*Article* **Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture**

**Nur Hidayah Zulaikha Othman Zailani <sup>1</sup> , Normawati M. Yunus 1,\* , Asyraf Hanim Ab Rahim <sup>1</sup> and Mohamad Azmi Bustam <sup>2</sup>**


**Abstract:** Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO2 capture. This work presents the synthesis, thermophysical properties and the CO2 absorption of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids (PILs), namely 2-ethylhexylammonium pentanoate ([EHA][C5]), 2-ethylhexylammonium hexanoate ([EHA][C6]), 2-ethylhexylammonium heptanoate ([EHA][C7]), bis-(2-ethylhexyl)ammonium pentanoate ([BEHA][C5]), bis-(2-ethylhexyl)ammonium hexanoate ([BEHA][C6]) and bis-(2-ethylhexyl)ammonium heptanoate ([BEHA][C7]). The chemical structures of the PILs were confirmed by using Nuclear Magnetic Resonance (NMR) spectroscopy while the density (*ρ*) and the dynamic viscosity (*η*) of the PILs were determined and analyzed in a range from 293.15K up to 363.15K. The refractive index (*n*D) was also measured at T = (293.15 to 333.15) K. Thermal analyses conducted via a thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC) indicated that all PILs have the thermal decomposition temperature, Td of greater than 416K and the presence of glass transition, Tg was detected in each PIL. The CO2 absorption of the PILs was studied up to 29 bar at 298.15 K and the experimental results showed that [BEHA][C7] had the highest CO2 absorption with 0.78 mol at 29 bar. The CO2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.

**Keywords:** ammonium-based protic ionic liquids; density; viscosity; refractive index; phase transition; thermal expansion coefficient; standard entropy; lattice potential energy; CO2 absorption
