POLYDISPERSE ETHOXYLATED FATTY ALCOHOL SURFACTANTS AS ACCELERATORS OFCUTICULAR PENETRATION .1. EFFECTS OF ETHOXY CHAIN-LENGTH AND THE SIZEOF THE PENETRANTS

The effects of polydisperse ethoxylated fatty alcohol (EFA) surfactant s on the penetration of six organic compounds varying in size (molar v olumes, 107-282 cm(3) mol(-1)) and lipophilicity (log K-ow 0.8-6.5) we re investigated using astomatous isolated cuticular membranes (CM) of Citrus and pear leaves. Mobilities of model compounds in CM were measu red by unilateral desorption from the outer surface (UDOS). Rate const ants (k) obtained in these experiments are directly proportional to d iffusion coefficients and, in the absence of EFA, k values decreased by a factor of 52 when molar volumes increased only 2.64-fold. Under U DOS conditions using micellar surfactant solutions as desorption media , surfactants are sorbed in the CM and the volume fractions sorbed wer e found to decrease from approximately 0.062 to 0.018 when the average number of ethoxy groups (nE) increased from 5 to 17. In the presence of the EFA surfactants in the CM, solute mobilities increased markedly though this effect diminished with increasing nE. Surfactants with nE = 17 affected solute mobilities only marginally. Surfactant effects o n solute mobility increased with the size of the solutes leading to al most identical mobilities of the model compounds. With the current ran ge of our model compounds, lipophilicity increased with increasing mol ar volumes, though evidence is presented showing that the mobilities o f solutes depend on their molar volumes while lipophilicity has no eff ect. Effects of micellar aqueous solutions of polydisperse surfactants on solute mobilities followed the pattern observed with monodisperse ones. Experiments simulating foliar uptake (SOFU), by applying 5-mu l droplets of solute and increasing amounts of surfactant (1-20 g litre( -1)) to the outer surface of the CM and then monitoring the rates of a ppearance at the inner surfaces of the CM, were carried out with fluor omethylphenyl)-5-phenoxy-5H-1,2,3,4-tetrazole (WL110547; log K-ow = 3. 6) and cyanazine (log K-ow = 2.1) and the surfactants 'Genapol' C-050 (GP C-050, nE 5) and 'Genapol' C-200 (GP C-200, nE 17). In the applied range, uptake increased with increase in amount of both surfactants a nd both surfactants were more effective than polyethylene glycol (PEG 400). The smaller, more lipophilic GP C-050 was able to increase the r ates of uptake of both compounds greatly, while GP C-200 had a less, t hough observable, effect, similar to the pattern observed in the UDOS experiments. The results indicated that GP C-050 penetrated the cuticl e rapidly while sorption of GP C-200 was slower, though better from a concentrated residue in SOFU experiments than from aqueous solution in UDOS experiments. This induced some acceleration effects by GP C-200 in SOFU experiments, but these could be countered by reductions in con centrations, and hence driving forces, for penetration at higher level s of application.