Electric Double Layer Structures at the Water and Electrode Interfaces of a Fluorous Solvent Studied Using Molecular Dynamics Simulation

Electric double layers (EDLs) at a liquid|liquid interface between a fluorous solvent (F) and water (W) were examined using molecular dynamics (MD) simulation, where methyl nonafluorobutyl ether (HFE-7100), a hydrofluloroether, used as an F contained a hydrophobic ionic liquid (IL, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide) as a supporting electrolyte to polarize the F|W interface electrochemically. The F molecules were found to be accumulated on the noncharged F|W interface; however, on the charged interface, they were replaced with IL ions that formed a monolayer-like structure, nevertheless maintaining the F accumulation below the ionic monolayer. The MD simulation using a different hydrofluoroether (HFE-7300) revealed that the distributions of IL ions and F molecules in the EDL varied with their structures. Since the F–W two-phase system in our simulation box was sandwiched between the two graphene (G) electrodes to polarize the F|W interface, we were able to investigate the EDL at the electrode interface of F. Ionic multilayers were formed at the F|G interface, expelling the F molecules from the interface even though F was a dilute solution of IL. These accumulation and depletion behaviors at the water and electrode interfaces of F, combined with recent neutron reflectometry results, demonstrate that the EDL structures in F are strongly affected not only by the phase-boundary potential difference but also by the relative polarity of F molecules and IL ions. These findings can help design and control the EDL structures in F, contributing to the electrochemical application of F molecules at liquid/liquid and solid/liquid interfaces.