Elucidating the Role of Hydrogen Bond Donor and Acceptor on Solvation in Deep Eutectic Solvents Formed by Ammonium/Phosphonium Salts and Carboxylic Acids

Abstract

Deep eutectic solvents (DESs) constitute a rapidly emerging class of sustainable liquids that have been widely studied and employed in chemical separations, catalysis, and electrochemistry. The unique physico-chemical and solvation properties of DESs can be highly tailored by choosing the appropriate hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD). Understanding the role of the HBA and HBD on the multiple solvation interactions in DESs is important to enable their judicious selection for particular applications. This work constitutes the first study to exploit chromatography to measure solute-solvent interactions of DESs using a wide array of known probe molecules. The constituent components of 20 DESs, formed by ammonium and phosphonium-based salts and carboxylic acids, are systematically modulated to delineate the contribution of the HBA and HBD towards individual solvation properties. Solute-solvent interactions measured in this study are used to interpret and explain the performance of DESs in desulfurization of fuels and extraction of natural products. The results from this study can be used to predict and understand the performance of DESs in various chemical processes where solvation interactions heavily influence outcomes.

Investigating the Variation in Solvation Interactions of Choline Chloride-Based Deep Eutectic Solvents Formed Using Different Hydrogen Bond Donors

“…In order to preserve the integrity of the DES stationary phase throughout the study, GC inlet/detector temperatures less than 150 °C and oven temperatures below 70 °C were maintained. 40 However, some HBDs examined in this study had slightly higher vapor pressures, necessitating the use of lower oven temperatures. Therefore, all system constants were measured at 30, 40, and 50 °C.…”

“…The DESs within both classes exhibited significant dipolarity (s-term), hydrogen bond basicity (a-term), and dispersive-type + ][Cl − ] DESs are attributed to longer alkyl chain substituents on the cation, as is consistent with previous studies. 28,29,40 The differences in solvation interactions between the two DES systems lead to some probe molecules exhibiting longer chromatographic retention factors, as shown in Tables S5 and S6. Alcohols interacted less on [Ch + ][Cl − ] DESs due to their lower hydrogen bond basicity but interacted much more strongly with basic probe molecules, such as N,N-DMF and N,N-DMAC, compared to [N 4444 + ][Cl − ] DESs (Table S7).…”

“…39 IGC has previously been used to investigate the solvation characteristics of phosphonium and ammonium DESs, and the results have shown significant variation in solvation properties between different eutectic mixtures within the same class of DESs. 40 To date, the solvatochromic approach has provided a limited understanding as to the role of HBA and HBD on overall solvation characteristics for choline chloride-based DESs.…”

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…For example, type III DES have a quaternary ammonium salt as HBA, and the HBD is an organic molecule, whereas type V DES have organic molecules as the HBA and HBD. 1,2 Many of the studies in the current literature involve type III DES with choline chloride ((2-hydroxyethyl)-trimethylammonium chloride) or tetrabutylammonium chloride as the HBA, 6,18,19,[23][24][25][26][27][28][29][30] but there has also been an increase in the study of type V DES. 14,31,32 Given the number of HBA and HBDs available, the possible binary (and ternary) DES combinations is extremely large, but there are no models capable of predicting what combinations will result in DES.…”

“…1 In principle, the properties of a DES depend upon the choice of HBA and HBD, which should make it possible to "tune" the properties of a DES. For example, incorporating a chiral HBA or HBD results in a chiral DES solvent which can be utilized in asymmetric synthesis, [33][34][35][36][37] chiral separations, 24,38 and circularly polarized light emitting materials. [39][40][41] The role of chirality in chemistry and biochemistry highlights the importance of developing new chiral DES solvents However, the large number of possible chiral HBA and HBDs and lack of predictive capability make finding new chiral DES a time-consuming process of trial and error.…”