Aqueous Ion Mobility over a Broad Concentration Range

For concentrations between dilute and the highly concentrated limit of almost 5 M, we compare our explicit-water molecular dynamics simulations of LiH_{2}PO_{4} (which dissociates into H_{2}PO_{4}^{-} anions relevant to biochemical processes and Li^{+} cations relevant to battery technology) to our pulsed-field gradient NMR measurements of ion diffusion, and find compensation between electrostatic and osmotic forces. The significance is that, noticing that the Kirkwood equation holds when using its exact solution but seemingly is violated when making the traditional approximation of using total force relaxation time in place of the memory relaxation time, we explain slower translational diffusion with increasing ion concentration as a dynamical effect arising from growing memory relaxation time. Physically, 2 orders of magnitude separate the timescales of electrostatic and osmotic forces from the total force such that dynamical correlations between force components lead to concentration-independent total force variance and force relaxation time.