Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy

Abstract

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids (RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium ([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]), was investigated by the sum frequency generation vibrational spectroscopy (SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH 3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the (N)-CH 3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up (N)-CH 3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.ionic liquid, sum frequency generation vibrational spectroscopy, interface, orientation

Strong Propensity of Ionic Liquids in Their Aqueous Solutions for an Organic-Modified Metal Surface

“…We rely on the SAM, since we wish to measure the electrostatic environment with a well-calibrated probe and reasonable orientation information with respect to the surface. One may directly measure the IL vibrational signatures at the surface, which is possible, albeit with some degree of complexity, and have been reported previously. − However, in that case, two important factors are highly uncertain. The first is the orientation of the IL molecules near the surface.…”

Toward Unraveling the Puzzle of Sum Frequency Generation Spectra at Interface of Aqueous Methanol Solution: Effects of Concentration-Dependent Hyperpolarizability

“…The SFG-VS setup used in the current study was based on a Nd:YAG laser (PL2143, EKSPLA) system with a repetition rate of 10 Hz and a pulse width of 23 ps, the details of which have been previously described. ,− Briefly, a visible beam with its wavelength fixed at 532 nm and a tunable IR beam were arranged in a copropagating configuration and then were spatially and temporally overlapped on the sample surface. The resultant sum frequency signals with the frequency of ω SF = ω Vis + ω IR were collected in the reflection geometry by a monochromator (MS3501, SOL Instruments) and a photomultiplier (Hamamatsu, R585).…”

Liquid/Vapor Interface of Dimethyl Carbonate–Methanol Binary Mixtures Investigated by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulation

“…SFG-VS has been extensively employed to study the liquid/vapor interfaces of water, pure organic liquids, water–organic binary mixtures, water–ionic liquid mixtures, etc. ,− These efforts have revealed important physicochemical properties of the liquid surfaces, such as the molecular orientations within the interfacial regions and surface adsorption free energies, providing detailed experimental evidence that can be compared with the results from the MD simulations or other theoretical methods. ,− But as for the organic–organic binary mixtures, the existing investigations are mostly limited to few examples of the solid/liquid interfaces. − The explorations of the competitive adsorption and molecular structures at the liquid/vapor interfaces of multicomponent organic mixtures have still been scarce, both experimentally and theoretically.…”