A structural study of lyotropic liquid crystal phases formed by aqueous solutions of fluorinated surfactants

Abstract

The binary lyotropic liquid crystalline system Caesium Perfluorooctanoate (CsPFO) / heavy water ( 2H20) has previously been reported to exhibit three mesogenic phases. These were, as a function of decreasing temperature at fixed concentration and pressure isotropic, nematic and lamellar. The mesogenic unit in all three mesophases was reported to be a discrete discoidal micelle. In this project the binary system, as well as the ternary systems upon the addition of an electrolyte and a cosurfactant, have been studied using optical microscopy, small angle neutron and X-ray scattering (SANS and SAXS respectively) and 2H nuclear magnetic resonance (NIMR). The liquid crystalline phases have been characterised in all cases and their structure investigated.
In the binary and ternary systems only three mesophases were observed, as previously reported. On the addition of a third component to the binary system the phase transition temperatures increased in proportion to the amount of added component. In both the nematic and lamellar phases features in the scattering patterns change on the increase of electrolyte concentration. This is attributed to a change in the structure of both the phases. The results are discussed in the light of the
published structures, and a possible alternative. An alternative description of the mesophase behaviour is proposed.
SANS scattering experiments from the binary system, in particular the lamellar phase, in conjunction with the conclusions of the electrolyte experiments have shown that the reported structure of the mesophases, whilst not unreasonable, requires the invocation of physically unlikely interactions to sustain it. The new results are tested against other structural models and alternative structures proposed.
The addition of cosurfactant is less straight forward. It appears to have a profound effect on the structure of the mesophases, the reason for which is, as yet, equivocal.