SURFACTANT DIFFUSION AND DYNAMIC SURFACE-TENSION IN SPRAY SOLUTIONS

In spray application, reflection (rebound) and wetting difficulty can hinder droplet retention by plant surfaces. Surfactants are usually added to formulations to help overcome these problems, but are sometimes ineffective. Some workers have found dynamic surface tension (DST) to be more reliable than equilibrium surface tension (EST) as a measure of surfactant effectiveness. Diffusion-controlled surfactant adsorption generally produces the quickest surface tension reductions as newly generated interfaces age. A diffusion model was developed for DST of three surfactants representing those commonly used in agrochemical formulations: Tween 20, Triton X-100, and Triton X-405. A bulk-solution diffusion model was developed which, together with the DST model indicated apparent diffusive surface-layer thicknesses of a few micrometers. Apparent diffusion coefficients were determined from DST data obtained by oscillating-jet and maximum bubble pressure methods for aqueous solutions. Good agreement of the models with DST data indicated that adsorption processes for the surfactants were diffusion-controlled. Surface ages required to achieve EST for the surfactants were greater than time intervals observed for droplet deformation and recovery in impaction.