Ionic liquids are (ILs) akin to common salts such as sodium chloride (table salt), but unlike table salt which melts at approximately 800° C, ILs are liquid below 100° C with many of them being liquid at room temperature. This behavior is due to ILs being composed of bulky asymmetric anions and cations, which due to stearic effects prevent ILs from readily forming crystal structures. Because ILs are liquid at room temperature and contain no solvents, they have several unique properties including negligible vapor pressure, high thermal stability, and moderate conductivity. As such ILs have found use in a variety of applications such as energy storage, solvents in extractions and separations, and lubrication. Our work investigates effects of co-solvents on interfacial IL behaviors. We present in situ analysis of IL thin films, analyzed by electrochemistry and surface sensitive spectroscopic techniques such as infrared reflection absorption spectroscopy (IRRAS), and spectroscopic ellipsometry. We previously presented results showing IL films’ long ranged ordering in these thin film environments. This significantly changes interfacial molecular ordering, as evidenced by significant changes in the spectral absorption profiles. Here, we show data for IL systems in the presence of co-solvents and analyze changes in the dynamics of the interfacial region’s ordering process. Specifically, we study 1-alkyl-3-methylimidazolium trifluoromethanesulfonate ([CnMIM][OTf]) ILs with varying alkyl chain lengths and varying amounts of co-solvents (water, acetonitrile). Special attention is given to the effects of decreasing viscosity on the rates of diffusion and ordering of the films. We find that the length of the alkyl and the concentration of co-solvent in the IL film dramatically alter the electrochemical transport properties of the film, with longer alkyl chains exhibiting slower diffusion through the IL film. We also find that the rate at which the mixed IL-co-solvent film matures increases dramatically when compared to a neat IL film. We believe these results hold valuable information useful in the design of new classes of ILs to better fit their applications in energy storage, extraction, and lubrication.

Dilution effects on the behavior of thin ionic liquid films probed with electrochemistry and vibrational spectroscopy

2019 Fall National Meeting

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