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

228 7 Accelerators Increase Solute Permeability of Cuticles
Effect on mobility k/k control
100
10
1
0.1
iprovalicarb
control
C 12 E 8
C 12E 6
C 12E 2
C 12E 4
methyl glucose
control
0 50 100 150 0 50 100 150
Concentration in wax (g/kg)
C 12E 8
C 12E 6
C 12E 4
C 12E 2
Fig. 7.16 Logarithms of the effects (k/kcontrol) of monodisperse alcohol ethoxylates on rate constants
of penetration k across Stephanotis CM of iprovalicarb (blue symbols) and methyl glucose
(red symbols) as a function of the concentration of the alcohol ethoxylates in Stephanotis wax.
Dotted lines represent 95% prediction intervals for the regression lines. Data from Shi et al. (2005a)
diffuse in the amorphous wax phase, where diffusion of lipophilic solutes such as
iprovalicarb takes place. They act as plasticisers, and accelerate their own diffusion
and the diffusion of other solutes which can access the waxy pathway.
The fact that DESU and TBP, which are very lipophilic compounds lacking surface
activity, do not enhance penetration of methyl glucose indicates that these
plasticisers are spatially separated from methyl glucose during penetration of the
cuticle. The plasticising effect of DESU and TBP on wax cannot affect diffusion
of methyl glucose in aqueous pores. Kww values measured with Stephanotis wax
(Table 7.1) are 692 and 2,626 for C12E 8 and C1266 respectively. Hence, these alcohol
ethoxylates are primarily sorbed in wax and cutin and only traces will be in the
aqueous pores. If methyl glucose does not dissolve in wax and cutin, C12E8 and
C1266 should not enhance rates of penetration of methyl glucose in CM. Yet they
do, and C12E 8 is more effective than C1266 even though its concentration in wax
is lower. An effect on aqueous pores can be ruled out, because the overwhelming
majority of these surfactant molecules are in lipid phases and not in water. How can
this be explained?
Alcohol ethoxylates are surfactants, with a lipophilic and a polar domain in the
same molecule, and this distinguishes them from DESU and TBP. With increasing
degree of ethoxylation their polarity increases, and statistically significant accelerating
effects on methyl glucose penetration have been observed only with the more
polar C12E 6 and C12E 8 representatives. This indicates that C12E 6 and C12E 8 must
at least partially diffuse in the same phase as methyl glucose, such that diffusion of
methyl glucose can be increased by C12E 6 and C12E 8 to some extent.