Thermodynamic Study of Amine-Based Deep Eutectic Solvents with EtOH.

The densities and viscosities of the three amine-based deep eutectic solvents (DESs), i.e., [EmimCl]-[MEA], [BmimCl]-[MEA], and [HmimCl]-[MEA], mixed with ethanol (EtOH), were measured from 288.15 to 323.15 K, and their enthalpies of mixing were determined at 298.15 and 308.15 K. Negative excess molar volumes across the entire compositional range indicate significant volume contraction and enhanced molecular packing, and their temperature dependence suggests the dominance of packing effects over hydrogen-bonding. Viscosity deviations show a characteristic U-shaped composition dependence, reflecting disruption of the original DES hydrogen-bond network by EtOH. The enthalpy of mixing changes from positive to negative as the DES composition increases, revealing strengthened DES-EtOH interactions. The distinct composition-dependent trends observed for these three (DES + EtOH) systems indicate that variations in alkyl chain length modulate molecular packing and interaction strength, resulting in nonuniform ordering patterns across thermophysical properties. Compared to water, which strongly restructures the DES hydrogen-bond network and produces significant exothermic mixing effects, EtOH exerts a milder structural and thermodynamic impact. This work provides reliable thermodynamic data and mechanistic insight for the design and application of amine-based DESs.