Philip Ball explains the paradox of dry water and powdered methane:
Methane gas hydrate forms naturally when water is mixed with methane at high pressure and low temperature. Huge deposits of the crystalline substance exist in the deep sea, where they could provide vast fuel reserves. But rising global temperatures increase the chances of the hydrate decomposing, releasing the greenhouse gas and accelerating further warming. This mechanism has been proposed as a cause of dramatic environmental change in the distant past.Using methane gas hydrate as a kind of solid methane for storage and transport has been mooted before. The Japanese company Mitsui Engineering and Shipbuilding has a pilot project for producing natural-gas hydrates on board ships that would then transport the gas from remote marine deposits — and using some of the stored gas to power the ships themselves.
The problem is that the hydrate forms only under cold, pressurized conditions, and then very slowly. Typically, a skin of the material forms at the surface of water and prevents further growth. The formation rate can be speeded up by vigorously mixing the gas with water, but that is costly and cumbersome.
Cooper and his colleagues have got round this problem by finding a way to break the water up into many tiny, stable droplets, massively increasing the surface area in contact with gas. They do this by converting water to dry water by stirring it up with a special form of silica, called hydrophobic fumed silica.
This consists of tiny grains of silica — the same basic material as sand — coated with a chemical layer that makes them water-repellent. The silica particles cover the surface of water droplets and stop them from coalescing.
“If you’ve ever seen water drops in dry dust, it’s the same thing”, says Cooper. “They form a ball with the dust on the surface.” The resulting dry water is a very odd substance. “It looks like a powder”, says Cooper, “but if you wipe it on your skin, it smears and feels cold” as the water is released.
The researchers found that their powder soaks up large quantities of methane at water’s normal freezing point, producing crystalline methane gas hydrate within the silica-coated drops. A litre of methane gas can be stored in about 6 grammes of the material. This storage capacity, they say, is very close to the target set by the US Department of Energy for such materials, and compares well with that of other candidate storage media.
And crucially, it is made from cheap raw materials, helping to make this method economical relative to other, more exotic potential methane-capture materials such as designed molecular frameworks.