Solvent Trade-Offs for Light-Driven CO2 Release: Photoacid Solubility and Thermodynamic Screening

Abstract Light-driven CO2 capture and release using merocyanine photoacids (MCHs) offers a potential alternative to thermally driven CO2 sorption regeneration. However, the low water solubility of MCH limits the photoacid and base concentration and therefore the light-responsive inorganic carbon pool. Here, we combine solubility measurements, flash differential scanning calorimetry (FDSC), electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) fitting of solubility data, and concentration-based equilibrium modeling, to examine aqueous mixtures containing methanol, ethanol, glycerol, or dimethyl sulfoxide (DMSO). Organic cosolvents increase MCH solubility but also alter CO2 solubility and carbonate speciation, reducing the inorganic carbon availability at higher cosolvent fractions. The model predicts a composition-dependent trade-off between photoacid solubility and the light-responsive inorganic carbon pool, with the most favorable screening regions occurring in water-rich to intermediate-composition mixtures. These results show that solvent selection based on MCH solubility alone is insufficient and provide a thermodynamic screening framework for photoacid-based CO2 capture systems.