Anomalous Diffusivity of Aqueous Polyatomic ions: A Mode-coupling Theory Study*

Unlike simple monatomic ions, biologically and industrially relevant complex polyatomic ions like nitrate, acetate etc. have not been studied in any great detail. Experiments have shown that some polyatomic ions with larger size show anomalously high diffusivity in water. We simulate nitrate and acetate ions in water to find that polyatomic ions exhibit jump rotations in water that is coupled to the jump dynamics of water. We show that the charge distribution decides the rate of rotational jump which is again coupled to their translational motion. A symmetric ion like nitrate undergoes faster rotational jump that enhances its diffusivity where an asymmetric ion like acetate with hindered rotational motion exhibits lower diffusivity.
We develop a mode-coupling theory (MCT) and find that the rotational jump motion of polyatomic ions offers an additional decay mechanism of the memory kernel and reduces the total friction on the ions while moving and enhances their diffusivity. The diffusivity predicted by MCT gives a satisfactory agreement with the experiment and simulation results. [P. Banerjee and B. Bagchi, J. Chem. Phys., 147, 124502 (2017); P. Banerjee, S. Yashonath, and B. Bagchi, J. Chem. Phys., 145, 234502 (2016); J. Chem. Phys., 146, 164502 (2017)]