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

Abstract

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.