The e †ects of polydisperse ethoxylated fatty alcohol (EFA) surfactants on the penetration of six organic compounds varying in size (molar volumes, 107È282 cm3 mol~1) and lipophilicity (log 0É8È6É5) were investigated using K ow astomatous isolated cuticular membranes (CM) of Citrus and pear leaves. Mobilities of model compounds in CM were measured by unilateral desorption from the outer surface (UDOS). Rate constants (k*) obtained in these experiments are directly proportional to di †usion coefficients and, in the absence of EFA, k* values decreased by a factor of 52 when molar volumes increased only 2É64-fold. Under UDOS conditions using micellar surfactant solutions as desorption media, surfactants are sorbed in the CM and the volume fractions sorbed were found to decrease from approximately 0É062 to 0É018 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 e †ect diminished with increasing nE. Surfactants with nE \ 17 a †ected solute mobilities only marginally. Surfactant e †ects on solute mobility increased with the size of the solutes leading to almost identical mobilities of the model compounds. With the current range of our model compounds, lipophilicity increased with increasing molar volumes, though evidence is presented showing that the mobilities of solutes depend on their molar volumes while lipophilicity has no e †ect. E †ects 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-kl droplets of solute and increasing amounts of surfactant (1È20 g litre~1) to the outer surface of the CM and then monitoring the rates of appearance at the inner surfaces of the CM, were carried out with 1-(3-Ñuoromethylphenyl)-5-phenoxy-5H-1,2,3,4tetrazole (WL110547 ; log and cyanazine (log and the sur-
) factants "GenapolÏ C-050 (GP C-050, nE 5) and "GenapolÏ nE 17). In the applied range, uptake increased with increase in amount of both surfactants and both surfactants were more e †ective than polyethylene glycol (PEG 400). The smaller, more lipophilic GP C-050 was able to increase the rates of uptake of both compounds greatly, while GP C-200 had a less, though observable, e †ect, similar to the pattern observed in the UDOS experiments. The results indicated that GP C-050 penetrated the cuticle rapidly while sorption of GP C-200 was slower, though better from a concentrated residue in SOFU experi-* To whom correspondence should be addressed.