Not a chemist. I'm trying to understand this abstract. In particular:

We found that guaiacol is preferentially associated with ethanol, and, therefore, primarily found at the liquid-air interface in mixtures that contain up to 45 vol-% of ethanol.

So guaiacol is amphipathic which means that is a largish molecule in which one end is hydrophilic and the other hydrophobic (or lilophilic). At least that's my understanding from Wikipedia. So, how did they arrive at the "therefore primarily found at the liquid-air interface" part? At the surface the molecule attaches one end to the alcohol and one end to the air?

Sorry for any naive interpretation of chemistry on my part.

Edit: Just run across this article discussing the matter.

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    $\begingroup$ Their meaning is that guaiacol is somewhat " extracted" from the bulk of whisky by the diluted, water richer layer formed upon dilution. Guaiacol moves towards the interface not because of air but because of the water. As you first sip and taste that layer, this is beneficial to the taste. Until here, it sounds nice. They also suggest that a prior dilution would affect the entire bottle, but I do not get the reasoning for this latter statement $\endgroup$ – Alchimista Aug 18 '17 at 15:37
  • $\begingroup$ Is the assumption that water is distributed preferentially on the surface, not throughout? Isn't water heavier than alcohol such that it would sink, even if added to the surface? $\endgroup$ – Al Lelopath Aug 18 '17 at 15:49
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    $\begingroup$ Yes, but not if you consider a glass situation. Especially with ice cube floating :) A water rich layer is surely atop. Again, I do not see the reasoning for the other conclusion... $\endgroup$ – Alchimista Aug 18 '17 at 16:56
  • $\begingroup$ It occurs to me that given a finite amount whisky in a glass and thus a finite amount of guaiacol, that the first half of a drink will have more guaiacol (and hence more flavor) and the second half of the drink will have less guaiacol (and hence less flavor) than if the guaiacol were evenly distributed. So I guess you just pour the second half down the drain? $\endgroup$ – Al Lelopath Aug 18 '17 at 20:15
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    $\begingroup$ I agree with your point. But surely the tasting happens at the first sip. If we have to take this seriously, the first sip might be anvenhanced gustative experience, but nevertheless the rest of the whisky is not totally depleted on guaiacol. Less seriously, after three on the rocks nobody cares :) $\endgroup$ – Alchimista Aug 19 '17 at 0:22

What the paper says is that in that range of concentrations, that the excess of alcohol at the air-liquid interface is enough to extract some of the guiacol from the bulk solution. At higher bulk concentrations of ethanol, there's no particular benefit for the guiacol to separate to the interface (there's enough ethanol in the bulk to keep the guiacol satisfied, so to speak). I forget whether the paper determined if there was a concentration (proof) below which the guiacol did not segregate, but if there was, it would be because the slight increase in alcohol concentration at the interface wasn't attractive enough. In other words the guiacol only is preferentially concentrated at the interface when the difference between the concentrations of alcohol at the interface to the concentration in the bulk makes it worth-while for it. (Worthwhile here means the difference in energy is large enough). Here's an analogy (My family would now roll their eyes, my analogies are lousy). A little kid is deciding whether to get up from his video game and go trick-or-treating. There's no candy in his house, and maybe he'll get a half dozen or a dozen pieces if he hits 20 or 30 houses/doors. Nah, he says, not worth it. But what if he's in a rich neighborhood? If he hits 10 doors, he's sure to get at least 2 dozen, maybe 3 or 4 dozen pieces of candy. Well, if his home hasn't much candy then he'll probably go, but if his home is chock-full of candy (rich neighborhood, remember?) then he may decide it's not worth it either. The choice he makes depends not just on the difference in the absolute amount of candy he will end up with, but also depends on how much he values an extra couple of pieces. It's non-linear, or as Liberals might say "context sensitive". (added: just to be clear, going trick-or-treating is analogous to concentrating at the air-liquid interface, not going is analogous to staying in the bulk phase.)

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  • $\begingroup$ Yes we knew it. Could you help me to understand why dilution (a bit more water than what is currently added) before bottling should be beneficial? It is a suggestion of the paper that I do not understand $\endgroup$ – Alchimista Aug 19 '17 at 0:25

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