Francesca Borghi1 Luca Bettini1 Paolo Milani1 Alessandro Podestà1

1, University of Milano, Milano, , Italy

Little is still known about the interfacial properties of very thin films of room-temperature ionic liquids (ILs) supported by flat solid surfaces; experimental investigations have been focused so far primarily to the air-(bulk)liquid, and (bulk)liquid-solid interfaces. Understanding the combined effects of surface interactions, presence of water in solution, long range electrostatic forces, and confinement on the structural rearrangement and on the interfacial properties of supported ILs films is a challenging task, which requires theoretical, computational, and experimental efforts. Advancements in this field would be beneficial for several applications where the liquid-solid interface plays a crucial role, such as in photo-electrochemical devices and in macro- and micro-mechanical systems, where ILs are used as lubricants. We present the results of experimental studies of the morphological and structural properties of nanometer-thick thin films of 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) deposited from highly diluted methanol solutions by drop-casting on flat and nanostructured silicon and carbonaceous surfaces. Atomic Force Microscopy (AFM) studies, including high-resolution imaging and nanomechanical tests, have been carried out on the thin IL layers. On smooth silicon surfaces, ordered lamellar nanostructures of mesoscopic area (1–100μm2) with a vertical structural periodicity have been observed at room temperature, while they are not observed on HOPG surfaces1,2. Nanomechanical investigations reveal that besides being vertically ordered, these structures also resist to normal compressive loads up to few hundreds of MPa. Beyond that limit, destructive penetration occurs in discrete steps. This observation suggest a solid-like character of the islands. The presence of nanostructured features on the flat solid surfaces, with the same chemical composition of the substrates, could in principle promote the solid-like terraces because of the increased area of interaction, or destroy them because of the introduction of a morphological disorder at the interface. We show how the presence of clusters of 6-20nm in diameters on the surface doesn’t prevent the formation of the IL solid-like layers, while they work as pinning points for the liquid droplets. Our findings highlight the potentialities of scanning probe techniques for the quantitative investigation of the interfacial properties of thin IL films. Moreover, the results of this study suggest that surface interactions and nanoscale confinement play an important role in the interfacial restructuring of the ILs, leading to modifications of the physico-chemical properties of the liquid that may have important consequences for the performance of those electrochemical and tribological devices employing these ionic compounds as electrolytes or lubricants.

1S. Bovio, et al. J. Phys. Chem. B 113, 6600 (2009).
2S. Bovio, et al. J. Phys. Condens. Matter 21, 424118 (2009).