Capacitance of electrolytes with hydration-mediated interaction in planar electric double layers

ABSTRACT

The theoretical model, which is based on the density functional approach, has been developed for studying the electrostatic properties of electrolytes with the soft, hydration-mediated ion–ion and ion–surface interactions. The theory approximates the hydration-mediated ion–ion interaction through the mean-field approximation. The results show that the theory leads to a good agreement with the simulation results and predicts the camel-to-bell shape transition for the charged hard-core Yukawa ions with the hydration-mediated interaction. The high $C_{diff}$ is observed at a low bulk concentration of ions and the camel-to-bell shape transition occurs when the bulk concentration rises to an appropriate value. The increase of an ion size shifts the maximum differential capacitance ($C_{diff}$) to a low surface charge density. The addition of a repulsive hydration interaction reduces the $C_{diff}$, whereas the attractive hydration interaction enhances the $C_{diff}$. The increase of ion–surface and ion–ion interactions decreases the $C_{diff}$ and shifts the maximum $C_{diff}$ to a higher surface charge density. The increase of hydration anion–anion repulsion decreases the $C_{diff}$ of a positively charged electrode and shifts its maximum to more positively charged surfaces.

GRAPHICAL ABSTRACT

Keywords:

• Hydration-mediated potential
• differential capacitance
• density functional approach

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was supported by a grant from 2019 Research Funds of Andong National University and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (no. 2018R1D1A1B07042326).