Thermodynamic analysis of diffusion of non-electrolytes across plant cuticles in the presence and absence of the plasticiser tributyl phosphate

Solute mobility in cuticular membranes (CMs) of 14 plant species (Citrus au rantium L., Citrus grandis L., Hedera helix L., Ilex aquifolium L., Ilex pa raguariensis St.-Hil., Malus domestica Borkh. cv. Golden Delicious, Populus alba L., Prunus laurocerasus L., Pyrus communis L. cv. Bartlett, Conferenc e and Gellerts Butterbirne, Pyrus pyrifolia (Burm. f) Nakai, Schefflera act inophylla (Endl.) Harms and Strophanthus gratus Baill.) was measured over t he temperature range 25-55 degreesC. The five organic model compounds diffe red in size (130-349 cm(3) mol(-1)) and cuticle/water partition coefficient (18-10(8)). For all individual CMs (n = 297), the data were plotted accord ing to the thermodynamic relationship between the preexponential factor (wh ich is proportional to entropy) of the Arrhenius equation and the activatio n energy (enthalpy) of diffusion (E-D). A strict linear correlation was obt ained, providing evidence that the five compounds diffused along the same l ipophilic diffusion path in all plant species tested. Extracting cuticular waxes from CMs of four plant species (Hedera, Pyrus, Schefflera and Stropha nthus) had no effect on the slope of the plot but a parallel displacement t owards 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 tribu tyl phosphate on solute mobility at various temperatures was measured for C M and MX membranes. The plasticiser increased solute mobility and E-D was r educed drastically for both membrane types. This plasticiser effect was alm ost completely reversible, when tributyl phosphate was desorbed from the me mbranes. For both, plasticised CM and MX, the thermodynamic correlation exi sts whereby all data points lie on the same line. The data are used to char acterise the lipophilic pathway across plant cuticles in terms of the free- volume theory.