(720f) In Situ Nanofrictional and Microstructrual Studies of Ionic Liquids at Neutral and Charged Surfaces Using AFM Colloid Probes

In situ changes of nanofriction and microstructures for ionic liquids (ILs, [BMIM][BF₄], [BMIM][PF₆]) at neutral and charged surfaces were investigated using colloid probe atomic force microscopy (AFM). The torsional resonance frequency was captured simultaneously when measuring the nanoscale friction force with AFM at a given normal load, which is taken as a measure for the contact stiffness, reflecting the in situ changes of IL microstructures. A higher nanoscale friction force was measured on neutral surfaces upon increasing the applied load, with simultaneously detected higher torsional resonance frequency, revealing a higher contact stiffness, indicative of a more ordered IL layer. The nanofriction of ILs at charged surfaces increases as the positive-bias voltage (0 V→8 V) increases, which shows a similar variation tendency under the negative-bias voltages (0 V→-8 V). Similar behavior was observed in the changes of the friction force with the surface bias at varying applied loads. The simultaneously recorded torsional resonance frequency in the IL increases as the positive- or negative-bias voltage increases, implying a higher contact stiffness. The stiffer ion layers at a higher positive- or negative-bias voltage correspond to more ordered ILs at interfaces. The molecular dynamics simulation reveals that the [BMIM]⁺ imidazolium ring lies preferentially flat at neutral surfaces, which favors the compact, ordered ion layers formation. In comparison, the parallelly “sleeping” structure II is more pronounced with the surface charging of either sign (8V, -8V). This implies stronger ion ordering at charged surfaces, which agrees well with the AFM-detected stiffer ion layers at surfaces with either positive- or negative-bias voltages. The in situ observations offer implications in IL-based applications in lubricants and batteries, as surface charges would influence the friction and microstructures close to the surface.