Probing Liquid Interfaces with Room-Temperature Ionic Liquids Using the Excited-State Dynamics of a Cationic Dye

Abstract

Interfaces with room-temperature ionic liquids (ILs) play key roles in many applications of these solvents, but our understanding of their properties is still limited. We investigate how the addition of ILs in the aqueous subphase affects the adsorption of the cationic dye malachite green at the dodecane/water interface using stationary and time-resolved surface second harmonic generation. We find that the interfacial concentration of malachite green depends crucially on the nature of both anionic and cationic constituents. This concentration reports on the overall charge of the interface, which itself depends on the relative interfacial affinity of the ions. Our results reveal that the addition of ILs to the aqueous subphase has similar effects as the addition of conventional salts. However, the IL cations have a significantly higher propensity to adsorb than small inorganic cations. Furthermore, the ILs constituents show a synergistic effect, as the interfacial concentration of each of them also depends on the interfacial affinity of the other.

Structure and Surface Behavior of Rh Complexes in Ionic Liquids Studied Using Angle-Resolved X-ray Photoelectron Spectroscopy

“…Interfacial properties of IL-based materials have been intensively studied under ambient conditions by means of various spectroscopic, microscopic, and scattering techniques, such as sum frequency generation (SFG) [31][32][33][34], second harmonic generation (SHG) [35][36][37], atomic force microscopy (AFM) [38][39][40] and spectroscopy (AFS) [41], scanning tunnelling microscopy (STM) [39,40,42], and X-ray reflectivity (XRR) [43,44] tech-Catalysts 2023, 13, 871 2 of 18 niques, to name a few. In addition, owing to the negligible volatility of ILs, ultra-high vacuum (UHV)-based surface science methods have also been successfully applied to access interfacial phenomena, for instance through low-energy ion scattering (LEIS) [45,46], mass spectrometry (MS) [47,48], high resolution electron energy loss spectroscopy (HREELS) [49], ultraviolet photoelectron spectroscopy (UPS) [49][50][51], metastable induced electron spectroscopy (MIES) [49,50], reactive atom scattering (RAS) [52][53][54], and UHV-based STM and AFM techniques [51,[55][56][57][58][59].…”