Deposition and wettability of [bmim][triflate] on self-assembled monolayers

The interfacial behavior of ionic liquids (ILs) affects their use in many applications including as solvents, electrolytes, and lubricants. Here, we report the wettability and deposition of 1-n-butyl-3-methylimidazolium triflate ([bmim][triflate]) on model surfaces generated by the assembly of thiol- and silane-based monolayers on gold and silicon, respectively. Advancing contact angles of [bmim][triflate] on -CF₃, -CH₃, -CH₂, -OH, and mixed -CH₃/-OH surfaces are found to be intermediate between those of water and hexadecane on the same surfaces and dissimilar to those of dicyclohexyl, which has a similar surface tension as [bmim][triflate] but only dispersive intermolecular interactions. Molecular simulations provide qualitative agreement with experimental contact angles and show that the hydroxyl surface disrupts electrostatic interactions between the ions of the [bmim][triflate] droplet, resulting in lower local surface tensions and increased area of contact. IL films as thin as ∼100 nm were prepared by casting [bmim][triflate] from ethanol at reduced pressures onto mixed -CH₃/-OH surfaces. Coherent films of [bmim][triflate] were achieved only beyond a critical surface energy corresponding to advancing [bmim][triflate] contact angles of ≤25° and advancing water contact angles of ≤70°. Comparison of infrared vibrational spectra of the IL as a thin film, versus those for thicker films and in the bulk, suggests an interfacially driven structuring of the ions in the thinnest films and is supported by simulations, which show that the CF₃ groups of the triflates orient away from the IL phase at the IL/ vacuum interface.