Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 1. Effects of ethoxy chain length and the size of the penetrants

The effects of polydisperse ethoxylated fatty alcohol (EFA) surfactants on the penetration of six organic compounds varying in size (molar volumes, 107–282 cm³ mol^-1) and lipophilicity (log K_ow 0·8–6·5) were investigated using 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 diffusion coefficients and, in the absence of EFA, k* values decreased by a factor of 52 when molar volumes increased only 2·64-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 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 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 effect. Effects of micellar aqueous solutions of polydisperse surfactants on solute mobilities followed the pattern observed with monodisperse ones. © 1997 SCI