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AChem's answer to Why does the graph of the electrical conductivity of sulfuric acid/water solutions have this knee in the ~85%-~92% range? includes this plot from Horace E. Darling in "Conductivity of sulfuric acid solutions" (Journal of Chemical & Engineering Data 9.3 (1964): 421-426.) and mentions:

There is a sharp increase in viscosity at 85%, which indicates there is a major structural change in sulfuric acid solution in the range 85-92%. Sulfuric acid forms a hydrate in this range. When the viscosity is high, the conductance goes down, there is a depression in the curve. This viscosity jump is causing the double hump. Once we are past the high viscosity range, conductance goes up again.

It is amazing how simple molecules do not stop from surprising us!

Das et al. (1997) Electrical Conductance and Viscosity of Concentrated H2SO4/H2O Binary Systems at Low Temperatures: Correlation with Phase Transitions (J. Phys. Chem. B 1997, 101, 4166-4170) do a thorough analysis and mention phase transitions and hydrate formation, but 25 years later are there more detailed molecular models of what is happening that might address the exact form of the hydrate(s) formed or any long-range correlations or ordering behind the peak in viscosity at a certain high concentration?

Question: What details are known or theorized as to what actually happens in cold sulfuric acid between 80 and 90%, what molecular changes cause the viscosity to skyrocket?


from Horace E. Darling in "Conductivity of sulfuric acid solutions" (Journal of Chemical & Engineering Data 9.3 (1964): 421-426.)

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The composition where we see this rapid rise in viscosity corresponds to one of several sulfuric acid hydrates, specifically the monohydrate formally rendered as $\ce{H2SO4•H2O}$. As shown in this colorful diagram from Maynard-Casely et al.1, this monohydrate corresponds to a maximum in the melting curve, indicating that in the cold acid this composition is organizing itself into a solid structure.

enter image description here

Figure 1. The binary phase diagram of sulfuric acid and water (after Beyer et al., 2003). This shows the region of solid sulfuric acid hydrates (red) and the regions where liquid sulfuric acid and solid water ice coexist (blue). The labelled vertical lines indicate the stoichiometric compositions for the observed hydrates in this system; sulfuric acid monohydrate (SAM) 84.5 wt%, sulfuric acid dihydrate (SAD) 73.1 wt%, sulfuric acid trihydrate (SATri) 64.5 wt%, sulfuric acid tetrahydrate (SAT) 57.6 wt%, sulfuric acid hemitriskaidecahydrate (SAH) 45.6 wt% and sulfuric acid octahydrate (SAO) 40.5 wt%.

Sulfuric acid is, of course a strong acid towards water with respect to its first deprotonation, forming $\ce{H3O^+}$ and $\ce{HSO4^-}$ ions (the second deprotonation, forming $\ce{SO4^{2-}}$ ions, predominates only in very dilute water solutions). As we see with some other strong acids such as perchloric acid, these ions can form a salt, whose composition matches the monohydrate ($\ce{H2SO4•H2O}=\ce{(H3O^+)(HSO4^-)}$). Taesler and Olovsson2 find this ionic structure from single-crystal XRD, with the additional feature that the bisulfate anions are polymerized by hydrogen bonding. Hydrates formed with more water are also known, including the octahydrare studied in the reference. But (with the additional water in their formulations) these involve either a more bulky, irregularly shaped cation than the monohydrate or have water molecules in their structures, both of which would favor a less stable and lower-melting solid than the monohydrate salt would be.

Reference

1. H. E. Maynard-Casely, H. E. A. Brand and K. S. Wallwork (2012). "Structure and thermal expansion of sulfuric acid octahydrate". J. Appl. Cryst45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

2. I. Taesler and I. Olavsson (1968). "Hydrogen bond stidies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Cryst. B24, 299-304. https://doi.org/10.1107/S056774086800227X.

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    $\begingroup$ Thank fo for such a thorough and definitive answer! Are the conclusions based on X-ray diffractions of solid hydrate crystals and simply assumed to apply to the liquid phase, or are there some measurements of the liquid that confirm this. I don't doubt the conclusion, but like the block quote in my question mentions: "It is amazing how simple molecules do not stop from surprising us!" $\endgroup$
    – uhoh
    May 21 at 22:12
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    $\begingroup$ The monohydrate has the expected ionic structure according to singke-crystal XRD,l, and there us also evidrnce that the anions are polymerized by hydrogen bonding. See Taesler and Olavsson (1968), _Acta Cryst. B24, 299-304, doi.org/10.1107/S056774086800227X. $\endgroup$ May 21 at 22:30
  • $\begingroup$ I'm curious about work that examines aqueous solutions in the liquid form. I understand that crystallographic studies of pure solid forms of various hydrate structures are incredibly helpful, but as a liquid there's no guarantee that what is present is not a mixture of several different ones in equilibrium. It's possible that there's no data on this but there might be, and certainly it's something that can be addressed theoretically. $\endgroup$
    – uhoh
    May 21 at 23:40
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    $\begingroup$ Liquids with high ion concentrations tend to have much higher viscosities than the pure solvent as they become saturated (which is what is basically happening here with the hydronium sulfate salt). We see this more commonly with sodium hydroxide solutions (protank.com/sodium-hydroxide for this effect with sodium hydroxide solutions); everything on either the ice or the solid sodium hydroxide saturation line is 20 cp or more versus 1 cp for water alone. $\endgroup$ May 22 at 0:17

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