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

212 7 Accelerators Increase Solute Permeability of Cuticles
As observed with alcohol ethoxylates (Table 7.1), partition coefficients for n-alkyl
esters are again a factor of about 10 higher for Stephanotis wax than for barley
wax. With increasing number of C atoms of the dicarboxylic acids and esterified
alcohols, Kwrec increases and the following regression equations were obtained for
barley (7.10) and Stephanotis (7.11):
logKwrec = 0.39Cx − 3.79 (barley wax) (7.10)
logKwrec = 0.42Cx − 3.20 (Chenopodium wax) (7.11)
This again is a quantitative structure–property relationship between log Kwrec and
the descriptor Cx. The addition of one C atom increases Kwrec by a factor of
about 0.4.
7.2 Plasticisation of Cuticular Wax: Evidence from Spectroscopy
The plasticising effect of accelerators on cuticular wax was shown for alcohol
ethoxylates and reconstituted barley wax using electron spin resonance (ESR)
spectroscopy (Schreiber et al. 1996b) and nuclear magnetic resonance (NMR) spectroscopy
(Schreiber et al. 1997). Specific ESR- and NMR-probes were mixed with
the wax which allowed direct insights into the molecular architecture of the wax
phase. The molecular environment of the probes in the absence or presence of a
plasticiser can be characterised.
In ESR experiments, spin labels are used which have a free unpaired electron in
a side chain, which can be monitored by ESR (Sackmann et al. 1973). About 0.1%
of the spin label 5-doxyl stearic acid (5-SASL) was added to barley wax, the solvent
was evaporated and the remaining mixture was reconstituted for the experiment.
Since the side chain of the fatty acid carrying the free electron is fairly bulky and
polar, the spin label will essentially be sorbed in the amorphous phase of reconstituted
wax. Further preparation of the wax samples is identical to that described for
the determination of Kww (Sects. 6.1.3 and 7.1.1) or D (Sect. 6.5.1), with the exception
that they had to be placed in small glass tubes for ESR experiments. In order
to test the plasticising effect of accelerators on the molecular architecture of barley
wax, alcohol ethoxylates were added to the wax samples. The exact amount
of each alcohol ethoxylate added to the wax was calculated according to (7.4).
This corresponds to the maximum amount of each alcohol ethoxylate which can be
sorbed to barley wax from an external aqueous solution with the alcohol ethoxylates
dissolved at any concentration above the cmc.
ESR spectra were measured at temperatures between −10 and 60 ◦ C (Fig. 7.4).
The characteristic parameter of the spectrum used to characterise the molecular
environment of the ESR-probe is the hyperfine splitting of the spectrum indicated
by the two arrows (Fig. 7.4). As long as the hyperfine splitting is visible, a significant
amount of the ESR-probe is trapped in a rigid environment, which prevents free
rotation of the probe around its own axis. With increasing temperature, the distance
of the hyperfine splitting (measured in Gauss) decreases. In parallel, the peaks of