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

8.2 Solute Mobility in Cuticles 239
available by an opening up or swelling of the polymer cutin once the low energy
sorption sites of about 30kJ mol −1 had been saturated. With tomato fruit MX,
the opposite effect was observed. Heats of sorption decreased initially until the
internal concentration reached about −18kJ kg −1 . All sorption sites seemed to be
equally accessible, but those providing higher heats of sorption were being saturated
first. At higher internal 4-NP concentrations heats of sorption increased strongly,
and this phenomenon can be explained by a cooperative effect between sorbate
molecules inside the cuticle. After all sorption sites in cutin have been saturated,
additional sorption occurs on the surface of 4-NP molecules already sorbed on
the polymer. In other words, initially solute–matrix interactions dominate, while at
high internal concentration sorbate–sorbate interactions become important and solid
4-NP precipitates between the polymer chains, which are forced apart. The polymer
swells.
The partial molar entropy measured for transfer from aqueous solution to the
cuticles has a negative sign (Fig. 8.2b). This loss of entropy can be attributed to
reduced mobility of 4-NP molecules and increased degree or order on sorption
on a solid substrate. The good correlation between enthalpy and entropy at all
concentrations, with both species and for CM and MX, indicates that the same
entropy–enthalpy relationship prevails for both species and for MX and CM. Sorption
sites having a low enthalpy result in lower order of sorbed molecules and vice
versa. Larger amounts of heat are evolved when solutes are sorbed at higher order
and reduced mobility. Whenever solute–solute interactions dominate, multilayers of
sorbate molecules may arise between the polymer chains of cutin.
It is tempting to extrapolate these data to the formation of embedded wax, that
is, to the generation of wax plates in cutin. Enthalpies of transfer to cutin are
very high when compared to the octanol water system, for which ∆HS between
−5 and −8kJ mol −1 were measured for substituted resorcinol (1,3-benzenediol)
monoethers (Sangster 1997). Sorption of 4-NP is driven by very high enthalpies, and
dipole–dipole interactions are important. It is unfortunate that we have no comparable
data for fatty acids, alcohols esters or alkanes. With these lipophilic solutes, van
der Waals forces are probably more important than dipole-dipole interactions. For
these reasons, extrapolation of the above data to formation of intracuticular waxes
should be looked at with extreme caution. These highly lipophilic compounds were
not included in this study because of their extremely low water solubility. Mixtures
of ethanol and water would have to be used to overcome this problem.
8.2 Solute Mobility in Cuticles
We have seen that partition coefficients generally decrease with increasing temperature
(Fig. 8.1). Permeance is the product of partition and diffusion coefficients
(2.18), and the question concerning the effect of temperature on diffusion coefficients
(D) arises. Diffusion is the consequence of Brownian motion of molecules,
and rates of diffusion are proportional to the frequency of jumps and the distance