Non-linear tricationic ionic liquid (NLTCIL) as a novel CO₂ absorbent: Investigating the effect of water with a dual MD/COSMO-RS approach

Abstract

This study leverages MD simulations and COSMO-RS analysis to thoroughly examine a nonlinear tricationic ionic liquid (NLTCIL) containing amine and hydroxyl functional groups, elucidating its intricate interactions with water and CO2. MD simulations reliably predicted physical properties, showing water decreases density while CO2 increases it. Microscopic structural analysis, through RDF, SDF, and CDF, revealed that while the strongest interactions in pure NLTCIL involve chloride anions and imidazolium ring hydrogens, water significantly disrupts these, forming extensive hydrogen bond networks and drastically reducing hydrogen bond lifetimes. CO2, in contrast, exhibited a minor structural impact, primarily occupying voids. 3D RDG distributions highlighted dominant van der Waals interactions around CO2, with water increasing weaker, non-specific interactions and reducing strong hydrogen bonds. TFI visualizations depicted stable CO2 interactions in pure NLTCIL (green/blue regions) but less stable, fluctuating ones (red regions) in water-containing systems, lowering CO2 absorption capacity. Sigma profiles and chemical potential overlaps revealed water's high polarity and strong hydrogen bonding with NLTCIL, outcompeting weaker CO2 interactions, explaining CO2's low solubility in water and humidity's negative impact on absorption. COSMO-RS analysis highlighted NLTCIL's potential for industrial gas separation due to significant CO2 solubility enhancement (up to tenfold at 10 bar) and high selectivity. However, humidity emerged as a critical challenge. The absorption mechanism was primarily linked to interactions at the N, O, and C sites of the cation, with the C-site potentially playing a more significant role. These findings provide crucial groundwork for designing humidity-resistant ionic liquids and optimizing CO2 absorption processes. © 2025 Elsevier B.V.