Solvents that change their hydrophilicity on addition and removal of CO₂ could eliminate the need to carry out energy intensive distillations on an industrial scale.

In industrial chemical processing, separating solvents from products is normally done by distillation, requiring the addition of volatile solvents and large amounts of energy. Now, Philip Jessop and colleagues at Queen's University in Kingston, Canada have developed a new type of solvent that can switch from hydrophobic to hydrophilic meaning it can be removed from products without distillation. 

Jessop's team first introduced the idea of solvents changing their solvation properties in 2005, when they discovered non-ionic and non-polar amidines transformed into an ionic and polar liquid hexylcarbonate salt after bubbling in CO₂ for an hour. After studying a variety of guanidines and amidines, they have now found that the solvent N,N,N'-tributylpentanamidine, which is normally insoluble in water switches to become completely miscible with water when CO₂ is added. 

Soybean oil is currently extracted from soybeans using hexane, which must then be removed by distillation. As an alternative, the switchable solvent in its hydrophobic form can be used to extract the oil. Then, adding carbonated water switches the solvent to the hydrophilic form and a biphasic system containing a layer of pure soybean oil and an aqueous layer is formed. After decanting the oil off, the CO₂ is easily removed from the aqueous solution by heating and the solvent switches back to its hydrophobic state allowing it to be separated from the water and reused.

'This technology could be useful in any such application, and probably many more that we haven't even thought about yet,' says Jessop. 

Nils Theyssen, an expert in alternative solvents at the Max Planck Institute for Coal Research, Mülheim, Germany feels this is a real step forward. 'A solvent with such an easy switch in hydrophilicity is unprecedented and deserves special attention, and the enormous application potential of this switchable-hydrophilicity solvent is documented in a most stimulating case study,' he says. 

Jessop's team are now working on developing cheaper switchable solvents and hopes the process could be scaled up in industry. 'We still have to do a proper analysis of the energy requirements and the environmental impacts of the new process compared to distillation,' he adds. 

Edward Morgan 

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