How does Super critical CO2 attract?

August 8, 2015 7:06 PM

I've been reading about SC CO2 extraction. CO2 is neutral in charge (as I have come to understand), so I am uncertain how it attracts the stuff (caffeine, cannaboids, antioxidants, et al.) that it draws out in this process. Does the charge change in the fluid state? You don't have to explain it like I'm 5, more like I took Chem 101 a long time ago.
posted by falsedmitri to Science & Nature (8 answers total)

It doesn't change in charge but it is a hydrophobic solvent with low surface tension, making it a good solvent for the compounds you ask about. Here's a Wikipedia link to get you started : https://en.m.wikipedia.org/wiki/Fragrance_extraction
posted by Tandem Affinity at 7:50 PM on August 8, 2015 [1 favorite]

Carbon dioxide is neutral in terms of ionic charge, but still exhibit a class of forces between each other known as van der Waal forces, which act between molecules. They are a result of how the molecule is bonded together, ie how uniform and symmetric the electron cloud densities are which hold the molecule together. In fluids, this class of forces is responsible for properties such as viscosity, vapor pressure, and wetting ability among others. In the supercritical phase, these forces become subdominant compared to the momentum associated with the molecule's motion, so the result is that the supercritical fluid achieves a much lower affinity for itself than it does with a solute. (The CO2 prefers to surround a caffeine molecule more readily than it does a single CO2.)
posted by incolorinred at 7:55 PM on August 8, 2015 [1 favorite]

Response by poster: @Tandem Affinity. The link you provide says CO2 in a non-polar compound has low surface tension and wets easily and you say that CO2 is a hydrophobic solvent. The "wets easily" and "hydrophobic" seem to contradict each other, that is, something that is hydrophobic would not wet easily, in my mind. Am I thinking about this wrong?
posted by falsedmitri at 8:00 PM on August 8, 2015 [1 favorite]

I think 'wet' is an imprecise verb, especially when talking about non-water solvents. I think they are trying to say that the liquid has low surface tension so it tends to adhere to surfaces and be absorbed more readily than other hydrophobic solvents. Incolorinred 's explanation explains this too at the molecular level. A liquid with high affinity for itself does not 'wet' things easily. Sorry if the link only caused you more confusion!
posted by Tandem Affinity at 8:08 PM on August 8, 2015 [2 favorites]

I also think I may have misspoke by saying 'hydrophobic' - 'nonpolar' is correct.
posted by Tandem Affinity at 8:40 PM on August 8, 2015 [1 favorite]

Response by poster: I think the things being extracted must be hydrophobic. So, a given liquid CO2 molecule is more attracted to a caffeine molecule than it is to another CO2 molecule. How do you then convince the caffeine molecule to leave whatever it might be attached to? Apparently, caffeine (et al) is hydrophobic and there is water in the coffee bean that it is attached to, but the attraction to the CO2 is stronger. Is this correct?
posted by falsedmitri at 8:40 PM on August 8, 2015 [1 favorite]

Hydrophobic refers specifically to how readily a material will create an interface with specifically water. So, in the case of a highly hydrophobic material, a water molecule will have a much higher attraction to another water molecule than it will the material. This is why water drops make large round beads on a Gortex layer- it being a very hydrophic surface. The opposite of that is a hydrophilic surface which water is much more attracted to than itself. This can be seen in materials that are very absorbent and wick water up quickly, like putting a paper towel over a spill.

Now for your coffee bean scenario- replace water with supercritical CO2. If we had a surface lined with caffeine molecules, the CO2 liquid would not make large beads on that surface, but rather spread out readily over the caffeine layer in a big puddle because it has a weak affinity for itself compared to with the caffeine. We say it "wets" the surface. In general, the same way the intermolecular forces would govern how a droplet would spread over a surface, they also govern solubility of things with each other based on their relative strength of attraction between the fluid and itself and the fluid and the thing being dissolved.

I think they remove the caffeine from the bean by letting the caffeine content leach out of the bean into the fluid CO2 solution and then modify the pressure slightly until the solubility of the caffeine molecule hits a critical point and precipitates out of solution where it can be collected and removed. I bet the process will also leach out tasty components from the bean as well which is why most decaffeinated coffee tastes slightly different than regular.
posted by incolorinred at 9:37 PM on August 8, 2015 [2 favorites]

Response by poster: Thanks, incolorinred, I understand now.
posted by falsedmitri at 9:22 AM on August 9, 2015 [1 favorite]