Small-Angle X-ray Scattering Study of the Amphiphilic Bulk Nanostructure of Tetraalkylammonium Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are low-melting mixtures, often prepared from a salt and a molecular hydrogen bond donor. Like ionic liquids, DESs that contain at least one sufficiently amphiphilic component can form bicontinuous nanostructures consisting of polar and nonpolar domains, although this has not been widely explored for many DES combinations. 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 the alkyl chain length and functionality of the hydrogen bond donor on the nature of the amphiphilic nanostructures formed. These findings demonstrated that the amphiphilic nanostructures of the DESs were not affected by the functional group on the hydrogen bond donor, with these nanostructures influenced primarily by both the absolute and relative alkyl chain lengths of the salt and hydrogen bond donor.

Green imine synthesis from amines using transition metal and micellar catalysis.

Imines are a versatile class of chemicals with applications in pharmaceuticals and as synthetic intermediates. While imines are conventionally synthesized via aldehyde-amine condensation, their direct preparation from amines can avoid the need for the independent preparation of the aldehyde coupling partner and associated constraints with regard to aldehyde storage and purification. The direct preparation of imines from amines typically utilizes transition metal catalysis and is often well-aligned with green chemistry principles. This review provides a comprehensive overview of transition metal catalysed imine synthesis, with a particular focus on the copper-catalyzed oxidative coupling of amines. The emerging application of micellar catalysis for imine synthesis is also surveyed due to its potential to avoid the use of hazardous solvents and intensify these reactions through reduced catalyst loadings and locally increased reactant concentrations. Future directions relating to the confluence of these two areas are proposed towards the more sustainable preparation of imines.

A kinetics study of copper-catalysed click reactions in ionic liquids.

Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions are of extensive interest in chemical synthesis. While the use of ionic liquids (ILs) as solvents for synthesis has been widely explored in recent years, the understanding of their influence on the mechanism and reactivity of CuAAC reactions remains poorly understood. Here, we investigate the kinetics of a phenylacetylene-benzylazide and acetylene-benzylazide CuAAC reaction to probe the influence of IL structure, including the role of the base used to promote the reaction and the importance of water content. The use of 'wet' ILs led to remarkable changes in the kinetic profile of the reaction by eliminating the initial induction period. The reaction rate was found to be dependent on the copper(I) source. The effect of an added base was also studied, with the use of a tertiary amine-bearing IL leading to high conversions in under 5 min at ambient temperature. The results of this study highlight the nature and complexity of CuAAC reactions in ILs. As more ILs are getting involved in industrial processes, the data obtained from this study are valuable for better understanding processes that affect the CuAAC reaction in IL media and for creating customized systems for organic synthesis, thus improving the efficiency and sustainability of such processes.

Mechanochemical destruction of per- and polyfluoroalkyl substances in aqueous film-forming foams and contaminated soil.

Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic chemicals of concern that exhibit extreme persistence within the environment and possess physicochemical properties that are resistant to targeted degradation. Comprising substantial concentrations of PFASs, aqueous film-forming foams (AFFFs) present a major exposure pathway to the environment having been applied to land at firefighting-training sites globally for decades. This has led to significant contamination of environmental media. Herein, we demonstrate that mechanochemical destruction (MCD) is an effective method for the destruction of PFASs in an AFFF concentrate and an authentic sample of PFAS-contaminated soil derived from a decommissioned firefighting training facility. Both targeted analysis and non-targeted analysis were used in this study to evaluate the degradation of PFASs in complex substrates during MCD treatment. Destruction efficiencies of target PFAS subgroups ranged from 99.88% to 100%. The only additive employed for MCD treatment was quartz sand, which was used only for the liquid AFFF sample, with no additives required for the destruction of PFASs in the contaminated soil. This confirms the viability of MCD for both the remediation of PFAS-contaminated land and the destruction of stockpiled AFFFs.

Solvent-Free, Ambient Temperature and Pressure Destruction of Perfluorosulfonic Acids under Mechanochemical Conditions: Degradation Intermediates and Fluorine Fate.

Perfluorosulfonic acids (PFSAs) are a recalcitrant subclass of per- and polyfluoroalkyl substances (PFASs) linked to numerous negative health effects in humans. Scalable technologies that effectively destroy PFSAs will greatly reduce the future health and ecological impact of these "forever chemicals". Herein, we show that several PFSAs undergo facile mechanochemical destruction (MCD) in the presence of quartz sand (SiO2). This process operates in the absence of solvent, at ambient temperature and pressure, generating a benign solid byproduct. Quantitative analysis of milled samples revealed high destruction efficiencies of 99.95% to 100% for five different PFSAs subjected to MCD conditions in the presence of SiO2 only. Extensive nontargeted analysis showed that, during degradation, other PFASs form and are ultimately destroyed upon extended mechanochemical treatment. Direct polarization (DP) and cross-polarization (CP) solid-state nuclear magnetic resonance (SSNMR) spectroscopy showed abundant silicon-fluorine (Si-F) bond formation post-MCD, indicating that fluorine was secured in a stable reservoir. Collectively, these results identified the degradation profile for an environmentally sound and effective PFSA degradation process that is amenable to scale-up.

The amphiphilic nanostructure of ionic liquids affects the dehydration of alcohols.

The effect of the amphiphilic nanostructure of ionic liquids on the dehydration of secondary alcohols to alkenes has been investigated. The influence of these nanostructures was inverted when an acid catalyst was added to the reaction. This phenomenon was ascribed to a balance between ion-solute interactions and the formation of solute-catalyst hydrogen bonds, highlighting the complex interplay between interactions and reaction outcomes in these nanostructured solvent systems.

Electrochemical Stability of Zinc and Copper Surfaces in Protic Ionic Liquids.

Ionic liquids are versatile solvents that can be tailored through modification of the cation and anion species. Relatively little is known about the corrosive properties of protic ionic liquids. In this study, we have explored the corrosion of both zinc and copper within a series of protic ionic liquids consisting of alkylammonium or alkanolammonium cations paired with nitrate or carboxylate anions along with three aprotic imidazolium ionic liquids for comparison. Electrochemical studies revealed that the presence of either carboxylate anions or alkanolammonium cations tend to induce a cathodic shift in the corrosion potential. The effect in copper was similar in magnitude for both cations and anions, while the anion effect was slightly more pronounced than that of the cation in the case of zinc. For copper, the presence of carboxylate anions or alkanolammonium cations led to a notable decrease in corrosion current, whereas an increase was typically observed for zinc. The ionic liquid-metal surface interactions were further explored for select protic ionic liquids on copper using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to characterize the interface. From these studies, the oxide species formed on the surface were identified, and copper speciation at the surface linked to ionic liquid and potential dependent surface passivation. Density functional theory and ab initio molecular dynamics simulations revealed that the ethanolammonium cation was more strongly bound to the copper surface than the ethylammonium counterpart. In addition, the nitrate anion was more tightly bound than the formate anion. These likely lead to competing effects on the process of corrosion: the tightly bound cations act as a source of passivation, whereas the tightly bound anions facilitate the electrodissolution of the copper.

Guanidinium solvents with exceptional hydrogen bond donating abilities.

Guanidinium chloride-based solvents have been prepared using deep eutectic solvent principles. Strong hydrogen-bond (H-bond) donating abilities are established based on a range of measures of solvent polarity, including a novel 31P NMR chemical shift method. The physical properties and origin of the strong H-bonding ability of these solvents have been explored.

Mixing divalent ionic liquids: effects of charge and side-chains.

We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion where two charged imidazolium groups in the cations are either directly bound to each other or linked by a single atom. We assessed the influence of the side-chain functionality and divalency on their physical properties and on the thermodynamics of mixing. The results indicate that shortening the spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge of the cations in the mixture.

Operationalization of a microbial electrolysis cell: The interaction of the primary factors for energy storage efficiency.

Microbial electrolysis cells have attracted attention as a method to enhance anaerobic digesters' performance. However, optimization of individual factors is not directly transferrable among systems as many are intimately linked and influenced by the system design, influent, and inoculum. To avoid this, here the effects and interactions between the relative electrode size, hydraulic retention time (HRT), and voltage imposed have been explored within a pair of otherwise identical reactors. Methane production has a positive correlation with the applied voltage, reaching 12.9 mLCH4 L-1h-1 with 10 days HRT and 1000 mV, also achieving 35% energy storage efficiency, despite the higher electrical input. Shorter HRTs led to bacterial washouts, reducing the methane production below 10 mLCH4 L-1h-1. Contour plots were constructed to relate the energy storage efficiency with operational conditions changes. These highlighted the benefits of using a relatively larger cathode than anode for improving energy storage efficiency.

Structural investigations of molecular solutes within nanostructured ionic liquids.

Ionic liquids (ILs) containing sufficiently long alkyl chains form amphiphilic nanostructures with well-defined polar and non-polar domains. Here we have explored the robustness of these amphiphilic nanostructures to added solutes and gained insight into how the nature of the solute and IL ions affect the partitioning of these solutes within the nanostructured domains of ILs. To achieve this, small angle X-ray scattering (SAXS) investigations were performed and discussed for mixtures of 9 different molecular compounds with 6 different ILs containing imidazolium cations. The amphiphilic nanostructure of ILs persisted to high solute concentrations, over 50 mol% of added solute for most 1-butyl-3-methylimidazolium ILs and above 80 mol% for most 1-decyl-3-methylimidazolium ILs. Solute partitioning within these domains was found to be controlled by the inherent polarity and size of the solute, as well as specific interactions between the solute and IL ions, with SAXS results corroborated with IR spectroscopy and molecular dynamics simulations. Molecular dynamics simulations also revealed the ability to induce π+-π+ stacking between imidazolium cations through the use of these added molecular compounds. Collectively, these results provide scope for the selection of IL ions to rationally influence and control the partitioning behaviour of given solutes within the amphiphilic nanostructure of ILs.

On the structural origin of free volume in 1-alkyl-3-methylimidazolium ionic liquid mixtures: a SAXS and 129Xe NMR study.

Ionic liquid (IL) mixtures enable the design of fluids with finely tuned structural and physicochemical properties for myriad applications. In order to rationally develop and design IL mixtures with the desired properties, a thorough understanding of the structural origins of their physicochemical properties and the thermodynamics of mixing needs to be developed. To elucidate the structural origins of the excess molar volume within IL mixtures containing ions with different alkyl chain lengths, 3 IL mixtures containing 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs have been explored in a joint small angle X-ray scattering (SAXS) and 129Xe NMR study. The apolar domains of the IL mixtures were shown to possess similar dimensions to the largest alkyl chain of the mixture with the size evolution determined by whether the shorter alkyl chain was able to interact with the apolar domain. 129Xe NMR results illustrated that the origin of excess molar volume in these mixtures was due to fluctuations within these apolar domains arising from alkyl chain mismatch, with the formation of a greater number of smaller voids within the IL structure. These results indicate that free volume effects for these types of mixtures can be predicted from simple considerations of IL structure and that the structural basis for the formation of excess molar volume in these mixtures is substantially different to IL mixtures formed of different types of ions.

The A Priori Design and Selection of Ionic Liquids as Solvents for Active Pharmaceutical Ingredients.

In this paper we derive a straightforward computational approach to predict the optimal ionic liquid (IL) solvent for a given compound, based on COSMO-RS calculations. These calculations were performed on 18 different active pharmaceutical ingredients (APIs) using a matrix of 210 hypothetical ILs. These results indicated that the 18 APIs could be classified into three distinct categories based on their relative hydrogen bond donating or accepting ability, with similar optimal IL solvent predictions within each class. Informed by these results, a family of strongly hydrogen bond donating ILs based on the N-alkylguanidinium cation were prepared and characterized. The solubility of the APIs in each of these classes was found to be qualitatively consistent with the predictions of the COSMO-RS model. The suitability of these novel guanidinium salts as crystallization solvents was demonstrated by the use of N-butylguanidinium bis(trifluoromethanesulfonyl)imide for the purification of crude fenofibrate using dimethylsulfoxide as an antisolvent, which resulted in good yields and excellent purities. Finally, a simple descriptor based model is proposed to suggest the best IL solvent for arbitrary APIs.

Linking the structures, free volumes, and properties of ionic liquid mixtures.

The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [C4C1im]+ cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and 129Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and 129Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.

A structural investigation of ionic liquid mixtures.

The structures of mixtures of ionic liquids (ILs) featuring a common 1-butyl-3-methylimidazolium ([C4C1im](+)) cation but different anions have been investigated both experimentally and computationally. (1)H and (13)C NMR of the ILs and their mixtures has been performed both on the undiluted liquids and those diluted by CD2Cl2. These experiments have been complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These techniques have identified the formation of preferential interactions between H(2) of the imidazolium cation and the most strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion to interact above the imidazolium ring through anion-π(+) interactions has been identified. The modelling of these data has identified that the magnitude of these preferences are small, of the order of only a few kJ mol(-1), for all IL mixtures. No clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the reorientation of the cation within a randomly assigned network of anions. π(+)-π(+) stacking of the imidazolium cations was also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily disrupted by the introduction of small proportions (<50 mol%) of a weakly coordinating anion due to their propensity to form anion-π(+) interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit ideal, or nearly ideal, behaviour.

Self-association during heterogeneous nucleation onto well-defined templates.

We investigated the interplay between self-associates in solution and surface templating by studying the crystallization behavior of isonicotinamide (INA) and 2,6-dihydroxybenzoic acid (DHB) in the presence of self-assembled monolayers (SAM). The end group of the SAM as well as the hydrogen-bonding capabilities of the solvent and self-association of INA and DHB were found to be important in polymorph crystallization on SAMs. In the case of INA in ethanol, both chain and dimer self-associates are present in the solution. In the absence of SAMs the polymorph form II (dimer structure) is the crystallization outcome. In ethanol the 4-mercaptopyridine and 4-mercaptobenzoic acid SAMs organize INA chain associates at the template surface and enable the crystallization of form I while the 16-mercaptohexadecanoic acid SAM results in the crystallization of form II. Raman spectroscopy suggests that molecular interactions between INA and the SAM are responsible for the formation of specific polymorphs. XRPD results in the identification of the orientation of the crystal on the surface that further verified the results obtained by Raman spectroscopy. In nitrobenzene and nitromethane INA associates in solution only as chains and crystallization results in the formation of form IV and form I, respectively (both chain forms). The crystals formed in the bulk solution and on SAMs were the same, which seems to indicate that the self-association in nitrobenzene and nitromethane is not influenced by the presence of templates. In the case of DHB in toluene and chloroform, all three SAMs nucleated only one type of polymorph (stable form 2). In the case of toluene the polymorphic outcome was stable form 2 instead of metastable form 1, which is favored in toluene in the absence of the SAMs. Again, Raman spectroscopy and XRPD suggest that DHB-SAM molecular interactions may be responsible for the formation of form 2.

Steric, hydrogen-bonding and structural heterogeneity effects on the nucleophilic substitution of N-(p-fluorophenyldiphenylmethyl)-4-picolinium chloride in ionic liquids.

The nucleophilic substitution of N-(p-fluorophenyldiphenylmethyl)-4-picolinium chloride was investigated using water and a range of alcoholic nucleophiles in ionic liquid solvents. The reactivity patterns across the nucleophiles examined could be attributed to steric factors, which mediated the relative nucleophilicities. Reducing the hydrogen-bond acidity of the ionic liquid cation was found to generally increase the rate of reaction, however, the magnitude of this rate effect could be influenced by the steric bulk of the nucleophile and the structural heterogeneity of the ionic liquid. Preferential solvation phenomena in binary mixtures of ionic liquids were examined and suggest that the mechanism behind the hydrogen-bond solvation phenomenon arises from direct cation-mediated, rather than indirect anion-mediated, effects.

Controlling hydrolysis reaction rates with binary ionic liquid mixtures by tuning hydrogen-bonding interactions.

The ability of a binary ionic liquid (IL) system consisting of a phosphonium transition state analogue (TSA) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf(2)]) to accelerate the rate of the well-studied hydrolysis of a tert-alkyl picolinium salt by influencing the solvent structure was investigated. A significant rate enhancement was observed in the presence of the TSA; however, comparison with other cations illustrated that this enhancement was not unique to the chosen TSA. Instead, the rate enhancements were correlated with the dilution of hydrogen bonding by the added cations. This phenomenon was further examined by the use of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BMMIM][NTf(2)]) as a cosolvent and the use of Reichardt's dye to measure the extent of hydrogen bonding on solutes in these systems. The rate increases are rationalized in terms of weaker hydrogen bonding from the solvent system to water.