Dynamics in Ionic Liquid Thin Films

The structure and dynamics at liquid interfaces are different from those in bulk liquids. In conventional solvents, this interfacial effect dies off rapidly: when a liquid molecule is further than ~3 nm from a surface, it behaves like in the bulk. But in ionic liquids, ions that are hundreds of nanometers from the surfaces can be affected. In order to measure how deep the interfacial effect can propagate in ionic liquids, we made flat ionic liquid thin films, and measured the dynamics in them.

Making Ionic Liquid Thin Films

The substrates that supports ionic liquid thin films are 3 mm thick CaF2 windows, with a ~ 100 nm thick silica layer coated on its top surface. The surface is functionalized, turning terminal OH groups into head groups, where enhance the affinity between the surface and ionic liquids. Then a methanol solution of the ionic liquid is spin coated . The thickness of the films is determined by the concentration of the IL in methanol solution.

By first

See details.

Dependence of Film Thickness and IL Structure

We found that. The thickness of the films are determined by the concentration of ionic liquid in methanol.

Dependence of Surface Charge Density (Controlling the Dynamics in IL Films)

We found that the larger, but even is neutral, the dynamics is still slower than the bulk.

Please read our papers for the details:

[1] Jun Nishida, John P. Breen, Boning Wu, and Michael D. Fayer, "Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid.", ACS Cent Sci, 2018, 4, 1065 - 1073, doi: 10.1021/acscentsci.8b00353.

[2] Boning Wu, John P. Breen, and Michael D. Fayer, "Structural Dynamics in Ionic Liquid Thin Films: The Effect of Cation Chain Length.", J. Phys. Chem. C., 2020, 124, 4179-4189, doi: 10.1021/acs.jpcc.9b11871.

[3] Boning Wu, John P. Breen, Xiangyu Xing, and Michael D. Fayer, "Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization.", J. Am. Chem. Soc. accepted, doi: 10.1021/jacs.0c03044 .