A Student’s Exploration of Supercritical CO₂

Is It A Gas? A Liquid? What Is This Thing?!

When Mr. Red first began to successfully create scCO₂, I couldn’t tell if it was a gas or a liquid.

From what I knew about CO₂, I knew that it often experienced sublimation, or the process of going from a solid state directly to a gaseous state (used in products such as dry ice). Mr. Red clarified, and I confirmed with further research, that scCO₂ is referred to as supercritical fluid, a term that frees it from the limitations of the categories of ‘gas’ or ‘liquid.’ In fact, scCO₂ seems like it can behave as one or the other. If a chamber containing scCO₂ is turned and moved, the visible substance will move along with it (like a liquid) but the non-visible part acts as a gas and doesn’t. Technically though, this supercritical CO₂ is not, and never will be, a liquid as long as it stays supercritical: the temperature of the scCO₂ is higher than the critical temperature of regular CO₂, meaning that there is too much energy for the molecules to stay together as a liquid. 

Below is what scCO₂ can look like just after it has started to shift from a solid state to this new state of matter. As you can see, it looks almost exactly like a gas, or some kind of smoke, which is weird knowing that it is still referred to as a fluid. Interestingly enough however, there are still some scientists who will refer to scCO₂ as a gas-liquid hybrid, which is something I would personally do if I were to ever work with this substance. 

How Is This Useful?

Apparently scCO₂ is used in a lot of different things. For example, starting in the 1970s, it has been used to extract caffeine from coffee beans by tripling the caffeine’s solubility and allowing further extraction. It is also used in cleaning supplies as a substitute for solvents. ScCO₂ is an eco-friendly, non-toxic and non-flammable substance that has many, many different uses.

What’s the difference between scCO₂ and regular liquid CO₂, though? Why go through all the effort to make it?

Well, scCO₂ has a higher diffusivity rate, lower viscosity and surface tension, and gas-like diffusion properties, though liquid-like density. Regular CO₂ can’t compare to this! 

Potential Dangers

While I did mention that scCO₂ is non-toxic in the previous section, it should be noted that it does come with dangers. It’s not the fluid itself that is dangerous, but more so the process of making the scCO₂ in the first place.

To make scCO₂, you need, well, CO₂, which Mr. Red acquired in the form of dry ice. Then, you need to pressurize it and heat it up past its critical point, which is around 31˚ C, in a chamber. This heat (which naturally causes expansion) and the pressure put on by the chamber used to make the scCO₂ create a potentially dangerous situation.

In the video, Mr. Red had a leak in the chamber during his first attempt making scCO₂, but because he found it early and was only making a small amount, thankfully there were no disasters. However, if larger amounts were being made, the process may not be as safe.

Knowing what goes into the process of making scCO₂, and the dangers of potential pressure explosions, I am currently conflicted about whether the mass-creation of scCO₂ is truly safe, despite all the claimed benefits of the substance itself. This worry could apply to things across the production board though, from oil to plastics, so I digress. 

Conclusion

Sources

Information and Helpful Links

• NileRed Video: https://www.youtube.com/watch?v=JslxPjrMzqY
• Caffeine Extraction and scCO2: https://www.futurelearn.com/info/courses/everyday-chemistry/0/steps/22338#:~:text=Supercritical%20CO2%20has%20been,solubility%2C%20hence%20accelerating%20the%20extraction
• Cleaning Supplies and scCO2: https://www.unitechannemasse.com/co2.html#:~:text=The%20properties%20of%20Supercritical%20CO,to%20the%20material%20undergoing%20treatment
• NY Health on Dry Ice: https://www.health.ny.gov/publications/7081/#:~:text=Dry%20ice%20can%20be%20a,and%20in%20some%20cases%2C%20die

Images

• NASA’s Mars exploration goals: https://mars.nasa.gov/#red_planet/1 
• Mars’s northern ice cap: https://mars.nasa.gov/news/1001/nasa-orbiter-penetrates-mysteries-of-martian-ice-cap/
• The Perseverance Mars rover: https://mars.nasa.gov/mars2020/multimedia/images/?t=347&&start=1

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