  Electrostatic correlations between ions dissolved in water are known to
impact their transport properties in numerous ways, from conductivity to ion
selectivity. The effects of these correlations on the solvent itself remain,
however, much less clear. In particular, the addition of salt has been
consistently reported to affect the solution's viscosity -- but most modelling
attempts fail to reproduce experimental data even at moderate salt
concentration. Here, we use an approach based on stochastic density functional
theory, which accurately captures charge fluctuations and correlations. We
derive a simple analytical expression for the viscosity correction in
concentrated electrolytes, by directly linking it to the liquid's structure
factor. Our prediction compares quantitatively to experimental data at all
temperatures and all salt concentrations up to the saturation limit. This
universal link between microscopic structure and viscosity allows to shed light
on the nanoscale dynamics of water and ions in highly concentrated and
correlated conditions.
