Sorption and diffusion of an ethoxylated stearic alcohol and an ethoxylated stearic amine into and through isolated plant cuticles

Abstract

Sorption and transfer of an ethoxylated stearic alcohol and an ethoxylated stearic amine into and through plant cuticles were investigated using enzymatically isolated tomato and pepper fruit cuticles. Sorption was rapid during the first hour and equilibrium was nearly achieved within 48 hours for amine and 72 hours for alcohol. Sorption was greater for the alcohol and there was a relatively small difference between the two plant species. Cuticle/water partition coefficients (K cw) determined at two surfactant concentrations (0.1 and 1 mM) were greater at the lower concentration. K values for alcohol were 239 and 205 at 0.1 mM, and 84 and 88 at 1 mM for tomato and pepper, respectively. Desorption of both surfactants from cuticles during washing with water was very slow and was not complete. After 3 hours washing, 81% and 85% of amine, and 85% and 87% of alcohol were still retained by tomato and pepper fruit cuticles, respectively. Transfer measurements through cuticles expressed by flux measurements and permeability coefficients (P) showed that the surfactant penetration increased linearly in relation to time after a steady state was reached; the linear regression coefficient was generally between 0.98 and 1 for individual measurements. The transfer was more rapid for alcohol for the three tested concentrations (0.1, 1 and 5 mM). For pepper cuticles, the difference observed between the two surfactants was greater at the lower concentration. Data obtained were used to calculate diffusion coefficients (D) both from flux measurements and hold-up times. Diffusion was greater for alcohol which means that this surfactant has a better mobility than amine within cuticles. Thus, cuticular permeability to the two surfactants was quantitatively analysed with the determination of the three characteristic parameters: P, K and D.