Who I'm Reading Today

https://www.science.org/content/blog-post/what-s-happening-water-surface-anyway

The whole “Are there weird electric fields at the surface of water droplets” question is not getting any easier to understand. I last wrote about this topic here). There are several subsidiary questions that go off in different directions: does unusual chemistry actually happen at air-water interfaces (and is this more prominent, as you might expect if true, in small droplets that are mostly surface area?) Or are these reports (mostly) experimental artifacts? If it does happen, what’s the mechanism? Does it have to do with unusually high electric fields (and field gradients) as some have proposed, or is it something else? Is it specific to air-water interfaces, or does it happen in the vacuum as well? And as for the later, how about under mass spec conditions (with electric charges all over the place?)

This latest paper will not douse the flames. They’re using a technique called “vibrational sum frequency generation” spectroscopy which seems well suited to studying surface phenomena. The technique probes changes in the OH stretching band in water’s IR spectrum, and the wavenumber values you get are well known to be correlated with local electric field strength. The authors take pains to go into detail about what is meant by “electric field strength”, to their credit - is it from a roughly static arrangement of water molecule dipoles at the surface, or is it the sum of a bunch of dynamic fluctuations (in the hydrogen-bonding network,e etc.)? But no matter how you get there, the arguing is most vigorous over the idea that such fields can be large enough to change chemical reactivity, and there are very passionate advocates on both sides of that question.

The authors are studying extended thin films of water by SFG spectroscopy, and they make predictions of what they should see for the OH stretches under different conditions. Specifically, there should be a red-shifted zone under any sort of relatively high and relatively persistent electric field at the surface (as opposed to the spectra of bulk water), and they go into detail about what the different sorts of fields (and mechanisms for generating them) might mean to the spectra.

Now, people have of course tried to estimate these field strengths before, through other techniques, and some measurements come in at 10 to 20 MV/cm, which is pretty strong stuff. Values of this size, the authors note here, should be very apparent in these SFG measurements (and you can indeed see things shift around dramatically in SFG with small amounts of ionic solutes in water or with changes in pH). But they don’t see it!

These findings indicate that exceptionally large interfacial fields, as sometimes invoked to explain microdroplet reactivity, are not supported by our SFG results. Within the framework of vibrational SFG, we conclude that there is no evidence for such fields at the neat air−water interface. The interpretation of interfacial electric field-driven catalysis in microdroplet experiments must therefore be reconsidered in light of the absence of spectroscopic field signatures at water interfaces.

Yeah, it seems as if it should be. But as the authors note, you don’t have to reach for electric field explanations to deal with unusual interface chemistry - there could be altered solvation chemistry, redox pathways, evaporative concentration and other factors at work, and these aren’t mutually exclusive, either. So this certainly doesn’t mean that all the reports of interesting surface chemistry are wrong - but people may have been reaching for the wrong way to explain them. Let’s see how this goes over!

https://www.science.org/content/blog-post/what-s-happening-water-surface-anyway