Regensburg 2002 – wissenschaftliches Programm

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CPP: Chemische Physik und Polymerphysik

CPP 15: Poster: Dünne Filme, Monolagen und Oberfl
ächen

CPP 15.10: Poster

Mittwoch, 13. März 2002, 16:30–18:00, B

2H NMR investigations of internal dynamics and structures of surfactant layers adsorbed in porous silicas — •Ying Qiao and Monika Schönhoff — Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam/Golm, Germany

Adsorbed nonionic surfactant layers are investigated by using 2H solid NMR. α or β or γ position deuterated dodecyl penta (ethylene oxide), C12E5, was adsorbed from its aqueous solution onto the surfaces of various porous silicas of different pore sizes (1.88–24nm) and shapes (vermiform, hexagonal tubular, ink-bottle). All surface aggregate spectra show isotropically averaged Lorentzian (hyper-Lorentzian for pore size <5nm ) line shape, with relaxation rates R2 of several kHz. The structural aggrangement is represented in terms of relative order parameter profile Srel of the C-2H bond. The order of magnitude of Srel is about 1–1.05, which suggests the structures of the surface aggregates are close to bilayers. The size dependence and shape dependence of R2 and Srel are discussed. The most interesting discovery is that the simple, increasing relationship between Srel and distance of the 2H label position from the head group remains true only for pore sizes above about 5nm. For pore sizes below about 5nm (about one C12E5 chain length), β position has more mobility than both α and γ positions. This may suggest the existence of a phase change induced by the confined geometry. We thank Prof. G. H. Findenegg, Dr. B. H. Han and Dr. J. G. Jia for providing the porous silicas.