Details of what actually happens in cold sulfuric acid between 80 and 90%; what molecular changes cause the viscosity to skyrocket?

Question

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?

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Answer 1

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.

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 octahydrate studied in reference 1. 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. Cryst. 45, 1198-1207. https://doi.org/10.1107/S0021889812037752. (also accessible in researchgate)

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