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 (CO₂) reversibly. ILs are highly attractive solvents for CO₂ 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 ([N₂₂₂₂][OL]), tetraethylammonium caprylate ([N₂₂₂₂][Cap]), and tetraethylammonium acetate ([N₂₂₂₂][OAc]), and employed the aqueous solutions of these AAILs to absorb CO₂ via vapor-liquid equilibrium (VLE) experiments. It has been observed that the CO₂ uptake value gradually increases as the chain length of the AAILs increases, which follows the order: [N₂₂₂₂][OAc] < [N₂₂₂₂][Cap] < [N₂₂₂₂][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 CO₂ absorption reveal that [N₂₂₂₂][OAc] and [N₂₂₂₂][Cap] mainly absorb CO₂ via a physical absorption process, and [N₂₂₂₂][OL] absorbs CO₂ through a chemical absorption process. Before and after CO₂ capture, the turbidity values of [N₂₂₂₂][OAc] and [N₂₂₂₂][Cap] in the aqueous medium are almost unchanged, whereas the [N₂₂₂₂][OL] aqueous solution turns turbid after CO₂ capture, which corroborates the NMR data. The AAILs are highly reversible with regard to the absorption and desorption of CO₂, and we can reuse them up to several cycles. The transparent micellar solution of [N₂₂₂₂][OL] transforms into a turbid vesicular solution after capturing CO₂. 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 CO₂ capture by [N₂₂₂₂][OL] because of the increase in the hydrophobicity of the vesicular aggregates compared to the micellar system.