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Ionic Liquids Confined to Thin Films: Structure and Lubricity

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Room temperature ionic liquids (ILs) are salts in the liquid state under ambient conditions. Recently many stable and benign ILs have been developed, and these present new possibilities as electrolytes in applications such as solar cells, supercapacitors (double-layer capacitors), fuel cells and electro-actuators. The structure of ILs at interfaces is not yet well understood because the standard theories used to explain the nature of dilute electrolytes do not apply at such high ion concentrations. I will present recent measurements of the interaction force between atomically smooth solid surfaces across confined films of ILs which reveal structural and dynamic characteristics. We observe oscillatory ‘structural forces’ due to a layered arrangement of ions at the surfaces. By changing the nature of the cation, it is possible to alter the layer structure from alternate cation-anion ‘stripes’ to cation ‘bilayers’ in toe-to-toe orientation1. We have measured the viscosity and lubrication characteristics of these films as a function of their thickness, with sub-molecular resolution down to a single ion layer, demonstrating strikingly low friction coefficients between the surfaces and ‘quantized’ friction coefficients dependent on the number of ion layers in the film2, 3.

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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