Tailoring Solvent Polarity via Molecular Polarity Index for Energy-Efficient Biphasic CO2 Capture with Enhanced Mass Transfer

Abstract The development of high-performance biphasic absorbents is limited by empirical screening and the difficulty of balancing CO2 capacity, phase behavior, and regeneration energy. Here, the molecular polarity index (MPI), a descriptor of molecular surface polarity, is exploited to rationalize and shortlist physical solvents according to their compatibility with the polar species generated upon CO2 absorption. Built around 1-(2-aminoethyl) piperazine (AEP) as the reactive amine, a quaternary formulation incorporating ethylene glycol, dimethyl sulfoxide (DMSO), and diethylene glycol diethyl ether (DGDE) was assembled. MPI analysis showed that the polarity gap between DGDE and the highly polar carbamate/protonated-amine products drives phase splitting, whereas DMSO tunes the local polarity to retain the ionic products within a compact-rich phase. The optimized absorbent confined the rich phase to ∼53% of the total volume, concentrated over 96% of captured CO2 therein, and reached a loading of 0.58 mol/mol. 13C NMR confirmed that DMSO partitions chiefly into the lean phase, shielding it from thermal regeneration. Consequently, the regeneration duty dropped to 1.72 GJ/t CO2, while the absorption rate rose to 25.2 mmol/(m2·s), 2.05 times that of MEA. Coupling polarity matching with multicomponent tuning thus offers a transferable route to low-energy, mass-transfer-intensified CO2 capture.