Thermodynamic Properties, Excess Properties, Structural Studies, and Hydrogen Bonding Interaction of Methyldiethanolamine + 1,2-Propylenediamine Mixtures

Density (ρ) and viscosity (η) of 1,2-propylenediamine (PDA) and N-methyldiethanolamine (MDEA) mixtures with various mole ratios were measured at 298.15–318.15 K under atmospheric pressure. Excess and thermodynamic properties were calculated based on ρ and η. The correlations among ρ, η, and mole fractions were evaluated using the Jouyban–Acree and McAllister models. The temperature-dependent behavior of the ρ and η values was modeled by applying the Arrhenius equation and the least-squares method. Moreover, the Gibbs free energy (ΔG^*E), viscosity deviation (Δη), and excess molar volumes (V_m^E) of the mixtures were fitted using the NRTL model, the Wilson model, and the Redlich–Kister (R–K) equation. Computational chemistry analyzed hydrogen bonding interactions between PDA and MDEA. Furthermore, the molecular structure of the mixtures was analyzed through Fourier-Transform Infrared (FTIR), and the intermolecular interaction structures of MDEA and PDA were investigated by Raman and nuclear magnetic resonance (NMR) hydrogen spectroscopy, thereby determining the physical anchoring effect of MDEA on PDA molecules. The discussion of the results identifies that the structure of hydrogen bonds is −O–H···NH₂-. The CO₂ absorption capacity of the MDEA + PDA mixture reached 10.11 mmol/g_PDA at a 1:1 molar ratio, a 45.7% increase compared with pure PDA.