In-situ surface-enhanced Raman scattering on imidazole-based ionic liquids at variable temperatures

Abstract

Ionic liquids (ILs) are unique solvents with a broad liquid temperature range. However, little has so far been done to investigate the ionic structures of ILs as a function of temperature. This can be performed by surface-enhanced Raman scattering (SERS) technique to effectively investigate the ionic structure and interface reaction of ILs at different temperature. Herein, in-situ SERS spectra of 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) and 1-carboxyethyl-3-methylimidazolium chloride ([HO2CEtMIm]Cl) ILs adsorbed on silver substrates at variable temperatures were recorded to study the adsorption behaviors and enhancement mechanisms. The results suggested interaction between [EMIm]Cl and silver substrate through N-atoms with the imidazole ring standing on the substrate. The enhancement mechanism of silver substrates toward [EMIm]Cl originated from the combination of electromagnetic enhancement (EM) and chemical enhancement (CM). At temperatures of 25–200 °C, no significant changes in the adsorption behavior and ionic structure of [EMIm]Cl on the silver substrate surface were observed. However, a further temperature rise resulted in changes in SERS spectral intensity of [EMIm]Cl owing to the aggregation of silver particles, as well as the increment in the amount of adsorbed molecules. Besides, [HO2CEtMIm]Cl interacted with the silver substrate through the carboxyl group, resulting in a molecular configuration that greatly influenced the adsorption behavior of [HO2CEtMIm]Cl. Compared to [EMIm]Cl, the imidazole ring of [HO2CEtMIm]Cl was located farther away from the substrate surface, leading to weaker SERS enhancement. Finally, the temperature variation did not affect the adsorption behavior and ionic structure of [HO2CEtMIm]Cl on the substrate surface. Overall, these results look promising for future studies of ionic structure and interface reaction of various ILs for advanced synthesis processes.