Activation Energy of Ion Desorption at Ionic Liquid/Pt Electrode Interfaces: A Sum-Frequency Generation Vibrational Spectroscopic Study

Electrolyte/electrode interfaces of room-temperature ionic liquids (RTILs) exhibit hysteretic responses to different applied potentials owing to the differences in the ion adsorption/desorption processes; the ion desorption requires excess potential, which reflects the activation energy of ion desorption. Thus far, the contributions of the ion adsorption energy and the activation barrier for ion desorption toward the ion-dependent excess potential have not been quantified. Herein, we report on our infrared-visible sum-frequency generation vibrational spectroscopy study of the hysteretic responses of the anion adsorption/desorption at Pt electrode interfaces using neat, binary, and diluted RTILs composed of 1-butyl-3-methylimidazolium cations ([C4mim]+) and bis(trifluoromethanesulfonyl)amide ([TFSA]-) and trifluoromethanesulfonate ([OTf]-) anions. Experimental results are compared to the theoretical calculations for the electric double layer model. The hysteretic response of the RTIL/Pt interface derives predominantly from the activation energy of anion desorption, which causes the negative excess potential required for anion desorption. A comparison of the anion adsorption/desorption behaviors of neat RTILs with those of binary and diluted RTILs reveals that the large activation energy of anion desorption at the neat RTIL/Pt interface originates largely from the activation barrier for restructuring ionic layering in the diffuse layer.

Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study

Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([C_nmim]⁺; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]^-) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA]^- anions decreases as the alkyl chain length increases, whereas the SF signal from the r⁺ mode (the terminal CH₃ group) of the [C_nmim]⁺ cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA]^- anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r⁺ mode of [C_nmim]⁺ does not experience destructive interference because the peak position of the r⁺ mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.

Ordering and Nonideality of Air-Ionic Liquid Interfaces in Surface Second Harmonic Generation

The air-ionic liquid interface for a series of ionic liquids involving imidazolium cations [C_nmim] with different alkyl chain lengths (n = 2, 4, 6, 8, 10, and 12) and the same [NTf₂] imide anion has been studied by polarization-resolved second harmonic generation (SHG). An increase as a function of the alkyl chain length of the orientational parameter reveals the increasing ordering of the air-pure ionic liquid interfaces although it is not possible to disentangle the change in mean tilt angle from a change in the tilt angle probability distribution width. Besides, the study of the air-mixed ([C₁₂mim])_x([C₂mim])_1-x[NTf₂] ionic liquid interface clearly demonstrates the interfacial nonideality of the mixed ionic liquids. The long alkyl chain cation perturbs the interface as seen from the orientational parameter and displaces the short alkyl chain one for bulk mixture contents as low as 10%. At higher long alkyl chain cation bulk mixture contents, the interface behaves close to a pure long alkyl chain ionic liquid.

Tuning Water Networks via Ionic Liquid/Water Mixtures

Water in nanoconfinement is ubiquitous in biological systems and membrane materials, with altered properties that significantly influence the surrounding system. In this work, we show how ionic liquid (IL)/water mixtures can be tuned to create water environments that resemble nanoconfined systems. We utilize molecular dynamics simulations employing ab initio force fields to extensively characterize the water structure within five different IL/water mixtures: [BMIM + ][BF 4 - ], [BMIM + ][PF 6 - ], [BMIM + ][OTf - ], [BMIM + ][NO 3 - ]and [BMIM + ][TFSI - ] ILs at varying water fraction. We characterize water clustering, hydrogen bonding, water orientation, pairwise correlation functions and percolation networks as a function of water content and IL type. The nature of the water nanostructure is significantly tuned by changing the hydrophobicity of the IL and sensitively depends on water content. In hydrophobic ILs such as [BMIM + ][PF 6 - ], significant water clustering leads to dynamic formation of water pockets that can appear similar to those formed within reverse micelles. Furthermore, rotational relaxation times of water molecules in supersaturated hydrophobic IL/water mixtures indicate the close-connection with nanoconfined systems, as they are quantitatively similar to water relaxation in previously characterized lyotropic liquid crystals. We expect that this physical insight will lead to better design principles for incorporation of ILs into membrane materials to tune water nanostructure.

Effects of Water on Solvation Layers of Imidazolium-Type Room Temperature Ionic Liquids on Silica and Mica

Effects of the addition of water on solvation layers of imidazolium-type room temperature ionic liquids (RT-ILs) have been studied through force curve measurements of atomic force microscopy (AFM). Two kinds of RT-ILs were employed in this study; one is a hydrophilic RT-IL (1-butyl-3-methylimidazolium tetrafluoroborate, BmimBF4), and the other is a hydrophobic one (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EmimTFSI). These RT-ILs form solvation layers on hydrophilic solid substances (i.e., silica and mica) in the absence of added water. The addition of water into BmimBF4 resulted in the disruption of the solvation layers and then the formation of an interfacial water phase on silica. In contrast, the formation of the interfacial water phase was not evidenced on mica because of the absence of hydrogen-bonding sites on the mica surface. Interestingly, the addition of water into EmimTFSI induced the formation of the interfacial water phase on the two solid surfaces. In the EmimTFSI system, importantly, significantly greater adhesion forces were observed on silica than on mica. This reflects the different formation mechanisms of the interfacial water phase on the two solid surfaces. We conclude that the hydrogen bonding is a key factor in determining whether water molecules can be adsorbed on the solid surfaces, but it is also necessary to take into account the hydrophilic/hydrophobic nature of the RT-ILs.