Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 70

Fig. 3.5 Determination
of the reaction model
for vaporization of
2,2′-bipyridyl. (Reproduced
from Vecchio et al. [ 23 ] with
permission of Elsevier)
has been determined by using the technique described in Sect. 2.2.2. According
to Fig. 3.5 , the best-fitting model of this process is N13, which is g ( ʱ ) = ʱ (i.e.,
f ( ʱ ) = 1) or zero order [ 23 ]. This is obviously consistent with the basic assumption
of Eqs. 3.5 and 3.9. However, as already mentioned, this is not always the case. For
example, both vaporization and, especially, sublimation of ammonium nitrate dem-
onstrate clear deviation from the zero order to decelerating type of kinetics [ 24 ].
The rate of diffusion of the condensed substance vapor in the surrounding gas is
a very important factor when measurements are conducted under the conditions of
regular thermal analysis experiments. The surrounding gas is a purge gas, such as
nitrogen, that is delivered to the sample at an ambient pressure and a certain flow
rate. If the forming vapor diffuses too slowly, the surrounding gas may become
saturated with it. The local vapor pressure may start approaching its equilibrium
values that would promote the reverse reaction of condensation. That is why the rate
of vaporization or sublimation should be measured at sufficiently fast flow rates that
would secure efficient removal of the forming vapor and suppress its condensation.
The effect of the purge gas flow rate on vaporization of methyl salicylate has
been demonstrated by Cheng et al. [ 25 ]. It has been found that a systematic increase
in the flow rate of nitrogen resulted in a small but systematic shift of TGA mass loss
curves to lower temperature. This effect is typical to find in reversible processes
[ 26 ]. The isoconversional activation energies of vaporization also have demonstrat-
ed a systematic shift as illustrated in Fig. 3.6 . It is seen as an increase in the flow
rate causes a systematic decrease in the activation energy of vaporization, bringing
its value closer to the reference value of the vaporization enthalpy (52 kJ mol −1 ).
The rate equations 3.5 and 3.9 rely on the mass loss that makes TGA a method of
choice for measuring the kinetics of vaporization and sublimation. However, DSC