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Usually, metal-ions in terrestrial chemistry will be solvated - either by gas-phase solvent molecules or by liquid-phase solvent molecules. But the interstellar medium (ISM) is never in local thermodynamic equilibrium. Moreover, the solvation of any system is driven by the system's proclivity to attain thermodynamic stability. But given the lack of local thermodynamic equilibrium in the ISM, solvation should not at all be an important factor in the ISM. Is it right or wrong? Any reference from the Astrochemistry/Astrophysics community will also be highly appreciated.

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  • $\begingroup$ I'm not exactly seeing what you'd imagine as solvation in sth like this. $\endgroup$
    – Mithoron
    Jul 13 '17 at 11:53
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Per Wikipedia, the upper bound on the particle density in the interstellar medium is approximately $\pu{E6}$ molecules per cubic centimeter, in "cool, dense regions."

In the context of terrestrial aqueous solvation, where the concentration of water is approximately $\pu{55M}$, the particle density is on the order of $\pu{3.3E22}$.

So, even if every single particle in the vicinity of a metal ion in the ISM were a water molecule, the ISM would be roughly $\pu{E18}$-fold more dilute in neighboring water molecules as compared to aqueous solution.

No, metal ions in the interstellar medium will never be solvated.

Note that this does not imply that it is impossible for aquo-complexes of transition metal ions to occur in the ISM, given their significant affinity for ligands in the first coordination shell and for the appreciable organizing influence (thanks, MaxW!) such ions have on more distant coordination shells. It's just highly improbable that one would find a metal ion associated with a sufficiently statistically large number of water molecules to be considered solvated.


Further, to address the portion of the question about thermodynamic equilibrium: I don't think that's a particularly significant factor, no. On Earth, it's frequently the case that ions are solvated but not in thermodynamic equilibrium: the proton gradient that is sustained in the aqueous milieu of biological organisms to drive ATP synthase is one of the most significant examples that comes to mind. Thus, an absence of thermodynamic equilibrium does not necessarily entail a lack of solvation.

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  • $\begingroup$ It is even more improbable. Solvation isn't just about ligands. Because of the dipole moment of water there will be a number of "spheres" (layers)of oriented water molecules around the complex before the water molecules are oriented randomly. $\endgroup$
    – MaxW
    Jul 13 '17 at 3:36
  • $\begingroup$ your answer is enlightening. I fully get your point. Can you please tell if the qualitative thermodynamic argument I listed - ISM not being in thermodynamic equilibrium; can also be a suitable reason for the lack of solvation in the ISM? $\endgroup$
    – RaRa
    Jul 13 '17 at 5:37
  • $\begingroup$ Interstellar medium is in equilibrium all right (when they say it isn't, they mean an entirely different thing, mostly related to radiation). But this has nothing to do with solvatation. With densities that low, the equilibrium state would not be solvated either. $\endgroup$ Jul 13 '17 at 7:41

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