Exchange dynamics of surfactants in adsorption layers investigated by PFG NMR diffusion

Surfactant adsorption layers on polystyrene latex particles are investigated concerning their equilibrium exchange dynamics with surfactant molecules in solution. Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) self-diffusion measurements of the nonionic surfactant C12(EO)5 are performed, making use of the difference of the diffusion coefficients of adsorbed and free surfactant in solution. On variation of the gradient pulse spacing the echo decay data show a transition from fast to slow exchange, giving the possibility of a quantitative evaluation. A two-site model is applied to fit the data and obtain average residence times of the surfactant in both sites. The results are consistent with diffusion-controlled adsorption of surfactant monomers and additional micellar contributions to the exchange, whereas no evidence for kinetic barriers is found for these nonionic surfactants on the relevant timescale (10 ms).