Crystal structures of protic ionic liquids

Abstract

Protic ionic liquids (PILs) are a class of ionic liquids that have an available proton for hydrogen bonding, offering unique physicochemical properties with applications in various fields, such as catalysis, energy storage, and separation processes. Despite their potential, the crystal structures of PILs remain underexplored due to their innate preference for amorphous phases, even as solids. This study investigates the crystal structures and phase behavior of a series of 24 alkylammonium-based PILs (with melting points up to 425 K) using synchrotron x-ray powder diffraction. Of these, 18 were able to be crystallized, and full crystal structures were obtained for 5. Our results reveal that the PILs investigated can be broadly divided into two groups, those with hydrogen-bonded networks dominating and those with a lamellar structure, separated into hydrophobic and hydrophilic regions. For both groups, the bonding dominant in the crystalline phase is similar to that of the liquid state. We discuss the challenges in crystallizing PILs and the approaches that can be used, laying the groundwork for further exploration of their structures.