by John Timmer

May 2010

from arstechnica Website

It may look like ice, but it burns.
Pacific Northwest National Lab

News reports of the failed attempt to contain the oil-spewing equipment on the bottom of the Gulf of Mexico have referred obliquely to things like "ice crystals" or an "icy slush" clogging the hardware that was intended to cap the leak.

Anyone who is paying attention would recognize that there's a bit of a problem here, in that, even at the temperatures and pressures of the ocean at the site, the water there is very much in its liquid phase, as are the hydrocarbons that are spewing through the leak. The methane that caused the original explosion remains gaseous down to -161°C.

The "ice" that's forming is actually a solidified mixture of methane and water called a clathrate. Clathrates have also been in the news because of a potential role in climate change, so it seems like an opportune time to explain what they are.

The first thing that you might consider them is counterintuitive. At the conditions in which they form, methane is a gas, water a liquid. Somehow, they come together to form a solid. The key to understanding why is the small size and nonpolar (hydrophobic) nature of methane. Because water is very polar, the two substances don't mix well; forcing them to mix is very energetically unfavorable.

As a result, when a nonpolar molecule is placed in water, the water molecules tend to form a cage around it by hydrogen bonding with each other.

This isn't especially favorable on the atomic scale, since water molecules don't normally settle down and form stable connections with partners when it's in the liquid form. But, by locking the nonpolar substance in place, they limit its contact with the rest of the liquid, which ends up being favorable overall.

Methane is special because of the small size of its molecules.

It's easy for the hydrogen bonding network of water that surrounds it to fold back on itself and form a complete cage that traps some methane inside of it. This locks a collection of water molecules into a stable structure that looks a lot like the one present in ice. Since the water is already locked in place as part of a cage, it's actually better (from an energetic standpoint) to form additional cages by growing them off the side of an existing one.

As more and more cages get added on, the end result is an ice-like solid that stores a significant amount of methane.

This won't happen under many conditions. If the pressure is too low, the gas won't sit still long enough; if the temperature is too high, the water won't. However, the pressures and temperatures that prevail in many parts of the oceans are perfectly suited to the growth of methane clathrates. And, based on the fact that the explosion that led to the leak seems to have been caused by natural gas from the oil well, there's no shortage of methane there, either.

The solutions that are now being considered are based on the energetics described above.

The simplest to understand is the option of placing a heating element inside the box, which would raise the temperatures above the point where clathrates are stable. An alternative is to pump a partly polar molecule, like ethanol or methanol, into the site. Because these can interact with both water and methane, they keep the stable cage structures from forming.

Both of those aren't long-term solutions, since they require constant attention, but they could serve as stop-gaps until a permanent fix is put into place.

Why clathrates could be a big deal

It doesn't take an oil leak to trigger clathrate formation.

Bacteria in ocean sediments produce methane all the time, and a lot of that ends up trapped in deep ocean clathrates. Many estimates of their volume indicate that there's more methane in clathrate form than we can possibly ever hope to extract through traditional drilling. It's not an obvious source of energy, however, given that nobody has figured out how to recover significant quantities of it.

But scientists are watching these clathrates carefully, because they could play a major role in future climate change.

As the atmosphere continues to warm, some of that heat inevitably gets transferred to the oceans, which will slowly heat up as well. Since clathrates are very sensitive to temperature, there's a chance that some of them will reach a point where they're unstable at the prevailing temperatures, and they'll melt, releasing methane into the atmosphere.

This is especially true in polar regions, where low temperatures have allowed clathrates to form at relatively low pressures.

Methane is considered to be a more potent greenhouse gas than carbon dioxide. It only stays in the atmosphere for a few years on average, but the majority is removed by oxidation, which converts it into carbon dioxide. So there's a chance that clathrates could accelerate and/or enhance any climate change induced by high levels of atmospheric carbon dioxide.

So far, however, we don't know whether this is taking place. There have been a few reports of substantial methane releases in the Arctic, but we have not studied the issue long enough to know whether these are unusual or represent a common occurrence.

Clathrates may be an interesting oddity of physical chemistry, but this is one science fan that hopes they get out of the news soon—and stay out.