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

7.1 Sorption of Plasticisers in Wax and Cutin 207
K ww (wax/water partition coefficient of C 12E 8)
10 2
10 1
cmc
10
Aqueous concentration of C12E8 (mol/L)
−6 10−5 10−4 10−3 10−2 Fig. 7.1 Wax/water partition coefficient (Kww) of octaethylene glycol monododecyl ether (C12E8)
for the system reconstituted barley wax/water. Error bars represent 95% confidence intervals.
Redrawn from Schreiber (1995)
the wax when external concentration of 14 C-labelled C12E8 was increased (Fig. 7.1).
Kww is calculated in the usual way as the ratio of the internal and external concentrations
of C12E8 (6.1). Above the cmc, the partition coefficient Kww is constant
and independent of the external concentration (Fig. 7.1). This is due to the fact that
above the cmc all additional surfactant molecules form micelles, and the concentration
of free C12E8 molecules dissolved in water remains constant. Adding more
surfactant increases the number of micelles, but there is no increase of C12E8 sorbed
in the wax. Correct Kww values are only obtained when cmc is used as external
concentration, not total surfactant concentration.
The formation of micelles also affects the sorption of lipophilic solutes in wax.
The Kww measured with 14 C-labelled pentachlorophenol (PCP) is about 3,500
(Table 6.2 and Fig. 7.2). It is not affected by C12E8 as long as the cmc is not reached
(Fig. 7.2). If the C12E8 concentrations exceeds the cmc, micelles are formed and
apparent Kww of PCP decreases, because PCP dissolves not only in the wax but also
in lipophilic micelles (PCP is solubilised). Thus, above the cmc PCP is in equilibrium
between lipophilic wax, lipophilic micelles and the aqueous phase. Increasing
the number of micelles results in increasing amounts of PCP solubilised in micelles,
and less is sorbed in wax. This leads to a decrease of the apparent Kww. This should
be taken into account when calculating partition coefficient of surface active compounds
or of lipophilic compounds in the presence of micellar solutions of surface
active compounds.
Kww have been determined for a large number of monodisperse alcohol ethoxylates
and reconstituted waxes from three different species (Table 7.1). Partition