneous mixture, or one in which the individual components cannot be separated through simple physical means, unlike heterogeneous mixtures. When something is ‘clear,’ that just implies that it is see-through (Figure 1). It does not imply that the oil is ‘pure’ or only contains the desired components intended. The extract could be perfectly ‘clear’ and contain a toxic amount of pesticides. An assay, or an analytical procedure, is completed in an accredited laboratory to examine a mixture or a chemical to determine its purity.

Figure 1: Cuvettes containing various oil extract solutions that are considered clear, but the chemical compositions are unknown.

Potency

Everything in the universe is made of chemicals, and what determines whether a chemical substance is considered ‘healthy’ for you or non-toxic is its dosage level. Even drinking too much water at once can upset the normal balance of salt levels in the blood, leading to life-threatening effects. The term potency is so important because it refers to the dose necessary for a substance to exert its effect on the body. Describing potency in terms of weight of the drug or chemical substance in reference to the weight of a person’s body is more useful than just saying “a potency of 96%.” Typically, potency and purity are considered interchangeable in the cannabis industry, but they are quite different in the correct context.

Polarity

The concept of polarity is probably the most important concept for understanding how the extraction process works. If you have heard the terms ‘polar’ (hydrophilic, lipophobic) and ‘nonpolar’ (lipophilic, hydrophobic) thrown around when referring to cannabinoids and terpenes, then the first lesson is to realize that it’s not a black or white notion, but more of a spectrum (no pun intended). Polarity is a continuum.

Unfortunately, you cannot determine the polarity of a chemical substance without looking at its molecular structure. There is the polarity between two atoms connected by a chemical bond or the polarity of the overall molecule. Defining either as ‘polar’ or ‘non-polar’ is like imagining a game of tug-of-war, but the atoms are the players. Atoms like oxygen and fluorine more easily attract electrons in a bond and in a molecule. They have more muscle-power in this tug-of-war and polarize the electrons towards them compared to other atoms like carbon or hydrogen. As a result, the game is not evenly matched and the chemical bond is considered ‘polar.’ If two oxygen atoms were playing against each other, they are evenly matched, and the chemical bond would be considered ‘non-polar.’ Atoms like carbon and hydrogen are evenly matched in their strength and as a result, they tend to be more ‘non-polar’ when bonded together.

As mentioned before, the tug-of-war concept for polarity can be applied to the overall molecule, not just individual chemical bonds. Simply looking at an accurate 3D drawing of a molecule like water, ethanol, or CO2, will tell you if it is more ‘polar’ or ‘non-polar’ just by its overall symmetry (Figure 2). In the case of bigger molecules, the categories of ‘polar’ and ‘non-polar’ are even more blurred and are in most cases just defined relative to other molecules and their polarity.

Figure 2: Electrostatic maps of non-polar molecule CO2 (left) and polar molecule H2O (right) and their molecular dipole moments, or the tug-of-war game inside a molecule.

The majority of the molecules found in cannabis extracts are considered on the ‘non-polar’ side of the polarity spectrum due to the abundance of carbon and hydrogen in their structure. The presence of any oxygen atoms only increases the polar-character of a hydrocarbon-type molecule. Take for instance the structures of CBD-A vs CBD (Figure 3), where the former contains more oxygen atoms because it has not been decarboxylated yet. CBD-A is considered more polar relative to CBD, but referring to CBD-A as a ‘polar’ molecule wouldn’t be very accurate.

Figure 3: Molecular structures of CBDA compared to CBD, which has lost the carboxylic acid functional group as the byproduct of CO2 through the decarboxylation process.

But be aware, some molecules are large enough that they can possess a clear ‘polar’ side and ‘non-polar’ side. These substances are said to be amphiphilic. In fact, you use one every day (at least we all hope so, anyway). Soap! (Figure 4) The use of amphiphilic molecules is also the best way to create water-soluble CBD or THC products, as you will find out why below.

Figure 4: The structure of a stearate soap molecule.

Solubility

The intricacies of solubility are built on the concepts of polarity and intermolecular interactions (or forces). Talking about solubility is usually in reference to a solvent, usually a liquid or supercritical fluid, and a solute, which can be a solid, liquid, or gas. The act of a solute dissolving into a solvent to become solution (or homogeneous mixture) is called solvation. As a result, the composition of the solvent and solute is uniform throughout, the particles are not visible with the naked eye, the solute(s) will not settle out after a period of time, and the components can’t be separated by any conventional physical means, like a filter or centrifuge. Some common liquid solutions include carbon dioxide (CO2) gas dissolved in water to make carbonated water, alcoholic beverages as solutions of ethanol in water, and sugar (sucrose) and table salt (sodium chloride) dissolved in water. The most relevant solution is cannabis oil, which has hundreds of solutes, dissolved in a solvent like ethanol, compressed liquid butane, or supercritical carbon dioxide.

Choosing the right solvent for a solute is as simple as following the “like dissolves like” motto, which is to say that compounds of similar polarities will more easily mix to make a solution. This saying derives from the idea that molecules that can interact with each other through similar intermolecular forces will have better solubility. Particularly, the ability of a molecule to participate in hydrogen bonding has a large impact on its solubility and miscibility, even compared to substances that appear to have the same polarity or dielectric constant. Overall, paying attention to the functional groups (–OH, –COOH, –NH2, etc.) on a molecular structure helps determine these types of intermolecular interactions. As a result, you will be able to understand why THC-A is much more water-soluble than THC.

Other parameters that greatly influence solubility include temperature, pressure, and the concentration of solute. Warmer temperatures intuitively increase the solubility of a solute in most cases (gases and some salts being the most common exceptions), whereas cooling a solvent prevents solutes from dissolving or causes compounds to precipitate out of solution. In the cannabis industry, this is called winterization, which is necessary for precipitating lipids out of extract oil. If the cooling process is slow enough, crystallization of the solute can occur, which is how pure CBD is isolated from a pentane solution. It’s important to note that the solubility of a compound must be listed with an associated temperature at which it was measured, otherwise comparison to other compounds is not especially useful.

Supercritical fluids are different than normal liquid solvents. The changes in temperature and pressure for a supercritical fluid have a much more complex effect on solubility due to the nature of the solvent’s changing density, which is a unique characteristic that can change the polarity of the solvent itself.

Figure 5: A Qualitative pressure-temperature phase diagram of an arbitrary substance showing the changes in density if the particles in each phase state. Source: J. Mater. Chem. 9, 67–75 (1999). 10.1039/A804964E

Finally, at some point the solvent has a limit for the amount of solute it can dissolve or interact with at a molecular level. In other words, the concentration of solute in the solvent has a limit, which is usually how the solubility of a chemical is listed (e.g., mg of solute in mL of solvent).

Extraction

Extraction is simply the application of solubility, except in this case it is used as a separation method. When making coffee or tea with hot water, you’re extracting out the water-soluble flavor compounds from a solid matrix: roasted, ground beans or dried leaves, respectively. In this case, water is the solvent and the delicious flavor compounds, antioxidants, and caffeine are the solutes pulled out of the solid phase and dissolved into a new solution. Just like with cannabis, the size of the ground coffee beans and the temperature of the solvent (and solute) greatly affect the rate of extraction and the chemical profile extracted. Unfortunately, the mechanisms and parameters used for cannabis extraction utilizing ethanol, hydrocarbons (butane), or supercritical CO2 are complex enough that they need their own full discussion.

At Spektrum, our team of engineers, chemists, and designers take all these chemical concepts into consideration when designing the best way to extract hemp and cannabis efficiently and at large-scales. We pride ourselves in avoiding unrealistic, imprecise claims and are committed to using the most realistic and accurate terminology when referring to our extraction equipment. As a result, you can always expect quality products from Spektrum Cannabis Technologies.

By: Dr. Meghan McCormick, Ph.D., Process Lead

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