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We have investigated the structure and phase behavior of biocompatible, aqueous deep eutectic solvents by combining choline acetate, hydrogen aspartate, and aspartate amino acid salts with water as the sole molecular hydrogen bond donor. Using contrast-variation neutron diffraction, interpreted via computational modeling, we show how the interplay between anion structure and water content affects the hydrogen bond network structure in the liquid, which, in turn, influences the eutectic composition and temperature. These mixtures expand the current range choline amino acid ionic liquids under investigation for biomass processing applications to include higher melting point salts and also explain how the ionic liquids retain their desirable properties in aqueous solution.

The liquid structures of six choline carboxylate/amino acid ionic liquids (bio-ILs) and their mixtures with water and various n-alkanols have been investigated by small-angle X-ray scattering (SAXS). The ILs exhibit long-range amphiphilic nanostructure comprised of polar and apolar domains that can be controlled by choice of anion, and which is tolerant to water dilution. Mixtures with n-alkanols can lead to marked changes in domain size and ordering. Utilising the Teubner-Strey model, we find amphiphilicity factors in many of these mixtures are comparable to those observed in conventional microemulsions, and that cooperative assembly in bio-IL/alkanol mixtures can enhance amphiphilicity, with potential to improve performance in a range of applications.

Millions of tons of solvents are produced annually and are ubiquitous across many industries ranging from coatings, paintings, textiles, pharmaceuticals, extraction of resources, to chemical synthesis. The global market for solvents is projected to rise as solution processes are typically safer, more efficient, and easier to handle on an industrial scale. Therefore, the search and use of greener solvents are fundamental to our endeavour for a sustainable future. Ionic liquids (ILs) present exciting opportunities as next-generation solvents due to their designer characteristics and high recyclability. However, concerns remain around their toxicity and biocompatibility, and the cost of production is often high due to the low atom economy. Recently, ILs derived from biomolecules have emerged as biocompatible solvents with negligible toxicity and straightforward preparation. Choline amino acid ILs (ChAAILs) are a representative family of biocompatible ILs. They exhibit anion-controlled solvent properties and have been extensively studied for applications including biomass processing and CO2 capture. This review discusses the structure–property relationships of ChAAILs and their mixtures, especially with water. This lays the foundation for the rational design of ChAAIL-based solvent systems with optimised physical properties and reduced costs for specific applications. [ABSTRACT FROM AUTHOR]

The structure of the ionic liquid choline lysinate ([Ch][Lys]) and its water mixtures, including a dissolved model lignin residue, guaiacol, is revealed at atomic and nanoscale resolution using neutron diffraction. The performance of [Ch][Lys] for biomass pretreatment depends on both its detailed H-bonding capacity and bicontinuous polar/apolar nanostructure, which is stabilised by lysinate adopting an H-bonded cyclic conformer that enhances its amphiphilicity. These features are heightened by the addition of water, which forms nanoscale pockets but also extends the H-bonding network within the IL polar domains while largely preserving its bicontinuous nanostructure, rendering the ionic liquid–water mixture an effective solvent for organic molecules bearing various functional groups. A model representative lignin compound, guaiacol, is found to H-bond through its phenol group to both the anion and water, destabilising the lysinate cyclic conformer and altering the liquid nanostructure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Low-q peak in SAXS is due to self-assembly of components, not the IL nanostructure. • At high dilution, key inter-ion interaction is the hydroxyl to carboxylate H-bond. • Inter-anion H-bond enables extended anion network in [Ch][Lys] mixtures. • Liquid structure can be engineered via abundance of H-bond donors. Choline amino acid ionic liquids ([Ch][AA]ILs) have shown outstanding capacity for extraction and biomass pre-treatment but their very high viscosity renders them difficult to handle and inhibits their large-scale application. As a result, [Ch][AA]ILs are commonly mixed with a low-viscosity molecular solvent such as water to form hybrid solvents, which preserves the favourable characteristics of ILs while enhancing physical properties. Here we investigate the structure of binary mixtures of choline lysinate and phenylalaninate with water and primary alcohols. Contrast-variation neutron diffraction and computational modelling shows the nanostructure of both [Ch][AA]ILs can be engineered by mixing with alcohols up to high concentrations, and that the abundance of hydrogen-bond donors is a critical determinant of the overall liquid structure. This provides a new strategy to engineer and fine tune solvent nanostructure for the design of application-specific, biofriendly hybrid solvent systems. [ABSTRACT FROM AUTHOR]

[Display omitted] Popular deep eutectic solvents (DESs) typically lack amphiphilic molecules and ions and therefore do not have the useful self-assembled nanostructures prevalent in many ionic liquids. We hypothesise that nanostructure in DESs can be induced via an amphiphilic hydrogen bond donor (HBD), and that nanostructure becomes better defined with HBD chain length. The structure of DESs formed from choline chloride mixed with either butyric acid (ChCl/BuOOH) or hexanoic acid (ChCl/HeOOH) in a 1:4 M ratio were studied using atomic force microscopy (AFM) imaging, force curves, and friction measurements combined with bulk rheology. DESs formed with both the C 4 and C 6 acids are nanostructured. As the length of the acid group is increased from C 4 to C 6 , AFM images reveal the nanostructure becomes larger and better defined due to the longer acid chain, and AFM force curves show the interfacial nanostructure extends further from the surface. Self-assembled nanostructure in these systems is a consequence of choline cations, chloride anions, and acid alcohol groups clustering together due to electrostatic attractions and hydrogen bonding to form polar domains. Acid alkyl chains are solvophobically excluded from the polar domains and aggregate into apolar domains. [ABSTRACT FROM AUTHOR]

Coal is a major resource in Australia, both for power generation and as an export good, however its environmental impact is of significant concern. The structure of coal is inherently complex and contains many heteroatoms, which negatively affect the combustion efficiency and cause pollution. This study reveals that pretreatment of two types of Australian coal, a lignite coal and a thermal bituminous coal, with low cost choline amino acid based ionic liquids, markedly reduces the particle size and heteroatom content. This method is rapid (3 h) and proceeds at a low temperature of 70 °C. FTIR, NMR and TGA analysis revealed that pretreated coal has reduced aliphatic carbon, carbonyl groups and sulphur compared to untreated coal. Choline arginate was the best performing pretreatment agent for both types of coal. This is attributed to multiple hydrogen bond acceptor sites on the arginate anion, which facilitates strong interaction with the coal surface. These results open new pathways towards more energy efficient and cleaner coal, using low cost ionic liquids. [ABSTRACT FROM AUTHOR]