Mobilities of Organic Compounds in Reconstituted Cuticular Wax of Barley Leaves: Effects of Monodisperse Alcohol Ethoxylates on Diffusion of Pentachlorophenol and Tetracosanoic Acid

Effects of monodisperse alcohol ethoxylates on mobilities of ¹⁴C-labelled pentachlorophenol (PCP) and tetracosanoic acid (C₂₄AC) in reconstituted cuticular wax of barley leaves were measured. Depending on the respective alcohol ethoxylate investigated, the diffusion coefficient (D) of PCP in barley wax was increased by factors ranging from 3·3 to 19·6, whereas D of C₂₄AC, was increased by factors varying between 22 and 315. In order to analyse the relationship between the concentration of surfactants in the wax and their effects on D, the amounts of alcohol ethoxylates dissolved in the wax at equilibrium with external concentrations well above the critical micelle concentration (CMC) were determined. Wax/water partition coefficients (K_ww) of the alcohol ethoxylates were about one order of magnitude lower than cuticle/water partition coefficients (K_cw), which is a consequence of the semi-crystalline structure of the wax compared with amorphous cutin. Correlations between effects on D and maximum amounts of alcohol ethoxylates dissolved in the wax were obtained indicating an unspecific wax/surfactant interaction. This was solely dependent on the amount of surfactant sorbed to the wax, leading to increased mobilities of pesticides in the wax. Applying ESR-spectroscopy, which gave an insight into the molecular structure of the wax, supported this interpretation of an unspecific plasticising effect of the alcohol ethoxylates on the molecular structure of the wax. The results obtained in this study are in good accordance with the results obtained in a recent study investigating the effects of the same group of alcohol ethoxylates on mobilities of pesticides in isolated, but intact, cuticular membranes of Citrus. This demonstrates that the investigation of isolated and subsequently reconstituted cuticular wax is a useful model system analysing the mechanisms of the surfactant interaction with the transport-limiting barrier of plant cuticles.