Abstract
Ionic liquids which are often classified as low melting point salts have received significant attention from research groups and industries to be used in a wide range of applications. Many of these applications require thorough knowledge on the thermophysical properties of the ionic liquids before utilizing their full potentials in various fields. In this work, a series of alkylammonium cation and carboxylate anion-based room temperature protic ionic liquids (PILs) were synthesized by varying length of alkyl chain of the cation from diethyl to dibutyl combined with pentanoate, hexanoate and heptanoate anions. These ammonium-based PILs named as diethylammonium pentanoate [DEA][C5], diethylammonium hexanoate [DEA][C6], diethylammonium heptanoate [DEA][C7], dibutylammonium pentanoate [DBA][C5], dibutylammonium hexanoate [DBA][C6] and dibutylammonium heptanoate [DBA][C7] were characterized using Nuclear Magnetic Resonance (NMR) spectroscopy. The thermophysical properties of the PILs namely density, dynamic viscosity and refractive index were measured and analyzed. Density, ρ and dynamic viscosity, η were determined at T = (293.15 to 363.15) K and refractive index, nD was measured at T = (293.15 to 333.15) K. The fitting parameters are proposed for the empirical correlations of density, dynamic viscosity and refractive index. The values of thermal expansion coefficient, αp, molecular volume, Vm, standard entropy, S° and lattice potential energy, Upot also have been calculated by using the specified equations. The thermal decomposition temperature, Td was also determined using a thermogravimetric analyzer (TGA) while the differential scanning calorimetry (DSC) technique provided the glass transition, Tg, melting point, Tm and crystallization, Tc temperatures of the PILs. The experimental results revealed that the dependency of the experimental values namely the ρ, η, nD, and Td on the alkyl chain of the anion, size of the cations and the temperature of measurement.
Highlights
Ionic liquids (ILs) are molten salts generally made from large organic cations and organic or inorganic anions
We report the synthesis of six new ammonium-based protic ionic liquids (PILs), namely diethylammonium pentanoate [DEA][C5], diethylammonium hexanoate [DEA][C6], diethylammonium heptanoate [DEA][C7], dibutylammonium pentanoate [DBA][C5], dibutylammonium hexanoate
In order to remove traces of water that might be coming from surrounding atmosphere or from the starting reagents, the synthesized PILs were heated at 70 ◦ C under vacuum approximately for 8 h upon the completion of the reaction [35]
Summary
Ionic liquids (ILs) are molten salts generally made from large organic cations and organic or inorganic anions. These ions interact poorly with each other which causes the salts to exist in liquid forms at room temperature or below 100 ◦ C. Processes 2020, 8, 742 flammability, high specific solubility, wide liquid range, high electrical conductivity and good thermal and chemical stability [1,2,3,4,5]. These attractive properties have driven researchers to utilize. Since PILs can be synthesized by the transfer of protons from Brønsted acids to Brønsted bases, PILs can be useful solvents for acid catalyzed reactions such as Diels–Alder reaction [11,12,13]
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