Water and Solute Permeability of Plant Cuticles: Measurement and ...

Problems 257
E D (kJ/mol)
DH S (kJ/mol)
100
80
60
40
20
0
−20
−40
−60
−80
−20
PVB
EC
CA
PEMA
PVC
NY
PVA
DH S = − 42.7 + 0.05 E P (r 2 = 0.008)
E P (kJ/mol)
rubber
E D = 42.3 + 1.05 E P (r 2 = 0.71)
−10 0 10 20 30 40
Fig. 8.10 The relationship between ED and ∆HS with EP. Data from Table 8.2 were plotted
of water at low partial pressures, where clustering of water and swelling of the polar
polymers are not involved.
The above data obtained with homogeneous polymers leave little doubt that
temperature effects on water permeability of cuticles cannot be analysed if only
activation energy for penetration (EP) is known. The problem cannot be solved by
analogy, because cuticles are heterogeneous, and data on temperature effects on
water vapour sorption in waxes, cutin and MX have not yet been determined. Sorptive
properties of polar polymers in the MX can be obtained by comparing sorption
in cutin and in MX. Some of these aspects have been discussed when dealing with
sorption of 4-nitrophenol in CM and MX (Sect. 8.1), but comparable data for water
vapour sorption as affected by temperature are not available. Such measurements
can be made. They may be difficult, but there are no insurmountable obstacles. As
long as these data are not available, speculations concerning the magnitudes of ED
and ∆HS in CM should be avoided.
Problems
1. What is the difference between the partition coefficient K and the parameter k
of the Freundlich isotherm? For n = 1 calculate k when K = 90.
2. When all sorption sites are occupied K = 1. What is Gibbs free energy change
in this situation?
3. How large are ∆H and T ∆S when ∆G = 0?
4. Given the data of Fig. 8.2b, what is ∆S when ∆H = −30kJ mol −1 ?
PE