EXCHANGE DYNAMICS OF SURFACTANTS IN ADSORPTION LAYERS INVESTIGATED BYPFG NMR DIFFUSION

Surfactant adsorption layers on polystyrene latex particles are invest igated concerning their equilibrium exchange dynamics with surfactant molecules in solution. Pulsed field gradient (PFG) nuclear magnetic re sonance (NMR) self-diffusion, measurements of the nonionic surfactant C-12(EO)(5) are performed, making use of the difference of the diffusi on coefficients of adsorbed and free surfactant in solution. On variat ion of the gradient pulse spacing the echo decay data show a transitio n from fast to slow exchange, giving the possibility of a quantitative evaluation A two-site model is applied to fit the data and obtain ave rage 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 evi dence for kinetic barriers is found for these nonionic surfactants on the relevant timescale (10 ms). (C) 1998 Elsevier Science Inc.