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).