Interfacial structure of protic and aprotic ionic liquid-DMSO-Li salt mixtures near charged and neutral electrodes: A Molecular Dynamics study

In this paper we report classical Molecular Dynamics simulations of the interfacial structure of two ternary mixtures, based on both a protic (ethylammonium nitrate, EAN) and an aprotic (1-ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4) ionic liquid. A molecular solvent (dimethylsulfoxide, DMSO) and a lithium salt with common anion are the rest of the components of the mixtures. The simulations were performed with implicit graphene walls, both neutral and charged with a surface charge density of ±1 e/nm2. Density, charge and electrostatic potential profiles as well as integral capacitances were calculated for all systems. For both liquids, the evolution of the density profiles throughout the DMSO concentration range are in accordance with previously characterized bulk properties of such systems. In the case of charged interfaces, the adsorption of Li+ cations into the negative electrode was found to be possible for the protic liquid, but unfavourable in the case of the aprotic one. Moreover, the probability distribution functions for the orientation of all molecular species near the interfaces were computed, and they indicate a tendency of the solvent molecules to form a dense layer at the interface separating the ionic liquids from the electrodes. The influence of hydrogen bonds in determining the dissimilitudes between protic and aprotic mixtures is highlighted.