Question: How do your crystals work (how do the store the gases)?

Michael Wharmby answered on 14 Jun 2011:

My crystals have very, very small holes in them – about 1/5000th the width of a human hair. When I make them they are occupied by water/air molecules. I can remove these molecules by heating the crystals under vacuum. Now there are big (to a molecule) spaces in the structure which really don’t want to be empty. I then expose my crystals to a fixed pressure of gas of my choice (often nitrogen or carbon dioxide, but also methane, hydrogen, water…). The gas molecules very quickly fit into the holes, because there is an energy gain for them to be inside (lots of interactions with the walls of the holes) and I can then ‘watch’ the gas going in by seeing the change in pressure of the gas.
I make the crystals selective to certain gases by changing the metal that makes the structure – for example magnesium has really good interactions with carbon dioxide so I use that a lot at the minute.