Physicochemical properties of deep eutectic solvent choline chloride: Propionic acid (ChCl/PA DES) and its binary solutions with 1-butanol as cosolvent

Deep eutectic solvents (DESs) are considered suitable replacement for conventional organic solvents, and the prospects of their use in pharmaceutical and biomedical fields are growing. A detailed knowledge of their physicochemical properties and understanding about structural behavior are particularly important for academic and industrial applications. In the present study a deep eutectic solvent is prepared by combination of choline chloride (ChCl) and propionic acid (PA) designated as ChCl/PA DES. Physicochemical properties density (ρ), speed of sound (u) and dynamic viscosity (η) of ChCl/PA DES and its binary mixtures with cosolvent n-butanol are measured in the temperature range (293.15 – 333.15) K for all compositions x₁ = 0 – 1 (x₁ is mole fraction of ChCl/PA DES). The density data is appropriately fitted with a second-degree polynomial equation in T. The volumetric properties, excess molar volume (V^E) and isentropic compressibility deviation (∆κ_S), of ChCl/PA DES and its binary solutions show negative deviation from ideal behavior. The V^Evsx₁ curves exhibit a minimum at x₁≈0.35 which becomes deeper with increasing T. Curves corresponding to excess partial molar volume of the DES and 1-Butanol also cross each other at x₁≈0.35 which supports dominance of packing effect over specific interactions. Similar behavior in viscosity deviation (Δη) is also observed but at x₁≈0.6 which is again supportive of the volumetric results. Lattice energy (U_pot), molar entropy (S⁰) and intermolecular free length (L_f) are calculated to explore behavior of the derived thermodynamic properties. Comparison of the results of temperature dependence of transport property (η) with Vogel Fulcher Tammann (VFT) and Arrhenius equations reveals that the VFT equation satisfactorily explains the relationship between dynamic viscosity and T. To model the properties, we treat DES as a pseudo pure fluid and the DES + 1-butanol mixture as a pseudo binary system. The Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state was employed as a fitting approach for modeling the experimental density of ChCl/PA DES + 1-butanol mixtures. The theoretical models named Schaaff's Collision Factor Theory (SCFT) and Nomoto's Relation (NR) were used to compute the speed of sound (u) for the mixtures from theoretical point of view. Finally, dynamic viscosity (η) was modeled using Free Volume Theory (FVT). This theory was applied as a fitted method using only three fitted parameters across the entire range of temperature and liquid mole fraction of DES.