Thursday, June 13, 2013

Squeezing harder makes it bigger

That sounds like trying to squash political dissent, but it is how some materials tested at Argonne behave. The article is here. They squeezed (at 9,000 - 18,000 times atmospheric pressure) zinc cyanide in the presence of various fluids (e.g. water and methanol), and some combinations doubled the original material volume. Somehow the force rearranged the bonds to force fluid into the lattice, and in some cases the new lattice structure stayed the same size even after the pressure was removed. Compare with wet crepe paper, which gets a lot smaller when squeezed.

In one case (isopropanol was the fluid) the product was amorphous rather than crystalline, and shining X-rays on it caused it to shrink somewhat. This has to make X-ray crystallographic analysis complicated.

Although ScitechDaily suggests that this might have application in health care, zinc cyanide is not ideal, being poisonous and explosively reactive. But maybe there are some other substances that will do this too.

The 2-fold pressure-induced percentage volume expansion is 50–100 times larger than that previously observed in zeolites due to superhydration or cation migration (100% cf. 0.8–2.5% volume expansion).(35) While the increase in volume may seem surprisingly large, the resulting new phases contain guest-filled pores, such that the overall system volume (solid sample and fluid medium) is reduced. This eliminates inefficiencies in space filling associated with the interpenetrated structure (Zn(CN)2-I).

So it makes sense, but it isn't ordinary.

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