*2.4. Viscosity (η) Measurement*

Viscosity is one of the important properties that governs the potential applications of any solvents, and it is largely influenced by intermolecular interactions namely hydrogen bonding, dispersive forces and columbic interactions [64]. The experimental data and the plots with standard errors for viscosity values are available in Table S2 and Figure S14, respectively, in the Supplementary Materials. The viscosity was measured in a temperature range of 293.15 to 363.15 K and graphically shown in Figure 4. The viscosity of all ammonium-based PILs decreased exponentially with an increase in temperature in each PIL as depicted in Figure 4. For example, the viscosity of [EHA][C5] at 293.15K is 45 times larger than at 363.15K. In another study, Liu et al. performed dynamic density measurement on three series of ILs, namely N-alkylpyridinium bis(trifluoromethylsulfonyl)imide ([Cnpy][NTf2], *n* = 2, 4, 5, 6), N-alkyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([Cn3mpy][NTf2], *n* = 2, 3, 4, 6) and N-alkyl-4-methylpyridinium bis(trifluoromethylsulfonyl) imide ([Cn4mpy][NTf2], *n* = 3, 4, 6) within a temperature range of T = (283.15 to 353.15) K [60]. They suggested that the dynamic viscosity increases with the extension of the alkyl side chain of the cation for the three series of pyridinium-based ILs. However, in this work, PILs with [EHA] cation display a higher viscosity value than PILs with [BEHA] cation. Basically, the van der Waals attraction between the aliphatic alkyl chain affects the viscosity values of the PILs [41,53]. However, the water content of the PILs may also affected the observed viscosity results. Furthermore, PILs with [BEHA] cation displayed a marginal increment in the viscosity values as the alkyl chain length of the anion increased.
