Thermophysical behavior and molecular interactions in acetonitrile-tertiary alkanolamine mixtures.

The thermophysical behavior and molecular origins of dynamic viscosity (η) non-ideality in binary mixtures of acetonitrile (ACN) with six tertiary alkanolamines, namely dimethylethanolamine (DMEA), dimethylpropan-1-olamine (DMPA-1), dimethylpropan-2-olamine (DMPA-2), methyldiethanolamine (MDEA), ethyldiethanolamine (EDEA), and butyldiethanolamine (BDEA), were investigated over the full composition range at 303.15-323.15 K and 0.10 MPa. Viscosity deviations (Δη) and activation free energies of viscous flow were evaluated to elucidate intermolecular interactions. All systems exhibited predominantly negative Δη values with minima at intermediate compositions, indicating disruption of self-associated alkanolamine networks upon mixing with ACN. The magnitude of the negative deviation followed the order DMPA-2 < DMEA < DMPA-1 < BDEA < EDEA < MDEA, highlighting the influence of hydroxyl functionality, molecular architecture, and steric effects. Increasing temperature reduced the magnitude of non-ideality, consistent with weakened intermolecular interactions. Fourier-transform infrared (FTIR) spectroscopy of the representative ACN + MDEA system and density functional theory (DFT) calculations confirmed hydrogen bonding and electronic charge redistribution. Incorporation of newly optimized -CH2N interaction parameters enabled accurate viscosity prediction using the UNIFAC-VISCO (UVM) and UNIFAC-THERMO (UTM) models, yielding overall mean absolute relative deviations (MARDs) of 6.08% and 5.14%, respectively. These findings provide molecular insight into viscous-flow behavior in non-aqueous ACN-alkanolamine mixtures.