Molecule of the Day

Everyone has heard of superglue. If it weren’t so entrenched in everyone’s mind as something that existed, you’d be absolutely amazed. Whenever I am at Wal-Mart, a drug store, a dollar store, or anywhere that has the six two-gram tubes of superglue for a dollar, I compulsively grab at least one, usually shaking my bemused shopping mate or getting one on the phone (”Guess what I just found at Wal-Mart?!” “The six tubes of superglue again, huh?” “Don’t patronize me, it’s AMAZING!”). I have tolerant friends.

Superglue is a trade name (probably a generic name, by now, I guess, considering my vast superglue collection is definitely not all marketed by the same company) for a class of adhesives called “cyanoacrylates.” The superglue you buy at the store is typically either methyl or ethyl cyanoacrylate. Another cyanoacrylate adhesive, 2-octyl cyanoacrylate, is available to seal wounds. I’ve heard it’s commercially available as “dermabond,” but I’ve never seen it, even in the suture cabinet back when I volunteered in a hospital for a few years. You can be sure I will own some someday.

The explanation I’ve always been given is that the octyl group makes the superglue bond a little more flexible, which is desirable in wound closure. Superglue is very strong, but poor against shear force. That is, pulling against superglue won’t break the bond very easily. Pulling along the plane of the bond does. During my superglue adventures I have glued my thumb and forefinger together…a few times. Rubbing from side to side tends to release this bond relatively easily.

Also remarkable about superglue is that it is so acidic. We don’t usually think of carbon atoms as being acidic, but the cyano (the C triply bonded to the N) and ester groups are sufficiently electron-withdrawing that it loses a proton pretty easily. Stabilized or not, carbanions (negatively charged carbons) are still great bases, so one cyanoacrylate molecule attacks the next, and so on, in a chain reaction, resulting in a long polymer of polycyanoacrylate.

Perhaps the coolest use for superglue, and one that many of you will be familiar with, is developing latent fingerprints. There is usually enough residual water (or protein, or something basic) in fingerprints to set off cyanoacrylate polymerization. Superglue is also fairly volatile before polymerization (evaporates easily), so if you put an item with a putative fingerprint in a chamber with an itty bit of cyanoacrylate and warm it, the superglue will polymerize around the fingerprint, leaving a negative image. I’m sure I will have an experiment day and this will end up on the site sooner or later.

Night!