Detailing molecular interactions of ionic liquids with charged SiO2 surfaces: A systematic AFM study

It is crucial to understand the behaviour and interfacial interactions as well as properties of ionic liquids (ILs) at electrode surfaces on the molecular level for developing IL-based electrochemical energy storage devices including supercapacitors and batteries. In this work, a colloid probe atomic force microscopy (CP-AFM) -based experimental approach is presented to determine the molecular interaction forces between ILs and differently charged SiO2 microspheres. The effects of structural variations in ILs and the nature surface charges of SiO2 on the molecular interaction force are systematically studied. The surface charges of SiO2 were achieved by grafting quaternary ammonium and –COOH, –NH2 groups. The determined molecular interaction force is found to be strongly dependent on the surface charge, in which, the force enhances at a more negatively charged surface. Furthermore, the ILs with longer alkyl chains on cations exhibit stronger molecular interaction forces with the charged SiO2. These reported experimental results on the molecular level provide new insights for model development and molecular simulations of ILs interacting with charged surfaces and guide the design of ILs-based supercapacitor and battery systems.