Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 130

0.00

200 -0.05

-0.10

0 04060

T / o C

150

100

0.0 .2 0.4 0.6 0.8 1.0

α

Fig. 3.56 Conversion dependence of the effective activation energy for demixing of an aqueous
solution of triethylamine that takes place on heating. The inset shows a typical DSC curve of the
process measured at heating rate 1 ﾰC min −1

the fit appears rather high, 117 3 kJ mol −1 . The activation energy of diffusion in
liquids is about the same value as those of viscous flow [ 148 ]. For small spherical
molecules, the latter value is about one third of the vaporization enthalpy [ 149 ],
i.e., a few tens of kilojoules per mole. The obtained value is more characteristic
of large molecules [ 148 ]. However, triethylamine in water exists as relatively big
hydrated molecule, (C 2 H 5 ) 3 ﾷ 2H 2 O [ 150 ], the diffusion of which is likely to involve
the breakage and restoration of hydrogen bonds that can create a significant energy
barrier. Note that a solution of triethylamine in water has viscosity that is about four
times larger than that of water [ 150 ]. It should also be noted that the activation ener-
gies of viscous flow in aqueous solutions of some amino acids have been reported
[ 151 ] to be around a 100 kJ mol −1 .

Fig. 3.57 Temperature
dependence of the effective
activation energy derived
from E ʱ versus ʱ dependence
(Fig. 3.56 ). The solid line is
a theoretical fit by means of
Eq. 3.86

250

200

150

100

292 294 296 298 300 302 304

T / K