A MECHANISTIC APPROACH TOWARDS SURFACTANT/WAX INTERACTIONS - EFFECTS OF OCTAETHYLENEGLYCOLMONODODECYLETHER ON SORPTION AND DIFFUSION OF ORGANIC-CHEMICALS IN RECONSTITUTED CUTICULAR WAX OF BARLEY LEAVES

Mechanisms of surfactant/wax interactions which contribute significantly to the acceleration of rates of cuticular uptake of agrochemicals have been investigated. Experiments were carried out using isolated and subsequently reconstituted barley wax. As C-14-labelled model compounds, the monodisperse alcohol ethoxylate octaethyleneglycolmonododecylether (C(12)E(8)), the aromatic compound, pentachlorophenol (PCP), and the aliphatic tetracosanoic acid (C(24)AC; representing a typical wax constituent) were used. Each of the three compounds represented one part of the complete interacting system: surfactant, pesticide and wax. Partitioning of PCP was independent of simultaneously present C(12)E(8) below its critical micelle concentration (CMC), but decreased linearly above the CMC. The maximum increase in the mobility of PCP was by a factor of about four by C(12)E(8), whereas the maximum increase of the mobility of C(24)AC was about forty-fold. Effects on the mobility of PCP and C(24)AC were independent of C(12)E(8) concentrations above the CMC, but they decreased significantly with decreasing C(12)E(8) concentrations below the CMC. A common model usually applied in life sciences for the analysis for dose-response curves, relating the observed effects to the logarithms of the applied concentrations, was successfully fitted to the results obtained here. This indicated that the observed effects were a function of the relative, rather than absolute, concentrations of the surfactant within the wax. Furthermore, it was discovered that surfactant-induced acceleration of the diffusion of PCP and C(24)AC was completely reversible if C(12)E(8) was removed from the system. Thus, accelerating effects on diffusion only occurred if distinct amounts of surfactant were present within the wax. This, finally, led to a tentative model, explaining the effect of C(12)E(8) on wax structure as an unspecific plasticising interaction between surfactant and wax. Rapid induction and complete reversibility of the effects indicated that the assumption of irreversible alterations of the wax structure (e.g. solubilisation of wax or destruction of crystalline wax domains) was not necessary in order to explain the surfactant-induced acceleration of rates of cuticular permeation of pesticides, when studying absorption/desorption from non-evaporating aqueous solutions. However, such changes may still occur when evaporation of the water in spray deposits concentrates surfactants sufficiently for them to become wax solvents.