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

4.1 Water Permeability of Synthetic Polymer Membranes and Polymer Matrix Membranes 63
calculated by multiplying Kwv by 23.05 × 10 −3 kgm −3 , the water vapour concentration
of air at 100% humidity and 25 ◦ C (3.20). Concentration of water in synthetic
polymers in equilibrium with 100% humidity (or p/p0 = 1) ranged from 0.65 to
168kgm −3 .
Pwv was obtained by multiplying Pw by 43,384 (3.11). Permeances of the selected
MX membranes ranged from 1.2 × 10 −3 to 2.5 × 10 −4 ms −1 . In order to compare
synthetic polymers and MX membranes, both types of membranes must have the
same thickness. Permeances for the synthetic membranes were calculated as P/ℓ
for a thickness of 3µm, which is similar to thicknesses of the MX membranes in
Table 4.1. As these synthetic polymers are homogeneous, this is perfectly legitimate.
Permeances of MX membranes are similar to those calculated for the polar polymers
EC, CA, PMA, PEMA and Nylon. Permeances of PP, PE, PVA and PET membranes
are considerably lower.
Comparing diffusion coefficients meets with some difficulties (Table 4.1). Becker
et al. (1986) determined D values for Ficus and Citrus MX using the hold-up time
method (2.5), while Chamel et al. (1991) estimated D from sorption isotherms
(2.33), and their D are considerably higher than those of Becker et al. (1986).
Chamel et al. (1991) also determined water vapour sorption in CM and MX gravimetrically.
Sorption in MX and CM was similar or identical because most sorption
sites (dipoles) are contributed by cutin and polar polymers, and access of water to
Table 4.1 Water permeability at 25 ◦ C of selected synthetic polymer membranes and plant polymer
matrix membranes isolated from astomatous leaf surfaces
Polymer Pwv D(m 2 s −1 ) Kwv Cw Pwv (ms −1 ) Jwv
(m 2 s −1 ) (kgm −3 ) (ℓ = 3µm) (gm −2 h −1 )
PP a 1.9 × 10 −11 4.9 × 10 −13 39 0.90 6.3 × 10 −6 0.52
PE a 3.3 × 10 −11 1.2 × 10 −12 28 0.65 1.1 × 10 −5 0.91
PVA b 3.7 × 10 −11 5.1 × 10 −15 7,269 168 1.2 × 10 −5 1.00
PET a 1.3 × 10 −10 2.7 × 10 −13 484 11.2 4.3 × 10 −5 3.57
Nylon b 3.3 × 10 −10 1.2 × 10 −13 2,750 63 1.1 × 10 −4 9.13
PEMA c 2.9 × 10 −9 1.1 × 10 −11 264 6.1 9.7 × 10 −4 80.5
CA b 9.3 × 10 −9 3.1 × 10 −12 3,000 69 3.1 × 10 −3 257.3
EC d 1.9 × 10 −8 1.9 × 10 −11 1,000 23 6.3 × 10 −3 522.8
Ficus MX e 1.4 × 10 −9 1.8 × 10 −14 7.8 × 10 4 1.8 × 10 3 2.5 × 10 −4 20.7
Ficus MX f – 4.1 × 10 −13 2,000 34 - −
Citrus MX e 5.2 × 10 −9 6.0 × 10 −15 8.7 × 10 5 2.0 × 10 4 1.8 × 10 −3 149.3
Citrus MX f – 2.6 × 10 −14 2,828 41 - −
Pyrus MX g – – 4.7 × 10 −3 390.0
Hedera MX g – – − – 1.2 × 10 −3 99.6
a Rust and Herrero (1969)
b Hauser and McLaren (1948)
c Stannett and Williams (1965)
d Wellons and Stannett (1966)
e Becker et al. (1986)
f Chamel et al. (1991)
g Schönherr and Lendzian (1981)