Clathrates 4/12/23

Science with Richard Bleil

Let’s talk about a different type of compound, clathrates.  Or inclusion compounds.  Or non-stoichiometric compounds. 

My very first research project, which was kind of like an undergraduate senior research project, but I never took it as a course or received credit for it, was on gas hydrate clathrates.  Gas is simple enough to understand.  Of primary interest at the time were gases like hydrogen or methane or other gaseous compounds that could be used as fuel.  Hydrate was a specific type of lattice, primarily ice crystals. 

A clathrate forms when a molecule or element, like methane and hydrogen, called the “guest” becomes trapped in the cavities created by the crystal lattice of the “host”.  Recently I spoke of how water freezes, where it is most dense at four degrees Celsius (roughly 39 degrees Fahrenheit).  Here the water molecules huddle together, moving slowly, and are as close to one another as they will ever be under normal atmospheric pressure.  As it freezes, it expands.  This means that as the temperature continues to drop, the molecules begin forming bonds with surrounding molecules, and spread themselves a little bit farther apart from one another in order to form a proverbial tetrahedral arrangement (think of a pyramid with a three sided base, or if you’re familiar with fantasy dice, the four sided die).  This arrangement allows for the maximum number of bonds with surrounding water molecules.  But as it does so, and the water expands, there are voids, or “cavities”, in the crystal lattice. They kind of look like little cages in the crystalline structure.

This is where molecules and elements will become “trapped”.  These trapped gases are not chemically bound to the water in the ice crystal in any normal sense of the term, but neither can they escape.  They’re held because they’re simply too large to fit between the bonds and atoms holding the crystal together.  They’re basically stuck there until the ice melts.  This is what we call a clathrate.

It’s sometimes called a “non-stoichiometric” compound because there is no hard and fast rule as to the number of gas molecules will become trapped.  In traditional compounds, according to Dalton’s Atomic Theory, it requires two or more elements to bond together in fixed whole-number ratios to form compounds.  In water, two hydrogen atoms will bond to each oxygen atom to form the molecule.  Change the elements, or change the ratio, and you have an entirely different compound.  For example, if you bind hydrogen to oxygen in a ratio of 2:2, you form hydrogen peroxide.  Water is necessary for every cell to live, while hydrogen peroxide kills them.  But in gas hydrate clathrates, the ratios are usually reported in percentages, and can vary depending on the pressure of the gas when the clathrate formed.

It’s been suggested that clathrates are how frogs survive the freezing process in the winter.  As they freeze, and their normal life functions cease, oxygen becomes trapped in ice crystals in their brain.  As the spring comes, those ice crystals thaw, releasing oxygen straight into their brains and “jump starting” the life process for the frog once again.

As you sip a cool refreshing beverage with ice in it, that ice, no doubt, is a clathrate.  A certain amount of various gases from the atmosphere (nitrogen, oxygen) will become trapped in the ice lattice as the water freezes.  You would not be able to recognize a clathrate from a pure crystal just by inspection.  However, these clathrate compounds are being studied in arctic ice today.  As the warming atmosphere causes the loss of more ice in the poles, scientists are extracting core samples of ever older ice from the caps as the newer ice melts off.  As they melt this older ice, the gases released is a reflection of the chemical composition of the earth’s atmosphere as it existed long ago. 

The idea of clathrates (although using compounds other than water as any crystal forms cavities) has been proposed to be able to safely deliver otherwise dangerous gas fuels over long distances, although I doubt that it ever will come to pass.  Trapping these gases in a crystal lattice, the gases would be in a form no different than salt.  The problem is forming the clathrates (which would have to be under high temperature and high pressure, the temperature so the host starts in liquid form and pressure to increase the percentage of lattice cavities filled).  On the other side, releasing the gases (requiring high temperature, again, to melt the host crystal) and the fact that the efficiency will leave much to be desired since it would take far more of the host molecule to form the clathrate than of the gases you could incorporate.  Still, recently there have been advances in clathrates in sutures, especially internal ones.  Antibiotics were added to internal sutures that are designed to dissolve, and as they do so, the clathrates also dissolve releasing the “guest” antibiotic molecules as they do. 

It feels like my blogs often end too abruptly.  I should have a cool catch phrase to add to all of my blog posts, so let me end this in the way I’ve recently seen in a movie.  Cool catch phrase here!