Three-dimensional electric double layer at solvent-free liquid/liquid interface between two immiscible ionic liquids

Abstract

The structure of the electric double layer (EDL) at a new type of solvent-free liquid/liquid interface between two immiscible ionic liquids (ILs), trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide (TOMAC4C4N) and ethylammonium nitrate (EAN), was explored using molecular dynamics (MD) simulation. The analyses of the charge density distribution and radial distribution function at the IL/IL interface found that the smaller ions constituting the hydrophilic EAN phase are inserted into the interfacial "pockets" formed by the capillary wave, whose minimum wavelength is determined by the sizes of the larger ions constituting the hydrophobic TOMAC4C4N phase. The larger ions at the interface were coordinated by the smaller ions across the interface, forming overscreening ionic layers because of the absence of solvents. This specific interfacial ion coordination led to a three-dimensional EDL, in which the smaller counter ions in the interfacial "pockets" interact closely with the larger ions. This contrasts with the case at the conventional oil/water interface where ions in water phase are hydrated and cannot insert into the "pockets", forming a two-dimensional EDL that is insensible to the interfacial roughness. Simulations for various types of liquid/liquid interfaces found that this unique EDL structure is not specific to the IL/IL interface, rather the amphiphilicity of smaller ions and the interfacial structure of the larger ions plays a key role. Moreover, when a smaller ion in the hydrophilic IL phase is transferred to the hydrophobic IL phase, the ion is often accompanied by other smaller 2 ions, forming a "ionic liquid finger".