Room-temperature ionic liquids (RTILs) are new materials with fundamental importance for energy storage and active lubrication. They are unusual liquids, which challenge the classical frameworks of electrolytes, whose behaviour at electrified interfaces remains elusive, with exotic responses relevant to their electrochemical activity. Using tuning-fork-based atomic force microscope nanorheological measurements, we explore here the properties of confined RTILs, unveiling a dramatic change of the RTIL towards a solid-like phase below a threshold thickness, pointing to capillary freezing in confinement. This threshold is related to the metallic nature of the confining materials, with more metallic surfaces facilitating freezing. This behaviour is interpreted in terms of the shift of the freezing transition, taking into account the influence of the electronic screening on RTIL wetting of the confining surfaces. Our findings provide fresh views on the properties of confined RTIL with implications for their properties inside nanoporous metallic structures, and suggests applications to tune nanoscale lubrication with phase-changing RTILs, by varying the nature and patterning of the substrate, and application of active polarization.
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L.B. and A.S. thank B. Rotenberg, B. Cross and E. Charlaix for many fruitful discussions. J.C., A.N. and A.S. acknowledge funding from the European Union’s H2020 Framework Programme/ERC Starting Grant agreement number 637748 - NanoSOFT. L.B. acknowledges support from the European Union’s FP7 Framework Programme/ERC Advanced Grant Micromegas. L.B. acknowledges funding from a PSL chair of excellence. The authors acknowledge funding from ANR project BlueEnergy.
The authors declare no competing financial interests.
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Comtet, J., Niguès, A., Kaiser, V. et al. Nanoscale capillary freezing of ionic liquids confined between metallic interfaces and the role of electronic screening. Nature Mater 16, 634–639 (2017). https://doi.org/10.1038/nmat4880
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