Water self diffusion in caesium pentadecafluorooctanoate (CsPFO)/H2O and CsPFO/CsCl/H2O and its relationship to structure

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Abstract
The lamellar phases 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, Lf of the caesium pentadecafluorooctanoate (CSPFO)/H~O and CSPFO/Cscl/H20 systems have been studied using tH-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 ± 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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