J. Phys. II France
Volume 3, Numéro 12, December 1993
Page(s) 1735 - 1746
DOI: 10.1051/jp2:1993227
J. Phys. II France 3 (1993) 1735-1746

Water self diffusion in caesium pentadecafluorooctanoate (CsPFO)/H 2O and CsPFO/CsCl/H 2O and its relationship to structure

M. C. Holmes, P. Sotta, Y. Hendrikx and B. Deloche

Laboratoire de Physique des Solides CNRS, LA 0002, Université Paris-Sud, 91405 Orsay, France

(Received 16 July 1993, accepted 10 September 1993)

The lamellar phase of some surfactant water systems are not infinite, continuous lamellar planes but scattering experiments show that they contain intra-planar structure. This is believed to be water filled defects with highly curved edges. Here, oriented samples of the defected lamellar phase, L $_{\alpha}^{\rm H}$ of the caesium pentadecafluorooctanoate (CsPFO)/H 2O and CsPFO/CsCl/H 2O systems have been studied using 1H-NMR pulse field gradient measurements of the water self diffusion. The water coordination number per surfactant in the lamellar phase is found to be $4.8 \pm 0.2$ and obstruction factors for water diffusion parallel and perpendicular to the lamella normal are obtained. The latter confirm the defected nature of the lamellar phase. Using structural data from neutron scattering, theoretical obstruction factors have been calculated with no adjustable parameters and agree well with experiment. Both water diffusion and neutron scattering are sensitive to the same defect structure. Although tending to confirm a model of surfactant continuous lamellae broken by elongated water filled defects, the diffusion measurements cannot unequivocally distinguish between different structural models for the defects. The measurements do show that addition of surfactant or of electrolyte leads to the "healing" of the defects and an approach to a classical lamellar phase structure.

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