Ions at electrochemical interfaces: from explicit to implicit molecular solvent descriptions

Abstract: We investigate the interplay between electronic screening inside a metal and screening by a polar molecular solvent, focusing on their impact on the charge induced by an ion and the solvent structure at the interface. To that end, we consider atomistically resolved electrodes within the Thomas-Fermi model of screening and describe the molecular solvent either explicitly via classical molecular dynamics or implicitly using Molecular Density Functional Theory (MDFT). Specifically, we examine the effect of screening by tuning the Thomas-Fermi screening length $l_\text{TF}$, the ion charge by considering Na$^+$ and Cl$^-$ and the solvent nature by studying water and acetonitrile. Consistently with our previous findings without solvent, $l_\text{TF}$ significantly affects the charge distribution inside the metal. However, $l_\text{TF}$ has no significant impact on the interfacial solvent structure, suggesting that its effect on the charge distribution induced inside the metal by the ion is essentially due to how the metal responds to the (same) external charge distribution, including the solvent, even though the coupling between both sides of the interface may play a secondary role. Furthermore, MDFT accurately reproduces fine details of the interfacial solvent structure around the ion at a fraction of the computational cost of MD simulations. These results highlight the relevance of MDFT as a powerful tool to model electrochemical systems at the molecular level.