Interfacial properties and nanostructures of ionic liquids




Chen, Zhichao (Ph. D. in chemical engineering)

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Virtually, every investigation and application of ionic liquids (ILs) involves gas-liquid, liquid-liquid and liquid-solid interactions. Therefore, understanding the behavior of ILs at those interfaces is critical. In this work, such things are investigated in both macroscopic and microscopic aspects by looking into the impact of chemical structures of ILs on their interfacial properties and nanostructures. On a macroscopic level, we studied the interfacial properties of protic and aprotic ILs with N-alkylimidazolium and 1-alkyl-3-methylimidazolium as cations and bis(trifluoromethylsulfonyl)imide, methanesulfonate, and trifluoromethanesulfonate as anions. The surface tension of these ILs is measured. The contact angle measurements are performed at 293.15 K on three solid substrates. Dispersive and non-dispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most interesting result is that the protic ILs have lower surface tension and smaller contact angles than the equivalent aprotic ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. On the other hand, from a microscopic view, we report the layered structures of four ILs on both unbiased and biased highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy (AFM). The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C₄F₉SO₃]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis-(trifluoromethylsulfonyl)imide ([MTBD][Tf₂N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C₄F₉SO₃]). On an unbiased surface, these ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]+ and [MTBD]+ have similar orientation at the surface but [MTBD]+ forms a thinner layer compared to [emim]+ on both HOPG and Pt(111). When the potential is applied to the solid surface, the very first ion layer near the solid surface has the opposite charge of the potential applied to the electrode; i.e., the first layer is a cation when a negative potential is applied and it is an anion layer when a positive potential is applied. We observe oscillating ion layer profiles up to 1 nm away from the solid surface. [emim]+ and [MTBD]+ cations are also found to have a preferably flat and parallel structure to the solid surface.


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