Thermophysical properties of ammonium and hydroxylammonium protic ionic liquids

• Density, viscosity and sound velocity measured for five ammonium and hydroxylammonium based protic ionic liquids.
• Experimental density and viscosity data estimated using Gardas and Coutinho model and Vogel–Tamman–Fulcher equation.
• Effects of cation, anion and alkyl chain length on studied properties have been discussed.
• The intermolecular interactions were analyzed on the basis of derived properties.

In this work, five protic ionic liquids having propylammonium, 3-hydroxy propylammonium as cations and formate, acetate, trifluoroacetate as anions have been synthesized. Thermophysical properties such as density (ρ), viscosity (η) and sound velocity (u) have been measured at various temperatures ranging from (293.15 to 343.15) K at atmospheric pressure. The experimental density and viscosity were fitted with second order polynomial and Vogel–Tamman–Fulcher (VTF) equations, respectively. Also experimental densities were correlated with the estimated density proposed by Gardas and Coutinho model. The coefficient of thermal expansion (α) and isentropic compressibility (βs) values have been calculated from the experimental density and sound velocity data using empirical correlations. Lattice potential energy (UPOT) has been calculated to understand the strength of ionic interaction between the ions. Thermal decomposition temperature (Td) and glass transition temperature (Tg) along with crystallization and melting point were investigated using TGA and DSC analysis, respectively. The effect of alkyl chain length and electronegative fluorine atoms on anionic fragment as well as hydroxyl substituent on cationic side chain in the protic ionic liquids has been discussed for studied properties. The effect of ΔpKa over the studied properties has also been analyzed.