Inclusion Compounds 5/5/22

Science with Richard Bleil

John Dalton laid the foundation for modern chemistry when he introduced his Atomic Theory in 1808, the guiding principles still used today.  For the most part, he brought a lot of elements together that were already in play in one concise theory, and largely based on the “Atomist’s” beliefs, a philosophical movement from ancient Greece (circa 500 BC).  Three of the tenants postulated the existence of atoms, the likeness of like element atoms and the differences between different element atoms.  Pretty basic stuff.

The big postulate states that atoms from two or more different elements will combine in fixed, whole-number ratios to form compounds (and every other material in existence).  This is the heart of chemistry.  First, it demands that to form a compound, you need at least two different elements, and that the ratio is always fixed whole numbers (because you can’t have a part of an atom).  So, water has two hydrogens and one oxygen.  Change that ratio, like two hydrogens and two oxygens, and you have a very different compound, in this case hydrogen peroxide.  The ratio of elements is called the “stoichiometric ratio”. 

Being that this is such a critical key to chemistry, it may surprise the reader to learn that there are “compounds” that do not follow this rule.  They are called “non-stoichiometric compounds”, such as, for example, inclusion compounds.  I don’t usually think of these as actual compounds since they do violate Dalton’s law.  They always strike me as more of a specialized type of mixture since the ratio of components is variable, but I’m not here to debate terminology.  That’s a matter for the International Union of Pure and Applied Chemistry (IUPAC), the governing body in chemistry responsible for things like names.  I’m more interested in the science.

In 1962, the book “Clathrate Inclusion Compounds” was published, the first definitive book on clathrate compounds and still considered to be the authoritative text on these compounds today.  It was published by Hagan in you want to look it up.  What’s surprising is that it was written and published by Mary Martinette Hagan.  In 1962, women were becoming more mainline in business, but were still fairly uncommon in the sciences, especially the physical sciences such as chemistry.  But not only was she a trail blazer female chemist, but her full name was Sister Mary Martinette Hagan.  She was a nun as well as a chemist. 

One of my first projects (kind of like an undergraduate senior project, but senior projects were not required at my institution, and I never signed up for a capstone course) was clathrate inclusion compounds.  It was in the field of statistical thermodynamics (theoretical chemistry, even as an undergraduate).  But before I discuss that, I suppose we should talk about what clathrates are, but, before we can discuss clathrates, we must discuss crystalline structure.

Crystals are formed by regular repeating patterns of the compound or ions that make up the crystal.  Sugar, for example, is cubic because, at the atomic level, the sugar molecules arrange themselves like little cubes.  They then continue to grow until those cubes are large enough to be visible.  (Rock Candy is a fun and delicious way to see these formations.)  But within these crystals are repeating empty spaces, like little pockets between the molecules or ions.  This is why ice expands on freezing.  As the water arranges itself in its final structure, it creates little lattice pockets that kind of look like little cages.

Sometimes, other elements or compounds can become “trapped” in these little pockets.  It’s been suggested that frogs may survive freezing in the winter because oxygen becomes trapped in the crystal lattice of water as it freezes.  Once the water thaws, the oxygen is released providing a rapid source of oxygen for the frog’s brain. 

These compounds are easy to form.  Simply solidify the compound forming the crystal while under high pressure of the gas to be trapped in these crystalline cages.  It’s been suggested that methane might be safely transported as clathrates.  In the clathrate form, it would look, feel and be almost as safe as sugar crystals.  Once melted, the methane would be released.  But while it’s easy to form these compounds, it’s very difficult to get a constant fixed ratio.  For example, if you are trapping argon in ice, you might have between, oh, say 70 and 80% inclusion.  The ratio is not fixed, so it’s non-stoichometric.

My project was to try to figure out how the trapped gases like spending their time in their isolation chamber.  Are they clinging to a wall?  Sitting calmly in the middle?  It was a fun project to do, and I worked with a great adviser who was excellent at providing credit and motivation (the two go hand-in-hand).  And the short answer; if they have room, they hang around by the wall. 

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