Wee Mama will be available to answer questions after 8:30 CDT.
We've all been looking on with horror as the oil gushes out in the gulf, like a black cloud spreading out and covering the sea. From some comments and diaries, it seems that many folks are confused about what exactly the oil is and how to think about what can and what cannot happen. This diary is offered as an introduction to the chemistry of carbon and how it interacts with other molecules. It will cover the carbon-carbon bond, the hydrogen bond and why oil and water don't mix, evaporation, aromatic hydrocarbons, how detergents like Corexit work, where hydrocarbons go, and a little bit about toxicity.
The carbon-carbon bond
All molecules are made up of atoms connected by chemical bonds. A chemical bond is a pair of electrons shared between two atoms. In a carbon-carbon bond, where two carbon atoms are part of a molecule, the electrons are shared equally between the two atoms. Hydrogen is fairly similar to carbon in its ability to hold onto electrons so a carbon-hyrdogen bond is also fairly equally balanced.
If we start to count up possible hydrocarbon molecules the simplest one is one carbon, C, with all its bonds filled with hydrogen, H. This is methane, CH4. The link to Wikipedia shows four ways to draw methane, a simple pyramid shaped molecule.
Each of these ways of drawing molecules is useful, but none of them is completely accurate. Starting at the top left picture, the diagram shows the atoms with single letters for the elements, C for carbon and H for hydrogen. It also shows the bond angles, the three-dimensional shape of the bonds between atoms. Moving clockwise, the picture called a ball and stick diagram is a way to preserve these geometric relationships. Black is conventionally used for carbon and white for hydrogen. Beneath the ball and stick diagram is one which takes more effort to show bond length as well as bond angles. Moving to the left is a diagram called a space filling diagram which comes the closest to showing how a methane molecule occupies space. It still shows the boundaries as hard surfaces, which is somewhat misleading because the electron clouds have some fuzziness to them. However, all four of these representations will be used again for other molecules and it is useful to keep in mind what they can (and cannot) show about molecular structure.
Methane, a small and light molecule that makes up natural gas, is the smallest saturated hydrocarbon. Two carbons together can make ethane, C2H6, and three can make propane, C3H8. So far, as you can see if you look at the wiki links, all of these compounds can only be put together in a single way. However, with four carbons, branched compounds start to be possible. Butane, C4H10, is the first of these compounds with more than one form or as we say isomer; butane has only two isomers, a straight ("normal") molecule and a symmetrical branched one.
Hydrocarbons can have many, many more carbons than butane. Octane has eight and is abundant in gasoline but a large range is possible. The larger the number of carbons, the more possible ways to put them together. Take a look at the isomers of octane; dodecane (12 carbons) has 355!
The hydrogen bond and why oil and water don't mix
Oil and water don't mix, but why? It goes back to the carbon-hydrogen bond. Carbon and hydrogen share their electrons fairly evenly, so the bond is symmetrical. Water on the other hand is made of hydrogen and oxygen, and oxygen is much better at holding onto the electrons than carbon is. The covalent bond in water is so uneven that it is called polar, with oxygen slightly negatively charged and hydrogen slightly positive. In a water molecule, the two hydrogens are at an angle so not only is each bond polar but the whole water molecule has a more positive and a more negative side.
Opposites attract (sometimes) and the partial positive of the hydrogens tends to stick to the partial negative of an oxygen in another molecule of water. This weak interaction is called a hydrogen bond. Each water molecule can make up to four H bonds, so liquid water is a constantly shifting mass of water molecules loosely bonded to other water molecules.
Hydrocarbons have nonpolar bonds. They can't H bond to water; they don't bond to each other. The result is that hydrocarbons don't interact with water much. They are left outside that shifting network of water molecules and end up hanging out with each other by default, rather like unsocial nerds at a prom, left in a corner by the cool kids but not really talking to each other either. This is called the hydrophobic effect. It's a bit of a misnomer: hydrocarbons don't fear water. They don't love water. They just aren't that into water.
Evaporation
Water as we meet it every day is a liquid - cohesive enough to have a fixed volume but loosely tied together so that it flows. Octane is a liquid, too. When a molecule of water or octane breaks loose from the other molecules in the liquid and flies off on its own as a gas molecule, we say it evaporates. One measure of how easy or hard it is for a molecule to evaporate is to look at its boiling point. Water boils at 100°C. Octane boils at 125°C but it is about eight times as heavy! It is that network of hydrogen bonds that holds water together so well.
All of the hydrocarbons lighter than octane boil at lower temperatures than water. Methane, almost identical in mass to water, boils at -161.6°C. We say that the simpler hydrocarbons are volatile; they easily evaporate and once in the atmosphere they remain there until they break down. Again, this is all due to the non-polar character of the carbon-hydrogen bond. A complete series is here. Notice that the really large hydrocarbons are solid at room temperature.
Aromatic hydrocarbons
So far we've talked about molecules that have a single bond connecting any two atoms. But suppose two atoms shared four electrons? We call that a double bond. Carbon and oxygen can form double bonds, but hydrogen can't. Saturated hydrocarbons have all single bonds, and are saturated with hydrogen. Hydrocarbons with some double bonds have fewer hydrogen atoms than the same number of carbons in a saturated one. A molecule can have just one double bond, like propene (the -ene ending is the clue that there is a double bond in the molecule). Having just one double bond doesn't make a lot of difference to how molecules behave. Having two, though, and especially having two separated by one single bond makes a big difference. We call such double bonds separated by a single bond conjugated bonds. In a linear molecule, they can make it absorb light much better than a similar saturated molecule.
Conjugated bonds get especially significant in a molecule that has closed onto itself to make a ring. Benzene, C6H6, is the classical example of these, a hexagonal ring where all the electrons are shared equally by all the carbons. With the right number of carbons we call these rings aromatic (many of the smaller one do have scents). The aromatic hydrocarbons are especially important for health reasons. Those hexagonal rings can slip in between the base pairs of DNA. When they do, there is a risk of offsetting the next round of DNA synthesis and causing a mutation. This is one of the ways in which aromatic hydrocarbons (but not simple hydrocarbons) can cause cancer. Some aromatic hydrocarbons common in petroleum include benzene, toluene and polycylic aromatic hydrocarbons (PAH) which have two or more rings fused into one molecule. These are especially good mimics of DNA base pairs (and may have helped life get started, oddly enough).
How detergents (like Corexit) work
Oil and water don't tend to mix (it takes the high pressure and low temperatures near the bottom to make methane clathrates, for instance). Detergents contain surfactants, molecules that can bridge the gap. Part of the surfactant will be hydrophobic (either saturated hydrocarbon or something else that doesn't mix with water) and part of it will include other atoms like oxygen. That part of the molecule will be polar or even charged, so it will interact with water. The nonpolar part of the molecule can stick its toe into the cluster of hydrophobic hydrocarbon. The end result is to allow water and oil to mix in small droplets called micelles. You see this in the kitchen. Shake up oil and vinegar; they separate almost immediately. Add a teaspoon of mustard and shake. Now the cloudy mixture (called an emulsion) remains mixed, because the mustard is acting as surfactant.
Corexit EC9500A, the dispersant being used in the gulf, has several components. The surfactant in it is butanedioic acid, 2-sulfo-, 1,4-bis(2-ethylhexyl) ester. Its ability to disperse oil is very similar to the way shampoo works. Its melting temperature is 153-157 °C so it cannot evaporate and it cannot enter the water cycle or come down as rain.
Where hydrocarbons go
Oil has a complicated mixture of hydrocarbons and different parts have different fates. The lightest molecules are volatile. When they evaporate they remain in the atmosphere until they oxidize from exposure to UV light and oxygen. Less volatile ones may remain in the water column until they are degraded by microorganisms or adsorb onto some solid particle and fall down. The largest molecules are left behind as these smaller ones are removed and can form tarballs, solid masses of asphaltenes, the large and sometimes aromatic molecules that are too large to evaporate and insoluble in water. The conjugated systems in unsaturated hydrocarbons make them more vulnerable to UV light. When microorganisms consume hydrocarbons they often use oxygen to metabolize the hydrocarbon so oxygen depletion may result.
A little bit about toxicity
There is a principle as old as the Greeks:
The dose makes the poison
My toxicologist friend put it more concretely:
Too much of anything - salt, sugar, water or mother love - will kill you
For all of the components of oil there is some level at which they are toxic. Toxicity can also vary with how long the exposure lasts. However the pattern of exposure is very variable and for many of the oil components, humans will not be exposed at those levels. In some cases not even organisms in the environment will be so exposed. Take for example methane. As a non-reactive gas, at certain levels methane can kill by taking up the volume occupied by oxygen. However these levels are not being seen, even in the immediate vicinity of the BP gusher. At the other end of the size scale, asphaltenes are non-toxic because they are solid at the temperatures at which we will handle them.
In the intermediate size range the toxicity of oil molecules will vary with their properties. The volatiles like propane and butane resemble methane; they would only harm if they displace oxygen and there is not enough of them to do that. The saturated hydrocarbons like hexane tend to have fairly low acute toxicity (though some can be a mild anesthetic when inhaled at high enough concentration). N-hexane has chronic effects at levels of ~500 ppm; these levels will almost certainly not be reached on a chronic basis from this spill because of their continuing evaporation and dispersal. Levels these high are seen in manufacturing settings where bulk solvents are used.
The aromatic hydrocarbons like benzene present specific risks. However benzene is a trace component of the oil that is flowing out, and we have multiple environmental exposures to benzene from other sources. Whether benzene from the gulf gusher will have actual health consequences is not clear yet, and won't be until we know more about concentrations and durations of exposure.
A more detailed discussion of oil toxicity is found in this link from the ROVs. The specific government limits are:
The EPA requires that spills or accidental releases into the environment of 10 pounds or more of benzene be reported to the EPA.
The Occupational Safety and Health Administration has set an exposure limit of 500 parts of petroleum distillates per million parts of air.
Yet more information is available through the National Library of Medicine.
Previous motherships and ROV's from this extensive live blog effort may be found here.
