Volumetric, Viscometric, and Molecular-Level Characterizations of Choline Chloride/Butyric Acid Deep Eutectic Solvent (ChCl/BA DES) with 1-Butanol from 293.15 to 343.15 K

This study reports the measurement of density (ρ) and dynamic viscosity (η) of choline chloride and butyric acid deep eutectic solvent (ChCl/BA DES) and investigates the physicochemical properties of neat DES and its binary mixtures with 1-butanol for the entire range of composition (x1) at T = 293.15–343.15 K. The ρ data is fitted by the first-degree temperature polynomial equation, and the fourth-degree mole fraction polynomial equation adequately describes both ρ and η. Excess molar volume (VE) shows a positive deviation with a maximum at x1 ≈ 0.45. The viscosity deviation (Δη) exhibits a minimum at x1 ≈ 0.55, confirming the mixture stability in the region. Excess properties (VEand Δη) are correlated by the Redlich–Kister polynomial equation (R–K). Temperature dependence of η is compared by Vogel–Fulcher–Tammann and Arrhenius equations. Molecular dynamics simulations revealed that 1-butanol disrupts the ChCl-BA hydrogen bond network, reducing H-bond populations by ∼ 50%. Radial distribution function revealed weakened OH···COOH interactions with increasing temperature (RDFmax∼ decreasing from 1.42 to 1.29). Quantum chemical calculations identified seven low-energy conformations (ΔE < 10 kcal/mol), with QTAIM confirming strong choline-butyrate hydrogen bonding (ρ = 5.7 × 10–2 a.u.). Perturbed chain statistical associating fluid theory accurately describes the density in the pseudo binary ChCl/BA DES + 1-butanol mixture and successfully reproduces the qualitative trends in the excess molar volume.