## 2 Materials and Methods  
### Synthesis and Initial Characterization of Chemicals  
The reagents 1-methylimidazole (>99%), 1,2-dimethylimidazole (98%), propyl iodide (98%), ammonium hexafluorophosphate (99.5%), and ammonium tetrafluoroborate (99.5 wt%) were purchased from Fluka and used as received. Deionized H\({}_{2}\)O was obtained with a Millipore ion-exchange resin deionizer. MPrImX and MMPrImX (with X = [I\({}^{-}\)], [BF\({}_{4}\)\({}^{-}\)], and [PF\({}_{6}\)\({}^{-}\)]) were synthesized as follows. A mixture of 1-methylimidazole (7.78 mL, 100 mmol) and propyl iodide (9.74 mL, 50 mmol) was heated at 120 \({}^{\circ}\)C for 3 min. During this period, it was additionally exposed to microwaves. The yield of the reaction was 87%. In the next step, the reaction mixture was evaporated at reduced pressure. Then diethyl ether (5 \(\times\) 20 mL) was used to wash the product several times in order to remove any excess propyl iodide. Thereafter, the product was dried under vacuum for 8 h to remove the solvent, yielding a product with sufficiently high purity. The [BF\({}_{4}\)\({}^{-}\)] ILs were prepared employing the same procedure. On the other hand, the [PF\({}_{6}\)\({}^{-}\)] ILs were obtained through a metathesis anion exchange reaction from IL iodide. They were washed with distilled water several times, yielding a 2-phase mixture: the ionic liquid at the bottom and an aqueous solution at the top. The former was separated from the latter by centrifugation (3000 rpm) for 60 s. A rotary evaporator was used to remove residual volatile compounds from the ionic liquid phase at reduced pressure. Then the liquid was washed with fresh ethyl acetate (100 mL). The same protocols were followed to synthesize the methylated ILs, using 1,2-dimethylimidazole as the precursor. In order to remove water-soluble impurities and obtain sufficiently high purity, all ILs were washed with high-purity water under stirring. Then the 2 phases were separated and dried during continuous agitation under vacuum conditions (\(\approx\)10 Pa) at 70 \({}^{\circ}\)C over several hours to reduce the water content and volatile compounds. Prior to use, all of the ILs were dried on a high-vacuum line (\(p<10^{-5}\) bar) for 4 days at \(\sim\)40 \({}^{\circ}\)C. The water content was measured by coulometric Karl Fischer titration (Metrohm 831). The obtained values are summarized in Table 1. The drying step is important, as the [BF\({}_{4}\)\({}^{-}\)] and iodide ILs are hydrophilic and significant amounts of water would have an impact on the interionic interactions. \({}^{1}\)H NMR (400 MHz), \({}^{13}\)C NMR, \({}^{31}\)P, and \({}^{19}\)F (100.6 MHz) spectra were recorded on a DRX 400 MHz spectrometer. The chemical shifts (\(\delta\)) are given in ppm and referenced to the internal solvent signals, namely TMS (tetramethylsilane), H\({}_{3}\)PO\({}_{4}\), and CFCl\({}_{3}\). An initial set of IR spectra for identification purposes was recorded on an FT-IR Perkin-Elmer Spectrum BX spectrophotometer with a resolution of  
Figure 1: Chemical structure of (**a**) cations and (**b**) anions. The blue numbers in (**a**) indicate the atom numbering used in the discussion. 4 cm\({}^{-1}\) in the range 4000-650 cm\({}^{-1}\). The IR spectra discussed in the Results section were recorded with higher resolution, as described in Section 2.3. The structures of the obtained products were confirmed using \({}^{1}\)H, \({}^{13}\)C, \({}^{19}\)F, \({}^{31}\)P-NMR, and FT-IR spectroscopy. The data also confirm the absence of significant impurities, e.g., residuals of the reactants or byproducts.