Im am geochemist, dealing with isotopes such as 35Cl and 37Cl.

For some model I'm trying to make I need to be able to estimate what is the difference of partial molar volume between dissolved 35Cl and 37Cl in water (as chloride Cl-).

Huge literature exists concerning the partial molar volume of Cl- and other ions, but I can't find anything on specific studies on variations of volume on different isotopes. As in isotopes geochemistry we're often looking a permil variations of ratios, any tiny difference in molar volume of 35Cl and 37Cl would have an importance for me.

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    $\begingroup$ I suspect the difference will be quite small, likely immeasurably so, and that there are many other larger uncertainties to take into account before this effect. $\endgroup$ – Nicolau Saker Neto Apr 26 '16 at 22:34
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    $\begingroup$ While there are some large chemical differences between deuterium and protium, these are mainly related to vibrational mode differences given the (relatively large) mass differences. By the time one gets to chlorine those are pretty much gone away. I see no reason to think that there would be a large difference in molar volume between the two isotopes. $\endgroup$ – Jon Custer Apr 26 '16 at 23:03
  • $\begingroup$ Again, I'm measuring some 0.1 permil variations of chlorine isotopes ratios. So any percent/ permil variation of the molar volume would be of importance $\endgroup$ – Tom Giun Apr 26 '16 at 23:57
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    $\begingroup$ As I said, I expect it to be immeasurable, and attempting to find the difference may lead to cases of false precision. I would be surprised if the difference was higher than ~1 ppm (absolute guess here, though). The best case to investigate would be with either hydrogen or lithium cations, as both are light and thus differences between isotopes are enhanced. Unfortunately there are probably many complicating factors involved with $\ce{H^+}$, so looking into a difference between $\ce{^{6}Li^+}$ and $\ce{^{7}Li^+}$ may be your best bet. $\endgroup$ – Nicolau Saker Neto Apr 27 '16 at 10:23

I don't have an exact answer for chloride specifically, but consider the theoretical equations in the following two articles:

Isotope Effects on Molar Volume and Surface Tension: Simple Theoretical Model and Experimental Data for Hydrocarbons J. Chem. Phys. 44, 457 (1966)

We assume the molecules are held together in a condensed phase by van der Waals "bonds" or covalent bonds represented by 6-12 or Morse potential-energy functions, V(x). For monatomic molecules x is taken to represent the distance between molecular centers.


Lattice Constants of Separated Lithium Isotopes J. Chem. Phys. 27, 1030 (1957)

which finds atoms of Lithium-6 $4 \times 10^{-4}$ times more separated than those of Lithium-7 and also cited to a reference which considers lithium-6 versus lithium-7 fluoride-19.


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