Thermodynamic analysis of diffusion of non-electrolytes across plant cuticles in the presence and absence of the plasticiser tributyl phosphate
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Solute mobility in cuticular membranes (CMs) of 14 plant species (Citrus aurantium L., Citrus grandis L., Hedera helix L., Ilex aquifolium L., Ilex paraguariensis St.-Hil., Malus domestica Borkh. cv. Golden Delicious, Populus alba L., Prunus laurocerasus L., Pyrus communis L. cv. Bartlett, Conference and Gellerts Butterbirne, Pyrus pyrifolia (Burm. f.) Nakai, Schefflera actinophylla (Endl.) Harms and Strophanthus gratus Baill.) was measured over the temperature range 25–55 °C. The five organic model compounds differed in size (130–349 cm3 mol−1) and cuticle/water partition coefficient (18–108). For all individual CMs (n = 297), the data were plotted according to the thermodynamic relationship between the preexponential factor (which is proportional to entropy) of the Arrhenius equation and the activation energy (enthalpy) of diffusion (ED). A strict linear correlation was obtained, providing evidence that the five compounds diffused along the same lipophilic diffusion path in all plant species tested. Extracting cuticular waxes from CMs of four plant species (Hedera, Pyrus, Schefflera and Strophanthus) had no effect on the slope of the plot but a parallel displacement towards higher entropy was observed with these polymer matrix (MX) membranes. This displacement is interpreted as a temperature-independent tortuosity factor directly related to entropy. The influence of the plasticiser tributyl phosphate on solute mobility at various temperatures was measured for CM and MX membranes. The plasticiser increased solute mobility and ED was reduced drastically for both membrane types. This plasticiser effect was almost completely reversible, when tributyl phosphate was desorbed from the membranes. For both, plasticised CM and MX, the thermodynamic correlation exists whereby all data points lie on the same line. The data are used to characterise the lipophilic pathway across plant cuticles in terms of the free-volume theory.
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