Seeing at the surface

One of the quirks of chemistry is that we measure the bulk solution, but the reaction often happens on a surface - whether it's precipitation, dissolution, or catalysis.

While we usually calculate reaction rates based on the concentrations found in the bulk solution, in a case of a surface reaction, that's out where no reaction is happening! The equations for reaction rates empirically account for this in the constants, where the reaction rate is rolled together with the rate the reagents diffuse toward the surface to react and the rate the reaction products diffuse away from the surface and out of the way.

But in terms of designing a reaction, controlling it to get, say, the product we want instead of a byproduct if there are two possible reactions, or speeding it up or slowing it down, what happens at the surface can be key. And whether we watch or measure, knowing what happens at the surface is the first step toward changing what happens at the surface.

Of course, this isn't only about designer reactions. The new pH tip that measures within 0.1mm from the surface could help with things like learning what's actually happening on a rusting surface in terms of where acidity gets more concentrated and what effect that has both on how fast the metal rusts and where it rusts first and fastest.

That isn't as flashy as the videos of molecules mid-reaction, but just as useful to science.

I was very interested to find out that videos of molecules have actually been around for a decade, even if the images were grainy. Depending on which of the two molecule imaging links above you click on, the researchers are using X-ray or electron beams instead of visible light to illuminate the molecules, because visible light has a wavelength too much longer than the size of the molecule. If the wave is longer than the object, it's just not going to reflect off of it in any useful way; it'll just wash over it instead.