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

9.8 Measuring Solute Permeability 273
drying is not known exactly. As the area is not needed to calculate rate constants
(5.1), this is no problem. Adding a surfactant to the donor solution improves contact
between the aqueous solution and the lipophilic waxy cuticle. This greatly increases
rate constants (Fig. 5.6).
Surfactants and other adjuvants affect partition coefficients (Sect. 7.1) of
lipophilic solutes. If surfactants and solutes are both contained in the donor, data
interpretation can be difficult, because adjuvants affect both partition coefficients
and solute mobility in the limiting skin. These effects can be separated by adding
the adjuvants to the desorption media. When solutes are desorbed, the adjuvants
penetrate the CM from the outer surface and quickly accumulate in the limiting skin
and the sorption compartment. They accumulate in the CM, from which they cannot
escape (Fig. 9.5). There is no effect of adjuvants on driving force, because solute
concentration in the soco is the driving force (Fig. 9.5). The adjuvant concentration
in the CM can be calculated from the partition coefficient and the concentration in
the receiver (Schönherr et al. 2001; Shi et al. 2005a, b). If solute mobility in the control
(no adjuvant) is smaller than in the treatment (adjuvant), the adjuvant is called
accelerator (Chap. 7).