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

Chapter 6
Diffusion of Non-Electrolytes
6.1 Sorption in Cuticular Membranes, Polymer Matrix, Cutin
and Waxes
Chemicals are characterised by their physicochemical properties such as molecular
weight, volatility, ionisation constants, water solubility and solubility in lipids.
Agrochemicals and environmental xenobiotics are often lipophilic, and these compounds
are sorbed and accumulate in lipophilic compartments of the environment,
such as the organic fraction in soil, the storage lipids of man, animals or plants.
Rates of diffusion across cuticles and other membranes are proportional to the partition
coefficient (2.17) of the compound, which is a measure for differential solubility
in lipids and water (2.12). Thus, when analysing sorption in and penetration through
cuticles, partition coefficients for the substances must be known or measured.
6.1.1 Definition and Determination of Partition Coefficients
The octanol/water partition coefficient (Kow) of a compound is most commonly
used. It relates the equilibrium concentrations in n-octanol and water (6.1). Since
concentration is the ratio of amount (M) over mass (m), this equation can be written
in terms of ratios of amounts and masses. Molal concentrations (mol kg −1 ) are used
for both phases, because they do not depend on temperature. These units cancel,
and partition coefficients are dimensionless. By convention, concentration in lipid
phases are in the numerator:
Kow = Coctanol(mol kg −1 )
Cwater(mol kg −1 ) =
M octanol
m octanol
Mwater
mwater
= Moctanol
Mwater
× mwater
. (6.1)
moctanol
Kow-values have been measured and tabulated for a large number of substances
(Hansch and Leo 1979; Leo et al. 1971; Sangster 1997). A partition coefficient of
L. Schreiber and J. Schönherr, Water and Solute Permeability of Plant Cuticles.
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