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

112 4 Water Permeability
a Pwv of 4.23×10 −11 m 2 s −1 . This is nearly identical to polyethylene. Polyethylene
and parafilm of 333 nm thickness would have a permeance (Pwv = Pwv/ℓ) of
1–1.27 × 10 −4 ms −1 , and for a 1.1µm thick membrane permeance would be 3.0–
3.84 × 10 −5 ms −1 . Comparing these values with those shown in Table 4.9 leaves no
doubt that a hydrophobic membrane having Pwv similar to PE or parafilm cannot
account for most of the permeances observed especially not for those of leaf CM.
Pwv measured for a liquid paraffin (hexamethyl tetracosane) is even 100 times
higher (3.3 × 10 −9 m 2 s −1 ) (Schatzberg 1965), and it can be ruled out that liquid
cuticular waxes might contribute to barrier properties. Citrus wax is in the fluid state
when T > 70 ◦ C (Reynhardt and Riederer 1991). At 25 ◦ C Pw was 5.9 × 10 −10 ms −1 ,
and at 55 ◦ C it had dropped to 2×10 −8 ms −1 (Haas and Schönherr 1979). This is not
too far from Pw measured for the polymer matrix, which is 2.5×10 −7 ms −1 . Clearly,
we need a solid wax to account for water permeability of cuticular membranes.
Fox (1958) studied water vapour transmission of thin cellophane films coated
with paraffin waxes. Waxes were applied at temperatures above the melting point,
and solidified at various temperatures. The composite membranes were stored at
23 ◦ C or 35 ◦ C before vapour transmission rates were determined at 23 ◦ C and 50%
humidity using a cup method (Sect. 9.7). Data obtained with a paraffin wax having
a melting point of 62 ◦ C are shown in Table 4.11.
In the original paper, wax load was given as lb/ream. These figures were converted
to gm −2 using 453.6 g per pound and 278.71m 2 per ream. Depending on
type of paper the ream can have variable size. The ream of glassine paper consists
of 500 sheets of 24 × 36 inches (Scott et al. 1995). Water vapour transmission rates
reflect permeability of the wax films because cellophane has very high water permeability.
WVTR depended on temperature of water used for cooling the melt and
on duration of storage of solidified wax. Rapid cooling at 1.7 ◦ C resulted in high initial
WVTR, which decreased to about one half during 4–5 days storage. At higher
cooling temperatures initial WVTR were lower, and decreased to 0.02gm −2 day −1 .
Longer storage (38 days) had little effect, but with some waxes WVTR dropped to
0.01gm −2 day −1 . After 4–5 days storage at 35 ◦ C, permeance (Pwv) ranged from
2.2 × 10 −8 to 8.9 × 10 −8 ms −1 . This is lower by at least one order of magnitude
than Pwv for CM (Table 4.9).
Table 4.11 Water vapour transmission rate (WVTR) of cellophane membranes coated with molten
paraffin wax and cooled at various temperatures either in water or air. WVTR rates measured at
23 ◦ C and 50% humidity
Cooling T( ◦ C) Wax ℓ of WVTR WVTR Pwv (ms −1 )
load wax (first day) (after 4–5 days)
(gm −2 ) (µm) (gm −2 day −1 ) (gm −2 day −1 )
1.7 (water) 11.7 12.9 0.15 0.08 8.9 × 10 −8
7.2 (water) 10.9 12.0 0.12 0.03 3.3 × 10 −8
12.8 (water) 9.6 10.6 0.05 0.03 3.3 × 10 −8
18.3 (water) 9.8 10.8 0.07 0.02 2.2 × 10 −8
23.0 (air) 10.7 11.8 0.09 0.02 2.2 × 10 −8