Nematic–lamellar tricritical behavior and structure of the lamellar phase in the ammonium pentadecafluorooctanoate (APFO)/water system

Abstract

Articles you may be interested inTricritical behavior of the nematic to smectic-A phase transition in the binary mixture of liquid crystalThe electric conductivity of the lamellar smectic, the micellar nematic, and the isotropic micellar solution of ammonium perfluorononanoate in water 2H NMR spectroscopy has been used to map a high-resolution ( ± 0.04 K) phase diagram for the ammonium pentadecafiuorooctanoate(APFO)/heavy water system. It is qualitatively similar to that for the CsPFO/heavy water system. In particular, it exhibits a disco tic micellar nematic phase N [t intermediate to an isotropic micellar solution phase 1 and a lamellar phase for weight fractions of APFO between 0.395 (¢ = 0.278) and 0.589 (if; = 0.455) and temperatures between 292.10 and 338.10 K The N IJ to lamellar transition crosses over from second to first order behavior at Ii tricritical point similar to the superftuid transition in 3He/ 4 He mixtures. X-ray scattering experiments show there to be no dramatic change in the structure ofthe micelle at the I to N I) and the N 6 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 ri to the lameUar 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 disco tic 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 of2H quadrupole splittings of heavy water. Similar measurements for deuterated ammonium ions show a preference for cOllnterion binding to sites oflowest 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. I. INTRODUCTIONphasesN B • The nature of the isotropic-to-nematic transition is fairly well established, but the transition from the nematic to the smectic (translationally ordered) phase is not. The general perception is that this will involve a dramatic change in aggregate structure, viz. discoidal micelles into the infinite bilayers of lamellar phases or canonic micelles into the infinite cylinders of hexagonal phases. Indeed, current statistical mechanical models 5 predict that coupling between the size and the long-range orientational ordering of the micelles in the nematic phase leads to their "explosive" growth, However, the fa...

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“…Equation (15 ) shows that the critical temperature increases with b 1 5 bÕ mg p (14 b) increase of the concentration of surfactant. Thus the conditions a 1 5 0 and b 1 5 0 correspond to the Landau Rosenblatt [8] pointed out that the supercooling temwhere perature of the isotropic phase strongly depends on the concentration of surfactant and decreases with concen-…”

“…They observed a vanishing has been found in many binary systems. A large number of experimental studies are devoted to the nature of the latent heat and coexistence range at low concentration on the rst order N+ D -I transition line, indicating the NI transition in LLCs [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The nematic phases of all LLCs are formed by the interaction of non-spherical existence of a Landau point.…”

Surfactosomes: a novel approach to the reconstitution and 2‐D crystallisation of membrane proteins

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