Cosolvent-tuned interactions in ionic liquids: A vibrational and quantum-chemical study of ethylene glycol ratio effects

• FT-IR analysis reveals a systematic weakening and reorganization of the intrinsic anion-cation H-bonds, ionic network disruption and reduced viscosity with increasing EG concentration, showing a correlation with the EG molar ratio. • Theoretical analyses (optimized geometries, interaction energies, QTAIM, RDG-NCI) support experimental findings, indicating that stronger EG–anion and EG–cation H-bonds progressively alter and weaken the intrinsic anion-cation interactions. • MESP surface results reveal enhanced polarizability and the creation of favorable CO₂ binding sites as EG concentration increases. • An IL–EG molar ratio of 1:4 is identified as optimal for enhanced CO₂ absorption, whereas higher ratios lead to EG self-association, the formation of molecular domains which could reduce the absorption. • Combined experimental and theoretical analysis indicate that optimizing IL–EG ratio is crucial for balancing viscosity, CO₂ absorption, and regeneration performance in DAC applications. Ionic liquids (ILs) are attractive media for CO 2 capture but remain limited by viscosity and cost. Blending ILs with ethylene glycol (EG) is a practical route to mitigate these constraints, yet the molecular origins of cosolvent effects and their dependence on composition are not well resolved. We combine Fourier-transform infrared (FT-IR) spectroscopy with quantum-chemical (DFT) analysis to elucidate how the IL:EG molar ratio modulates intermolecular interactions and electronic structure. Computed vibrational frequencies enable mode assignment and deconvolution of overlapping bands, revealing systematic, ratio-dependent shifts and broadenings in (i) EG O–H stretching, (ii) cation and EG C–H stretchings (imidazolium C2–H, C4–H, C5–H, methyl and ethyl groups, -CH2 of EG), (iii) anion signature modes (e.g., CN motifs), and (iv) EG C–O and C–C stretchings, consistent with the redistribution of hydrogen-bonding networks. Molecular electrostatic potential (MESP) maps quantify attenuation of extreme potential regions with increasing EG, indicating progressive screening of cation–anion electrostatics. Quantum Theory of Atoms in Molecules (QTAIM) identifies emergent bond critical points between EG and the IL ions, while Reduced Density Gradient–Noncovalent Interaction (RDG–NCI) analysis differentiates strong directional hydrogen bonds from dispersive contacts across compositions. Together, these results show that EG fraction controls a switch from predominantly ion–ion to mixed ion–EG coordination, altering local polarity and polarizability that underlie the observed FT-IR trends. The framework provides composition–structure–spectrum relationships that can guide rational selection of IL–EG ratios to balance favorable molecular interactions with practical performance targets in scalable CO 2 capture systems.