Water-lean solvents are a promising technology for capturing acid gases like carbon dioxide (CO2). In situ liquid time-of-flight secondary ionization mass spectroscopy (ToF-SIMS) is used to study a representative solvent N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) with different CO2 loadings to reveal the complex solvent structure upon CO2 capture. Characteristic peaks of 2-EEMPA, such as m/z- 215 C11H23N2O2- (deprotonated 2-EEMPA) and m/z+ 217 C11H25N2O2+ (protonated 2-EEMPA), are detected due to acid gas uptake. Also, solvent molecules and carboxylate ion pairs, such as m/z– 259 C12H23N2O4– [(deprotonated 2-EEMPA∙∙∙CO2)] and m/z+ 261 C12H25N2O4+ (protonated 2-EEMPA∙∙∙CO2), are observed. Interestingly, more than one CO2 molecule can be captured per each solvent molecule as evidenced in SIMS mass spectra, for example, m/z– 321 C13H25N2O7– [(deprotonated 2-EEMPA)∙∙∙2CO2∙∙∙H2O], m/z+ 305 C13H25N2O4+ [(protonated 2-EEMPA)∙∙∙2CO2], m/z– 389 C17H29N2O8– [(deprotonated 2-EEMPA)∙∙∙3CO2∙∙∙3CH2], and m/z+ 373 C16H25N2O8+ [(protonated 2-EEMPA)∙∙∙3CO2∙∙∙2C]. However, the monomer of 2-EEMPA and CO2 seems to be most prevalent. Furthermore, solvent clusters are detected in loaded solvents, for instance m/z+ 433 C22H49N4O4+ [(2-EEMPA)2∙∙∙H] and m/z+ 646 [(2-EEMPA)3-2H], while capturing CO2 at different amounts. Relative abundance of cluster ions provides a semi-qualitative venue to assess the free energies of gas capture energetics, indicating the relative stability trend within the same solvent system, previously impossible. These observed ion clusters are verified with molecular modeling, where dimer, trimer, and cluster ions are validated for their presence either due to weak molecular interactions or hydrogen bonds. In situ molecular imaging of ionic liquids and molecular modeling reveals that the acid gas capture mechanism by ionic liquids includes both physical adsorption and chemical bonding with multiple reaction pathways, engaging cluster formation and alteration of solvent structures.
