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?