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

210 7 Accelerators Increase Solute Permeability of Cuticles
barley wax depends only on Ey (Burghardt et al. 1998). The contribution of Cx
to maximum amounts of alcohol ethoxylates sorbed in the wax disappears from
regression equations, because the increase in Kww by the addition of one C atom is
compensated by the decrease of the cmc (Burghardt et al. 1998, 2006):
logC max
wax = 1.29 − 0.08Ey (barley wax) (7.4)
A similar equation was obtained for Chenopodium wax (Burghardt et al. 2006).
logC max
wax = 1.61 − 0.104Ey (Chenopodium wax) (7.5)
Maximum amounts of surfactant molecules which dissolve in wax are given by the
product of the cmc and the Kww. In most experiments, concentrations of the alcohol
ethoxylates far above the cmc are used. The variability in maximum amounts of
alcohol ethoxylates sorbed in reconstituted waxes is much lower than is suggested
by the tremendous variability of Kww (Table 7.1).
These considerations are valid only for aqueous solutions of alcohol ethoxylates.
The situation changes when treatment solutions dry, which is the situation after
spray applications in the field. Structures of these neat surfactant residues on the
cuticle and penetration into the cuticle have not been investigated.
7.1.2 Sorption of Alcohol Ethoxylates in Polymer Matrix
Membranes
Partition coefficients of alcohol ethoxylates for polymer matrix membranes (MX)
Kmxw of Citrus aurantium (Riederer and Schreiber 1995) and Prunus laurocerasus
(Burghardt et al. 2006) have also been determined. An equation which relates Cx
and Ey to Kmxw for Citrus MX has been established.
logKmxw = −1.78+0.52Cx − 0.17Ey
(7.6)
This equation resembles (7.1) and (7.2). For each C atom log Kmxw increases again
by a factor of 0.52, and for each E unit log Kmxw decreases by a factor of 0.17
(7.6). This is excellent evidence that the amorphous environments in cutin and wax
in which alcohol ethoxylates are sorbed have similar physicochemical properties.
Cutin and waxes are composed of methyl and methylene groups with small amounts
of oxygen (Chap. 1). Kmxw values for alcohol ethoxylates are highly correlated with
Kww values (Fig. 7.3), but the former are higher by about one order of magnitude.
Linear regression equations quantitatively account for this correlation.
Equation (7.7) applies to barley wax, and Citrus MX (Burghardt et al. 1998) and
(7.8) to Chenopodium wax and Prunus MX (Burghardt et al. 2006):
logKww = −1.06+1.00log Kmxw
logKww = −1.04+1.05log Kmxw
(7.7)
(7.8)