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

7.2 Plasticisation of Cuticular Wax: Evidence from Spectroscopy 213
a b
10 Gauss
−10° C
0° C
10° C
20° C
30° C
40° C
50° C
60° C 10 Gauss
−10° C
0° C
10° C
20° C
30° C
40° C
Fig. 7.4 ESR (electron spin resonance) spectra of 5-doxyl stearic acid measured between −10
and 60 ◦ C in reconstituted barley wax in (a) absence or (b) presence of the alcohol ethoxylate
triethylene glycol monohexyl ether (C6E 3) at a concentration of 10g kg −1 . Arrows indicate the
hyperfine splitting. Redrawn from Schreiber et al. (1996b)
the hyperfine splitting continuously lose intensity until they finally disappear completely
at high temperatures. A homogeneous ESR spectrum is obtained at high
temperature (Fig. 7.4). This indicates that the molecular environment of the ESRspectrum
is fluid, allowing free rotation of the label around its own axis without any
restriction.
The molecular environment in which the spin label is dissolved in barley wax is
rigid between −10 and 40 ◦ C (Fig. 7.4a). A transition occurs between 40 and 50 ◦ C,
and at 50 ◦ C the spin label can freely rotate around its own axis. Increasing the
temperature provides the energy for the wax molecules forming the amorphous wax
phase to become fluid. This microscopic transition in the amorphous wax phase,
where sorption and diffusion of lipophilic molecules takes place, is mapped by the
spin label.
If C6E 3 is present in the wax (1%), this transition from a rigid to a fluid environment
takes place at much lower temperatures (Fig. 7.4b). At 10 ◦ C the spin label
is still restricted in free rotation, and the hyperfine splitting is visible. At 20 ◦ C
the transition from a rigid to a fluid environment can be seen in the spectrum, and
already at 30 ◦ C the molecular environment of the spin label is fluid and free rotation
of the label is possible. The alcohol ethoxylate plasticises the amorphous phase
of the wax, and it is fluid already at room temperature. In the absence of the plasticiser,
the temperature needs to be 20–30 ◦ C higher for the same degree of fluidity.
Thus, adding an alcohol ethoxylate to cuticular wax mimics the effect of higher
temperatures.
A series of alcohol ethoxylates was tested using ESR spectroscopy, and all were
active as plasticisers compared to the control (Fig. 7.5). The plasticising effect on