I recently went into a old fashioned type of sweet shop which had a vast range of sweets in jars and some in packets. My wife reminded me about the joys of “space dust” which was the old name of a popping candy which will fizz and pop as you eat it. This made me think about it, a US patent (4,289,794) is the patent for this sweet. A later patent is for food which contains gases in the form of clathrates.
The early patent was for a sugar which contains very small bubbles of carbon dioxide. But the later patent is for foods which contain carbon dioxide which is not in bubbles but spread in a more homogenous manner through the solid. Many crystalline solids contain molecules of solvent in the crystal lattices, but sometimes rather than a “solvent” molecule it is a molecule or atom of a gas.
The classic example of a clathrate is one of a noble gas and water. Recently L. Yang et. al. published a paper (Proceedings of the National Academy of Sciences, 2009, 106(15), pages 6060 to 6064) in which they give details of the crystal structures of xenon and water clathrates. Before we get stuck in with this complex solid it is better to think about normal ice as published by J.D. Bernal and R.H. Fowler, Journal of Chemical Physics, 1933, 1, pages 515 to 548. In this paper by Bernal the arrangement of atoms in normal hexagonal ice is published. The oxygen oxygen distance in normal ice is 2.765 Å. The arrangement of the oxygens is very much like the carbon atoms in diamond. Here is a small part of the crystal lattice of ice.
In the xenon ice clathrate the oxygen oxygen distances are slightly shorter than those in the ice and the oxygens are arranged in tetrahedrons. Here is a picture of a unit cell of the xenon / water clathrate. As a result of the fact that the xenon is so heavy I suspect that it not possible to locate the hydrogen atoms on the water molecules.
Now a close up of the central part of the unit cell is shown below. It is clear that the xenon atom is at the centre of a cage of oxygens, as the oxygens are sp3 type atoms none of the orbitals of the oxygen can point towards the major lobes of the oxygen’s orbitals.
I will comment more on clathrates another day.