Shurui MIAO1, Haihui Joy JIANG1, Silvia IMBERTI2,4, Rob ATKIN3, Gregory WARR1
1School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, Australia
2STFC, ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Didcot OX11 0QX, Didcot, United Kingdom
3School of Molecular Sciences, The University of Western Australia, Perth, Australia
4ACS Publications, ACS International Ltd., Oxford, United Kingdom
The controlled addition of water to ionic liquids (ILs) is often used as a tool to regulate the physical properties of many ILs without significantly affecting their solvent properties.[1] At low concentration, trace water is often seen as an impurity that can convolute the measured physical properties such as viscosity and melting point of ILs and deep eutectic solvents (DESs).[2, 3] However, water is also a paragon among hydrogen-bonding species and it is biofriendly. Its potential as a hydrogen bond donor (HBD) in biocompatible DES formulation has been largely overlooked. To design fully biocompatible DESs, the ionic component should also be bio-friendly. Choline amino acid salts stood out as promising candidates as both ions are derived from biomolecules and shown to be non-toxic.[4] In this study, we have investigated the structure and phase behaviour of biocompatible, aqueous deep eutectic solvents by combining choline acetate, hydrogen aspartate, and aspartate with water (as the sole molecular HBD).[5] Using contrast-variation neutron diffraction, interpreted via computational modelling, we show how the interplay between anion structure and water content affects the structure of hydrogen-bond networks in the liquid, which in turn influences the eutectic composition and temperature. These mixtures expand the current range of choline amino acid ionic liquids under investigation for biomass processing applications. This study also demonstrates how choline amino acid ionic liquids retain their selective lignin solubility as aqueous solutions and the concentration-dependent role of water as a molecular additive.[4]
[1] J. Shi, K. Balamurugan, R. Parthasarathi, N. Sathitsuksanoh, S. Zhang, V. Stavila, V. Subramanian, B.A. Simmons, S. Singh, Understanding the role of water during ionic liquid pretreatment of lignocellulose: co-solvent or anti-solvent?, Green Chemistry, 16 (2014) 3830-3840.
[2] X.-Q. Meng, K. Ballerat-Busserolles, P. Husson, J.-M. Andanson, Impact of water on the melting temperature of urea plus choline chloride deep eutectic solvent, New Journal of Chemistry, 40 (2016) 4492-4499.
[3] O.S. Hammond, D.T. Bowron, K.J. Edler, The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution, Angewandte Chemie International Edition, 56 (2017) 9782-9785.
[4] S. Miao, R. Atkin, G. Warr, Design and applications of biocompatible choline amino acid ionic liquids, Green Chemistry, 24 (2022) 7281-7304.
[5] S. Miao, H.J. Jiang, S. Imberti, R. Atkin, G. Warr, Aqueous choline amino acid deep eutectic solvents, The Journal of Chemical Physics, 154 (2021) 214504.