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

The structure of the electric double layer (EDL) at the liquid/liquid interface between hydrophobic and hydrophilic ionic liquids (ILs), trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide (TOMAC4C4N) and ethylammonium nitrate (EAN), was explored using molecular dynamics (MD) simulations. Analyses of the charge density distribution and radial distribution function at this solvent-free IL/IL interface revealed 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 counterions in the interfacial "pockets" interact closely with the larger ions. This contrasts with the case of the conventional oil/water interface, where ions in the water phase are hydrated and cannot be inserted into the "pockets", forming a two-dimensional EDL that is insensitive to the interfacial roughness. Simulations of various types of liquid/liquid interfaces revealed that this unique EDL structure is not specific to the IL/IL interface: the amphiphilicity of smaller ions and the interfacial structure of the larger ions play 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 ions, forming an "ionic liquid finger".