A small angle neutron scattering study of the lamellar phase of caesium pentadecafluorooctanoate (CsPFO)/1H-1H- perfluoroheptan-1-ol/ ²H ₂O

(Received 8 March 1993, revised 24 May 1993, accepted 7 June 1993)

Abstract
The ionic surfactant system CsPFO/ ²H ₂O has been extensively studied in recent years. It exhibits a defected lamellar phase (L ) at low temperature or high surfactant concentration followed by nematic (N ) and isotropic (L ₁) phases as temperature increases or surfactant concentration decreases. Previous work on the lamellar phase, mainly by X-ray and neutron scattering techniques, has revealed an intra-lamella structure of water filled defects although the precise nature of these defects remains controversial. The addition of the cosurfactant 1H-1H-perfluoroheptan-1-ol to the binary system has been studied by optical microscopy and SANS. As the cosurfactant is added the phase transition temperatures increase and the nematic phase is replaced by a broad two phase coexistence region. SANS shows that the reflection from the intra-lamella structure weakens and shifts to lower Q. The number density of defects decreases with increasing cosurfactant although their shape remains roughly constant being elongated holes. Using these results and those for the binary system a more precise picture of the defect structure, which is consistent with theoretical modes of the defected lamellar phase, has been obtained.