Influence of alkyl substituent on thermophysical properties and CO<inf>2</inf> absorption studies of diethylenetriamine- based ionic liquids

Diethylenetriamine (DETA) based protic ionic liquids (ILs) containing different carboxylate as anions have been synthesized and characterized by using NMR, ATR-FTIR, and mass spectroscopic techniques. The alkyl chain of carboxylate anion varied from formate (For) to octanoate (Oct). The synthesized DETA based ILs were subjected to CO2 absorption studies, and the results revealed that the CO2 absorption increases with increasing alkyl chain length of anion, and maximum CO2 absorption capacity were observed for [DETA][Oct]. The CO2 absorption results were confirmed by FTIR spectroscopic studies by monitoring the carbamate peak formation at 1634 cm−1. Furthermore, the physicochemical properties such as density (ρ), polarity, speed of sound (u), viscosity (η), and refractive index (nD) were determined at different temperatures from 293.15 to 333.15 K and pressure (0.1 MPa). The thermal stability of synthesized ILs was established using TGA analysis. The derived thermodynamic properties such as thermal expansion coefficient (α), isentropic compressibility (βS), intermolecular free length (Lf), lattice potential energy (UPOT), molecular volume (V), standard entropy (S°), activation energy (En), molar refraction (Rm) and free volume (Vf) were calculated from experimental data to predict the intermolecular interactions and space between constituent ions in the ILs and it was found that [DETA][Oct] having highest CO2 absorption capacity due to its higher free volume and basicity compare to other ionic liquids.