van Laar in his famous work "On the Critical Temperature and Pressure of Mercury and Phosphorus" (KNAW, Proceedings, 20 I, 1918, Amsterdam, 1918, pp. 138-148)
http://www.dwc.knaw.nl/DL/publications/PU00012205.pdf
has claimed that “the liquid Mercury is a fortiori bimolecular, the saturated mercury vapour on the other hand will be Hg1 only at lower temperatures, however, it in consequence of the decreasing volume, it will become Hg2 in ever increasing degree”
p.140
At the same time, the DIPPR database provides the Noble gas temperature-independent value for the ideal gas heat capacity of Mercury.
I would very appreciate additional information on this issue.
Dear Ilya,
I am not sure that Hg forms Hg2 in the sense that Hgn with n > 2 does not exist. If the vapour density is high enough, a nonmetal–metal transition occurs. Look up the work of Friedrich Hensel on this subject, e.g., R. Redmer, F. Hensel, and B. Holst (eds.), Metal-to-nonmetal transitions, Springer, Berlin 2010.
Dear Ulrich,
Thank you a lot. Although the book itself is unavailable for me, the metal – nonmetal issue seems to be important direction for search. Thank you again. And in fact they are things previously unknown for me, such as the Landau and Zeldovitch hypothesis and the possible existence of the second critical point whose thermodynamic parameters differ from those of the ordinary one.
http://arxiv.org/pdf/1302.3937.pdf
Finally, the question I intend to find an answer is what should be the theoretically correct number of segments (m) of Mercury while implementing models such as SAFT. Should it be just 1 or other variants are possible.
It seems to me that the existence of Hg clusters in vapor phase has been already confirmed experimentally. Possibly corresponding references are contained in the book by Hensel et. al. (this book has been already mentioned above by Ulrich Deiters). I can recommend the article Pahl E. et. al. // THE JOURNAL OF CHEMICAL PHYSICS 132, 114301 (2010).
Its url
http://scitation.aip.org/content/aip/journal/jcp/132/11/10.1063/1.3354976
In this article there is the calculation of potential curve for Hg2 and discussion of Hg-n.
In an older reference, on the other hand, Norrby [J.Chem Educ. 68 (2), 110-113 (1991)] states that mercury "is unique among metals by being almost entirely monomeric in the gas phase Hg(g)," a fact that he attributes to the comparative weakness of the Hg-Hg bond, and that also provides the explanation for Hg (unlike Au or Ag) being a liquid rather than a solid at normal conditions.
Dear Evgeniy and Claudio, thank you a lot, it is very important information!
In particular, the last sentance of the Lars J. Norrby's paper: "Mercury could in the vein of this analogy be called a pseudo noble gas." Perhaps it explains why DIPPR implements Cpo=5/2*R for this compound.
As seems, a fact that Mercury is a mixture of Hg1 + Hgx (x=2,...) is sometimes overlooked.
Dear Ilia, Mercury in the vapor phase possesses dimers, trimers and larger clusters. It is seen from its thermophysical properties, reflected in the Vargaftik's Handbook. I plan to describe the Hg clusters' properties in my next book "Structural Thermodynamics of Real Gases".
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