Effect of the Alkyl Chain Length of the Cation on the Interactions between Water and Ammonium-Based Ionic Liquids: Experimental and COSMO-RS Studies

To improve the understanding of the molecular interactions of water with tetraalkyl ammonium-based ionic liquids (ILs) such as tetra­methyl­ammonium hydroxide, tetra­ethyl­ammonium hydroxide, tetra­propyl­ammonium hydroxide, and tetra­butyl­ammonium hydroxide, thermophysical properties such as density (ρ), speed of sound (<i>u</i>), viscosity (η) and refractive index (<i>n</i>_D) were measured and a computational study using COSMO-RS was performed. The derived properties such as excess volumes (<i>V</i>^E), deviation in isentropic compressibilities (Δκ_<i>s</i>), deviation in viscosities (Δη), and deviation in refractive indices (Δ<i>n</i>_D) under the same experimental conditions for these systems were also estimated. The observed <i>V</i>^E and Δκ_s values are negative over the entire composition of ILs at all investigated temperatures, whereas Δη and Δ<i>n</i>_D values are positive under the same experimental conditions. These results reveal that the ammonium-based ILs significantly affect the intermolecular interactions between the solvent molecules. The computational study allows a qualitative analysis of the results in terms of the ion–dipole interactions, ion-pair formation, and hydrogen bonding between ammonium-based ILs and water