Effect of Anion Chain Length on the Reversible Absorption of CO₂ by Aliphatic Acid-Based Ionic Liquids and Transition from Micellar to Vesicular Aggregates

Ionic liquids (ILs), which are commonly referred to as ″green solvents”, are excellent sorbents that can absorb carbon dioxide (CO 2 ) reversibly. ILs are highly attractive solvents for CO 2 absorption due to their tunable chemical structures, low vapor pressure, and high thermal stability. Here, we have synthesized three different aliphatic acid-functionalized ionic liquids (AAILs), tetraethylammonium oleate ([N 2222 ][OL]), tetraethylammonium caprylate ([N 2222 ][Cap]), and tetraethylammonium acetate ([N 2222 ][OAc]), and employed the aqueous solutions of these AAILs to absorb CO 2 via vapor–liquid equilibrium (VLE) experiments. It has been observed that the CO 2 uptake value gradually increases as the chain length of the AAILs increases, which follows the order: [N 2222 ][OAc] < [N 2222 ][Cap] < [N 2222 ][OL]. The carbon capture process is analyzed using turbidity, viscosity, and nuclear magnetic resonance (NMR) spectroscopy studies. The NMR spectra of AAILs before and after CO 2 absorption reveal that [N 2222 ][OAc] and [N 2222 ][Cap] mainly absorb CO 2 via a physical absorption process, and [N 2222 ][OL] absorbs CO 2 through a chemical absorption process. Before and after CO 2 capture, the turbidity values of [N 2222 ][OAc] and [N 2222 ][Cap] in the aqueous medium are almost unchanged, whereas the [N 2222 ][OL] aqueous solution turns turbid after CO 2 capture, which corroborates the NMR data. The AAILs are highly reversible with regard to the absorption and desorption of CO 2, and we can reuse them up to several cycles. The transparent micellar solution of [N 2222 ][OL] transforms into a turbid vesicular solution after capturing CO 2 . From the steady-state and time-resolved fluorescence studies, we have seen that the emission intensity and the fluorescence lifetime of the hydrophobic dyes are enhanced after CO 2 capture by [N 2222 ][OL] because of the increase in the hydrophobicity of the vesicular aggregates compared to the micellar system.