In a literal sense, we live in a material world. Our made world is constructed from and uses a vast range of different materials in ever more ingenious combinations. Some are natural, some are synthetic, some we have used for centuries, others are ‘new kids on the block’. The advent of a new type of material is particularly interesting to those of us who teach science or design & technology and liquid sponges fall into that category. Not only are liquid sponges a new type of material their behaviour is highly counterintuitive. It is relatively easy to understand the behaviour of a solid sponge – a flexible material, highly porous because its structure contains lots of holes each of which can hold small quantities of liquid. When the sponge is squeezed the air in the holes is pushed out and on release liquid is drawn into the holes. So, sponges can be used to mop up spilled liquids. But a liquid sponge? Liquids are incompressible, composed of a maelstrom of molecules, tightly packed together, tumbling over one another. To be sure there are small spaces between the molecules, holes if you like, but they are forever moving and changing places. How can they possibly soak anything up?
Enter Stuart James who started with a solid material with a cage like structure giving it holes and simply melted it to give, he hoped, a liquid which still retained the cages and would have holes. But it didn’t work out. Then he tried dissolving the solid in a solvent to give a solution but that didn’t work because the solvent molecules filled up all the holes. Eventually he tried a different sort of material with a cage structure and a different solvent whose molecules were too big to go into the holes and voilà he had prepared the first ever liquid sponge or, as they are usually called, porous liquids. Jarad Mason developed the idea further by developing materials with a cage like structure that dissolved in water but had cages that were water repellent. This meant the holes didn’t get filled up with the water that was being used to dissolve the material. These sorts of materials are biocompatible which means they could be used in humans to, for example, oxygenate their blood if they are having breathing difficulties.
Katherine Sanderson writes about this beautifully in the latest edition of New Scientist (The unlikely rise of liquid sponges 11 March 2023) explaining in everyday language (doughnuts, spaghetti and dinner plates) just how they work. Do read her article. Importantly these remarkable materials are of more than academic interest. Here are just some of the possible uses as indicated in Katherine’s piece.
Capturing Carbon Carbon-capture-and-storage technology is used to try to sequester greenhouse gases produced at fossil fuel power plants and other high emitting industries, such as steelworks. Porous liquids could be a cheaper, more efficient way of soaking up the carbon dioxide than the current technology.
Purifying Crude Oil The process of separating crude oil into all its components – natural gas, petrol, bitumen and more – currently relies heavily on distillation which requires a huge amount of energy to heat up the mixture. Porous liquids could be an alternative, lower-energy separation technology.
Harvesting Xenon Xenon is a rare gas used in physics experiments as an anaesthetic and in lights. We currently obtain it by liquefying air and then distilling it, an energy-intensive process. Porous liquids could be used to separate xenon from nuclear waste instead.
Katherine’s piece would make an excellent reading homework for learners in KS4 and the basis for a technological perspective reader.
As always comments welcome