Showing papers on "Phase (matter) published in 1979"

Theory of Systems of Rodlike Particles: I. Athermal systems

Abstract: An improved treatment of the lattice model proposed in 1956 is presented for a system of hard rods, with axial ratio x, dispersed in a diluent. Interparticle forces, apart from repulsions on contact, are deliberately ignored within the scope of the present paper. The equilibrium distribution of orientations of the rodlike particles in a phase with nematic order is derived and explicitly taken into account. Thermodynamic properties in the asymptotic limit of high degree of order reduce approximately. to those obtained previously. Calculations carried out for equilibrium between anistropic (nematic) and isotropic phases yield ν′x/νx = 1.4653 for the ratio of volume fractions in the respective phases in the limit x → ∞. The product xνx, expressing the combined covolume of solute species in the isotropic phase at coexistence is 7.89 in the same limit. The calculated critical value of the axial ratio for coexistence of the two phases in the neat liquid is xcrit, = 6.417. Results are compared with thos...

Review of binary alloy formation by thin film interactions

Abstract: Formation and properties of binary compounds made by thin film interactions at temperatures well below the melting point of the various components were recently studied. Several aspects concerning such compounds are still, however, unclear; among which are (a) the mechanism responsible for the growth, (b) which phase is formed first, and (c) why not all the compounds predicted by the phase diagram were observed. We review the literature and suggest that the processes occurring at the interfaces control the phase formed. This conclusion is supported by new experimental data concerning the stability of the various phases as a function of the temperature and/or the sample composition.

Phospholipid lateral phase separation and the partition of cis-parinaric acid and trans-parinaric acid among aqueous, solid lipid, and fluid lipid phases

Abstract: The partition of cis-parinaric acid (9,11,13,15-cis, trans, trans,cis-octadecatetraenoic acid, cis-PnA) and trans-parinaric acid (9,11,13,15-all-trans-octadecatetraenoic acid, trans-PnA) among aqueous, solid lipid, and fluid lipid phases has been measured by three spectroscopic parameters: absorption spectral shifts, fluorescence quantum yield, and fluorescence polarization. The solid lipid was dipalmitoylphosphatidylcholine (DPPC); the fluid lipid was palmitoyldocosahexaenoylphosphatidylcholine (PDPC). Mole fraction partition coefficients between lipid and water were determined by absorption spectroscopy to be for ci--PnA, 5.3 X 10(5) with a solid lipid and 9 X 10(5) with fluid lipid and, for trans-PnA, 5 X 10(6) with solid lipid and 1.7 X 10(6) with fluid lipid. Ratios of the solid to the fluid partition coefficients (Kps/f) are 0.6 +/- 0.2 for cis-PnA and 3 +/- 1 for trans-PnA. A phase diagram for codispersions of DPPC and PDPC has been constructed from the measurements of the temperature dependence of the fluorescence quantum yield and polarization of cis-PnA and trans-PnA and their methyl ester derivatives. A simple analysis based on the phase diagram and fluorescence data allows additional calculations of Kps/f's which are determined to be 0.7 +/- 0.2 for the cis probes and 4 +/- 1 for the trans probes. The relative preference of trans-PnA for solid phase lipids and its enhanced quantum yield in solid phase lipids make it sensitive to a few percent solid. The trans probes provide evidence that structural order may persist in dispersions of these phospholipids 10 degrees C or more above their transition temperature. It is concluded that measurements of PnA fluorescence polarization vs. temperature are better suited than measurements of quantum yield vs. temperature for determining phospholipid phase separation.

Dynamics of the n‐decylammonium chains in the perovskite‐type layer structure compound (C10H21NH3)2CdCl4

Abstract: The two structural phase transitions in the perovskite layer structure compound (C10H21NH3)2CdCl4 at Tc1=35 °C and Tc2=39 °C have been studied by x‐ray diffractional, calorimetric and dielectric measurements, proton NMR spin–lattice relaxation and second moment investigations, 35Cl and 14N quadrupole resonance spectroscopy as well as group theoretical considerations. The results show that the transition (Tc1) from the low temperature phase (P21/n) to the intermediate temperature phase (Pmnn) is basically a dynamic order–disorder transition of rigid alkyl chains, whereas the transition (Tc2) from the intermediate to the high temperature phase (Amaa) is connected with a melting of the alkyl chains. In the intermediate phase the rigid alkyl chains are flipping around their long axes between two equivalent orientations separated by 90°. The two transitions are somewhat analogous to the ones found in the lipid bilayer membranes and can be described by order parameters used for smectic liquid crystals.

Percolative conduction in microemulsion type systems

Abstract: In the presence of a suitable combination of surfactants (an alkaline metal soap and a medium chain length alcohol, for example), water and oil type organic liquids can form transparent compounds of low viscosity that have been labelled “microemulsions”, with little or no mechanical agitation (spontaneous emulsification) (refs 1–3). Depending on the chemical nature of the surface active agents and the relative constituent proportions, water-in-oil (w/o) or oil-in-water (o/w) systems can be obtained. (Because of the similarity between tertiary solutions of inverted micelles and w/o microemulsions, the term ‘inverted micellar solutions’ has also been suggested4–7.) Microemulsions have been investigated by many scientists interested in liquid state and surface physicochemistry and by many technologists foreseeing numerous applications in industry8,9. It has been suggested9–13 that conductivity and permittivity studies could provide, along with other techniques, valuable information about the structure and phase behaviour of microemulsions which are considered to consist of dispersions between a few tens and a few hundreds of angstroms in diameter globules made up of an inner spherical core surrounded by a concentric shell of mixed surfactant and cosurfactant1–10. The experiments reported here show that the conductive behaviour of certain microemulsion systems can be accounted for using the percolation and effective-medium theories14–16 that have depicted transport properties and continuous metal–non-metal transitions in disordered materials with microscopic inhomogeneities associated with, for example, density, composition15 or bonding configuration17 fluctuations. This result, which could help in understanding the structural behaviour of micro-emulsions, is considered in connection with the postulated existence in liquid systems of equilibrium bicontinuous structures, a state described by Scriven18 as ‘related to ordinary liquids as porous media are to homogeneous solids’.

Phase transitions in BaMnF4

Abstract: BaMnF4 is a pyroelectric ferromagnet which displays a number of unusual physical characteristics, some of which are unique. It has the only known continuous antiferroelectric phase transition. It has two- and three-dimensional antiferromagnetic ordering temperatures. It is a weak ferromagnet with ferromagnetism caused by the linear magnetoelectric effect-the only case yet known. It exhibits dielectric anomalies at its Neel temperature. At high temperatures its dielectric constant diverges with increasing temperature; a ferroelectric phase transition would occur if the crystals did not melt first. At high temperatures BaMnF4 is also an anisotropic ionic conductor. The antiferroelectric phase is incommensurate. The incommensurate phase appears unusual in that its translation vector is temperature-independent. Sound velocity measurements made near the antiferroelectric phase transition temperature demonstrate the presence of and characteristics for a relaxational mode; this mode couples strongly to transverse acoustic phonons and is probably the 'phason' predicted theoretically for incommensurate lattices. This article reviews theory and experiment for this unusual material, including neutron, Raman and Brillouin scattering; X-ray, dielectric and conductivity measurements; magnetic resonance and susceptibility studies.

Micellar Solubilization of Proteins in Aprotic Solvents and their Spectroscopic Characterisation

Abstract: The quaternary ammonium salt methyl-trioctylammonium chloride enables the transfer of α-chymotrypsin, trypsin, pepsin and glucagone from water to cyclohexane. Reversed micelles, whose polar core solubilizes both protein and water, are probably formed in the apolar phase. The influence of various parameters on the phase transfer (concentration, pH, solvent, temperature, etc.) has been investigated. Absorption, fluorescence and circular dichroism studies of the biopolymers in the cyclohexane system have been carried out. For trypsin and chymotrypsin, the CD. signal in the 200 nm region is very similar in water and in cyclohexane, which suggests that the polypeptide folding is not substantially different in the two phases. The fluorescence quantum yield is always much larger in the cyclohexane phase than in water. The longer wavelength region of the UV. absorption spectrum is slightly red-shifted relative to water, and a band at 225 nm, probably arising from the aromatic chromophore, is apparent in the organic phase. Reasons for these spectral perturbations are discussed. The enzymes transferred from water into cyclohexane phases can be continuously retransferred into a second water phase. The possible relevance of this ‘double transfer’ as a model for the vectorial transport of biopolymers or a separation technique is discussed.

Spinodal decomposition (phase transitions via unstable states)

Abstract: Considerable deviations from equilibrium conditions are observed for nonstatic, first-order phase transitions. In some cases, unstable (labile) phase states may precede the onset of the phase transition. The relaxation of the system is then accompanied by an enhancement of random inhomogeneities and the appearance of a modulated intermediate structure. This mechanism of the initial stage of a phase transition is called spinodal decomposition (SD). Theoretical and experimental studies of SD in two-component systems are reviewed in this paper. Thermodynamic stability and the possibility of SD in one-component liquid-vapor systems and an alternative nucleation mechanism are discussed. The phenomenological theory of SD is based on the Ginzburg-Landau expression for the free energy of an inhomogeneous system. A linearized diffusion equation is derived, for which thermodynamically unstable states have exponentially increasing solutions for the Fourier components of composition. Subsequent refinements of SD theory take into account thermal fluctuations and involve the derivation of the kinetic equation for the distribution functional. Diffraction methods are the most effective in the experimental study of SD in alloys, glasses, and binary liquid mixtures. So far, the agreement between theory and experiment must be regarded as only qualitative.

The description of the smectic A and C phases and the smectic A–C phase transition of TCOOB with a diffuse‐cone model

Abstract: A previous paper demonstrated that the orientational disorder present in all smectic A phases causes most molecules to make fairly large angles with the normal to the smectic plane. Thus, the molecular directions form a diffuse cone around the layer normal. It is now argued that even the preferred directions of the molecules in a smectic A phase will make a nonzero angle with the layer normal, and from the data examined, this angle, ϑm, appears to be about 10–15 ° for the compound TCOOB (trans‐1,4‐cyclohexane‐di‐n‐octyloxybenzoate). The smectic A‐to‐C phase transition is found to have two aspects in the diffuse‐cone model: the distribution of the molecular directions around the layer normal loses its rotational symmetry at the phase transition temperature TA–C, and the angle ϑm starts increasing sharply at a temperature Tt. In most cases these two temperatures are probably equal. The increase of ϑm with decreasing temperature in the smectic C phase of TCOOB can be described by a (Tt−T)0.35 dependency superimposed on a much smaller linear temperature dependency already present in the smectic A phase.