PFG-NMR Diffusion as a Method To Investigate the Equilibrium Adsorption Dynamics of Surfactants at the Solid/Liquid Interface

Abstract

The equilibrium dynamics of surfactant exchange between an adsorption layer on polystyrene latex particles and an aqueous solution were investigated for the nonionic poly(ethylene oxide) surfactant C12(EO)5. In this system, surfactant molecules occur in different sites, i.e., in solution and as adsorbed surfactant, where the surfactant in each site exhibits a different diffusion coefficient. NMR diffusion experiments using pulsed field gradients (PFG) and a stimulated echo sequence were performed. On variation of the gradient pulse spacing, Δ, the transition from slow to fast surfactant exchange, as compared to Δ, can be covered, and the region of intermediate exchange is described by a two-site model of exchange-coupled diffusion. Exchange−diffusion experiments on adsorbed surfactant were performed for several surfactant concentrations with varying gradient spacing. Applying a two-site model, we obtained the average residence times of the surfactant in the monolayer and in solution. Error analyses of the...