Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 71

Fig. 3.6 Effect of the
nitrogen gas flow rate on
the activation energy of
vaporization of methyl
salicylate. Numbers by
symbols represent the flow
rate in mL min −1 . (Data taken
from Cheng et al. [ 25 ])

65

60

50
100

55

150

200 250

52 kJ mol -1

50

0.0 0.2 0.4 0.6 0.8 1.0

α

can be used in such studies as well. Figure 3.7 shows a dependence of the effec-
tive activation energy estimated from DSC data on vaporization of water [ 27 ]. The
isoconversional values of E ʱ are practically independent of ʱ in a rather wide range.
The average E ʱ value in the range ʱ = 0.4-0.6 is 43.80 ﾱ 0.03 kJ mol −1 . The value
is somewhat larger than the reference value [ 2 ] of the enthalpy of vaporization of
water, 40.7 kJ mol −1 . It should be noted that this reference value corresponds to the
enthalpy of vaporization at 100 ﾰC, i.e., the boiling temperature, which is a common
way of reporting the enthalpies of vaporization. However, under the conditions of
DSC runs (open pan, flow of nitrogen), water is completely vaporized by 60 ﾰC.

Fig. 3.7 Isoconversional
values of the activation
energy for vaporization
of water. Solid line
represents the average of
the E ʱ values in the range
of ʱ = 0.4-0.6. Dash line
denotes the enthalpy of
water vaporization at 100 ﾰC.
(Reproduced from Prado
and Vyazovkin [ 27 ] with
permission of Elsevier)