Life on earth is damp: if it doesn’t occur in water, it still occurs because of water. There is life in bone-dry deserts and in mines miles under the ground; all that life is composed of cells, and a cell is essentially a tiny bag of water containing stuff.
The problem with water is that all sorts of things will not dissolve in it, as you can observe when you do the washing up: you can’t clean a greasy pan properly and even with washing-up liquid, most of the leftovers end up as lumps instead disappearing into the suds.
Even so, those lumps and grease were once living creatures. Life revolves around water but also includes substances that do not mix well with water: hydrophobic substances. This may seem strange, but biologists and chemists had a good explanation for that: cells produce soapy substances, just like washing-up liquid, to encase the hydrophobic substances.
However, that is not the entire story, explains researcher Young Hae Choi of the Institute of Biology. Those soap bubbles are useful for transporting substances that are not soluble in water, but how does a plant produce them? And the amounts are wrong: "For example, plants of the Sophora species produce a poorly-soluble compound in their flowers called rutin. The dry weight of the flower might be composed of up to thirty per cent rutin – that’s simply too much to process with soapy substances." There must be more to it.
In 2011, Choi, his professor Rob Verpoorte and their colleagues from the Delft University of Technology published a paper on a collection of substances in plants that supply the solution. The generic name for them is NADES, short for Natural Deep Eutectic Solvents - "Natural", because they occur naturally and "solvents" because they are solvents. "Deep eutectic" refers to an amazing quality of some of the substances: if you mix them in the right proportion, the joint boiling point and melting point differ to those of the separate parts.
To elucidate that point, Choi shows a picture of some jars containing white powder. One contains sucrose: ordinary sugar from sugar beet. The other contains malic acid, a simple compound produced by plants in relatively large quantities. If you mix the two powders together in a ratio of one to one, the mixture will become a liquid, and that liquid will be a solvent. "We’ve known about these eutectic liquids for nearly a hundred years, from synthetic chemistry," says Choi. "We thought: why shouldn’t they occur naturally?"
It turns out that plants are bursting with NADES, according to the article the Leiden-Delft team is to publish shortly in the specialist journal Analytica Chimica Acta. All sorts of sugars, acids and other compounds found in plants form eutectic solvents when mixed together. Choi’s student Yuntao Dai is the first author.
This knowledge is incredibly important to people who want to know how a plant works, but "regular" chemists might be able to use it too, because working with large amounts of chemical substances is terribly impractical when they are solid. It is much easier if they are liquid, but then you have to dissolve the substances in something first. Paints, drugs, cosmetics – there are several industries making lots of money that use large quantities of solvents, and many of those solvents are toxic. Sugar and malic acid are biologically degradable: just add gelatine and you can make sweets. Who knows, perhaps the NADES could lead to "greener" chemistry: "There are nice potentials out there", the researcher adds.
A French cosmetics corporation is already paying for permission to use NADES in its products. According to Choi, several other companies are interested too, but he does not want to say anything while they are still negotiating.
He mentions another application: the extraction of chemical substances from plants. Many plants produce substances that are interesting to people - as medicine, for instance. "The great thing is that you can extract certain substances from plants very precisely, depending on which solvent you use." Has he had any calls from Colombia yet, where a major industry extracts substances from coca plants? He starts to laugh: "No, and you need some technology before you can do that. This gimmick is still very new." Later on he adds: "Science is like a knife in your kitchen drawer. You can use it to stab someone or to make your dinner."