A Small Angle X-ray Scattering Study to Understand the Effect of Co-Solvents and Composition on the Amphiphilic Nanostructures of Ionic Liquids and Deep Eutectic Solvents.
Navjot K. KAHLON1,2, Emma L. MATTHEWMAN1,2, Tamar L. GREAVES3, Cameron C. WEBER1,2
1School of Chemical Sciences, The University of Auckland, Auckland, New Zealand
2MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand
3School of Science, RMIT University, Melbourne, Australia
In this ever-evolving field of neoteric solvents, both ionic liquids (ILs) and deep eutectic solvents (DESs) are specifically interesting due to their unique, tunable physicochemical properties. ILs are low-melting salts, often defined as melting below 373 K, forming liquids composed purely of ions. Of particular importance to this study is the propensity of many ILs to develop dichotomous, amphiphilic nanostructures in the presence of at least one amphiphilic ion. This arises from the strong cohesive interactions between the charged moieties of the constituent ions (forming polar domains), leading to a solvophobic exclusion of (usually cation) alkyl chains into apolar regions. DESs are low melting eutectic mixtures, i.e., mixtures that have a melting point lower than either of their components, typically formed from a hydrogen bond donor and acceptor. Given the weaker cohesive interactions between the molecular components and ions of DESs, their ability to develop similar nanostructures to ILs is quite fascinating. Yet the nanostructural organization of far fewer DESs has been investigated compared to ILs. Such amphiphilic ILs and DESs will be referred to here as nanostructured ionic solvents (NISs). These inherently structured soft matter systems can place electronic or steric constraints on reactants, transition states, or products, thus potentially affecting organic reactivity and reaction rates. This, as well as implications of these structures for a multitude of applications may require the NISs to be mixed or diluted by reagents or cosolvents, hence a systematic exploration into the resilience of these nanostructures is essential.
Towards developing a deeper understanding of how robust these amphiphilic nanostructures are and their effect on solute partitioning, we employed small angle X-ray scattering (SAXS) to examine the nanostructures of neat NISs and mixtures of selected NISs with 8 different molecular solutes/co-solvents. These solutes include water, 2-propanol, acetone, dimethyl sulfoxide, ethyl acetate, cyclohexanol, cyclohexanone, and cyclohexyl acetate to model a range of reagents varying in terms of properties such as functional groups, hydrophobicity, and molecular size. The NISs include over 20 DESs which vary systematically in the degree of amphiphilicity of their constituents. So, going from “solute in NIS” to “NIS in solute” in 10 mol% increments, 9 different mixtures were prepared for each NIS-molecular solute system. Also, the influence of the ratio of the constituents in these DESs was explored. This presentation will highlight the key findings from this systematic study and discuss what these outcomes mean for the robustness of the nanostructure of these solvents and for the partitioning of solutes.