ABSTRACT
The effects of surfactants on solute (2,4-D) and the water permeability of isolated cuticular membranes (CMs) were measured. In analyzing the data, surfactant effects on partition coefficients (K) and on diffusion coefficients in cuticles (D) are distinguished. Surfactants in donor solutions at concentrations above the critical micelle concentration (cmc) slow the penetration of solutes which are soluble in surfactant micelles. This effect is due to the reduced partition coefficients (cuticle/water) of solutes when the water contains surfactant micelles, and is called K-depression. Surfactants may also increase the mobility (D) of solute and water molecules in cuticles. This effect was studied using unilateral desorption of solutes from the outer surface (UDOS). In these experiments, 2,4-D sorbed in CMs must diffuse through a thin layer located at the outer surface of the CM. This skin of the CM is the layer that limits the velocity of desorption of 2,4-D. Rate constants of desorption using an inert phospholipid suspension and micellar surfactant solutions are compared. With surfactant solutions, rate constants of desorption were larger by a factor of up to about 40. Surfactant effects on rate constants of desorption increased with time and depended on the initial permeability of the CMs. This effect of surfactants is due to an increased mobility of 2,4-D in the limiting skin of the CM and requires the presence of surfactants in the CM. Our data and analysis show that activator surfactants must overcompensate K-depression by increasing diffusion coefficients in the limiting skin of the CM. Since activator surfactants must penetrate into cuticles, their effects depend on external concentrations, cuticle/water partition coefficients, and the diffusion coefficients of the surfactants. It is argued that activation of solute diffusion in CMs is due to increased segmental chain mobility in cutin and amorphous regions of soluble cuticular lipids (wax). Since water and solute permeation across cuticles are limited by the same barrier, the effects of surfactants on water and solute permeability are similar. Both the transpiration test and UDOS are suitable for screening surfactants and formulations for the activation of cuticular penetration of solutes.
