Münster 1999 – wissenschaftliches Programm

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CP: Chemische Physik

CP 33: Poster: Dynamik molekularer Systeme

CP 33.7: Poster

Montag, 22. März 1999, 18:00–20:00, R52/R72

2H-NMR Relaxation Study of Surfactant Order in Surface Aggregates and Bulk Structures — •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

Nonionic surfactant surface aggregates on colloidal silica are investigated by 2H-NMR on selectively deuterated C12(EO)5. Local molecular order and timescales of surfactant motional modes in surface aggregates are compared to different types of bulk aggregates. Surface aggregates show an isotropically averaged lorentzian line shape, relaxation rates R2 are on the order of several kHz, indicating a slow motion correlation time of t = 2 us. Possible mechanisms of isotropic averaging in surface aggregates are discussed. Relative order parameter profiles Srel are obtained for surface and different bulk aggregates from quadrupolar splittings and 2H-relaxation, rsp.. An increase of Srel with 2H label position was found for all bulk aggregate types, with the slope depending on aggregate curvature. The comparison to surface aggregates gives an indication of their curvature. All results are consistent with the formation of large anisotropic surface micelles, with motions slowed down compared to bulk micelles, and isotropic averaging achieved by exchange and/or dynamic micelle re-formation processes.