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

206 7 Accelerators Increase Solute Permeability of Cuticles
effects of adjuvants which occur on the outer surface of the cuticle (spreading,
adhesion, inhibition of desiccation of the spray droplets, partitioning between the
deposit and the cuticle) will not be considered, although they can significantly affect
foliar penetration (Baur 1998, 1999; Baur et al. 1997b, 1999; Tadros 1987).
7.1 Sorption of Plasticisers in Wax and Cutin
Any molecule acting as a plasticiser on the transport-limiting barrier of the cuticle
must be soluble in wax and cutin. Differential solubility is best described as partitioning
between the lipophilic wax or cutin and the external aqueous phase (6.1).
Cuticular waxes form the transport-limiting barrier of the cuticle (Chaps. 4 and 6),
and the partition coefficient wax/water (Kww) for plasticisers must be known. Most
of the work on the effect of plasticisers on permeability of cuticles has been conducted
using non-ionic alcohol ethoxylates, which are also effective surfactants.
They consists of a lipophilic long-chain fatty alcohol having a variable number of
carbon atoms (Cx) and a polar head composed of ethylene oxide (–O–CH2–CH2–)
units (Ey). The lipophilic Cx is linked to the hydrophilic Ey by an ether bond. In
addition to alcohol ethoxylates, n-alkyl esters, lacking surface activity have also
been investigated. They are composed of short-chain α,ω-diacids of 8 or 10 carbon
atoms. Phosphate esterified to short-chain alcohols having 1–4 carbon atoms also
have good accelerating activity.
7.1.1 Sorption of Alcohol Ethoxylates in Wax
Surface active compounds exhibit a characteristic behaviour when dissolved in
water. At low concentrations, surfactant molecules tend to accumulate at the air/
water interface (Adamson 1976). This leads to a decrease of surface tension of water,
and this is the reason why surfactants improve wetting of hydrophobic surfaces.
With increasing concentrations in water, surface tension decreases, and at a certain
concentration called the critical micelle concentration (cmc), surfactant molecules
aggregate and form micelles within the aqueous solution (Schick 1987). At the cmc
the minimum surface tension is obtained, and if more surfactants are added they
form additional micelles. Micelles are spherical aggregates of surfactant monomers
with the hydrophobic carbon tails buried in the centre of the micelle while the polar
head groups are in contact with water. The presence of micelles affects partition
coefficients.
Technical surfactants are polydisperse, that is, they are mixtures of molecules
of different linear alcohols to which varying numbers of ethylene oxide groups are
linked. In most of our work we have used monodisperse surfactants consisting of
a specific alcohol (for instance dodecanol) and a specific number of ethylene oxide
groups (–CH2CH2O–). Below the cmc, increasing amounts of C12E8 are sorbed in