Design and molecular modelling of phenolic-based protic ionic liquids

Abstract Five new phenolic-based protic ionic liquids (PILs) were successfully synthesised via neutralisation reaction. The synthesised PILs were characterised using spectral analyses such as 1H NMR, FTIR and thermogravimetric analysis. The effects of alkyl chain length and temperature towards the physical properties of the PILs were comprehensively investigated as well. The alkyl chain length of the cation and temperature significantly influenced both the density and viscosity of the PILs. As the alkyl chain length elongated from 2-hydroxy-N-methylethanaminium salicylate (2HMES) to 2-hydroxy-N-propylethanaminium salicylate (2HBES) the density decreased from 1158.40 to 1110.60 kg.m−3 (at 293.15 K) and viscosity increased from 541.69 to 1383.00 mPa.s (at 293.15 K). Moreover, the density and viscosity of the PILs declined steadily as the temperature elevated from 293.15 to 363.15 K. Furthermore, the structural conformation of the PILs from the spectral analyses was further validated by the density function theory (DFT) calculation. Based on the optimised structure from the computational study, the most favourable interaction occurred between the –NH and –COO groups of the ion-pairs resulted from the transfer of hydrogen atom from acid to base.