The excess thermodynamic properties of binary mixtures comprising water and three choline chloride (ChCl)-based deep eutectic solvents (DESs): ChCl:urea (1:2), ChCl:1,2-propanediol (1:3), and ChCl:1,3-butanediol (1:3) were systematically investigated. Density and viscosity were measured over the entire composition range (DES mole fraction x1= 0 to 1) at temperatures from 313.15 K to 328.15 K under atmospheric pressure. From these data, key excess properties were derived, including excess molar volume ( \({V}_{\text{m}}^{\text{E}}\) ), partial molar volume ( \({\overline{V} }_{\text{m},\text{i}}\) ), isobaric expansion coefficient (αₚ), viscosity deviation (Δη), and excess Gibbs free energy of activation for viscous flow \((\Delta {G}^{*\text{E}}\) ). The Redlich–Kister equation was employed to correlate the composition dependence of \({V}_{\text{m}}^{\text{E}}\) and Δη with high accuracy (R2 > 0.998). Fourier Transform Infrared (FTIR) spectroscopy was used to probe intermolecular interactions. The results showed consistently negative \({V}_{\text{m}}^{\text{E}}\) values across all systems and temperatures, indicating significant volume contraction upon mixing due to enhanced hydrogen bonding and molecular packing. The viscosity deviation Δη and positive \(\Delta {G}^{*\text{E}}\) values further confirmed the dominance of specific interactions, such as hydrogen bonding, which were corroborated by characteristic FTIR peak shifts. The magnitude and temperature sensitivity of these excess properties depend strongly on the hydrogen bond donor structure and temperature. These work provided fundamental insights into molecular-level interactions governing the non-ideal behavior of aqueous DES mixtures, which are essential for the rational design of DES-based systems in applications such as separations, reactions, and material synthesis.
