2
$\begingroup$

In water, distillation causes fractioning of the $\ce{^{18}O}/\ce{^{16}O}$ oxygen isotopes (Wikipedia).

Does something similar happen to the $\ce{^{13}C}/\ce{^{12}C}$ and possibly $\ce{^{14}C}$ isotopes in alcohol when it is distilled out of a watery brew?

$\endgroup$
7
  • 2
    $\begingroup$ Looking at the percentage differences of the different isomers may be instructive. $\endgroup$
    – TAR86
    Commented Oct 11, 2019 at 5:04
  • $\begingroup$ Yes, It does. But the effect would be weaker, because of smaller relative mass difference of isotopes, and of the ethanol mokecule. Additionally, different mass of C atoms has much lower impact on ethanol intermolecular bonding, than has O mass on water bonding. $\endgroup$
    – Poutnik
    Commented Oct 11, 2019 at 7:34
  • $\begingroup$ @Poutnik Selection among ethanol molecules might be limited, since most of ethanol evaporates out of the brew and water (with a bit of ethanol) stays behind. When evaporating water partially, as with Oxygen, all molecules are subject to selection. I wonder if fractioning still takes place for carbon. $\endgroup$
    – HannesH
    Commented Oct 11, 2019 at 22:33
  • $\begingroup$ The first ethanol fractions are enriched by 12C. As most of ethanol is evaporated, the last fractions are in contrary impoverished, with total balance about neutral. $\endgroup$
    – Poutnik
    Commented Oct 12, 2019 at 4:00
  • $\begingroup$ @TAR86 Can you expand on that? How would the enrichements of isomers relate to that of isotopes? $\endgroup$
    – HannesH
    Commented Oct 12, 2019 at 18:10

1 Answer 1

2
$\begingroup$

Does the distillation of alcohol cause the fractioning of isotopes[...]?

The short answer is it depends.

  1. Distillation of ethanol from water can be done nearly to completion. There are published methods for determining isotopic composition of sugars in fruit juices which involve fermentation of the juice to yield ethanol, followed by nearly quantitative distillation of the ethanol from the resulting brew. Then, the 13C content of the ethanol is measured. If the ethanol distillation step (or fermentation step) substantially altered the isotopic composition of the sugars, this method would not be suitable. The main thing which determines the isotopic composition of the ethanol in this case is the type of photosynthetic metabolism used by the plants from which the juices or syrups are derived. Quoting from the linked paper:

    The fruit juices or maple syrups are completely fermented with yeast, and the alcohol is distilled with a quantitative yield (>96%). The carbon-13 deviation (delta13C) of ethanol is then determined by IRMS. This parameter becomes less negative when exogenous sugar derived from plants exhibiting a C4 metabolism (e.g., corn or cane) is added to a juice obtained from plants exhibiting a C3 metabolism (most common fruits except pineapple) or to maple syrup.

  2. If distillation is not done to completion, there can be a fractionation. In fact this has been measured fairly accurately in 1990 for distillation of pure ethanol. Here is a figure from that paper. enter image description here

    It plots the ratio $R_D$ of 13C to 12C of the distillate ($D$) normalized to the same ratio $R_Q$ for the starting material $Q$, as a function of the fractional yield or progress $\rho$ of the distillation (so $\rho = 0$ corresponds to no distillation and $\rho = 1$ corresponds to complete distillation). It also plots the same curve for the remaining liquid $L$.

    • As you can see, at $\rho = 0$, when only a few molecules of ethanol have distilled, the liquid (unsurprisingly) has an unchanged isotopic composition relative to the starting material, and the tiny amount of vapor that does come over has an isotopic composition that is enriched in $\ce{^{13}C}$. That's right, for pure ethanol at least, distillation is a process with an inverse isotope effect.
    • As $\rho \rightarrow 1$, the isotopic content of the distillate tends to 1.0, consistent with the idea that for quantitative distillation, isotopic composition doesn't change.
    • At 50% completion, the distillate has about 0.2% more $\ce{^{13}C}$ than the starting material, and the remaining liquid has about 0.2% less $\ce{^{13}C}$ than the starting material. So even though the composition does vary, the effect isn't very big.

Does the distillation of alcohol cause [...] carbon-12 enrichment?

No, contrary to what I would have guessed before I read the literature, distillation of alcohol can cause carbon-13 enrichment.

$\endgroup$
2
  • $\begingroup$ Great answer, and surprising indeed. Any thought on how this inverse isotope effect could come about? $\endgroup$
    – HannesH
    Commented Feb 16, 2020 at 17:43
  • $\begingroup$ User @Paul supplied a nice answer and comment pointing to this 1970 paper discussing the inverse isotope effect for 13C-methanol, where is also seen. $\endgroup$
    – Curt F.
    Commented Feb 16, 2020 at 21:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.