A Pseudo Hydronium Solvate Ionic Liquid and Taxonomy of the Allied Protic Media

A ternary equimolar mixture of water, 18-crown-6-ether (18C6), and (+)-10-camphorsulfonic acid (CSA), [H₂O]/[18C6]/[CSA] = 1/1/1 by mole, has been prepared and characterized as a "pseudo hydronium solvate ionic liquid (IL)." In this system, 18C6-coordinated water is only partially protonated by CSA, resulting in a coexistence of 18C6-solvated hydronium (H₃O⁺) ions and neutral [H₂O·18C6] complexes. Unlike previously reported protic media formed with superacids, which undergo complete proton transfer to form solvate ILs, this system retains a significant fraction of neutral species due to the moderate ΔpK_a between CSA and water. Spectroscopic measurements including diffusion coefficient assessments reveal that proton transport proceeds via a Grotthuss-like mechanism involving neutral complexes, distinguishing this medium from both vehicle-type and fast-proton-diffusive solvate ILs. Analysis using dimensionless parameters-self-diffusion coefficient and molar conductivity ratios-offers a quantitative framework for profiling ionicity and transport behavior in such systems. These findings provide fundamental insights into proton conduction mechanisms in pseudo-ionic environments and establish a rational basis for designing new proton-conducting materials. The chiral nature of the CSA^- anion further expands the potential of this system for applications in asymmetric catalysis, electrochemical sensing, and photoelectrochemical energy conversion devices.