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Three-dimensional electric double layer at the solvent-free liquid/liquid interface between two immiscible ionic
Kazuma Yamaguchi1, Kazuya Kobayashi1, Kosuke Ishii2
1Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan. nishi.naoya.7e@kyoto-u.ac.jp.
Physical Chemistry Chemical Physics : PCCP
|June 16, 2026
Summary
Molecular dynamics simulations reveal a unique three-dimensional electric double layer (EDL) at liquid/liquid interfaces. Smaller ions from hydrophilic ionic liquids (ILs) form overscreening layers within interfacial pockets, unlike conventional oil/water interfaces.
Area of Science:
- Physical Chemistry
- Materials Science
- Computational Chemistry
Background:
- The electric double layer (EDL) structure at liquid/liquid interfaces is crucial for electrochemical applications.
- Conventional oil/water interfaces exhibit a two-dimensional EDL, limited by ion hydration and interfacial roughness.
Purpose of the Study:
- To investigate the EDL structure at a solvent-free ionic liquid/ionic liquid (IL/IL) interface.
- To compare the EDL structure at IL/IL interfaces with conventional oil/water interfaces.
Main Methods:
- Molecular dynamics (MD) simulations were employed to model the interface between hydrophobic trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide (TOMAC4C4N) and hydrophilic ethylammonium nitrate (EAN).
- Analyses included charge density distribution and radial distribution functions to characterize ion behavior and interfacial structure.
Main Results:
- A three-dimensional EDL was observed at the IL/IL interface, characterized by smaller hydrophilic ions inserting into interfacial pockets formed by capillary waves of larger hydrophobic ions.
- Overscreening ionic layers formed due to the absence of solvent, enabling close interactions between counterions and larger ions.
- This unique EDL structure, driven by ion amphiphilicity and interfacial ion organization, was also observed at other liquid/liquid interfaces.
Conclusions:
- The EDL structure at IL/IL interfaces differs significantly from conventional interfaces, forming a three-dimensional structure due to specific ion interactions and the absence of solvent.
- The amphiphilicity of ions and the organization of larger ions at the interface are key factors determining the EDL structure.
- The phenomenon of "ionic liquid fingers" was observed when smaller ions transferred between phases, highlighting complex interfacial dynamics.
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