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

230 7 Accelerators Increase Solute Permeability of Cuticles
a plasticising activity. Effects of plasticisers without surface activity (e.g., DESU
or TBP) on cuticular transpiration have not been published. Rates of penetration
of salts which exclusively diffuse in aqueous pores (Chap. 5) were not affected by
plasticisers (Sect. 5.3.1).
Problems
1. What is the cmc and how does it affect the sorption of surfactants and lipophilic
solutes to wax?
2. Alcohol ethoxylates have the general formula CxEy. Cx indicates the number of
C atoms or lipophilic CH2 groups of the alcohol. Ey refers to the number of
the polar ethylene oxide units. How is the wax/water partition coefficient Kww
affected by Cx and Ey?
3. What is the Kww of C12E 8 in barley wax?
4. How much C12E8 is sorbed in barley wax when equilibrated with an aqueous
solution above the cmc?
5. What is the Kww for a solute in barley wax when the Kmxw of this substance is
250 in Citrus MX?
6. Which property of waxes is studied by ESR and NMR? How is wax structure
affected by alcohol ethoxylates?
7. Mobility of lipophilic solutes in wax and cuticles is increased by the addition
of alcohol ethoxylates. The effect can be reversed when alcohol ethoxylates are
desorbed from wax. Is this observation consistent with solubilisation of waxes?
8. Diffusion coefficients in reconstituted waxes can be estimated from molar volumes
of solutes using (6.23). For C12E 8, a D of 5.74×10 −20 m 2 s −1 is calculated.
This value is 150 times higher than D measured by desorption which is 8.5 ×
10 −18 m 2 s −1 . Can you explain this difference?
9. The n-alkyl esters DESU and TBP have pronounced accelerating effects on cuticular
penetration of the lipophilic solute iprovalicarb, but they were ineffective
with the polar compound methyl glucose. Can you explain this?
Solutions
1. The cmc is the maximum aqueous concentration of a surfactant. If surfactant
concentration is increased beyond the cmc, micelles are formed and the concentration
of free surfactant molecules does not increase. The maximum amount
of surfactant molecules sorbed in wax is reached at the cmc. Lipophilic solutes
which are sorbed in wax are also sorbed in micelles.Hence, apparent partition
coefficients (Kww) decrease when the surfactant concentration is increased above
the cmc, and micelles are formed which compete as sorbers with the wax.