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Analytical Chemistry

Watery Worlds

One of the themes of chemistry over the last few decades has been the ability to pay attention to increasingly fine-grained details. We have new instruments that measure smaller and smaller samples and in shorter slices of time, and we’re seeing that the world that we’ve been used to is just an averaged-out look at reality, which is really a superposition of many smaller, stranger worlds.

What we’re used to is compounds in solution in a flask, or solid samples in a bottle, but as you zoom in on these things it all gets odd. Think of a powder sample. Maybe it’s crystalline, but even if it’s amorphous there could be amorphous material interspersed with several different crystalline forms, present as microcrystals. Either way, imagine a cubic crystal sitting there, looking like a tiny grain of salt. The molecules in the middle of that crystal are actually having a very different experience than the ones on the surface faces, aren’t they? And those in turn are in a different world than the ones out on the exact corners, which are much more weakly bound to the lattice than the others – they will likely be the first to dissolve off if the crystal is dropped into a solution, for one thing.

Or think of a pure compound dissolved in a solution. For similar reasons, the molecules up near a liquid interface are also having a very different experience than the ones out in the middle are. That goes for the solvent-glass interface as well as the solvent-air one. There’s been a lot of study directed to these effects in recent years, but here’s a particularly dramatic example in a recent Nature Chemistry paper. The authors are looking at the photochemical reaction of phenol, which is known to ionize under ultraviolet light in water into a phenoxy radical, a solvated electron, and a proton. That reaction has been studied in detail in the bulk phase, but what about the surface layer? The paper uses a technique whose full name I am only going to type out once: ultraviolet-excited time-resolved heterodyne-detected vibrational sum frequency generation (UV-TR-HD-VSFG) spectroscopy. This allows spatial resolution a few molecule layers thick at the surface and femtosecond time resolution of changes in the OH stretching spectrum – just what you need to answer this sort of question.

The authors pick up transient signals that can (quite reasonably) be assigned to the hydrated electrons, protonated water molecules, and the phenoxy radical, and these things come on very, very quickly in the surface layers as opposed to the bulk phase: ten thousand times faster. In physical-organic-chemistry-geek terms, the potential energy surface of the phenol molecule must change pretty drastically, along with the gap between various excited states. If you use shorter UV wavelengths, you can get fast phenol photoionization with no problem, but these experiments were done with far less energetic 267nM light. But even that is apparently more than enough to set things off for phenol molecules up near the interface. The unique arrangement of the surface water molecules around a given phenol seems to stabilize the excited state much more than the bulk phase does.

There’s no reason that this sort of thing can’t apply to many other reactions – some of them will be far slower at the interface, and some far faster. Different products might be produced or inhibited as well, and all of this could (and should) vary depending on the solvents used, ionic strength, and other factors. And don’t think that this is some weirdo physical chemistry effect with no relevance to the real world, because the active sites of proteins are, in fact, another interfacial zone with bulk solvent. Enzymes indeed depend on arranging individual water molecules and amino acid side chains in ways that affect potential energy surfaces of their substrates and lower the energy gaps between their starting states and the transition states of the desired reactions.

What’s happening up at the water surface with phenol is a “wild-type” version of what goes on in an enzyme. If we understood these things better, we could design all sorts of odd nanoscale cavity reactors that would do unusual and selective things to molecules, but we’re not there yet. But we’re walking around, breathing, thinking, and writing and reading blog posts thanks to an untold number of those little nanoscale cavity reactors floating around in our cells. We’ll figure it out, eventually. . .



17 comments on “Watery Worlds”

  1. MattF says:

    Add to that the typical convoluted ‘wheels-within-wheels’ hierarchal structures of biological systems, and it’s a wonder that conventional chemistry-in-a-flask has any relevance at all in biology.

  2. anonymouse says:

    Solvated elections, hydrated elections…I was definitely hydrated when I watched the debates!

    1. albegadeep says:

      I understand lots of folks were ethylated.

  3. Nick says:

    As someone who did his dissertation on a new nonlinear spectroscopy technique, the one used here is beautiful!

  4. DH says:

    Might this work have implications for industrial chemistry as well? E.g., if a reaction goes thousands of times faster at the surface, then run the reaction in a fine aerosol mist rather than in a bulk liquid.

    1. LdaQuirm says:

      Hmmm. It’s not an obvious yes or no. Industrial production usually cares more about throughput than latency. A mist is by nature very low density. If the mist reacts 1000x faster, but the total volume of reagents in your reaction vessel goes down by 1000x, you’ve gained nothing. The throughput hasn’t improved.

      1. LdaQuirm says:

        On further consideration, you can get the same increase in surface area, with an insignificant decrease in volume, by doing the inverse: Aerating your mixture. A sonic mixer and bubbler ought to do it. Feels like it should be worth investigating. You are basically adding in activated nano-particles, only they are gas phase, not solid.

  5. Sim3 says:

    What does everyone think of the big covid surge in previously well doing s. Asian countries? Why and why now?

    1. DrOcto says:

      While I’d like to believe that it’s a just combination of relaxed restrictions, entitled behavior and lockdown fatigue, I’m also having trouble ruling out active sabotage. Hell, it might even be the weather for all I know.

      1. electrochemist says:

        Could be due to a monolayer of water adsorbed on their skins due to the rainy season….

    2. joh says:

      the slime mold connection it must be
      If you go small and find something fascinating , can a collective “soul” exist even at levels 10K smaller , at sub-cell level?

  6. David says:

    I find an interesting symmetry of sorts where more macro chemical effects are seen as a super position of processes/states akin to how many macro (and micro, I suppose) physical observations are understood to be super positions in the physics world.

  7. steve says:

    Wow. I’m just a lowly biologist who always assumed all these years it was the caffeine in my coffee that woke me up. Now I find out that it’s the unique arrangement of the surface water molecules around the sugar crystals that stabilize their excited state that must be doing the job.

  8. david says:

    Really nice post, thanks for sharing.

  9. Bruce says:

    So maybe we shouldn’t be running reactions in round-bottomed flasks, we should be running them in shallow pans.

    1. LdaQuirm says:

      What volume of the bulk do you think counts as “surface” One molecule deep? Two? Does this constitute a large enough percentage of the total to have a noticeable impact if you increase it 10-100 fold? My intuition says “no”, but I’ll yield to actual figures. (Of course if you add in nano-structures, that’s different. That creates a surface that scales with the cube, not the square.)

  10. Ben Gamari says:

    Small typo: “267 nM light” should read “267 nm light”, unless, of course, we are referring to some sort of photon trapping phenomenon ;-).

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