**4. Conclusions**

In this work, six new ammonium-based PILs have been successfully synthesized through a one-step procedure. The thermophysical properties including density, viscosity, refractive index and thermal stability have been measured. The experimental results revealed the dependency of the experimental values namely the *ρ*, *η*, *n*<sup>D</sup> and Td on the alkyl chain of the anion, size of the cations and the temperature of measurement. The phase transition analysis of the PILs yielded the glass transition temperature (Tg) and melting point (Tm) of the PILs studied. These synthesized ammonium-based PILs have been tested for their ability towards CO2 absorption in which [BEHA][C7] displayed the highest CO2 uptake in the experimental conditions signifying its capability to be a potential solvent in the application of CO2 capture. Future works should include CO2 desorption studies of the PILs for the purpose of recyclability and sustainability of the absorbents.

**Supplementary Materials:** The following are available online, Table S1: Density (*ρ*) values of PILs at temperatures (293.15–363.15) K, Table S2: Dynamic viscosity (*η*) values of PILs at temperatures (293.15–363.15) K, Table S3: Refractive index (*n*D) values of PILs at temperatures (293.15–333.15 K), Figures S1–S12: NMR analysis of the PILs, Figure S13: Plots of density (*ρ*) values with standard errors of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature, Figure S14: Plots of viscosity (*η*) values with standard errors of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature, and Figure S15: Plots of refractive index (*n*D) values with standard errors of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature.

**Author Contributions:** Conceptualization, N.M.Y. and N.H.Z.O.Z.; methodology, N.H.Z.O.Z. and N.M.Y. validation, N.H.Z.O.Z. and A.H.A.R.; formal analysis, N.H.Z.O.Z. and N.M.Y.; resources, N.M.Y. and M.A.B.; data curation, N.M.Y.; writing—original draft preparation, N.H.Z.O.Z. and A.H.A.R.; writing—review and editing, N.M.Y.; supervision, N.M.Y. and M.A.B.; project administration, N.M.Y. and M.A.B.; funding acquisition, N.M.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Yayasan Universiti Teknologi PETRONAS-Fundamental Research Grant (YUTP-FRG) (cost centre 015LC0-054) under research project "Design, Synthesis and Evaluation of Protic Ionic Liquids for CO2 Removal from Natural Gas", and the APC was funded by 015LC0-054.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study is available in this article and Supplementary Materials.

**Acknowledgments:** The authors would like to acknowledge the financial assistance and support provided by Universiti Teknologi PETRONAS and the Centre of Research in Ionic Liquids (CORIL), UTP. **Conflicts of Interest:** The authors declare no conflict of interest.
