Asymmetric Ions in Solution are Similar to Active Brownian Particles

Abstract: Molecular dynamics simulations of electrolyte ions with charge shifted by the distance $d$ from the ion geometrical center have shown that mechanical equilibrium in the ion's body frame is not established beyond a critical shift distance $d<sup>*$. A sharp crossover to a non-zero body-frame force occurs due to frustration of water molecules failing to compensate the electrostatic pull with a local density augmentation. A solution ion with asymmetric charge experiences a net body-frame force similar to self-propelled motion of active-matter Brownian particles. Thermal equilibrium and zero laboratory-frame force are still maintained, but the diffusion constant drops significantly ($\sim 400$ times) when the charge displacement crosses the threshold value $d<sup>*$. Alternative routes to the diffusion constant become inequivalent and the diffusion constant from the mean-squared displacement is much higher than those from velocity and force correlation functions.