Viscometric Properties and Thermophysical Modeling of Cyclic Ether + Alkanolamine Binary Liquid Systems from 298.15 to 323.15 K

Dynamic viscosities (η) of binary mixtures of 1,4-dioxane (DXN) and 1,3-dioxolane (DXL) with methylethanolamine (MEA), ethylethanolamine (EEA), dimethylethanolamine (DMEA), and diethylethanolamine (DEEA) were measured over the entire mole-fraction range at temperatures from 298.15 to 323.15 K and under 0.10 MPa. Viscosity deviations (Δη), Gibbs free energy of activation for viscous flow (ΔG≠), and excess free energy of activation (ΔG≠E) were evaluated to quantify nonideal mixing behavior. All systems exhibited negative Δη across the full composition range, with minima typically observed at alkanolamine mole fractions of 0.4–0.75. The magnitude followed MEA > EEA > DEEA > DMEA for both ethers, indicating progressive weakening of intermolecular association with increasing alkyl substitution at the amine nitrogen. Increasing temperature reduced the negativity of Δη and ΔG≠E, confirming thermally weakened mixed-liquid structures. Experimental η and ΔG≠ were correlated using the composition-dependent polynomial model, whereas Δη and ΔG≠E were fitted using the Redlich–Kister polynomial equation. The Jouyban–Acree model reproduced viscosity data with average relative deviations below 2.12%. Newly estimated CH2N interaction parameters enhanced group-contribution predictions, yielding average deviations of 3.85% (UNIFAC-VISCO) and 3.14% (UNIFAC-THERMO). These results provide quantitative insights into steric and hydrogen-bonding effects governing viscous flow in nonaqueous alkanolamine–cyclic ether systems.