Water lean solvents for point source capture of carbon dioxide generally exhibit a significant increase in their polarity and viscosity as the amount of chemically-bound CO₂ increases. Among these, N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2EEMPA), exhibits exceptional CO₂ capture properties that link to the liquid’s tendency to form various CO₂ concentration-dependent species, specifically molecular tetramers with 0 to 4 bound CO₂ molecules. Wide-angle x-ray scattering (WAXS) was used to probe the dimensions of these nanoscale clusters, which have a ~6 Å core and ~12Å diameter. Average density and WAXS measurements were carried out on 6 samples with controlled amounts of bound CO₂ ranging from 0 to 40 mol% CO₂, and generally show an overall density increase and appearance of a medium range order (MRO) structural peak, respectively. Our measurements provide estimates of the density (and thus the x-ray scattering length densities) of each composite species, allowing a more quantitative assessment of the corresponding structures obtained from WAXS. Furthermore, these results provide a critical reference baseline to deduce the volume fractions of each species from in-situ CO₂ pressurization WAXS experiments. Our time-dependent WAXS data clearly show structural transitions as CO₂ loading increases, including the rapid-onset of a cluster-cluster distance correlation. We will discuss the influence of water content on the molecular- and cluster-scale structural correlations of 2EEMPA from WAXS, NMR, and computational efforts by our team. Comparison of various structural models for the analysis of the x-ray scattering data, including peak-based models with/without the two-phase Teubner-Strey model will be discussed as well as comparisons with structural results obtained from theory. Understanding the link between structure and properties may offer insights to enabling efficient chemical conversion of captured CO₂, as well as determining effective approaches for direct air capture of CO₂ using water lean solvents.

Daniel Moran

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