Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study

In this work, we report the electric-field effects on ionic hydration of Cl^-, Na⁺, and Pb²⁺ using molecular dynamics simulations. It is found that the effect of weak fields on ionic hydration can be neglected. Strong fields greatly disturb the water orientation in the hydration shells of ions, though ion coordination number remains almost unchanged. Under strong fields, the first hydration shell of ions is significantly weakened and the ion-water interaction energy is dramatically reduced; surprisingly, the second hydration shells of Cl^- and Na⁺ are slightly structured because of the optimal water orientation; moreover, ionic hydration structures become asymmetrical along the field direction because of the uniformly aligned water dipoles. Compared with Na⁺ and Pb²⁺, the hydration of Cl^- is less disturbed by external fields, probably ascribed to the different water reorientation around anions and cations as well as the different structure-maker/breaker nature of the ions. Additionally, strong fields significantly enhance ion mobility and remarkably shorten the water residence time in the hydration shell. This work demonstrates that applying strong fields is an effective way to weaken ion hydration.