Understanding the Chemistry of Cannabis Oil – Part 2: Terms & Key Concepts

POSTED Sept. 14, 2020

Covering vapor pressure, boiling point, distillation, combustion, oxidation, and degradation.

Are you unclear about what distillation really means? Would you like to understand how terpenes, cannabinoids, and the different parts of cannabis are separated and purified? Do you want to know why the proper storage of extracted cannabis oil is so important? These questions can be more easily answered with a simple foundation of chemistry knowledge that many vendors and equipment sellers in the cannabis industry tend to gloss over or ignore. The following terms and concepts will help you become a smarter buyer and more confident consumer in the world of cannabis extraction. The discussion below is also a continuation of our first post in the series:  Understanding the Chemistry of Cannabis Oil – Part I: Terms & Key Concepts.

Vapor Pressure

Distillation is just a method of separating a mixture. To understand how distillation works, there are a few properties of liquid (and solid) substances that first must be recognized. For the sake of simplicity and relevance, we’ll discuss liquids, which are sometimes called solvents, like water or ethanol. Separating a mixture of liquids (including oils) via distillation relies on a chemical property called vapor pressure. The word ‘vapor’ is just referring to liquid molecules that have moved into the gas phase, or evaporation. ‘Pressure’ seems intuitive enough until it’s thought of at a molecular scale: pressure is created by the constant collision of gas or liquid molecules against the walls of a container, whether it be a bottle, a balloon, or even the room you’re sitting in. The best way to visualize vapor pressure is through a simplified example. First, imagine a half-empty bottle of water without a cap sitting in a room. After a few days you might see the level of water slowing going down, which you can guess is due to the water slowly evaporating up and out of the open bottle. If we wanted to prevent that, we just close it with a cap. Evaporation still occurs, but the vapor (gas) water molecules can’t escape and instead bounce around in the space above the liquid water (Figure 1). Eventually, some of the vapor water molecules will collide with the surface of the liquid water and condense back into their liquid form. This constant balance of evaporation and condensation is called an equilibrium, which is specifically defined for each type of liquid (or solid) substance. At this equilibrium point, the pressure exerted by the vapor molecules on the container walls and liquid surface is called the vapor pressure.

Figure 1: The movement of liquid and gas water molecules in an open (left) and closed (right) container.

The vapor pressure is important because it will tell you how quickly and easily a chemical substance will evaporate from its liquid or solid form. A higher vapor pressure means there are more molecules in the vapor phase that can exert pressure on the walls of the container. There are more molecules in the vapor phase because it’s easier for them to escape or evaporate from the liquid phase. A molecule’s ability to escape the liquid phase is related to the intermolecular forces present in the liquid, which circles back to the concept of polarity [discussed in Part I). Acetone, which makes up a large portion of nail-polish remover, is a perfect example of a liquid that has a higher vapor pressure than water. If you pour a dime-sized amount of acetone next to a similar sized amount of water, the acetone will evaporate very quickly relative to the water because it is considered a volatile solvent. When a liquid solvent is more volatile, it requires a lot less time and energy to evaporate it away.

As can be expected, the bottle of water sitting in a warm room would evaporate a lot more quickly compared to one sitting in the refrigerator. Temperature clearly affects the vapor pressure because heat (or energy) is needed for molecules in the liquid to break away from each other and enter the gas phase. In short, higher temperatures increase vapor pressures for all substances, just not in a linear fashion (Figure 2).

Figure 2: As the temperature increase, the vapor pressure of the substance also increases.

Boiling Point

Everyone who has attempted cooking has probably seen the effects of applying high heat to a pot of water in that it eventually starts to boil (at 100 °C or 212 °F). Boiling occurs when the vapor pressure above the liquid surface equals the ambient atmospheric pressure, which you could measure if you had a barometer in your kitchen. The bubbling you see at the bottom of the pot of water occurs because the heat source is at the bottom of the pot, causing evaporation and enough vapor pressure to push through the liquid to the surface. In short, if you apply enough heat (or energy) to molecules in a liquid, they now have enough kinetic energy to overcome 1) their intermolecular forces (aka, how molecules interact with each other) and 2) the pressure exerted by the atmosphere on the liquid surface.

Another familiar fact about cooking is that the boiling point of water changes with location, specifically elevation or altitude. For a reminder, the unit of one atmosphere (1 atm) is determined at sea level, and higher elevations cause a drop in pressure, and thus, a drop in the boiling point. As mentioned above, the boiling point occurs when the vapor pressure reaches the atmospheric pressure, which can be manipulated. Using a pressure cooker increases the atmospheric pressure dramatically, meaning water can reach temperatures of around 250 °F (121 °C), cooking food much more quickly. The opposite is true for a container or apparatus placed under vacuum where the pressure becomes almost negligible and liquids can boil and evaporate at much lower temperatures.

The dependence of the boiling point on the surrounding pressure can be shown with a nomographFigure 3 below illustrates how the measured boiling point of Δ9-THC can change with the extent of the vacuum applied. It can also be used to extrapolate what the boiling point of Δ9-THC would be (425 °C) at ambient atmospheric pressure (760 torr or mmHg).

Figure 3: A nomograph of Δ9-THC depicting the various boiling points at different pressures under vacuum.

Finally, melting point and boiling point measurements are a way to test the purity of a substance. Due to the mixture of various intermolecular forces, any impurities will change either value or muddle it into a temperature range instead of a precise point. A batch of CBD isolate can be tested for purity (~99.9%) if the melting point is measured at 67.5 °C (154 °F).

Distillation

Now that you are familiar with vapor pressure and boiling point, they can be utilized to separate the components of a mixture/solution, such as extracted cannabis oil. Distillation is a physical separation process of a liquid mixture based upon the components’ boiling points and their subsequent condensation into a separate container. The separation can be used for removing solvent and/or purification purposes, and the various designs are used at a range of temperatures and pressures for cannabis applications:

In a lot of cases, the distillation apparatus is held under vacuum, and remembering the trends relating boiling point to pressure (Figure 4), this is done for a few reasons. At lower atmospheric pressures, the liquid has a much lower vapor pressure, meaning the components can vaporize or boil at lower temperatures as well. Molecules like terpenes and cannabinoids therefore avoid the high heats that cause degradation (a sticky, burnt end-product), and the relative volatility of the components in the mixture can be fine-tuned separate at specific temperatures and pressures. Unfortunately, due to the complexity of the crude extract, data collection and fine-tuning each piece of distillation equipment is the best way to determine the ‘sweet spot.’

Figure 4: Summary of the trends relating vapor pressure, atmospheric pressure, temperature and intermolecular forces.

Something to keep in mind when applying vapor pressures in real-world distillations: mixing two or more components causes new cohesive and repulsive intermolecular forces that affect the individual vapor pressures (or a deviation from Raoult’s law). This is the basis for azeotropes, where the mixture of two or more liquids cannot be separated by simple distillation once they reach a certain ratio because a constant boiling point is reached. When an azeotrope is heated to boiling, the proportions of the components in the vapor are the same as the liquid. The best-known example is a mixture of 95.63% ethanol and 4.37% water (by mass), which boils at 78.2 °C, an important note to make when extracting cannabis with ethanol and distilling it for solvent recovery. This also happens with denatured ethanol, a mixture of ethanol, water, and n-heptane.

Combustion

Combustion (or an incomplete version of combustion) is what happens when you smoke flower or vape oil. The simplest chemical formula is when a carbon-containing molecule reacts with oxygen at high heat to create carbon dioxide and water. In reality, there will be other gases and incomplete combustion products created. But beware, caution must be taken when heating cannabis oil at extremely high temperatures since natural compounds in cannabis flower and oil can break down into carcinogens, like benzene (Figure 5).

Figure 5: Some high-temperature degradation products of common terpenes found in cannabis products (Source: https://doi.org/10.1021/acsomega.7b01130).

Oxidation Reactions and Degradation Pathways

Temperatures above freezing and exposure to light and oxygen are the main contributions to degradation of cannabis extracts. Oxygen and oxygen-containing components, including lipids, fatty acids, and other natural molecules found in cannabis, cause oxidation reactions and degradation pathways of the desired cannabinoids and terpenes. These molecules will steal electrons and break bonds in a relatively short time if extracts are not stored correctly. Air-tight, vacuum-sealed containers that are opaque will help remove oxygen and prevent UV light from speeding up the degradation of products. Storing any product in a freezer is always going to prevent natural decomposition over time (Figure 6). If an extracted product contains lipids or fats (not winterized or dewaxed), then applying an appropriate expiration date determined through regular testing would ensure a quality product is delivered to the consumer.

Figure 6: The dehydrogenation reaction of Δ9-THC into CBN is a well-known degradation pathway caused by heat and light over time.

At Evolution Built, 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 Evolution Built.

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

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