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

218 7 Accelerators Increase Solute Permeability of Cuticles
Effect (D/D control)
50
40
30
20
10
0
cmc
10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2
Aqueous concentration of C 12E 8 (mol/L)
Fig. 7.8 Effect on diffusion coefficients (D/Dcontrol) of C24Ac (tetracosanoic acid) in reconstituted
barley wax as a function of C12E8 concentrations in the external borate buffer. D/Dcontrol is 1 in
absence of C12E8 (blue symbol). Results obtained with various C12E8 concentrations are shown in
red. Error bars represent 95% confidence intervals. Data redrawn from Schreiber (1995)
effects on D varied greatly depending on type of surfactant and its concentration
in wax. Each alcohol ethoxylate has its specific cmc and Kww (Table 7.1). As a
consequence, surfactant concentration in wax and plasticising effects differed.
A plot D/Dcontrol vs internal concentration of the different alcohol ethoxylates
in wax resulted in a reasonable correlation (Fig. 7.9). This shows that the effects
measured for the different plasticisers are not a specific property of each alcohol
ethoxylate investigated. Instead, the magnitude of the plasticising effect of different
alcohol ethoxylates solely depended on the total amount of plasticiser present in
the wax. The plasticising effects of alcohol ethoxylates on D in wax are unspecific.
At the same concentration in wax they cause the same effect, and different effects
observed in the experiments are solely a consequence of differences in the cmc
and the Kww of the alcohol ethoxylates (Table 7.1). The same conclusions were
drawn from experiments analysing the effect of different alcohol ethoxylates on D of
C24Ac (Schreiber et al. 1996b) and bitertanol (Burghardt et al. 1998) in barley wax,
and of benzoic acid and bitertanol in Chenopodium wax (Burghardt et al. 2006).
7.3.3 Plasticising Effects of n-Alkyl Esters
The effect of n-alkyl esters (see Table 7.2) on D has been investigated using
reconstituted waxes from barley and Stephanotis leaves (Simanova et al. 2005).