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 distinguishing 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-2H2O (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 X 10(-5) cm3 g-1 of solution, a result at variance with a predicted decrease of 1 X 10(-7) cm3 g-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. No such changes are, however, detectable in the dilatometer experiments. This has led us to conclude that the excess specific volumes characterizing the nematic and lamellar phases in the densitometer experiments must be associated with some nonequilibrium state of the sample, induced by the vibration of the sample container. These effects are specific to the liquid-crystal phases, not to the isotropic solution phase, and must therefore arise because of the long-range structural organization of the former. It appears that the vibrating densitometer cannot be used to measure densities of amphiphilic liquid-crystal phases. Hysteresis and long relaxation times observed in electrical conductivity measurements in the lamellar phase are associated with partial focal conic textures; they do not arise from changes in aggregate structure.