The Basic Scientific Principles of Solvent Extraction for Hemp Processors

An overview of they physical and chemical properties that drive solvent extraction.

What is Botanical Extraction

Botanical extraction is the process of removing and concentrating one or more substances from a botanical material. Spices, hops, citrus and other fruits, herbs, hemp, and cannabis are all botanical materials that are commonly extracted. The products derived from these extractions range from vanilla extract to CBD oil.

The variety of products produced by botanical extraction has contributed to the diversity of extraction methods. Innovators have developed a wide range of extraction techniques. Each technique comes with drawbacks and benefits that need to be considered when choosing the best extraction method. It can be a daunting task, but this introductory guide will provide a basic overview of the scientific principles behind solvent extraction. Understanding these basic scientific principles is foundational to performing an astute review of the many solvents and extraction options available to you.

Two Primary Botanical Extraction Methods

At the broadest level, the two methods of botanical extraction are solvent extraction and mechanical extraction. This article focuses on solvent extraction, but it is important to understand the basic principles and differences between the two techniques.

Mechanical extraction refers to an extraction that is caused by mechanical force pressing a liquid out of a solid material. This method is usually reserved for either high value oils or for botanicals sources which have very high ratios of oil to solids. Extra virgin olive oils are produced in this manner inside giant screw presses. Citrus essential oils are produced in a similar manner by pressing citrus peels. Some mechanical extraction methods use heat to increase extraction efficiency.

The benefits of mechanical extraction include eliminating the need for flammable solvents and protecting temperature sensitive compounds. The downside of mechanical extraction is the extraction efficiency is much lower than solvent extraction. Even the most efficient expellers leave about 7% residual oil in the base material.

In hemp extraction, mechanical extraction is an acceptable method if:

Either the market value of the oil is so high it can cover the loss of some cannabinoids in the plant material, or
You are going to process the mechanically extracted material afterwards, recovering the residual oils in the plant materials.

Mechanical extraction is not advised if you are concerned about the loss of roughly 50% or more of the oil material that would be left in the spent material following extraction. Overall, mechanical extraction is suitable for extraction of fewer materials in fewer applications than solvent extraction.

The second method of extraction, and the focus of the rest of this article, is solvent extraction, which has much wider applications and higher possible efficiency than mechanical extraction. The multitudes of available solvents with distinct characteristics make the extraction of diverse compounds possible.

Solvent extraction provides higher throughput, lower energy consumption, and higher extraction efficiency than mechanical extraction.

Polarity And Solvent Extraction

Solvent extraction has thousands of applications. It is used in many industries including the petrochemical, pharmaceutical, food, and beverage industries. The principle behind solvent extraction is extremely basic. The goal is to use a liquid (solvent) to dissolve (solvate) a target molecule or group of compounds (solute) and to wash them out of the solid plant material. The solvent is then separated from the solute in order to concentrate the solute. This separation is usually achieved by evaporating (distilling) the solvent, which concentrates the solute. The distilled solvent is then condensed and recycled to conduct another extraction.

The effectiveness of a given solvent at dissolving your solute will depend on its polarity, the polarity of the compounds you seek to extract and the solubility (the amount of solute than be dissolved in the solvent) of the solute in the solvent. Polarity is a chemical property in which opposing sides of a molecule possess distinct positive and negative charges. The polarity of a molecule can range from very polar or slightly polar, to non-polar.

When it comes to solvent extraction, polar solvents can solvate polar solutes and non-polar solvents can solvate non-polar solutes. This phenomenon is commonly summarized and easily remembered as, “like dissolves like.”

"Like Dissolves Like"

We have all experienced the “like dissolves like” rule in our everyday life. For example, you can dissolve salt in water to high concentrations but salt is not soluble in vegetable oil. That is because salt is a polar molecule and so is water, they are “alike.” Vegetable oil however is non-polar which explains why it cannot act as a solvent for salt.

Solvation occurs when molecules of the solvent surround and encapsulate the molecules of the individual solute completely. This is driven by a number of chemical and physical forces which vary based on the polarity of the solvent and solute. Polar solvents for example, dissolve polar solutes via a magnetic attraction of the positive and negative charges of their molecules. A good everyday example is once again table salt (NaCl), which when dissolved in a polar solvent such as water, separates into Na+ and Cl- which carry a slight positive and negative charge. Water itself has a slight positive charge at one end and a slight negative charge on the other. These negative and positive charges orient themselves in such a manner that they are attracted to each other like miniature magnets, causing the Na+ and Cl- ions to be surrounded by the water and dissolved in it.

Figure 1: Table salt (NaCl) is dissolved by water. The Na+ and Cl- ions are surrounded by water molecules.

In contrast, oils and other lipids are non-polar and do not have areas of positive and negative charge. In keeping with the “like dissolves like” principle, non-polar substances such as oil dissolve well in non-polar solvents such as hexane. This process is driven by a different intermolecular force than the “magnetic” attraction between polar solutes and solvents.

The force that binds non-polar molecules is called the London Dispersion force. The London Dispersion force is a result of negatively charged electrons constantly moving in space around the nucleus of their atom, occasionally resulting in a temporary negative charge when all electrons are in the same area. This temporary charge may then attract nearby molecules and even induce a temporary charge in them.

In summary, once you understand the polarity of your target solute, you can begin to identify an ideal solvent for your extraction process.

Figure 2: Chart showing relative polarity of common solvents.

Other Factors that Affect Solubility

Solubility is affected by more than the polarity of the solute and solvent. By controlling and manipulating the parameters below, more efficient extractions can be achieved.

Temperature – As temperature increases, the solubility of solid solutes increases dramatically. The increased kinetic energy of a higher temperature solvent further breaks up the bonds holding the solute together, allowing more solute to be dissolved at a faster rate.A common example of this is dissolving sugar in water. In room temperature water, sugar dissolves until the solution is fully saturated. When the temperature of the water increases, the solubility also increases, allowing more sugar to dissolve in the warm water. This process also works in the opposite direction. Sugar will fall out of a saturated sugar-water solution as the solution cools.

Solvent Density – As solvent density increases, the solubility of most solutes also increases. This solubility increase is caused by the increased number of solvent molecules available to surround solute molecules. Solvent density becomes critical when the solvent is a gas that has been compressed into a dense liquid or supercritical fluid state in order to provide sufficient solvating power. Pressure increases the density of supercritical fluids dramatically, which is why many CO₂ extraction systems operate at pressures up to 5000 PSI.

Glossary

Distill: to purify (a liquid) by vaporizing it, then condensing it by cooling the vapor, and collecting the resulting liquid.
Polarity: a separation of electric charge leading to a molecule or its chemical groups having a negatively charged end and a positively charged end.
Solubility: the maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure.
Solute: a dissolved substance
Solvate: to enter into reversible chemical combination with (a dissolved molecule, ion, etc.).
Solvent: a liquid substance capable of dissolving or dispersing one or more other substances.
Solution: a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).