Münster 1999 – wissenschaftliches Programm

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CP: Chemische Physik

CP 24: Dynamik molekularer Systeme

CP 24.3: Vortrag

Donnerstag, 25. März 1999, 11:45–12:00, Phy

Surfactant Adsorption Dynamics in Colloidal Systems Investigated by PFG-NMR Diffusion — •Monika Schönhoff^1,2 and Olle Söderman ¹ — ¹Physical Chemistry 1, University of Lund, S-22100 Lund — ²MPI of Colloids and Interfaces, D-12489 Berlin

Investigations of surfactant adsorption dynamics at solid/liquid interfaces have so far involved methods to study slow nonequilibrium adsorption dynamics controlled by a surfactant depletion layer at the surface. For an understanding on a faster timescale, we investigate here the faster equilibrium exchange dynamics of two model surfactants between an adsorption layer on polystyrene latex spheres and the liquid phase, and show that NMR self-diffusion can give unique information about surfactant adsorption dynamics. NMR surfactant self-diffusion experiments using pulsed field gradients (PFG) are performed, varying the gradient pulse spacing. For the nonionic surfactant C12(EO)5 the echo decays cover the intermediate range between slow and fast exchange. A two-site model with the surfactant exchanging between an adsorption site with diffusion coefficient of the particle, and a free solution site is applied to analyse the data. By fits of the echo decay data the residence times in both sites are obtained. The results correspond to diffusion-limited exchange with additional contributions of exchange from micelles at higher concentrations. [1] J. Phys. Chem. 101 (41), 8237-42 (1997) [2] Magn. Reson. Imaging 16 (5-6), 683-685 (1998)