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

8.1 Sorption from Aqueous Solutions 237
partition coefficients was exclusively due to a temperature effect on solute activity
in the aqueous phase, that is, on water solubility of 4-NP. With increasing
temperature water solubility increased, and as a consequence partition coefficients
decreased. Sorption in tomato fruit MX differed. Here, k ′ increased with temperature
from about 5 to 14, showing that the number of available sorption sites increased
with temperature. The plot k ′ vs T has plateaus at 7–22 ◦ C and 22–32 ◦ C. A gradual
increase occurred between 22 and 47 ◦ C (Riederer and Schönherr 1986a). This
indicates structural changes in the cutin matrix, and the step character can be interpreted
as phase transitions which lead to a loosening of the polymer chains, with a
concomitant increase in number of sorption sites.
8.1.2 Thermodynamics of Sorption
Sorption in cuticles can be further characterised by Gibbs free energy (∆G) involved
in transfer of 1 mol of 4-NP from the aqueous phase to the cuticle phase.
∆Gsorption = −RT lnK (8.3)
∆Gsorption is proportional to ln K, and since ln K is always >1, Gibbs free energy
is always negative. This is expected for a spontaneous (exergonic) process. ∆Gsorption
was largest at low internal concentrations (about −12 to −10kJ mol −1 ), and
decreased with increasing internal 4-NP concentration to about −9 to −8kJ mol −1 .
At low internal concentration, ∆Gsorption was larger with MX than with CM (Riederer
and Schönherr 1986a). ∆Gsorption depends on heat of sorption (Hs) and entropy (Ss):
Substituting (8.3) into (8.4) leads to
∆Gsorption = ∆Hs − T∆Ss
lnK = ∆Hs
RT
+ ∆Ss
R
(8.4)
(8.5)
Plotting ln K vs T −1 (in Kelvin) results in a straight line having the slope ∆Hs/R,
and the y-intercept is ∆Ss/R. A large entropy is characteristic for sorption on solid
substrates, which leads to a higher degree of order. For ∆Gsorption to be negative, the
large T∆Ss term must be overcompensated by the heat of sorption (∆Hs). This was
the case at all temperatures and concentrations (Riederer and Schönherr 1986a).
With both species, a striking difference between CM and MX exists for the
effect of sorbate concentration on ∆Hs. At low concentration the heat of sorption
of CM was constant, and decreased only if the internal concentration exceeded
10 −2 mol kg −1 (Fig. 8.2a). Sorption sites in the CM seemed to be energetically
homogeneous as long as the internal concentration was