VOCs, or volatile organic compounds, just like BPA, TCE and DDT represent a growing number of TLAs — three-letter acronyms — for chemicals popularly understood to adversely affect human health or the environment. By abbreviating to VOC, we save our breath, page space and the confusing use of the word “organic,” which widely connotes universally wholesome, in a term (VOC) considered generally deplorable.
According to us chemists, “organic” simply means composed of mostly carbon, hydrogen and oxygen with some occasional nitrogen, sulfur or halogens. By this definition, BPA, TCE, synthetic fertilizers and pesticides would be considered “organic,” though organic farmers would avoid their use.
Given the perception of VOCs as universally abhorrent, sustainability minded consumers gravitate toward “no VOC” paint or “zero VOC ink.” A BPA-free label informs the consumer about the absence of a specific chemical, while a “No VOC” label necessarily is more of a catch-all, because VOCs are a class of compounds instead of a discrete chemical.
Any organic — according to the chemist, not the farmer — compound that will readily vaporize would be considered a VOC. Naturally, some are more hazardous than others. Typically benign pheromones — or airborne signaling molecules for living organisms — would be considered VOCs, but so too would many harmful solvents for chemical reactions or extractions.
Locally, Tom’s of Maine takes advantage of a technology that circumvents the need for hazardous VOCs to extract plant oils for use in their products. Instead of carcinogenic or otherwise harmful petrochemical-derived solvents — most of them VOCs — the technology uses an earth-abundant, benign yet extremely volatile organic compound: CO2.
To understand just how transformative this technology is in supplanting previous approaches, an analogy to vinaigrette salad dressing should help. If left to sit, the vinaigrette will separate into an oil layer and a vinegar layer — mostly water, with a little acetic acid. Components of the salad dressing will self-segregate into whichever layer they prefer. The salt, acetic acid and other highly polar compounds will end up in the water layer, while more greasy, fat-soluble compounds will prefer the oil layer.
When the compounds we wish to extract or react are insoluble in water — as is often the case with our oily, petrochemical-derived feedstocks — we often turn to VOCs to get the job done. These organic solvents need to be volatile so we can retrieve our desired material after they evaporate.
For example, you could extract the greasy oils of oregano into olive oil; but because olive oil isn’t volatile, you’ll have a hard time separating the two. This is fine for herbal remedies, but imagine a multistep synthesis of a pharmaceutical with an essential oil from oregano as the starting material. If you can’t isolate it after extraction, your synthetic route is toast, so we need something with the organic solvating properties of olive oil but which can readily evaporate. Traditionally, VOCs have been the answer.
VOCs, broadly speaking, aren’t just useful as solvents for extractions or a medium in which to run a chemical reaction. Paints, adhesives, markers, Wite-Out and many other products take advantage of VOCs that will evaporate, leaving behind a thin layer of a material that is often not soluble in water — i.e. “permanent,” because it won’t wash off under typical conditions.
Unfortunately, the utility of many VOCs as solvents often blinds us to their impact on human health and the environment. Take benzene, a solvent so good at dissolving greasy compounds that medical professionals used to wash their hands with it until we discovered that it was a carcinogen. We also used to use benzene — as well as chloroform — to extract caffeine
from green coffee beans in the decaffeination process.
Today, there are several tricks to get water to work for caffeine extractions, though the VOC extraction route has not been abandoned. Indeed, CO2, which is dirt cheap, simple, nontoxic and so volatile that it is a gas except under certain conditions, is now widely used for decaffeination. Under high pressure, CO2 can exist as a liquid capable of dissolving nonpolar, grease compounds. Simply release the pressure, and the CO2 vanishes into thin air, leaving behind the extracted material.
This high-pressure CO2 technology is what Tom’s of Maine takes advantage of to circumvent the use of VOCs more harmful than CO2, which is just about all of them. They employ CO2-extracted hop oils in their deodorants on the premise that if hops can preserve beer, perhaps they can protect our armpits from odor-causing bacteria. They also use CO2-extracted oils from chamomile flowers for their soothing properties.
As we become increasingly aware of the toxicological impacts of our modern industries, we need to consider more than the sourcing of our feedstocks and the properties of our products. The ways we choose to process our feedstocks into products play an important role in eliminating dangerous components from the waste stream. Replacing benzene or chloroform extractions with CO2 extractions substitutes the generation of hazardous VOC waste with CO2, the ultimate benign VOC.
Reuben Hudson is a postdoctoral fellow in chemistry at Colby College in Waterville.