Ammonium-based deep eutectic solvents for sustainable CO2 capture: insights from DFT, COSMO-RS, and MD simulations

Abstract

This study presents a comprehensive multiscale computational investigation into the effect of alkyl chain length on the CO₂ capture performance of ammonium-based deep eutectic solvents (DESs). Density functional theory (DFT), COSMO-RS calculations, and molecular dynamics (MD) simulations were employed to probe the molecular interactions as well as the structural and dynamical characteristics between CO₂ and three DESs containing lactic acid (LA) as the hydrogen bond donor. Interaction energy analysis and vibrational spectra revealed that, in all studied DESs, CO₂ preferentially interacts with the LA rather than with the anion or cation. COSMO-RS predictions confirmed that longer chains improve CO₂ solubility by increasing hydrophobicity and free volume. Furthermore, MD analysis showed that CO₂–LA interactions dominate, and longer chains reduce cation–CO₂ proximity due to steric effects. Structural and dynamic analyses, including RDF, SDF, Voronoi, and van Hove correlation functions, confirmed stronger CO₂ interactions, reduced ion mobility, and more extensive hydrogen bonding networks in longer-chain DESs. Spectral shifts further indicated physical absorption and increased cation involvement in CO₂ capture for longer chains. Finally, the findings demonstrate that extending the cation alkyl chain enhances overall CO₂ uptake through stronger dispersion forces, increased free volume, and more diverse hydrogen bonding. Increasing the alkyl chain length of the cation appears to reduce its interaction with CO₂, likely due to enhanced steric hindrance. However, as the alkyl chain length increases from DES(1) to DES(3), the overall CO₂ uptake improves. This enhancement is attributed to reduced cation–anion and cation–LA interactions caused by steric effects, which in turn increases the availability of the anion and LA to interact more effectively with CO₂.