Two Solvents for the Price of One

The prospect of using a single, non-volatile solvent that can reversibly mix freely with water is an interesting one to my mind, especially as chemists continue to look for process of lower environmental impact.

Philip Jessop et al (Green Chem, 2010, 12 (5), 809) describe an assessment of guanidine and amidine species with switchable hydrophilicity.  The group found that the amidine below displayed the best characteristics of the 9 compounds tested.  The basic premise is that under air (or nitrogen) the amidine is hydrophobic and so immiscible with water, however, under an atmosphere of carbon dioxide the amidine is protonated by the carbonic acid and is freely miscible.  The equilibrium below indicates the principle:

They demonstrated the potential utility by extracting soybean oil from soybean flakes.  The soybean flakes were extracted with the amidine and the extract solution then treated with water, resulting in a biphasic mixture of the water and solution of soybean oil in the amidine solvent.  Then carbon dioxide was bubbled through the mixture for 1.5h resulting in the shift of the equilibrium as above and leaving essentially pure soybean oil as a separate upper phase with the ‘extraction solvent’ having now been switched and dissolved in the water.  After phase separation to recover the soybean oil, the aqueous solution could be heated to 80ºC whereupon carbon dioxide was released and the amidine solvent separated as the upper layer for another cycle.

I was intrigued by the paper from the point of view of the synthetic chemist and the promise, for instance of product isolation simply by manipulating solvent polarity through the use of carbon dioxide.  I am sure others will see further opportunities.