Influence of the cation partner on levulinate ionic liquids properties

In the context of bio-based ionic liquids (ILs), in recent years levulinate ILs have been explored in various applications with appreciable performances. To expand their scope of use, seven selected levulinate ILs featuring different cationic moieties have been prepared and their physico-chemical properties compared. While all the ILs generated displayed only glass transitions at low temperature (<−60 °C), [C₄C₁Pyr]Lev presented melting and crystallization events. The thermal stability determined by TGA decreased in the order [P₈₈₈₁]Lev > [C₂C₁Im]Lev ≈ [C₄C₁Im]Lev > [C₂C₁Pip]Lev > [C₂C₁Pyr]Lev ≈ [C₄C₁Pyr]Lev > [N₈₈₈₁]Lev, and is in good agreement with the general trend observed for carboxylate ILs. In terms of viscosity, the LevILs showed a Newtonian behaviour when considering the shearing rates range, with a marked reduction of the viscosity at increasing temperatures. The glassy behaviour of LevILs was further ascertained applying the MYEGA viscosity-based model for the determination of the fragility index. The density of the LevILs was found to vary in the vicinity of 1 g / cm³ , and the systematic refractive index increase with increasing density was identified as a general trend. The refractive index was experimentally determined at five different wavelengths and several temperatures between 80 °C and 130 °C and followed the order [C₂C₁Im]Lev > [C₂C₁Pip]Lev > [C₄C₁Im]Lev > [C₂C₁Pyr]Lev > [C₄C₁Pyr]Lev > [P₈₈₈₁]Lev > [N₈₈₈₁]Lev. Overall, the possibility to tune the physico-chemical properties of ILs through structural variations of the cationic part has been proven valid for the LevIL family. When compared with the carboxylate ILs described in the literature, LevILs show great potential for further applicative studies.