Infinite dilution activity coefficient and thermo physical properties measurements of deep eutectic solvent mixtures

Abstract

Deep eutectic solvents (DESs) have emerged as promising alternatives to traditional organic solvents due to their environmentally friendly and sustainable nature. This study focused on the characterization of DES mixtures through the determination of infinite dilution activity coefficients and the measurement of their essential thermo-physical properties. DESs are formed by the complexation of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), resulting in unique solvent properties. In this research, a series of DES mixtures with varying HBA-HBD combinations were prepared and analyzed. The novelty of studying the infinity dilution activity coefficient and thermophysical properties of DES mixtures lies in the fact that DESs are a relatively new class of ionic liquids that exhibit unique properties and behaviors compared to traditional solvents. Understanding the behavior of DES mixtures at low concentrations is important for different applications such as separation processes and extraction. The thermal stability of the prepared DESs was ascertained via thermogravimetric analysis. Fouriertransform infrared spectroscopy (FTIR) was used to identify any possible shifts when the two compounds were assorted, and differential scanning calorimetry (SDT Q600 V20.9 Build 2D) was utilized to evaluate the formation of DESs using melting point. Furthermore, the density and sound velocity of the pure DESs under investigation were determined using an Anton Paar DSA 5000 M instrument. Additionally, surface tension measurements were conducted using a tensiometer (Attention Sigma 703D, Biolin Scientific, China). All data were collected at standard conditions of 298.15 K and 101.3 kPa. The infinite dilution activity coefficient, a crucial parameter for understanding solute-solvent interactions, was determined for several solutes across different DES mixtures using gas-liquid chromatography techniques. This encompassed the determination of for 34 solutes in DESs, specifically [ZnCl2][acetic acid] at ratios of 1:2 or 1:4, and [ZnCl2][phosphoric acid] at ratios of 1:2 or 1:2.5, spanning a temperature range of (313.15 to 353.15) K. These measurements shed light on the solvation behavior within the DESs, providing valuable insights into their potential applications in separation and extraction processes. Excess thermodynamic parameters, including partial molar excess enthalpies and Gibbs free energies ] were derived from the data. The usefulness of the researched DESs as extracting solvents for possible applications was evaluated by calculating and comparing their selectivity and capacity values with those reported in the literature for other DESs, ionic liquids, and sulfolane. In terms of separation performance, the results revealed that [ZnCl2][acetic acid] (1:2) exhibited superior selectivity and capacity for benzene/ethanol, cyclohexane/ethanol, and octane/acetonitrile systems, highlighting its potential in industrial separation processes. Conversely, [ZnCl2][phosphoric acid] (1:2) emerged as a promising solvent for replacing conventional solvents due to its outstanding separation parameters (capacities and selectivities) for components with closely matched boiling points. Despite the promise of deep eutectic solvents based on phosphoric acid, certain challenges, such as corrosion caused by phosphoric acid in certain industrial applications, limit its suitability for addressing industrial separation challenges. Moreover, this study expanded upon the investigation of the thermophysical characteristics of binary combinations consisting of [ZnCl2][acetic acid] or ([ZnCl2][phosphoric acid] (1:2) in a 1:4 ratio) and various alcohols (methanol, ethanol, and propanol), at different temperatures (293.15 to 313.15) K. To confirm the occurrence of intermolecular interactions within these mixtures, various thermodynamically relevant parameters, such as density and speed of sound, were employed to compute parameters including excess molar volumes ( E Vm ), isentropic compressibility ( ), intermolecular free length ( ), and deviation in isentropic compressibility (∆ ). The study further emphasized that ion-solvent and solute-solvent interactions prevailed over self-interactions, such as solvent-solvent and ion-ion or solute-solute interactions. Additionally, temperature exhibited a significant influence on all investigated thermodynamic parameters, as their values, (including E Vm , , ∆ , and ) were found to either decrease or increase with rising temperature. The Redlich-Kister polynomial expression was effectively employed to correlate excess molar volumes and deviation in isentropic compressibility. In addition, the perturbed chain statistical associating fluid theory equation of state (PC-SAFT EoS) was employed to model the densities of binary mixtures consisting of DES and 1-alkanol. Furthermore, utilized Schaaff's collision factor theory (SCFT) and Nomoto's relation (NR) to model the speed of sounds for these binary mixtures. This comprehensive study significantly contributes to the body of knowledge on DESs by elucidating their thermophysical properties, solvation behavior, and potential applications in separation processes. The findings not only enhance our understanding of DES mixtures but also offer valuable insights for designing more efficient and sustainable solvents for industrial and environmental applications. Future research could delve deeper into exploring the interactions and behaviors of DESs under varying conditions and in diverse applications to further harness their potential benefits.