Interpretation of density and conductivity measurements in the liquid-crystal phases of the cesium pentadecafluorooctanoate–water system and its implication for the structure of the lamellar phase

It has recently been argued [P. Photinos and A. Saupe, Phys. Rev. A 41, 954 (1990)], on the basis of density measurements (as measured by a vibrating densitometer) and the observation of long relaxation times in both these and electrical conductivity measurements, that the nematic-to-lamellar transition in the cesium pentadecafluorooctanoate(CsPFO)–water system involves a significant change in the aggregate structure, probably a transformation from discotic (disk-shaped) micelles to continuous lamellae. With a view to distingushing between these conclusions and previous ones, by the present authors [N. Boden et al., J. Phys. (Paris) 47, 2135 (1986)], which suggested the transition solely involves the onset of translational order without significant changes in micellar structure, we have made a comparison of the densities of a CsPFO${\mathrm{-}}^2$ ${\mathrm{H}}_2$O (w=0.406) sample as measured by both a vibrating densitometer and a classical dilatometer. The densitometer measurements exhibit discontinuities at both the isotropic-to-nematic and the nematic-to-lamellar transitions: at the isotropic-to-nematic transition, the specific volume increases by 7×${10}^{\mathrm{-}5}$ ${\mathrm{cm}}^3$ ${\mathrm{g}}^{\mathrm{-}1}$ of solution, a result at variance with a predicted decrease of 1×${10}^{\mathrm{-}7}$ ${\mathrm{cm}}^3$ ${\mathrm{g}}^{\mathrm{-}1}$ as calculated from the previously reported pressure dependence of the isotropic-to-nematic transition temperature [M. R. Fisch, S. Kumar, and J. D. Litster, Phys. Rev. Lett. 57, 2830 (1986)]; a large increase in specific volume is also observed at the nematic-to-lamellar transition.