A complete in-situ analysis of UV–vis and 2D-FTIR spectra of the molecular interaction between RO16 (azo dye) and synthesized ammonium-based ionic liquids

•Hydrophobic ammonium-based ionic liquids.•RO16 azo dye.•Liquid-liquid extraction process in different pH media.•Complete in-situ analysis of UV–vis and 2D FTIR.•Molecular interaction mechanism between RO16 (azo dye) and TRTMA (IL). The liquid-liquid extraction process of the azo dye RO16 from an aqueous solution by ionic liquids (ILs) was investigated. For this purpose, 5 immiscible and halogen-free-ammonium-based ILs were designed and synthesized. The extraction process in different pH media (2–12) at changing temperatures (10, 20, 30 °C) was carried out. The complete in-situ analysis of UV–vis and 2D FTIR spectra was performed to elicit the molecular interaction mechanism between RO16 (azo dye) and [TOMA]+[Tar]− (IL). The results show that the molecular structure of the ILs affected the extraction efficiency and depended on the number of functional groups. The analysis suggests not only the formation of hydrogen bonds between the IL and dye in acid medium, but also the formation of intermolecular interactions such as ion-dipole and dipole-dipole in alkaline medium.