Will It Go ‘Round in Circles: PAHs

What are PAHs?

PAHs, short for Polycyclic Aromatic Hydrocarbons, are groups of naturally occurring or man-made chemicals that result from the incomplete burning of many fuels such as coal, oil and even gas. When drawn, they look like multiple rings of benzene bonded together to form chains or sheets or… other odd shapes. You’re probably thinking, “How are rings going to hurt me?” Yet they do. They have been linked to cancer and can even hinder reproductive health. Clearly, in our current age and society the abundance of PAHs is potentially much higher than any previous generation. It is also necessary to notice that substances which produce the most PAHs are the compounds with the most moles of Carbon. Other than messing around, PAHs have two primary uses: research and dyes. They are also used to make plastics and pesticides. 

That’s Great, but what do we actually see?

PAHs in their base forms, because they are made of only carbon and hydrogen, are organic molecules and are thus nonpolar. Thus, they barely dissolve in water if at all, but they love to dissolve in fats, oils, and other organic solvents. PAHs are all solids, but their form depends on which method is used to crystallize them–some end up forming needles or plates. Their colors vary: for instance, anthracene is colorless in its purest state, benzo(a)pyrene is pale yellow, and pyrene can be either depending on the method it is crystallized.

 

I Thought Rings Were for Marriage

You’re probably thinking, “Oh, it’s an organic molecule, right? It can’t possibly be bad for me…” Apparently, the only type of rings the body tolerates come in the form of engagement and marriage. The body really doesn’t want PAHs inside. For starters, 200 of them are found in cigarette smoke, and many of these have the ability to hinder reproduction. Then they have the ability to indirectly cause cancer.

Benzo[a]pyrene is one of the worst PAHs there are; its effects have been well documented. Chimney sweeps often found themselves getting scrotal cancer due to this compound–in the mid 19th century. In today’s world, benzo[a]pyrene has been found in cigarette smoke, being one of the molecules that could potentially cause lung cancer, among others, by binding to DNA. In doing so, they may end up interfering with DNA replication. The mechanisms by which the DNA is metabolized are far too complex to be understood here (at least at the high school level for someone who hasn’t done any research), but know this: the body metabolizes it to form its toxic forms. Not only that, but it hampers your immune system. Overall, the body metabolizes benzo[a]pyrene and many other PAHS through microsomal enzymes, forming compounds that bind to DNA and introduce mutations.

And You Might Be Wearing Them Now

Most PAHS lack practical everyday uses, barring that some of them are toxic. Most of them that are used are used for research purposes only. Yet, as previously mentioned, some could be on you right now in your clothing. They were used in the process of making the color. The PAHs that have dyeing abilities include but are not limited to anthracene, carbazole, and pyrene. Anthracene is used to make red dyes, carbazole for violet, and pyrene for fluorescence. This technology has been used for quite a while; the patent for anthracene and its process was filed in 1925. Also, while its status as a PAH is debatable, naphthalene has been found in mothballs, a critical component of clothing storage, though it has been phased out because it has a tendency to catch on fire. But in the more practical sense, PAHs show up in many things derived from coal tars, including asphalt (the stuff cars drive on) and even cosmetics (things you put on your face).

 

And in a way that could actually benefit humanity, some PAHs are implemented into dye-sensitized solar cells. Dyes are used to boost output in otherwise unfavorable conditions while also diminishing costs by eliminating some of the expensive materials used in it. These dyes are often coated on the titanium dioxide found in the solar panel. The dyes can be based off of PAHs, for they give them superb photoconducting skills. For instance, carbazole is made electron-rich due to its nitrogen atom, making it useful for the  electron transfer necessary to induce current. And a dye known as TC501, bridged by anthracene, improved the open-circuit photovoltages and short-circuit photocurrent densities of a dye-sensitized solar cell, so much so that the solar conversion efficiency jumped to 7.03%, which is a relatively remarkable number. Many of the mechanisms by which PAHs assist in photoelectric conversions are specific to that PAH, and some of them are not yet fully understood. And speaking of extraterrestrial energy…

There’s Plenty of Space for This Poison Elsewhere

Naturally, we are able to observe things specifically on the planet Earth, otherwise we need either telescopes or overactive imaginations (we recommend some of both). Using this, science can now tell us that certain molecules also occur outside in the cold, not-empty void of interstellar space. The spectra of PAHs have been found in conjunction with large molecular and dust clouds; it is speculated that they form by photoionisation, which is also what causes hydrogen clouds to form in these places. Photoionisation, as the name may suggest, involves light providing the energy to ionise molecules and cause them to form into compounds. Sounds like a great way to cook

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For those who have an inexplicable fear of deep space, there’s something for you as well. PAHs have also been found in the atmosphere of Titan. Titan, as you should probably be required to know, is Saturn’s largest moon. It’s unique because it is the only moon to maintain a substantive atmosphere. To the average flying robot, it is an orange ball. Titan is a lot closer than all interstellar space, so we can accurately detect what is in the ball of Orange. It turns out that, besides a load of nitrogen (boring), there are PAHs as well. Those that the European Space Agency found are a bit more complex than the ones our beleaguered lungs are most familiar with, but they are still the same sort of molecules. It may be worth adding here that Titan also seems to have surface lakes of hydrocarbons, which may be of similar origin. Seti, or the Search for ExtraTerrestrial Intelligence, has found indications that these molecules, besides being distributed in gas clouds, are also found in interstellar dust and locked up in water ice. These are the basic ingredients in planet formation and by extension that of more complex organic forms.

Good Poison?

You learned earlier that PAHs often result from the burning of organic or fossil-derived materials. The existence of such complex  molecules in both interstellar clouds and the atmosphere of a moon suggest that maybe the molecules related to life are not so rare as we might’ve thought, and it certainly opens some new eyes and avenues for which to explore. Scientists have been forever intrigued by the presumably unique condition of life on the planet Earth. With the discovery of new worlds beyond our solar system, and the recent insights into the conditions on some places a bit closer, perhaps we will discover that, just maybe, we are not alone.

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