In case you were wondering, the evidence continues to accumulate that water is weird and that surface phenomena are weird, too. I realize that these are not exactly breaking news – honestly, water is one of the weirdest substances there is. I mean, molecular weight of 18 and it has a boiling point of 100C? And its solid phase is less dense than its liquid one? Most all that stuff comes down to its unique hydrogen-bonding capabilities, of course, but that’s where the surface/interface stuff comes in as well.
Because what happens when you come to the end of that hydrogen-bonded network? Something has to be left dangling in the breeze (or in the total lack of breeze, depending on the environment). It’s a general phenomenon. Think about a common salt crystal, with the sodium and chloride ions packed in that perfect cubic arrangement, each surrounded by the others – until you get to the edge. And suddenly those ions don’t have nearly as much support from the neighbors. Then what about the corners? At some point, no matter how you think about it, you’re going to have sodium or chloride ions that are out there even more exposed than the ones in the middle of the crystal surface, and they must have rather different properties compared to everything else in the crystal, too.
It’s the same for all crystals. Different crystal forms are going to have different sorts of edges, corners, and surfaces to them, and as the crystals themselves become smaller and smaller the percentage of them that are made up of edges and corners increases. Thus nanoparticles and their different properties. Nanoparticle catalysts not only have a lot more surface area, they have an increasingly different kind of surface area as the particles get smaller and smaller. Consider, say, finely ground platinum. Now, if you could somehow grind it down to single dispersed platinum atoms, those would have very different behavior than any sort of bulk metal, for sure (for one thing, there’s no silvery sea of delocalized electrons any more). How about two platinum atoms next to each other? Three? Six? A dozen? What forms do these clusters take, and when do you start getting separate populations of more-exposed and less-exposed atoms in them? At what point do you make the transition from single-atom quantum mechanical behavior to “tiny bit of bulk metal”? How bumpy is that changeover, and how many intermediate states are there? A lot of work has gone into these problems.
That situation with water is particularly acute because of the ridiculously strong intermolecular forces involved. Think about water molecules inside an enzyme cavity, for example – you certainly can’t think of them as a tiny amount of good ol’ liquid water from the tap. It’s already known that such “nanoconfined” water can have very unusual properties compared to the bulk phase, and this paper attempts to come up with a phase diagram of monolayers of such water in a graphene channel. It gets wild very quickly:
We find that monolayer water exhibits surprisingly rich and diverse phase behaviour that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400 kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials.
I can’t speak to how accurate the authors’ DFT calculations are, but this suggests at the very least that our intuitions about how confined nanoscale water might behave are worthless. And you don’t have to go to exotic graphene conditions to find this sort of thing. I wrote here about water microdroplets under electrospray mass spec conditions, which are going on constantly in every analytical chemistry lab, and the very odd reactions that can take place under such conditions. Here’s a new paper with more of this, showing that if you shoot mixtures of toluene and amines under such conditions, you get cation and radical reactions that couple the two, producing new benzylamines in measurable amounts. Hydroxyl radicals, solvated free electrons, and huge voltage gradients are all involved, and none of these are what you’d expect from a mixture of water, toluene, and diethylamine at room temperature, are they?
Here’s another new paper trying to get a theoretical handle on the situation, since (as they state truly) “the phenomenon is incompletely understood”. Their take is that the electrostatic potential is not only due to the air/water interface but also has some unusual contributions from the dynamics of the solvation shells in that environment:
Interestingly, these changes strongly affect the potential, while they influence to a much lesser extent its derivatives, i.e., the electric field. Different reagents will be subject to different potentials, and these potential differences in turn will be quite dependent on the position of the molecules relative to the GDS (Gibbs dividing surface). Hence, this factor is susceptible to play a key role in reactivity by dynamically influencing the relative electrophilicity, nucleophilicity, hardness, softness, chemical potential, and other properties of the partners.
So that’s going to be quite something to unravel, but there’s a lot to work with there as the different compounds are affected. It’s a different world down there, and I hope that we can learn to take advantage of it!
