Some Basics En Route to Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 4

Fig. 1.2 Evaluation of con-
version from TGA curve

m 0

m

m f

t 0 t t f

or two immiscible liquids, decomposition of solids to gases, reaction of gases with
solids and liquids, dissolution of solids, evaporation of liquids, sublimation, and
melting of solids are all examples of processes whose kinetics is heterogeneous.
Considering that for the heterogeneous kinetics the only concentration that mat-
ters is the concentration of the reactants in the interfacial reaction zone and that
this concentration is not readily measurable, it is only practical to switch from the
concentration to the extent of the reactant conversion ( ʱ ). This parameter simply
reflects the reaction progress from the initial state, i.e., before the reaction starts
( ʱ = 0) to the final state, i.e., when the reaction is complete ( ʱ = 1).
The conversion is readily determined as a fractional change of any physical prop-
erty associated with the reaction progress. When the process progress is monitored
as a change in mass by TGA, ʱ is determined as a ratio of the current mass change,
∆ m, to the total mass change, ∆ m tot occurred throughout the process:

mm
mm

−
−

∆
∆ tot

m
m

0
0

α= = ,

(1.4)

f

where m 0 and m f , respectively, are the initial and final masses (Fig. 1.2 ). In its turn,
when the progress is measured as a change in heat by DSC, ʱ is evaluated as a ratio
of the current heat change, ∆ H, to the total heat released or absorbed, ∆ H tot in the
process:

t

∫

( ddd Ht / ) t

∆
∆

H
H

t

α=

0

= ,

(1.5)

t

f

∫

( ddd Ht / ) t

tot

t

0

where d H /d t is the heat flow measured by DSC (Fig. 1.3 ).