Friday, April 09, 2010

A Neat New Set of Solvents

Probably because of graduate school work, I’ve always had an interest in solvents. Solvent usage is continually falling out of favor as it certainly is not “green” – the solvents themselves are generally hazardous to some degree, and even if degraded in a thermal-oxidizer, the resulting CO2 is not a desirable endstate in the current/anticipated regulatory environment.

Even before these newest trends away from solvents, I had noticed that most chemists didn’t really understand them in any quantitative sense – they were just a carrier matrix for whatever else of real importance was dissolved in them, and that a trial-and-error approach was sufficient.

This is a shame as solvents make it so much easier to work with polymer on a lab-scale. Extrusion coating is best done on large equipment – it is just too difficult to do it when you only have 10 grams of material, but if you dissolve those 10 grams, then you can use any of a number of drawdown techniques to make a coating.

All this is a big lead up to this: a report (open access) of a new[*] group of solvents (amidines and guanidines) that can be switched from hydrophilic to hydrophobic just be bubbling CO2 through it. (One of the solvents can even be switched back.) As you would expect, the CO2 is converted to carbonic acid. This then suddenly provides an easy handle to adjust solubility and precipitation. The paper provides an examples focusing on soybean oil extraction. The extracts are not polymers, but that doesn’t matter – the thermodynamics are the same regardless of the solvate. Bubbling CO2 is an easy switch to operate too. The most common alternative options are to evaporate the solvent off with heat or precipitate it out with the addition of a non-solvent. (With polymers, you can sometimes get cute by just adjusting temperature if the system has an upper or lower critical solution temperature (UCST/LCST).

[*}These aren't "new" solvents - they were all purchased from existing suppliers. This report just recognizes the previously unrecognized properties.


Andrew Sun said...

It may be better the solvent be switched by air not abundant in the atmosphere.

John said...

Good comment. Blame it on me, but I left out that it takes a good amount of CO2. They were bubbling 1 bar of pressure for an hour or more. I didn't see that they reported a specific amount of dissolved CO2 (or pH) - that would be helpful.