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

7.1 Sorption of Plasticisers in Wax and Cutin 211
K ww
10 5
10 4
10 3
10 2
10 1
10 0
10 −1
10−2 10−1 C 4E 2
10 0
10 1
C 8E 4
C 6E 3
10 2
C 12E 8
K mxw
C 10E 5
10 3
C 14E 7
C 12E 6
10 4
C 16E 8
10 5 10 6
Fig. 7.3 Correlation between wax/water partition coefficients (Kww) and cuticle/water partition
coefficients (Kmxw). Redrawn from Schreiber et al. (1996b)
Reconstituted waxes from barley and Chenopodium leaves are solid and partially
crystalline phases, and they offer fewer sorption sites for alcohol ethoxylates than
the amorphous MX of Citrus and Prunus; for this reason, Kww are lower than Kmxw.
A similar observation had been made with lipophilic solutes lacking surface actvity
(Sect. 6.1) where Kww and Kcw are compared.
These results are specific for barley and Chenopodium wax, and should not be
extrapolated to waxes of other species. With Stephanotis wax, considerably higher
Kww were observed (Table 7.1). Maximum concentrations (g kg −1 ) of alcohol
ethoxylates sorbed in Citrus MX (7.9) are a function of Ey (Riederer and Schreiber
1995), as was previously shown for wax [(7.4) and (7.5)].
logC max
wax = 2.07 − 0.044Ey
(7.9)
7.1.3 Sorption of n-Alkyl Esters in Wax
N-alkyl esters, lacking surface activity are lipophilic and have very low water
solubility. Therefore, 100g l −1 1,2-propanediol was added to water to obtain homogeneous
aqueous solutions without phase separation (Simanova et al. 2005).
Partitioning of n-alkyl esters between these external aqueous solutions containing
1,2-propanediol and reconstituted wax was measured with Hordeum and Stephanotis
wax. These partition coefficients are marked Kwrec (rec = receiver), instead of Kww.
Identical solutions of n-alkyl esters containing 1,2-propanediol were also used as
receiver solutions in transport experiments (Sect. 7.3.3).