A Microscopic Origin for the Breakdown of the Stokes-Einstein Relation in Ion Transport

Ion transport underlies the operation of biological ion channels and governs the performance of electrochemical energy-storage devices. A long-standing puzzle is that smaller alkali metal ions, such as Li⁺, migrate more slowly in water than larger ions, in apparent violation of the Stokes-Einstein relation. This anomaly is conventionally attributed to dielectric friction, viewed as a collective drag force arising from electrostatic interactions between a drifting ion and the surrounding solvent. Here, by combining nanopore transport measurements over electric fields spanning several orders of magnitude with molecular dynamics simulations, we show that the time-averaged electrostatic interaction between a drifting ion and its surrounding water molecules is not a drag force but a net driving force. By comparing charged ions with neutral reference counterparts, we reveal that ionic charge introduces additional Lorentzian peaks in the frequency-dependent friction coefficient. These peaks originate predominantly from short-range Lennard-Jones (LJ) interactions within the first hydration layer and constitute additional channels for energy dissipation, strongest for Li⁺ and progressively weaker for Na⁺ and K⁺. Our results demonstrate that electrostatic interactions primarily act to tighten the local hydration structure, thereby amplifying short-range LJ interactions rather than directly opposing ion motion. This microscopic mechanism provides a unified physical explanation for the breakdown of the Stokes-Einstein relation in aqueous ion transport.