Thermodynamic properties of deep eutectic solvents and organic solutes at different temperatures for separation of close boiling mixtures

Abstract

The environmental impact of volatile organic compounds coupled with its cost, stability, performance, turnability and versatility of conventional solvents and its effectiveness in the separation of close boiling point mixtures led to the growing interest in separation studies of deep eutectic solvents (DESs). DES have advantages over conventional solvents because of its distinctive properties such as thermal stability, easy and inexpensive methods to synthesise, low toxicity levels and biodegradability. DESs are recognized as possible alternatives to ionic liquids for diverse applications in the chemical industry for example in product development. The understanding of physical properties and intermolecular interactions is crucial for researchers and engineers to design more efficient and environmentally friendly processes for the separation of complex mixtures containing volatile organic solvents. This study focuses on the thermophysical and thermodynamic properties of deep eutectic solvents, categorized as type (III), for extraction or separation purposes. Conventional organic solvents, which are currently utilized in industrial processes for extraction or separation purposes are not environmentally friendly. The DESs were synthesized at 1:3 mole ratio of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD). The classified type (III) deep eutectic solvents under this study were: • DES1: 1-butyl-3-methylimidazolium chloride + ethylene glycol [BMIM]Cl: EG]. • DES2: 1-butyl-2,3-dimethylimidazolium chloride + ethylene glycol [BDMIM]Cl: EG]. The physical properties namely: densities, 𝜌, speed of sound, 𝑢, and refractive indices, 𝑛𝐷, for the binary mixtures [BMIM]Cl: EG + methanol or ethanol, and [BDMIM]Cl: DEG + acetic acid or propanoic acid were experimentally measured over the entire range of mole fraction, 𝑥𝐷𝐸𝑆 = (0-1) at 𝑇 = (293.15, 298.15, 303.15, 308.15, and 313.15) K and at atmospheric pressure. The measurements were conducted using an Anton Paar DSA 5000M and Anton Paar Abbermat 3200 refractometer. From the experimental data, excess thermophysical properties including excess molar volumes, vii 𝑉𝑚 E , isentropic compressibilities, 𝑘𝑠 , change in isentropic compressibilities, ∆𝑘𝑠 , intermolecular free length, 𝐿𝑓, and change in refractive indices, ∆𝑛𝐷, were calculated from the densities, speed of sound and refractive indices, respectively. The investigated properties of the deep eutectic solvent binary mixtures gave an insight into the types of molecular interactions in 1-butylmethylimidazolium chloride + ethylene glycol with methanol or ethanol or 1-butyl-2,3-dimethyl imidazolium chloride + ethylene glycol with acetic acid or propanoic acid at different experimental temperatures. The excess molar volumes, densities and refractive indices data was correlated with the application of the Lorentz-Lorenz equation. Density functional theory (DFT) was used to simulate the intermolecular interaction of deep eutectic [BDMIM]Cl: EG] + acetic acid or propanoic acid as well as [BMIM]Cl: EG] + methanol or ethanol binary mixtures. DFT calculations were employed to ascertain some physiochemical descriptors such as chemical potential (𝜇), electronegativity (𝜒), hardness (𝜂), the global electrophilicity index (𝜔) and softness (𝑆). The activity coefficients at infinite dilution (𝛾13 ∞) of the selected deep eutectic solvents with volatile organic solutes were also determined at different temperatures. The deep eutectic solvents used for the determination of the 𝛾13 ∞ were: • DES3: Tetrabutylammonium acetate with ethylene glycol, [TBN]AcO: EG]. • DES4: Tetrabutylammonium acetate with diethylene glycol [TBN] AcO: DEG] . The activity coefficients at infinite dilution was used as a pre-screening tool for the selection of possible entrainers and was calculated from retention data obtained from gas liquid chromatography (GLC) data. The GLC was operated at the temperature range of 𝑇 = (313.15 – 353.15) K and at 10 K interval. Thermodynamic properties such as excess enthalpies at infinite dilution, ∆𝐻1 E,∞, excess Gibbs free energies at infinite dilution, ∆𝐺1 E,∞, and excess entropy at infinite dilution, ∆𝑆1 E,∞, were computed from the activity coefficient at infinite dilution to further explain the types of intermolecular interactions between the solutes and the investigated DESs. Selectivity at infinite dilution (𝑆𝑖𝑗 ∞), and capacity at infinite dilution (𝑘𝑖𝑗 ∞) values were determined to evaluate the separation potential of the DESs. The data obtained from the spectroscopic techniques Fourier transform infrared (FTIR spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were used to validate the formation of DESs and the types of interactions arising between the HBD and HBA. Furthermore, evaluation of thermal stability for the prepared deep eutectic solvents was determined using differential scanning calorimetry (DSC)/ thermogravimetric analysis (TGA)