Interfacial structuring of non-halogenated imidazolium ionic liquids at charged surfaces: effect of alkyl chain length

Abstract

Control of the interfacial structures of ionic liquids (ILs) at charged interfaces is important to many of their applications, including in energy storage solutions, sensors and advanced lubrication technologies utilising electric fields. In the case of the latter, there is an increasing demand for the study of nonhalogenated ILs, as many fluorinated anions have been found to produce corrosive and toxic halides under tribological conditions. Here, the interfacial structuring of a series of four imidazolium ILs ([C n C 1 Im]) of varying alkyl chain lengths (n = 5, 6, 7, 10), with a non-halogenated borate-based anion ([BOB]), have been studied at charged interfaces using sum frequency generation (SFG) spectroscopy and neutron reflectivity (NR). For all alkyl chain lengths, the SFG spectra show that the cation imidazolium ring responds to the surface charge by modifying its orientation with respect to the surface normal. In addition, the combination of SFG spectra with electrochemical NR measurements reveals that the longest alkyl chain length (n = 10) forms a bilayer structure at all charged interfaces, independent of the ring orientation. These results demonstrate the tunability of IL interfacial layers through the use of surface charge, as well as effect of the cation alkyl chain length, and provide valuable insight into the charge compensation mechanisms of ILs.

Electroresponsive structuring and friction of a non-halogenated ionic liquid in a polar solvent: effect of concentration

“…24 By matching the scattering length density (SLD) of the bulk IL/solvent mixtures to the gold, the sensitivity of the reflected intensity to the potential-driven changes in the interfacial layer was dramatically improved compared to that afforded for pure IL systems. 18,25 For different potentials, systematic changes in the reflectivity fringes were observed. By invoking a single layer model, systematic changes in the relative populations of the IL ions in the boundary layer with applied potential were revealed.…”

“…In particular, ILs containing phosphorus-and boron-based ions have been identified as promising candidates for lubricant additives and have been shown to exhibit excellent electroresponsive and lubrication properties, both as neat lubricants and when dispersed at low concentrations in a base oil or greases. [11][12][13][14][15][16][17][18] Yet, a mechanistic understanding of how the interfacial structures and composition of IL boundary layers in a base fluid can be modified and controlled by an electric field (i.e., realising tribotronics), and importantly, how such behaviours may be affected by the bulk IL concentration, is still lacking.…”

Chiral Biobased Ionic Liquids with Cations or Anions including Bile Acid Building Blocks as Chiral Selectors in Voltammetry

“…[3][4][5][6][7] Very recent studies point to CILs, or even ILs with chiral additives, being of huge interest as chiral media for enantioselective electrochemistry and electroanalysis, too. [8][9][10][11] Actually, ILs are known to result in much higher and longer-range order at the electrode j solution interphase respect to the double layer structure of "classical" solvent + supporting electrolyte systems, [12][13][14][15][16][17][18][19][20][21][22][23][24] a peculiarity still holding in the presence of significant water amounts; [18] this feature can be regarded as an attractive tool for achieving high control in electron transfer processes, for both analytical and preparative applications. In particular, in the case of CILs such high order can promote effective transmission of the chiral information.…”

Observation of the Pockels Effect in Ionic Liquids and Insights into the Length Scale of Potential-Induced Ordering

“…32 This method, however, does not offer the spatial resolution necessary to study potential-induced ordering near IL−electrode interfaces. Sum-frequency generation 33,34 and surface-enhanced IR absorption 35 can look at the molecular orientation and structure at interfaces, but they are inherently surface-selective techniques and hence are unable to provide information on regions tens to hundreds of nanometers away from the surface.…”