Microheterogeneities in Ionic-Liquid-Methanol Solutions Studied by FTIR Spectroscopy, DFT Calculations and Molecular Dynamics Simulations

The interest in ionic liquids (ILs) is steadily increasing because of their fascinating physicochemical properties and because of their broad range of applications in synthesis, separation, catalysis and electrochemistry. However, the multiplicity of their uses strongly depends on a molecular understanding of their exceptional properties. One key to a better understanding of their unique properties are spectroscopic studies of ionic liquids in conventional organic solvents in combination with DFT calculations and molecular dynamics simulations. Therefore we investigated the mixtures of the imidazolium-based ionic liquid [C2mim][NTf2] with methanol. Caused by the amphiphilic character of methanol both liquids are miscible over the whole mixture range. The scope of this work is to study the changes in the IL network upon dilution and to investigate the formation of methanol clusters embedded in the IL matrix. The mixtures were studied by FTIR spectroscopy in the mid-infrared region. The formation of methanol clusters was studied from the OD stretching vibrational bands between 2300 and 2800 cm-1. The cluster populations of methanol could be derived from molecular dynamics simulations for the same mixtures. Weighting the DFT calculated frequencies by the cluster populations we could reproduce the measured spectra in the OD stretching region up to XMeOH=0.5. Above XMeOH=0.8, strong formation of self-methanol clusters takes place resulting in increasing diffusion coefficients related to decreasing dynamical heterogeneities. Thus we obtained a deep understanding of the solutesolvent and solutesolute interactions as well as information about the presence of microheterogeneities in the mixtures.