Fluctuation enhancement of ion diffusivity in liquids
The diffusivity of ions in liquid solutions is known either to decrease with an increase in the ion size or to have a single maximum depending on the ion size. This article presents evidence for the appearance of multiple maxima and thus multiple ion sizes with enhanced diffusivity. This is caused by destabilization of the ion solvation shell and only happens at the ion radii that correspond to changes of the ion coordination number, which happen with an increase in the ion size. Solvation shell fluctuations are activated which leads to additional ion motion with rearrangement of the shell. A theoretical model of this effect is derived and is shown to be in good agreement with molecular dynamics and experimental data. This result rewrites a long standing picture of ion transport as a function of ion size and introduces a new possibility in the search for highly conductive systems.