Prediction of the Basicity of Aqueous Amine Solutions and the Species Distribution in the Amine−H2O−CO2 System Using the COSMO-RS Method

A conductor-like screening model for real solvents (COSMO-RS) was applied to the study of the amine−H2O−CO2 system. pKa values for 25 amines including a variety of alkanolamines and some cyclic or aromatic amines were calculated by the COSMO-RS method coupled with the density functional theory (DFT). The predictions of pKa values were compared using different DFT levels for geometry optimization and also with those based on another solvation model (SM5.4/A). The DFT-COSMO calculation at the BP/TZVP level has shown a relatively good correlation with experimental values for the 25 amines (R2 ≅ 0.8) at a low computational cost. With this method, we have developed a calculation model to predict the equilibrium ratio between carbamate and bicarbonate anions in a CO2-loaded aqueous amine solution and have confirmed the validity of the prediction model by 13C NMR spectroscopy.