A small angle neutron scattering study of the caesium pentadecafluoro-octanoate/2H2O and two associated ternary systems

Abstract

Caesium pentadecafluoro-octanoate (CsPF0)/ 2 H 2 O has been extensively studied in recent years. It exhibits a lamellar phase to low temperature and high surfactant concentration and an isotropic micellar phase (L 1 ) to high temperature and low surfactant concentration. Between these two phases there is a narrow nematic phase which is composed of disk shaped micelles. Previous work on the lamellar phase with x-ray scattering, nmr and ionic conductivity has shown that the lamellar phase consists of bilayers which are broken by water filled defects. We have recently presented extensive SANS measurements on the lamellar phase of the binary system and with added electrolyte (CsCl) and cosurfactant (1H-1H-pernuoroheptan-1-ol). The results unequivocally show the lamellar phase to be surfactant continuous with elongated water filled holes which are differently affected by adding surfactant, cosurfactant or electrolyte. In this paper we summarize these results and further extend them because SANS also provides information from the scattering line shape about the positional order of the lamellar plane. This can be correlated with the inter-lamellar interaction and its modification by additives