Structure and Reactivity in Nanostructured Ionic Solvents

Abstract

Deep eutectic solvents (DESs) are low melting mixtures, often prepared from a hydrogen bond acceptor (HBA) salt and a molecular hydrogen bond donor (HBD) at a specific molar ratio. These unique solvents are of increasing research interest due to their potential for simple, 100% atom economical preparation from cheap, commercially available, environmentally benign components. Though DESs are now recognized as an individual class of solvents, they were previously categorized within the family of ionic liquids (ILs) due to similarities between these solvents including typical physical properties such as their low vapor pressures, wide liquid-state ranges, low flammability, and inherent electrical conductivity. However, ILs and DESs fundamentally differ in their chemical nature since ILs are pure liquid salts whereas DESs are mixtures. Like ILs, DESs that contain at least one sufficiently amphiphilic component can form bicontinuous nanostructures consisting of polar and non-polar domains. The ability of DESs to form amphiphilic bulk nanostructures similar to ILs is exciting given its implications in a wide range of applications, including catalysis, electrodeposition, chemical extraction, self-assembly, bioprocessing, energy storage, and lubrication. But these bulk nanostructures have not been widely explored for many DES combinations. So, the lack of characterisation of the nanostructure of DES hinders the development of essential structure-property relationships, which severely restrict the utility of this class of solvents. Here, the bulk nanostructures of DESs comprising tetraalkylammonium bromide salts (tetrabutylammonium bromide, tetraoctylammonium bromide and methyltrioctylammonium bromide) with alkanols and alkanoic acids of systematically varied chain lengths (C2, C6, C8 and C10) as hydrogen bond donors have been studied. Small-angle X-ray scattering techniques were used to identify the relationship between alkyl chain length and functionality of the hydrogen bond donor on the nature of the amphiphilic nanostructures formed. All the DESs investigated featured amphiphilic nanostructures, with the bulk arrangement varying between the three tetraalkylammonium salts used. Also, the findings from this study demonstrated that the amphiphilic nanostructures of the DESs were not affected by the functional group on the hydrogen bond donor but are primarily influenced by both the absolute and relative alkyl chain lengths of the salt and hydrogen bond donor. Due to the hygroscopic nature of the DES, the presence of water (at least in small quantities) is often unavoidable and may even be deliberately added to the DESs in order to leverage favourable physiochemical properties, such as lower viscosity, during an application. There have been several experimental and theoretical investigations on the hydration behaviour of DESs based on non-amphiphilic choline chloride (CholCl) salt, which highlighted the presence of two key regimes (association and hydration) with a distinct transition point between the two on the gradual hydration of DES. However, the amphiphilic DESs featuring polar-nonpolar domains could potentially lead to different solvation behaviour than the CholCl DESs or, at the very least, affect the transition point between the association and hydration regimes of DES-water mixtures. Apart from water, the solvation of other molecular or ionic solutes, co-solvents, or reactants can also be required in certain applications of structured DESs as reaction media or extraction solvents. Here, the resilience of the amphiphilic nanostructures of the tetraalkylammonium DESs was assessed through the addition of various solutes. The solutes include water (H2O), propan-2-ol/ isopropanol (IPA), acetone (Ace), dimethyl sulfoxide (DMSO), ethyl acetate (EtOAc), cyclohexanol (CyOH), cyclohexanone (Cyone), and cyclohexyl acetate (CyHxAc). The findings suggested the nanostructures to be remarkably resilient, with solute concentrations of 70 mol% or more tolerated before any breakdown. The partitioning behaviour of these solutes within the nanostructured domains was also analysed. Based on their nature (such as hydrogen bond accepting or donating ability or high dipolarity) and size, solutes exhibited different solvation affinity for the polar and nonpolar DES domains. Furthermore, the effect of the HBA:HBD stoichiometry on the DES amphiphilic nanostructure was also investigated using SAXS, which highlighted the sensitivity of these nanostructures to compositional changes. The overall findings illustrated the potential of these DES nanostructures to be fine-tuned through both salt: HBD composition and the deliberate addition of solutes for a specific application. The effects of the amphiphilic nanostructures of ILs have been previously explored for a small number of chemical processes, such as Diels-alder reactions, polymerization reactions, nucleophilic substitution reactions, condensation reactions, and elimination reactions. The results of those investigations highlighted the untapped potential of using structured media to control the course of reactions. It also encouraged the prospect of developing such reactions as probe systems to obtain nanostructural information, like the degree of domain segregation or the preferred orientation of reactive species in such solvents. Because the inherent structural organization in these soft matter systems (ILs or DESs) can potentially place electronic or steric constraints on reactants, transition states, or products, it can directly affect organic reactivity and reaction rates. Here, the kinetics of the inverse-electron demand Diels-Alder (DAinv) reaction were probed to develop structure-reactivity relationships with the tetraalkylammonium DESs investigated alongside selected molecular solvents and ILs. The key finding was the stronger effect of amphiphilic n-alkanols compared to the other solvent systems (amphiphilic ILs or DESs), including the corresponding alkanoic acids, on the reaction. The rationalizations were drawn based on solvent-solute interactions, solvophobic effects, and general polarity effects. It provided the basis for further in-depth investigations into the impact of domain structures and specific solute-solvent interactions on chemical reactivity.