Concentration-Dependent Nanoscale Network Connectivity in Water-Lean CO ₂ Capture Solvent Clusters Revealed by Structural Studies

Water-lean solvents for point source carbon dioxide capture generally reveal an increase in polarity and viscosity with higher CO₂ loadings. Among these, <i>N</i>-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) exhibits notable CO₂ capture properties due to its formation of CO₂-dependent molecular tetramers. Wide-angle X-ray scattering (WAXS) was used to examine the evolution of nanoscale clusters at different CO₂ loadings ranging from 0 to 42 mol % CO₂ and from -17 to 90 °C. Computational and experimental data were used to estimate the solvent density, cluster size, and cluster-cluster correlations of various tetrameric species, enabling quantitative assessment of WAXS-derived structures. Particularly, the Teubner-Strey thermodynamical model was used to investigate the microphase separation of our system, while the molecular nearest-neighbor behavior was obtained through scattering- and molecular-dynamics-derived structure factor analysis. The experimentally derived correlation length (ξ) and periodicity (<i>d</i>) of the CO₂-bound clusters of 2-EEMPA grow as the concentration of CO₂ increases, reaching ξ = 5.0 Å and <i>d</i> = 11.6 Å, in agreement with simulation results. Our findings capture the detailed structural changes that occur with CO₂ variation and compare them with chemical process simulations and physical properties measurements. The structural insights derived from this study provide crucial input to the modeling and design of the CO₂ capture solvents.