A small angle neutron scattering study of the lamellar and nematic phases of caesium pentadecafluoro-octanoate (CsPFO)/$^{2}$H$_{2}$O and CsPFO/CsCl/$^{2}$H$_{2}$O

The binary lyotropic liquid crystalline system caesium pentadecafluoro-octanoate (CsPFO)/water ($^2$H$_{2}$O) exhibits an isotropic micellar phase, a lyotropic nematic phase and a positionally ordered, lamellar phase whose planes contain substantial water filled defects. It has been suggested that the structural unit in all three phases is a disk shaped micelle which, as temperature is lowered, first become orientationalluy ordered and then positionally ordered onto planes. Increasing the surfactant concentration apparently causes the micelles to first increase in size and then decrease, explained by invoking an attractive inter-micellar interaction. In this small angle neutron scattering study, we present further results on the structure of the binary and the ternary systems, the latter containing added electrolyte. Our results call into question the previously proposed model of the lamellar phase and show that the form of the temperature dependence of the aggregate dimensions depends on the structural model that has been adopted. We show that a much more satisfactory picture of the lamellar phase is obtained if the shape of the aggregates is assumed to consist of continuous amphiphile layers pierced by irregular water filled holes ; increasing the CsPFO concentration then causes the amphiphile aggregates to grow. This structure is confirmed by the effect of adding electrolyte (CsCl) to the lamellar phase. Here, scattering shows explicitly that the water filled holes anneal out. The addition of electrolyte is also shown to cause changes of aggregate shape in the nematic phase. These changes in both binary and ternary systems are shown to be attributable to modifications of the intra-aggregate electrostatic head-group interactions