Protic Ionic Liquids Based on Decahydroisoquinoline: Lost Superfragility and Ionicity-Fragility Correlation

Physicochemical properties, ionicity, and fragility for protic ionic liquids (PILs) based on the protonation of the extremely fragile molecular liquid decahydroisoquinoline (DHiQ) by various Bronsted acids have been studied. The ionicity was evaluated using the Walden plot diagnostic, while the m-fragility (slope of T-g-scaled Arrhenius plot at T-g) was quantitatively measured by the Moynihan-Wang-Velikov variable scan rate, differential scanning calorimetry, method. DHiQ-derived PILs prove to cover the whole range of IL ionicities from poor IL to good IL, and even superionic, assessed from the Walden plot, depending on the choice of Bronsted acid. We find that the superfragile character of the parent DHiQ becomes completely suppressed upon conversion to ionic liquid, the initial value m = 128 sinking m = 45-91 for the ionic liquid. Such values are in the intermediate to fragile range. The DHiQ-based PIL showing superionic behavior, anion [HSO4-], proves to be the case with the lowest m value (m = 45) so far reported for either aprotic or protic ILs. Both low fragility and dry proton conductivity can be attributed to an extended hydrogen bond network that is set up by the hydrogensulfate anion. The good DHiQ PILs have m values similar to those reported for typical aprotic ILs (m = 60-80), while the poor DHiQ PILs in which proton transfer from acid to base is weak show some memory of the parent fragility. Thus, a correlation of ionicity with m-fragility is characteristic of this system. A range of noncrystallizing, and also nonglassforming, behavior is observed in this series of compounds, suggesting a possible test for ideal glassformer existence.