Dipolar solute rotation in ionic liquids, electrolyte solutions and common polar solvents: Emergence of universality

Here we develop a molecular theory attempting to answer the long-standing questions regarding the decoupling of dielectric friction from dipolar solute rotation in various liquid systems: ionic liquids, electrolyte solutions and polar solvents. Using both experimental and model solvent structure factors, we show insignificant dielectric friction and provide microscopic explanation for the experimentally observed domination of hydrodynamics in solute rotation in these media. Our analyses lead to a macro–micro relation indicating a quasi-universality in solute rotation for a wide variety of solute–solvent combinations. We demonstrate that both the quasi-universality and the domination of hydrodynamics originate from packing at liquid-like density.