Text S7. Possible factors affecting theTc
To further elucidate the reason for, the properties of amines with distinctively different functional groups (Table S1) were compared to identify possible factoring contributing to the significant difference in Tc values among amines.
Neither gaseous basicity (GB), aqueous basicity (Kb ) or the volatility of an RNC or its nitrate salt (psat and Kp , respectively) seems to directly determine the Tcof an RNC. For example, DMA and MEA have similar GB values yet distinctively different Tc . When compared with PZ, TMA has a similar Kb yet aTc that is ~ 28 K lower. PZ has a much higher GB than MEA but their Tc values are comparable. AN and MEA have similar saturation vapor pressures at room temperature yet distinctive different Tc , ruling out any direct contribution on Tc from the volatility of the amine. The solid/gas dissociation constants of the nitrates of alkylamines are about 10 times lower than that of ammonium nitrate yet they exhibit comparable Tc , suggesting that saturation condensation of nitrate salts alone may not explain the difference in Tc for Group I and II RNCs. Furthermore, the molar mass of IBA is about 50% more than that of MEA, but their Tc values are essentially the same, suggesting that Van der Waals forces do not contribute significantly here. It appears that the electron affinity of the functional groups on the amines has little effect on theTc . For example, the nitro group in AN-N and the methyl group in AN-M are electron withdrawing and electron donating groups, respectively. However, both RNCs showed little contribution to NA condensation at above 260 K.
One distinctive observation is that amines with only alkyl (e.g., MA) or aromatic (e.g., AN) substitutions showed much lowerTc than those with hydroxyl (–OH) groups (e.g., MEA). Furthermore, IBA and PZ, showed Tc values comparable to that of MEA, with the former having a similar chemical structure as MEA, while the latter having a second amine group (–NH–) in the ring (Table 1). Since MEA and PZ do not share the same functional groups, their high Tc values could not be explained alone by the presence of –OH groups in the chemical structure.
Another factor that may affect the Tc is hydrophobicity. AN and AN-M both contain an aromatic ring which is more hydrophobic than a methyl group. The decreasing solubility of amines in water results in less NA condensed and dissolved in the particle, greatly diminishing the condensation of NA compared with MA. In the case of AN-O, the –OH attached to the aromatic ring can form additional hydrogen bonds, which seems to offset the negative effect of the non-polar aromatic ring and increase the Tc to 289K (+15 °C).