Switchable ionic liquids are promising single component water-lean CO₂ capture solvents that can remove CO₂ from dilute and complex gas mixtures. These solvents switch their dielectric constant between non-polar and polar forms with CO₂ content as the chemical trigger. As the polar phase develops with increased CO₂ binding, structural heterogeneity of the nonpolar/polar system manifests itself through changes in the size, shape and orientation of liquid micro-domains. Control of such domains could hold the key to the design of fast, high capacity CO₂ capture solvents. For example, the aminopyridine-based solvent class shows an order of magnitude higher liquid-film mass transfer coefficients of CO₂ at solution viscosities 2-3 times that of other known solvents, yet the mechanisms for this behavior are not fully understood. We will discuss recent wide-angle x-ray scattering results which compare the CO₂ pressure (0-600 psig) and temperature (20 - 80 C) dependence of a low-angle intermediate range order (1-1.5 nm) feature, accompanied by slight modifications in nearest neighbor correlations (0.46-0.48 nm) in three related methylaminoethylpyridine (MAEP) solvents. By changing the position of the pyridine nitrogen to the ortho, meta, and para positions, the ratio and strength of inter- and intra-molecular hydrogen bonding may be controlled, which gives rise to different structural features, including possible long range molecular alignments that enable strong CO₂ uptake, and changes to the system’s viscosity. These systems will be compared to 2-EEMPA, another promising CO₂ capture and conversion solvent. The anisotropic local ordering of polar and nonpolar domains in the present system bears some similarities to the alkyl chain length-dependent molecular organization seen in many classic (e.g., imidazolium and pyrrolidinium based) ionic liquids, yet the mechanisms in this case for such ordering are different and tunable with CO₂ concentration.

Omar Avina

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