Relationships between Molecular Structure, Interfacial Structure, and Dynamics of Ionic Liquids near Neutral and Charged Surfaces

Room-temperature ionic liquids are being researched for a variety of applications ranging from electrochemistry, energy storage, to lubrication. In many applications with ionic liquids, key processes occur near surfaces but the impact of the complex interfacial structure of ionic liquids on its interfacial dynamics is unclear. This work uses molecular dynamics simulations to show (1) how the molecular structures of ionic liquids can affect its interfacial structure and (2) the impact of these interfacial structures on the dynamics of ionic liquids near neutral and charged surfaces. When the cation and anion molecules are small and similar in size (e.g., 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), laterally ordered structures form near neutral surfaces and the molecules have much lower diffusivities than that in the bulk. When the surfaces become charged, an order-to-disorder transition in the lateral structure increases the diffusivities. The presence of a long nonpolar tail (e.g., 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) causes the ionic liquid to be disordered near the neutral surfaces, and the molecules near the surfaces have diffusivities only slightly lower than that in the bulk. When the surfaces become charged, there is little change in the diffusivities for the systems with long nonpolar tails.