Concentration-Dependent Nanoscale Network Connectivity in Water-Lean CO2 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, N-(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 (d) of the CO2-bound clusters of 2-EEMPA grow as the concentration of CO2 increases, reaching ξ = 5.0 Å and d = 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.