Brewster angle microscopy of calcium oxalate monohydrate precipitation at phospholipid monolayer phase boundaries

The precipitation of calcium oxalate monohydrate (COM) at phospholipid monolayers confined to the air/water interface is observed in situ with the aid of Brewster angle microscopy. COM crystals appear as bright objects that are easily identified and quantified to assess the effects of different conditions on crystallization. Crystal precipitation was monitored at monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in liquid condensed (LC) and liquid expanded (LE) phases. Within the LC phase, higher pressures reduce the incidence of crystallization at the interface, implying that within this phase precipitation is enhanced by higher compressibility or fluidity of the monolayer. Precipitation at biphasic LC/LE and LE/gas (G) monolayers was also studied. COM appears preferentially at phase boundaries of the DPPC LC/LE and LE/G monolayers. However, when an LC/LE phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, crystal formation occurs away from the interface within the DPPC LC phase. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases.     

相图中紧邻相区及其边界的对应关系(Ⅱ)

根据相律导出紧邻相区内不同相的总相数Φ及其相边界的维数R₁的对应关系定理和它的六条推论。应用这个定理及其推论,可导出相图原理中的边界法则和相区接触法则,可阐明由简单相图单元构成复杂三元相图所必须遵循的十条经验规则,亦可说明一、二、三、四元相图中紧邻相区及其边界的相互关系,以及其他应用。     

Liquid to quasicrystal transition in bilayer water

The phase behavior of confined water is a topic of intense and current interest due to its relevance in biology, geology, and materials science. Nevertheless, little is known about the phases that water forms even when confined in the simplest geometries, such as water confined between parallel surfaces. Here we use molecular dynamics simulations to compute the phase diagram of two layers of water confined between parallel non hydrogen bonding walls. This study shows that the water bilayer forms a dodecagonal quasicrystal, as well as two previously unreported bilayer crystals, one tiled exclusively by pentagonal rings. Quasicrystals, structures with long-range order but without periodicity, have never before been reported for water. The dodecagonal quasicrystal is obtained from the bilayer liquid through a reversible first-order phase transition and has diffusivity intermediate between that of the bilayer liquid and ice phases. The water quasicrystal and the ice polymorphs based on pentagons are stabilized by compression of the bilayer and are not templated by the confining surfaces, which are smooth. This demonstrates that these novel phases are intrinsically favored in bilayer water and suggests that these structures could be relevant not only for confined water but also for the wetting and properties of water at interfaces.     

Molecular dynamics of a short-range ordered smectic phase nanoconfined in porous silicon

4-n-octyl-4-cyanobiphenyl has been recently shown to display an unusual sequence of phases when confined into porous silicon (PSi). The gradual increase of oriented short-range smectic (SRS) correlations in place of a phase transition has been interpreted as a consequence of the anisotropic quenched disorder induced by confinement in PSi. Combining two quasielastic neutron scattering experiments with complementary energy resolutions, the authors present the first investigation of the individual molecular dynamics of this system. A large reduction of the molecular dynamics is observed in the confined liquid phase, as a direct consequence of the boundary conditions imposed by the confinement. Temperature fixed window scans reveal a continuous glasslike reduction of the molecular dynamics of the confined liquid and SRS phases on cooling down to 250 K, where a solidlike behavior is finally reached by a two-step crystallization process.     

A density-functional theory study of the confined soft ellipsoid fluid

A system of soft ellipsoid molecules confined between two planar walls is studied using classical density-functional theory. Both the isotropic and nematic phases are considered. The excess free energy is evaluated using two different Ans?tze and the intermolecular interaction is incorporated using two different direct correlation functions (DCF's). The first is a numerical DCF obtained from simulations of bulk soft ellipsoid fluids and the second is taken from the Parsons-Lee theory. In both the isotropic and nematic phases the numerical DCF gives density and order parameter profiles in reasonable agreement with simulation. The Parsons-Lee DCF also gives reasonable agreement in the isotropic phase but poor agreement in the nematic phase.     

Recent developments in adsorption liquid chromatography (NP-HPLC) A review

The development of normal phase chromatography mainly in the past ten years is summarized. At first, the difference between normal-phase and reversed-phase chromatography is briefly discussed. According to Snyders theory of retention, some theoretical considerations are added. Most of the stationary phases synthesized in the past years and their field of use in NP-LC are given. They are sorted by linkage to silica and by their polar substituents. The effects resulting from variation of phases and eluents are discussed. Examples for group separation and other applications of NP-LC are given. Received: 10 December 1997 / Revised: 24 February 1998 / Accepted: 26 February 1998     

Recent developments in adsorption liquid chromatography (NP-HPLC) A review

The development of normal phase chromatography mainly in the past ten years is summarized. At first, the difference between normal-phase and reversed-phase chromatography is briefly discussed. According to Snyders theory of retention, some theoretical considerations are added. Most of the stationary phases synthesized in the past years and their field of use in NP-LC are given. They are sorted by linkage to silica and by their polar substituents. The effects resulting from variation of phases and eluents are discussed. Examples for group separation and other applications of NP-LC are given. Received: 10 December 1997 / Revised: 24 February 1998 / Accepted: 26 February 1998     

Field-induced reorientation dynamics in liquid crystalline hexatic phases

A smectic hexatic phase with tilted molecules is assumed to be confined between two plane boundaries. At a finite boundary distance two dielectric modes are excited by an a.c. external electric field. Using the elastic continuum theory, equations for the relaxation times and the corresponding dielectric susceptibilities are derived and discussed in the limit of high boundary distances and for different material parameters.     

Phase diagram and glass transition of confined benzene

We used differential scanning calorimetry, neutron scattering, and proton NMR to investigate the phase behavior, the structure, and the dynamics of benzene confined in a series of cylindrical mesoporous materials MCM-41 and SBA-15 with pore diameters, d, between 2.4 and 14 nm. With this multitechnique approach, it was possible to determine the structure and, for the first time to our knowledge, the density of confined benzene as a function of temperature and pore size. Under standard cooling rates, benzene partially crystallizes in SBA-15 matrixes (4.7 相似文献   

Shearing of nanoscopic bridges in two-component thin liquid layers between chemically patterned walls

Bridge phases associated with a phase transition between two liquid phases occur when a two-component liquid mixture is confined between chemically patterned walls. In the bulk the liquid mixture with components A, B undergoes phase separation into an A-rich phase and a B-rich phase. The walls bear stripes attractive to A. In the bridge phase A-rich and B-rich regions alternate. Grand canonical Monte Carlo studies are performed with the alignment between stripes on opposite walls varied. Misalignment of the stripes places the nanoscopic liquid bridges under shear strain. The bridges exert a Hookean restoring force on the walls for small displacements from equilibrium. As the strain increases there are deviations from Hooke's law. Eventually there is an abrupt yielding of the bridges. Molecular dynamics simulations show the bridges form or disintegrate on time scales which are fast compared to wall motion and transport of molecules into or from the confined space. Some interesting possible applications of the phenomena are discussed.     

Shift of the nematic–isotropic phase transition temperature in a thin layer of a metallomesogenic complex

The effect of boundaries on the nematic–isotropic phase transition temperature in a melt of a metallomesogenic complex was studied for the first time. This was done by comparison of the electro-optical constant of the isotropic phase with the dielectric and optical anisotropy of the nematic phase on the basis of the Landau–de Gennes theory. In a real experiment, the two liquid phases (nematic and isotropic ones) coexist in a range of several degrees around the transition. According to polarization microscopy data, the phase transition temperature decreases by more than 10°C as the metallomesogen layer thickness is reduced from 200 to 5 μm.     

Quasi-chemical treatment of intermediate phases appearing in nonstoichiometric compounds

A quasi-chemical treatment of the superlattice formation model is applied to intermediate phases appearing in nonstoichiometric compounds. Two kinds of interaction energy are introduced and both the intermediate phase and two-phase separation are described in a single formula. The order parameter and the free energy are obtained as a function of temperature and composition. The boundaries of phases are determined by the common-tangent method.     

Theory of phase diagrams described by thermodynamic potentials with T d symmetry

Phase diagrams of crystals induced by irreducible representations with symmetry group \(L = \bar 43m\) (T _d ) are constructed within the phenomenological theory of second-order phase transitions. A model of the Landau thermodynamic potential is studied, state equations of all symmetry-conditioned phases are obtained, and general conditions for their thermodynamic stability are formulated. Equations for the boundaries of phase areas and lines of phase transitions are obtained for the fourth order of expansion of the potential via components of the order parameter. Some types of the collapse of the multicritical point of the phase diagram for the eighth order of potential expansion are studied using computer calculations. The possible existence of phase diagrams that contain one or more triple points and areas of existence of three and four phases is shown for the first time for the potentials with the above symmetry. Examples are given of crystals that undergo phase transitions in the considered symmetry of the order parameter.     

Double-polyelectrolyte, like-charged amphiphilic diblock copolymers: swollen structures and pH- and salt-dependent lyotropic behavior

We consider a symmetrical poly(styrene- stat-(acrylic acid))- block-poly(acrylic acid), i.e., PSAA- b-PAA, diblock copolymer, with a molar fraction phi AA = 0.42 of acrylic acid, in the more hydrophobic PSAA statistical first block. We investigate its structural behavior at constant concentration in water using small-angle neutron scattering (SANS) by varying (i) the ionization of its acrylic acid motives via the pH by adding NaOH and (ii) the ionic strength of the solution by increasing the NaCl salt concentration c S. We present the resulting morphological phase diagram {pH, c S}, in which we identified two different lamellar phases presenting a smectic long-range order at small-to-intermediate ionizations and a spherical phase with a liquid-like short-range order at larger ionization. In the low-ionization regime, the first lamellar phase comprises a water-free PSAA lamellar core surrounded by a dense poly(acrylic acid) brush swollen with water. Its mostly hydrophobic core still being glassy, this phase is unable to reorganize and is frozen in. A detailed analysis of the SANS data shows the osmotic nature of the polyelectrolyte brush, in which the Na+ counterions are confined so that local electroneutrality is satisfied. Above the pH at which the PSAA statistical block starts ionizing, the PSAA lamellar core melts. The second lamellar phase identified then comprises a PSAA core thinner than that of the frozen-in previous phase, implying a significant increase of the core/water interface and a decrease of the brush surface density. The transition from the first lamellar phase to the second one can be quantitatively shown to result from the balance between the two contributions: (i) the extra interfacial cost between the thinner core and water and (ii) the associated gain in entropy of mixing for the counterions confined inside the brush. At even higher ionization, the diblocks finally form spherical objects with a very small, pH-dependent aggregation number and reach an apparent onset of self-association. When the highest ionization investigated is reached, the cores of these final spherical core-shell objects are found to contain a significant amount of water. We thereby demonstrate that at constant concentration, pH, and ionic strength both trigger a transition from frozen to molten hydrophobic phases as well as unexpected morphological transitions.     

Capillary condensation in a geometrically and a chemically heterogeneous pore: a molecular simulation study

A computer simulation study has been carried out, using an extended Gibbs ensemble Monte Carlo technique, to examine the influence of so-called geometric and chemical disorder on the thermodynamic behavior of simple fluids confined in porous media. The technique allows the equilibrium coexistence of gas and liquid phases to be calculated in a single run. The phase diagram of Lennard-Jones fluid has been calculated in a perfectly cylindrical pore as a reference. Some disorder is then introduced in the porous material, first by spatially modifying the external potential of the initially cylindrical pore, to imitate the geometric disorder of a more realistic pore (undulation, constrictions, etc.) and second by modulating the amplitude of the same initially cylindrical potential to reproduce the energetic disorder of realistic pores due to chemical variations along it. It is shown that the chemical disorder has a much stronger effect on the phase diagram of the confined fluid. The complete adsorption/desorption isotherms are also calculated to help in understanding the large effects of chemical disorder.     

THE THEORY OP BOUNDARIES IN ISOBARIC ISOPLETHS(Ⅰ)——REGULAR SECTIONS

According to the theory of the relation between neighboring phase regions {abbreviated to NPRs) and their boundaries in isobaric phase diagrams,we discuss systematically the relation between the characteristics of boundaries and the combinations of the phases of NPRs in isobaric isopleths.There are only three types and seven subgroups of boundaries of different character and the concrete characteristics of them have been determined.     

Phase discrimination by automated BKD

Automated backscatter Kikuchi diffraction in the SEM enables rapid discrimination of phases and the automated acquisition of phase-distribution maps at sub-micron resolution, if the phases belong to different Laue groups and/or if their lattice constants are significantly different. The acquired database can be further used for quantitative determination of phase contents, for construction of crystal orientation maps, for characterization of grain and phase boundaries, for crystal texture analysis on a grain-specific level by calculating pole figures and orientation density functions separately for each phase, and for orientation stereology in general. Pattern-quality maps provide a clear reproduction of the microstructure on a meso-scale level.     

Torsion-induced phase transitions in fluids confined between chemically decorated substrates

In this paper we investigate the phase behavior of a "simple" fluid confined to a chemically heterogeneous slit pore of nanoscopic width s(z) by means of Monte Carlo simulations in the grand canonical ensemble. The fluid-substrate interaction is purely repulsive except for elliptic regions of semiaxes A and B attracting fluid molecules. On account of the interplay between confinement (i.e., s(z)) and chemical decoration, three fluid phases are thermodynamically permissible, namely, gaslike and liquidlike phases and a "bridge phase" where the molecules are preferentially adsorbed by the attractive elliptic patterns and span the gap between the opposite substrate surfaces. Because of their lack of cylindrical symmetry, bridge phases can be exposed to a torsional strain 0相似文献   

Interface phenomena in (super)hard nitride nanocomposites: from coatings to bulk materials

Mechanical properties of nanocomposites usually surpass the mechanical properties of their micro-structured and single-crystalline counterparts. This is mainly due to an extremely high density of internal interfaces in nanocomposites like grain, crystallite and phase boundaries. When compared to diamond, carbides and borides, nitrides are of interest because of their high temperature oxidation resistance and compatibility with iron containing alloys. This tutorial review classifies the contributions of various internal interfaces to the hardness of the nanocomposites, and appreciates the outstanding role of partially coherent phase boundaries in the hardness enhancement. With selected examples of transition metal nitrides containing aluminium and silicon as well as of boron nitrides, it is explained how the nanocomposites with partially coherent phase boundaries and thus with enhanced hardness can be synthesised. As the possible ways of the formation of coherent phase boundaries, the local epitaxial growth of phases with limited mutual solubility, the production of supersaturated solid solutions followed by the segregation of elements during the spinodal decomposition and the incomplete phase transformation are discussed. The most important techniques, used for synthesis of nitride nanocomposites, like CVD, PVD, precursor-based methods, mechanical alloying and high-pressure-high-temperature synthesis are briefly reviewed. Besides, a short overview on hardness definitions and hardness measurements is included.