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

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.