**4. Conclusions**

In this study, the mechanism of CO2 sorption amino acid based NADES, the effect of the hydrogen bond acceptor and hydrogen bond donor on carbon dioxide absorption via natural deep eutectic solvent systems is presented for the first time. These results will assist the scientific community while designing NADES for gas sorption applications. Both experimental and DFT simulation for NADES, HBA, and HBD were studied for their CO2 sorption performances. Strong van der Waals forces were observed as binding characteristics of CO2 with all of the studied cases, which supports the physisorption behavior of the HBA + CO2, HBD + CO2, as well as NADES + CO2 experimental behavior that did not yield any adsorption-desorption hysteresis. Furthermore, it has been observed that HBA plays a leading role in CO2 solubility for NADES cases. Polarity unbalance of Be and ChCl based NADES cases causes strong negative deviations from ideality when mixed with their organic HBD. This effect was also observed on the CO2 absorption plots (Figure 2), at which the NADES solubility performances were pushed further down, yielding lower performance in comparison to both HBA and HBD. Furthermore, considering Be and ChCl cases, both the experimental sorption and theoretical interaction energies are close to each other and higher than that of the Al cases. This could be explained, due to both HBA forming a positive charge shielded by alkyl groups, forming stronger intra H-bonding between the HBA and HBD, which also leads to larger interactions with surrounding CO2 molecule. Furthermore, such strong H-bonding establishment in between the HBA and HBD, for all cases, but specific to Be and ChCl cases, negative deviations from ideality in mixture, can be correlated to higher CO2 affinity establishment. Additionally, the strong H-bonding between HBA and HBD leads to very strong NADES structure and, thus, no disruption effect has been noticed when the NADES compounds were exposed to the CO2 environment. Consequently, the characteristics of NADES has been preserved in these cases. For novel solvent design via DES or NADES, in order to confirm the leading effect of HBA for gas solubility, there is a need for a more systematic analysis, especially considering a wider range of HBA and HBD, as well as different gases.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2227-9717/8/12/1533/s1, Figure S1: title. CO2 capture performance for hydrogen bond acceptors and hydrogen bond donors prior to mixing to form deep eutectic solvent at 298.15 K isotherm with (mol CO2/mol solvent units).

**Author Contributions:** Conceptualization, T.A., S.A. and M.A.; methodology, S.A. and M.A.; software, S.A. and M.A.; validation, T.A., S.A. and M.A.; formal analysis, T.A., A.A., S.A. and M.A.; investigation, S.A. and M.A.; resources, A.A., S.A. and M.A.; data curation, M.A.; writing—original draft preparation, T.A., S.A. and M.A.; writing—review and editing, T.A., A.A., S.A. and M.A.; visualization, M.A.; supervision, A.A., S.A. and M.A.; project administration, M.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge SCAYLE (Supercomputación Castilla y León, Burgos, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
