Molecular interactions of binary mixtures of deep eutectic solvents with organic solutes
Authors:   , Molefi Reitumetse Precious 
Published: September 2024

Deep eutectic solvents are the new emerging solvents formed by the combination of hydrogen bond acceptor and hydrogen bond donor. This type of solvent is still under development for possible industrial application including in chemistry, biotechnology, and pharmaceutical processes. The deep eutectic solvents have attracted much attention because they are characterized as greener solvents when compared to the currently used volatile organic solvents. Deep eutectic solvents are to replace the ionic liquids, which are posed as green solvents, however, their toxic nature has turned to be its drawbacks. The deep eutectic solvents attracted great interest due to their unique properties such as biodegradability, thermal stability, less toxicity, easy and cheap to prepare. This work explores the activity coefficients at infinite dilution of deep eutectic solvents. The deep eutectic solvents under investigation were carefully synthesized using hydrogen bond donors (HBD) and hydrogen acceptors (HBA) at a specific ratio. The analysis of these prepared deep eutectic solvents were analysed using spectroscopic techniques (FTIR and NMR) to confirm the formation of deep eutectic solvents and the type of interaction occurring between the HBD and the HBA. Additionally, thermal stability of the prepared deep eutectic solvents was investigated. The DESs were then used to measure the activity coefficients at infinite dilution for volatile organic compounds (alkanes, alkene, alkynes, aromatic hydrocarbons, ketones, acetonitrile, tetrahydrofuran, alcohols and thiophene) using the chromatography technique. The activity coefficients at infinite dilutions were conducted over a range of temperature (313.15 - 353.15) K. The prepared deep eutectic solvents for this include.  DES1 {1- butyl-2,3-dimethylimidazolium chloride + ethylene glycol (1:3)}  DES2 {1-butyl-2,3-dimethylimidazolium chloride + diethylene glycol (1:2)}  DES3 {1-butyl-2,3-dimethylimidazolium tetrafluoroborate + ethylene glycol (1:3)}  DES4 {1 -butyl-2,3-dimethylImidazolium tetrafluoroborate + diethylene glycol (1:3)) The study also focuses on understanding the behaviour of these DES through a comprehensive analysis of their thermophysical characteristics and their ability to dissolve solutes at infinite dilution. The investigation revealed intriguing trends in the solvation behaviour of different classes of solutes within the DES. Alkanes exhibited higher activity coefficients, with a clear dependence on the alkyl chain length. Cyclic hydrocarbons showed distinct behaviour due to stronger interactions with the imidazolium ring. Alkynes demonstrated the lowest activity coefficients, attributed to the presence of triple bonds influencing solute-solvent interactions. Aromatic hydrocarbons exhibited unique solvation behaviour influenced by the delocalized pi- π- electrons on the benzene ring. To use the deep eutectic solvents at an industrial level, it is highly imperative to understand the intermolecular interactions and properties of the pure deep eutectic solvents and their mixtures with volatile organic solutes. The prepared deep eutectic solvents for thermophysical properties include.  DES5 (1-butyl-1-methylpyrrolidinium bromide + ethylene glycol)  DES6 (1-butyl-3-methylimidazolium chloride + ethylene glycol) Thermophysical properties, such as densities, speed of sound, and refractive indices were measured as a function of temperature. The study investigated the binary mixtures containing (DES5 + acetic acid or ethanol) and (DES 6 + acetic acid or ethanol). These were investigated at temperatures ranging between (293.15 and 313.15) K and at atmospheric pressure (0.1MPa) over a range of mole fraction (𝑥1= 0 to 1) as a function of DES. The measured property values were used to compute the excess properties such as excess molar volumes, isentropic compressibilities, deviation in refractive indices, deviation in isentropic compressibilities and intermolecular free length. The data obtained provides insights into the molecular interactions within the DESs, shedding light on their structural arrangements and overall stability. This study contributes to the fundamental understanding of deep eutectic solvents, offering a detailed exploration of their thermophysical properties and their solvation behaviour at infinite dilution. The findings have implications for the design and optimization of DESs for various applications, including their use as green solvents in chemical processes and separations.