Lynne Wong's PhD thesis

Moreover, if T β > T hm the process is enthalpy-driven, and if T β < T hm , the process is
considered to be entropy-controlled (Telis et al., 2000).
If the net isosteric heat of sorption q st is plotted as ordinate against the entropy of sorption
S d as abscissa, the slope of the plot gives the isokinetic temperature T β and the intercept
gives the free energy ΔG at the isokinetic temperature. The harmonic mean temperature
for this study can be calculated to be 320.23 K, since the isotherm was investigated at four
different temperatures of 30, 45, 55 and 60 °C.
4
T hm = − 1
− 1
− 1
− 1
(303.15) + (318.15) + (328.15) + (333.15)
= 320.23 K
The plots of the net heat of sorption q st (kJ mol -1 ) against the entropy of sorption S d (kJ mol -
1
K -1 ) for the nine cane components of R 570 aged 52 and 36 weeks are shown in Fig 6.10,
the coefficient of determination R 2 , the slope and the intercept for the enthalpy-entropy
relationship are shown in Table 6.5. The slope corresponds to the isokinetic temperature T
β, and the intercept, ΔG.
It can be seen from Table 6.5 that T β for all cane components aged 52 and 36 weeks is
greater than the harmonic mean temperature T hm of 320.23 K except in the case of stalk
pith aged 36 weeks, hence, all processes except stalk pith aged 36 weeks are enthalpycontrolled.
Hence the driving force for the adsorption of moisture on these fibres is the
strength of binding of water molecules to the surface of the fibre. The positive sign of ΔG
(except for the case of rind fines aged 36 weeks) indicates that the water sorption process
in the cane components is non-spontaneous, and that the enthalpy-entropy compensation
theory was satisfied.
Beristain et al. (1996) applied the enthalpy-entropy compensation theory to water
adsorption in starchy materials. Two isokinetic temperatures were observed, suggesting
that during the initial stages (at low water activity) the isotherm process was entropycontrolled,
whereas in the later stage, the process was controlled by changes in the
enthalpy of water. They also reported a spontaneous sorption isotherm for starch
materials.
258