Effect of Solid Substrates on the Molecular Structure of Ionic Liquid Nanofilms.

Fundamental understandings of the interfacial molecular structure of solid-confined ionic liquids (ILs) have significant impacts on the development of many cutting-edge applications. Among the extensive studies on the molecular structure at the IL/solid interface, direct observation of a double-layering quantized growth of [Cmim][FAP] on mica was recently reported. In the current work, the atomic force microscopy (AFM) results directly show that the growths of [Bmim][FAP] nanofilms on silica and amorphous carbon are different from the double-layering growth on mica. The growth of [Bmim][FAP] nanofilms on silica is dominated by the aggregation of the IL molecules, which can be attributed to the inadequate negative charging of the silica surface resulting in a weak electrostatic interaction between silica and the IL cation. [Bmim][FAP] on amorphous carbon shows a fairly smooth film for the thinner nanofilms, which can be attributed to the π-π parallel stacking between the cation imidazolium ring and the randomly distributed sp carbon on the amorphous carbon surface. Our findings highlight the effect of different IL/solid interactions, among the several competing interactions at the interface, on the resulting molecular arrangements of various IL.