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 inve stigated 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 s ite exhibits a different diffusion coefficient. NMR diffusion experime nts 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 Delt a, can be covered, and the region of intermediate exchange is describe d by a two-site model of exchange-coupled diffusion. Exchange-diffusio n experiments on adsorbed surfactant were performed for several surfac tant concentrations with varying gradient spacing. Applying a two-site model, we obtained the average residence times of the surfactant in t he monolayer, and in solution. Error analyses of the fitting procedure s were performed by Monte Carlo simulations of the experimental noise. With the PFG-NMR method, adsorption dynamics are studied under equili brium conditions. We found fast adsorption rates on the order of 10 ms , corresponding to diffusion-controlled adsorption. With increasing su rfactant concentration, the dynamics become faster, which we attribute to a large contribution due to exchange between micelles and the surf ace. PFG-NMR experiments can probe an exchange time scale as fast as m illiseconds, which has so far not been achieved for surfactants at sol id/liquid interfaces. They might therefore become an interesting tool to investigate equilibrium adsorption dynamics of surfactants beyond t he diffusion limit, where the adsorption is kinetically controlled.