Shots from The Battery

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CAUTION: EXTREMELY NERDY POST TO FOLLOW! Reading further may cause involuntary acquisition of useless knowledge. We do not assume responsibility for boredom or trivia related titillation.
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As some of you may know I am taking organic chemistry this year in college, and despite all of the fear mongering from my peers last summer, I’ve really enjoyed it so far! As the course has progressed, we’ve covered several interesting side-topics intended to put chemical concepts into “real world” context for us. Today’s lecture touched on one such topic, and I thought it would be fun to share with all of you. I’ve tried to keep concepts/terminology as simple as possible to allow everyone to understand the basics here, so if you happen to be a REAL chemist, please don’t beat me up too much for the minor inaccuracies in my analogies/artistry. Also anything I referenced that I thought a layperson might not recognize is clarified in ( ). All of that being said…

Have you ever wondered how Rain-X works?

Well, it turns out that the surface of glass is acidic, which for our purposes means that it has little hydroxyl (OH) groups that can lose their hydrogen (H) atoms. The presence of these hydroxyl groups also means that the surface of the glass can hydrogen bond (dashed lines) with water molecules (H-O-H) like so:

That hydrogen bonding is what makes the rain drops “stick” to your windshield! If you are a little rusty on your chemistry, it is probably easiest to imagine hydrogen bonding in terms of magnets. You know from experience with household magnets that opposite ends (N/S, +/-) attract each other, but can still be pulled apart pretty easily. That’s like hydrogen bonding! The hydrogen atoms are the (+) ends in this case, and the lone pairs of electrons (side-by-side dots) on the oxygen (O) atoms are the (-) ends. The attraction between these atoms is strong enough to keep the water droplets attached to your windshield (like opposing magnets), but weak enough that a little force from your wiper blades can easily separate the bonds and send the water droplets flying.

Rain-X prevents this interaction by deprotonating the hydroxyl groups and installing inert methylated silica groups. In other words, the H from the OH of the glass is taken away & replaced with the -Si- group as demonstrated in the picture below:

As you can see from the image, the silica group blocks the glass from binding to the water molecule, and without this bond the water rolls right off of the surface!

So why do you have to reapply Rain-X if the surface of the glass is no longer able to hydrogen bond with water? Over time the surface of the glass is exposed to contaminates from a combination of wind, dirt, rain, etc, and the process is reversed.