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At least 397 records · Page 22Linked to original sources
PubMed2005

Solid/solid phase transitions in confined thin films: a zero temperature approach.

We report a density functional theory study of confinement induced solid/solid phase transitions in a thin film (modeled as methane) at T=0. The solid film is confined by two graphite surfaces represented by a mean-field potential. As the wall separation is varied the structure of the confined film changes, which influences its density and the solvation force. Using the directly accessible grand canonical potential density we determine the stable phases and calculate the exact location of the phase transitions. We observe a series of phases having square and triangular symmetry. At low wall separations we find zig-zag buckling and an asymmetric buckled phase, whose structure is consistent with the strongest buckling instability of a triangular monolayer predicted by Chou and Nelson [Phys. Rev. E 48, 4611 (1993)] but, to our knowledge, has not been observed as a stable phase before. We find that the two-dimensional order parameters Psi(4) (square symmetry) and Psi(6) (triangular symmetry) show unphysical behavior in the transition region between square and triangular symmetry. Thus, in the present model they fail to predict the right location of the phase transitions.

Journal Article↗
PubMed1977

Cooperativity of the phase transition in single- and multibilayer lipid vesicles.

The effect of membrane morphology on the cooperativity of the ordered-fluid, lipid phase transition has been investigated by comparing the transition widths in extended, multibilayer dispersons of dimyristoyl phosphatidyl-choline, and also of dipalmitoyl phosphatidylcholine, with those in the small, single-bilayer vesicles obtained by sonication. The electron spin resonance spectra of three different spin-labelled probes, 2,2,6,6-tetramethylpiperdine-N-oxyl, phosphatidylcholine and stearic acid, and also 90 degrees light scattering and optical turbidity measurements were used as indicators of the phase transition. In all cases the transition was broader in the single-bilayer vesicles than in the multibilayer dispersions, corresponding to a decreased cooperativity on going to the small vesicles. Comparison of the light scattering properties of centrifuged and uncentrifuged, sonicated vesicles suggests that these are particularly sensitive to the presence of intermediate-size particles, and thus the spin label measurements are likely to give a more reliable measure of the degree of cooperativity of the small, single-bilayer vesicles. Application of the Zimm and Bragg theory ((1959) J. Chem. Phys. 31, 526-535) of cooperative transitions to the two-dimensional bilayer system shows that the size of the cooperative unit, 1/square root sigma, is a measure of the mean number of molecules per perimeter molecule, in a given region of ordered or fluid lipid at the centre of the transition. From this result it is found that it is the vesicle size which limits the cooperativity of the transition in the small, single-bilayer vesicles. The implications for the effect of membrane structure and morphology on the cooperativity of phase transitions in biological membranes, and for the possibility of achieving lateral communication in the plane of the membrane, are discussed.

Chemical Phenomena↗
PubMed2005

Toward selectivity control of a heme peptide electrode by modification with a phase-transition polymer.

Reduction currents for H2O2 at a heme peptide (HP)-modified electrodes are suppressed by inhibitors, such as imidazole derivatives. Although this inhibition effect allows determinations of the total inhibition ability of imidazole derivatives, it has no selectivity. In this study the selectivity control of HP-modified electrodes for imidazole derivatives was performed utilizing the thermoresponsive phase transition of poly(N-isopropylacrylamide), which was chemically immobilized on HP-modified electrodes. The inhibition ratios for imidazole derivatives appeared to be small at temperatures below the lower critical solution temperature (LCST), and to be large above the LCST. This change was ascribed to a steric hindrance caused by a phase transition of the polymer. On the other hand, the inhibition ratio for histamine, which has a larger molecular size relative to imidazole, was not significantly changed by the phase transition. Thus, the selectivity of the HP-modified electrode was found to be controllable using an immobilized phase-transition polymer.

Acrylic Resins↗
PubMed1989

Calorimetric studies of the effects of cholesterol on the phase transition of C(18):C(10) phosphatidylcholine.

Differential scanning calorimetry (DSC) has been employed to study the effects of cholesterol on the phase transition of C(18):C(10) phosphatidylcholine (C(18):C(10)PC). C(18):C(10)PC is an asymmetric mixed-chain phosphatidylcholine known to form mixed-interdigitated structures below the transition temperature and form partially interdigitated lipid bilayers above the transition. Three types of samples were used. The treated sample is the lipid dispersion that had undergone three freeze-thaw cycles and stored at 4 degrees C for more than 48 h. The untreated sample was made by vortexing the dry lipid in 50 mM KCl, without the above-mentioned pretreatment. The cold-treated sample was prepared by incubating the treated sample at -20 degrees C for 15 d. There is no apparent difference in the DSC curves between the treated and cold-treated samples. The data derived from the treated samples seem to be more reproducible. The DSC curves between the cholesterol/C(18):C(10)PC and cholesterol/symmetric diacylphosphatidylcholine mixtures are different in three aspects: overall appearance, the cholesterol dependence of delta H, and the effect of cholesterol on the maximal transition temperature Tm, the onset temperature To, and the completion temperature Tc. for both the treated and untreated samples, the total enthalpy change delta H of the phase transition of C(18):C(10)PC decreases with increasing cholesterol content, approaching zero at approximately 25 mol%. This level is lower than the total enthalpy changes reported previously for the cholesterol/symmetric diacylphosphatidylcholine mixtures. Both the heating and cooling thermograms show that Tm, To, and Tc decrease with increasing cholesterol content. The decreasing rates of these temperatures with cholesterol are in the neighborhood of -0.24 degree per mol% of cholesterol. This value is greater than those reported previously for cholesterol/symmetric diacylphosphatidylcholine mixtures. The phase transition between interdigitated lipid bilayers appears to be more sensitive to cholesterol than that between noninterdigitated lipid structures. The formation of highly ordered interdigitated lipid bilayers requires stringent structural conditions such as specific chain length differences and high molecular order. Apparently, in the presence of cholesterol, these stringent structural conditions are no longer satisfied. It is likely that cholesterol causes a local disordering effect on the gel phase of C(18):C(10)PC and that as a consequence the physical state of the gel phase changes continuously with the cholesterol content. The implication of the present study is that cholesterol may have a function in preventing lipids from forming highly ordered interdigitated structures in natural membranes.

Calorimetry, Differential Scanning↗
PubMed2002

Molecular dynamics simulation of spontaneous membrane fusion during a cubic-hexagonal phase transition.

We report a molecular dynamics simulation of the phase transition of monoolein from an inverted cubic phase to an inverted hexagonal phase. The transition proceeds via an intermediate structure consisting of water channels in a cubic geometry, in agreement with the predictions of the modified stalk theory (Siegel, 1999). Two mechanisms are identified by which the topology changes during the transition. Bilayer fusion proceeds via the formation of trans-monolayer contacts, whereas bilayer rupture is observed as a gradual thinning of each monolayer.

Biophysical Phenomena↗
PubMed2006

Quantum spin Hall effect and topological phase transition in HgTe quantum wells.

We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride-cadmium telluride semiconductor quantum wells. When the thickness of the quantum well is varied, the electronic state changes from a normal to an "inverted" type at a critical thickness d(c). We show that this transition is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. We also discuss methods for experimental detection of the QSH effect.

Journal Article↗
PubMed1998

[Organization of health care and health protection in the transitional phase of health care reform and under new legal regulations].

In Federation of Bosnia and Hercegovina, Health Reform is brought up, dictated after newly created post-war policy; fundamental laws in health are brought up, and changes in constitutional sense, brought the country in the phase of transition. Organization of health protection and health business had many disadvantages, what impacted its realisation in direction of satisfying needs and requirement of the population. This paper-work tries to connect existing situation and practice in organization of health care and health business, by using relevant rules which were not adequately and completely realised, and were without possibility of evaluation. The paper tries, by using new Law regulations in Health and Reform to come, to offer organizational solutions which could be more acceptable and realisable in this transitional period.

Bosnia and Herzegovina↗
PubMed2004

Static and dynamic phase transitions in multidimensional voting models on continua.

A voting model (or a generalization of the Glauber model at zero temperature) on a multidimensional lattice is defined as a system composed of a lattice, each site of which is either empty or occupied by a single particle. The reactions of the system are such that two adjacent sites, one empty, the other occupied, may evolve to a state where both of these sites are either empty or occupied. The continuum version of this model in a D-dimensional region with a boundary is studied, and two general behaviors of such systems are investigated, the stationary behavior of the system, and the dominant way of relaxation of the system toward its stationary state. Based on the first behavior, a static phase transition (discontinuous changes in the stationary profiles of the system) is studied. Based on the second behavior, a dynamical phase transition (discontinuous changes in the relaxation times of the system) is studied. It is shown that the static phase transition is induced by the bulk reactions only, while the dynamical phase transition is a result of both bulk reactions and boundary conditions.

Journal Article↗
PubMed2006

On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to Mnyukh.

The first (so-called) lambda transition in solids was found in the specific heat measurements for NH(4)Cl at 242 K by F. Simon in 1922 [Simon (1922). Ann. Phys. 68, 241-280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys. 7, 313-321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of "hybrid crystals" and "smeared transitions" can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4'-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' "hybrid crystal".

Journal Article↗
PubMed2001

Quantum phase transitions and the extended coupled cluster method.

We discuss the application of an extended version of the coupled cluster method to systems exhibiting a quantum phase transition. We use the lattice O(4) nonlinear sigma model in (1+1) and (3+1) dimensions as an example. We show how simple predictions get modified, leading to the absence of a phase transition in (1+1) dimensions, and strong indications for a phase transition in (3+1) dimensions.

Journal Article↗
PubMed1996

Laurdan fluorescence spectroscopy reveals a single liquid-crystalline lipid phase and lack of thermotropic phase transitions in the plasma membrane of living human sperm.

Membrane lipid phase(s), phase coexistence, and thermotropic phase transitions have been investigated in viable human spermatozoa using Laurdan fluorescence spectroscopy. Generalized polarization (GP) values derived from Laurdan excitation and emission spectra confirm that the sperm plasma membrane is a low polar, highly rigid (liquid-ordered) structure, and give evidence that, in the range from 10 degrees C to 42 degrees C, membrane lipids are in a single liquid-crystalline phase. The absence of phase transitions in the same thermal range argues against the hypothesis that the lipid domains previously detected on the sperm surface are produced by lipid lateral phase separation. The above findings are likely accounted for by the high cholesterol to phospholipid molar ratio found in the human sperm membrane. This is the first time that membrane lipid phase and polarity have been detected and quantified in living mammalian spermatozoa.

2-Naphthylamine↗
PubMed2004

Nonlinear transport near a quantum phase transition in two dimensions.

The problem of nonlinear transport near a quantum phase transition is solved within the Landau theory for the dissipative insulator-superconductor phase transition in two dimensions. Using the nonequilibrium Schwinger round-trip Green function formalism, we obtain the scaling function for the nonlinear conductivity in the quantum-disordered regime. We find that the conductivity scales as E2 at low fields but crosses over at large fields to a universal constant on the order of e(2)/h. The crossover between these two regimes obtains when the length scale for the quantum fluctuations becomes comparable to that of the electric field within logarithmic accuracy.

Journal Article↗
PubMedDate not supplied

[Observation of non-equilibrium phase transitions in a closed enzyme system. Intermittent regulation of the activity of lactate dehydrogenase and similar enzymes].

Kinetic models of closed enzymic systems which allow the existence of multi-steady-states were considered. In the basis of these models lies the assumption about the coexistence in solution of different substrate forms and their non-enzymic transitions one to another. If the forms of substrate have different affinity to the enzyme and, moreover, one of the forms inhibits the enzyme, then the enzymic activity in certain conditions may change discretely. The transition of the system incidentally from one steady state to another is similar to the first order phase transition. It is shown that for the enzymic reaction catalyzed by lactate dehydrogenase, the conditions may exist for the non-equilibrium phase transition. It is shown that the existence of the metastability region in the system causes the unusual shifts of the discrete transition from one experiment to another. A model of an allosteric enzyme that describes the discrete behaviour similar to the phase transition is also considered.

Catalysis↗
PubMed2004

Kinetic phase transition in A2 + B2 -->2AB reaction system.

Three lattice gas models for A2 + B2 -->2AB reaction system are studied by Monte Carlo simulation in two-dimensional triangular lattice surface. When both A2 and B2 adsorb and dissociate on the catalytic surface in the random dimer-filling mechanism or in the end-on dimer-filling mechanism, there is no reactive window in the reaction system and just a discontinuous phase transition appears from a "B+vacancy" poisoned state to an "A+vacancy" poisoned state. However, a reactive window appears when one of the two dimers, A2, dissociates in the random dimer-filling mechanism but another dimer, B2, is in the end-on dimer-filling mechanism and the system exhibits a discontinuous phase transition from the active reaction state to a B+ vacancy poisoned state and a continuous phase transition to an "A+vacancy" poisoned state. Furthermore, we show that the critical behavior of the continuous phase transition with infinitely many absorbing states belongs to the robust directed percolation universality class.

Journal Article↗