Here’s another one of those nanoscale articles that gives me a bit of a shiver, because it shows pictures of something that I had assumed was beyond our ability to see. The authors, from the Ruhr University in Bochum, are looking at a simple organic molecule (an azobenzene, shown at right), adsorbed onto a solid gold surface. Scanning tunneling microscopy reveals (as you’d guess) the two aryl rings to have rather different character – the salicylic acid one seems extended, since the pi-electrons are delocalized out into the carboxylic acid group, while the nitrophenyl actually seems to have a depression at its end, as the nitro group interacts with the adsorption substrate. So far, so good.
They then adsorb a small amount of water (actually deuterated water, to get better contrast) and look at where the individual water molecules end up. And they can see them associating with the test molecule in very defined ways – when there’s just one water, it’s always next to either the carboxyl group or the nitro group (never interacting first with just the phenol). That actually agrees with density function theory (DFT calculations), which confirms one’s chemical intuition that the hydroxy is already tied up in an intramolecule hydrogen bond with the carbonyl of the acid.
Shown at right are some examples of this. It’s remarkable that even at an adsorption temperature below 30K the individual water molecules are mobile enough to pick up their preferred spots. Pauling was right; the hydrogen bond does reign supreme. The next step was to increase the amount of water in the experiment, and that showed that (no matter what) the aryl rings, azo group, and salicylic OH just don’t attract waters on their own. Instead, further water molecules join the one next to the nitro group and make a hydrogen-bonded network around it.
Down at the salicylic end, things are messier. Once there’s a saturating number of waters around the carbonyl, the hydroxy can get involved, but there are a number of different configurations seen. As the solvation shell grows around the molecule, the final water distributions are a bit different in each case, and the waters around the outside increasingly resemble the bulk state. I have to say, I never expected to be able to see this happening directly, water by water. It’s true that this is a two-dimensional case, with that gold surface down there, but the test molecule is pretty two-dimensional, too, and you’d have to think that this is a good representation of solution reality. You have to wonder if this technique can also show waters interacting with two or more molecules of test compound that are already associated on the surface, to get some idea of how a solid sample is pulled apart by dissolution.
So that’s how it it happens. Individual features on a molecule pick up waters first, in a particular order, and picking up different quantities of water in the process. Then other water molecules join in around them, gradually building a solvation shell around the parts that don’t have any affinity for waters themselves. I look forward to seeing more studies in systems like this – having a mental picture of how this takes place is one thing, but real pictures are something else again!