PFG-NMR diffusion as a method to investigate the equilibrium adsorption dynamics of surfactants at the solid/liquid interface

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 C-12(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, Delta, the transition from slow to fast surfactant exchange, as compared to Delta, 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 fitting procedures were performed by Monte Carlo simulations of the experimental noise. With the PFG-NMR method, adsorption dynamics are studied under equilibrium conditions. We found fast adsorption rates on the order of 10 ms, corresponding to diffusion-controlled adsorption. With increasing surfactant concentration, the dynamics become faster, which we attribute to a large contribution due to exchange between micelles and the surface. PFG-NMR experiments can probe an exchange time scale as fast as milliseconds, which has so far not been achieved for surfactants at solid/liquid interfaces. They might therefore become an interesting tool to investigate equilibrium adsorption dynamics of surfactants beyond the diffusion limit, where the adsorption is kinetically controlled.