Now here’s a chemistry technique I don’t think I ever would have thought of. This new paper in Nature presents what could be a new way of doing multistep chemistry in a single vessel by the use of solvent layers of different densities in a rapidly rotating container (the examples below are in vessels are spinning at 5400 rpm). You inject these solvents from the central inlet and/or via channels running underneath the container. Folks who have spent a fair amount of money in bars might be reminded, with good reason, of a pousse-café.
The chart illustrates the solvent layers for the “e” example on the left, which are a mixture of aqueous and organic phases along a gradient of densities: W is water with sodium polytungstate (SPT), sodium metatungstate (SMT) or cesium chloride (CsCl), and these are thin clear “separator” layers between the organic phases. Those are dyed and are mixtures of tetrabromomethane (TBM), dibromomethane (DBM), 1,4-dibromobutane (DBB) and n-decane (D), and the whole assembly is stable over time. Meanwhile, the example on the right is all done in various mixtures of polyethylene glycol, methanol, and sodium iodide, and that one does gradually diffuse and blur.
OK, you are probably wondering why anyone would want to do this. Folks who are familiar with an old (and for many chemists, relatively obscure) technique called countercurrent chromatography might be getting some ideas, though: CCC is a separation technique that does a sort of continuous extraction between two immiscible solvent layers. There are several ways to realize this, some of which use this same sort of rotating-cylinder effect. My impression has always been that it needs a good deal of experimentation on the front end of the process, but when it works it can provide clean separations without your desired products ever touching a solid support that might degrade them.
So there are possibilities for (say) having a reaction take place in one solvent layer and allowing a differentially soluble product to be extracted out into another adjacent one (thus providing, in its slickest manifestation, a driving force for the reaction itself as you keep feeding in starting materials). The paper illustrates several multistep sequences that are performed across different solvent layers, where the intermediates alternate (for example) between aqueous-soluble and organic-soluble and work their way out towards the outer immiscible layer as the sequence proceeds. There are several interesting effects to do with the thickness of the various layers, the mixing between them that can be turned on and off by varying the rotation speed, and so on.
And you can do tricks that would otherwise be rather hard to pull off – one example is a simultaneous acid-base extraction, where a chlorinated solvent layer sits in between an acidic layer and a basic one, and the components introduced into the middle split off in both directions at the same time. Quinine nitrobenzoate, for example, disappears from the central organic layer and ends up as quinine hydrochloride in an aqueous HCl layer and sodium nitrobenzoate in an adjacent aqueous NaOH one. Similarly, the paper also demonstrates selective extraction of phenylalanine from a mixture of glucose and lactic acid (as one might find in a fermentation broth). These sorts of things could also be applied to biomolecules, inorganic compounds and clusters, and other species (the paper shows an example with silver nanoparticles which seems to be a real improvement over the known ways of handling them).
So while I don’t expect every bench chemist to jump at the chance to start revving up some rotating reactors, this should get some real interest from process chemists who are looking to optimize particular high-value reaction sequences and separations. It’s going to take a while for people to get the feel of this sort of reactor, but you can imagine situations where it could have some real advantages. . .
