NEMATIC LAMELLAR TRICRITICAL BEHAVIOR AND STRUCTURE OF THE LAMELLAR PHASE IN THE AMMONIUM PENTADECAFLUOROOCTANOATE (APFO) WATER-SYSTEM

H-2 NMR spectroscopy has been used to map a high-resolution (+/-0.04 K) phase diagram for the ammonium pentadecafluorooctanoate(APFO)/heavy water system. It is qualitatively similar to that for the CsPFO/heavy water system. In particular, it exhibits a discotic micellar nematic phase N(D)+ intermediate to an isotropic micellar solution phase I and a lamellar phase for weight fractions of APFO between 0.395 (phi = 0.278) and 0.589 (phi = 0.455) and temperatures between 292.10 and 338.10 K. The N(D)+ to lamellar transition crosses over from second to first order behavior at a tricritical point similar to the superfluid transition in He-3/He-4 mixtures. X-ray scattering experiments show there to be no dramatic change in the structure of the micelle at the I to N(D)+ and the N(D)+ to lamellar transitions. Nematic order parameters (orientational order parameters of the discoidal micelles) have been calculated from electrical conductivity measurements. Their variation with temperature in the nematic phase and across the nematic to lamellar transition are qualitatively consistent with the behavior expected for thermotropic calamitic liquid crystals undergoing an isotropic to nematic to smectic-A sequence of transitions. It is, therefore, concluded that the transition from the N(D)+ to the lamellar phase solely involves the propagation of long range positional ordering of the discoidal micelles into planes along the nematic director, that is, the lamellar phase is a discotic lamellar phase L(D). This result reinforces previous claims about the structure of the lamellar phase of the CsPFO system as opposed to the alternative scenario of a classical lamellar phase in which the bilayers are perforated by microscopic defects. This conclusion is supported by measurements of H-2 quadrupole splittings of heavy water. Similar measurements for deuterated ammonium ions show a preference for counterion binding to sites of lowest surface curvature. The fraction of counterions bound to the surface of the micelle is shown to increase as the temperature is lowered due to a growth in diameter and associated changes in surface curvature.