The
interactions between ionic liquids and their supports determine
many of their applications. The adsorption of the ionic liquid 1-ethyl-3-methylimidazolium
trifluoromethanesulfonate [C2C1Im][OTf] on Pd(111),
ordered Al2O3/NiAl(110), and Pd nanoparticles
supported on Al2O3/NiAl(110) was investigated
under ultrahigh vacuum (UHV) conditions using time-resolved infrared
reflection absorption spectroscopy (TR-IRAS). On Pd, the [OTf]− anion stands up with its CF3 group directed
toward the vacuum, whereas the anion is less clearly oriented on the
oxide. We also find that strong interactions of the IL with the Pd
result in migration of the IL from the oxide to the metal nanoparticles.
From a different angle: Thin films of functionalized ionic liquids are deposited on cerium oxides following a surface science approach. The functionalization of the alkyl chain changes its orientation with respect to the surface plane from normal to parallel. This then leads to a different surface chemistry at higher temperatures.
A total of 5-30 monolayer thick films of the ionic liquid (IL) [C2C1Im][OTf] were vaporized in vacuo onto an atomically clean Pd(111) single crystal surface at 220 K. Time- and temperature-resolved infrared reflection-absorption spectroscopy reveals growth, interactions with the metallic support, and the macroscopic phase behavior of the layer. At 220 K, the IL layer first grows in the form of a glassy phase. Crystallization of the IL was witnessed above a critical thickness of about 10 monolayers. On the basis of the known bulk crystal structure of the IL, we suggest the formation of well-oriented checkerboard-like crystalline film structures on the surface. The preferential orientation manifested by the crystal phase with regard to the macroscopic metallic surface is attributed to strong interactions between anionic headgroups and the metal.
The influence of confinement on the ionic liquid crystal (ILC) [C(18)C(1)Im][OTf] is studied using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The ILC studied is supported on Si-based powders and glasses with pore sizes ranging from 11 to 50 nm. The temperature of the solid-to-liquid-crystalline phase transition seems mostly unaffected by the confinement, whereas the temperature of the liquid-crystalline-to-liquid phase transition is depressed for smaller pore sizes. A contact layer with a thickness in the order of 2 nm is identified. The contact layer exhibits a phase transition at a temperature 30 K lower than the solid-to-liquid-crystalline phase transition observed for the neat ILC. For applications within the "supported ionic liquid phase (SILP)" concept, the experiments show that in pores of diameter 50 nm a pore filling of α>0.4 is sufficient to reproduce the phase transitions of the neat ILC.
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