Probing Interactions between Confined Ionic Liquids and Mesoporous Silica Using X-ray Photoelectron Spectroscopy

The interface of nanoconfined ionic liquid (IL) in mesoporous silica thin films was investigated by using X-ray photoelectron spectroscopy (XPS). The local environment of the ionic liquid 1-butyl-3-methylimidazolium chloride was examined as a function of the pore size of the silica substrate (3.5 and 8.5 nm diameter pores) and pore functionalization. Mesoporous silica thin films with accessible, vertically aligned pores were synthesized by dip-coating, and the thin film structure was characterized by grazing-incidence small-angle X-ray scattering. The IL was physically loaded into bare silica films or films covalently tethered with an IL-like functional group 3-methyl-1-[3-(trimethoxysilyl)propyl]-1<i>H</i>-imidazol-3-ium chloride, which contributed a positive surface charge to the otherwise negatively charged silica surface. XPS survey spectra of the IL-loaded film were coupled with selective etching to establish through compositional analysis that silica pores were loaded with IL, and the environment being probed contained both silica and bulk IL. The most notable features of the high-resolution spectra of IL in confinement were the introduction of a unique shielded nitrogen peak due to strong binding of the imidazolium ring with the pore wall and the pore-size dependent speciation of chlorine due to interactions with the silica pore wall or liberated protons from the surface. While bulk IL environments were more prevalent in the 8.5 nm pore silica films, the smaller 3.5 nm pore silica films and IL-functionalized 8.5 nm pore silica films exhibited surprisingly similar chloride speciation. The use of XPS to probe interfacial interactions between low-vapor-pressure solvents and nanoporous supports advances the design of confined ILs for catalysis and separations from knowledge of both the cation and anion electronic environments.