Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Phase Transition”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 523 records · Page 29Linked to original sources
PubMed1999

Continuous phase transition in a spin-glass model without time-reversal symmetry.

We investigate the phase transition in a strongly disordered short-range three-spin interaction model characterized by the absence of time-reversal symmetry in the Hamiltonian. In the mean-field limit the model is well described by the Adam-Gibbs-DiMarzio scenario for the glass transition; however, in the short-range case this picture turns out to be modified. The model presents a finite temperature continuous phase transition characterized by a divergent spin-glass susceptibility and a negative specific-heat exponent. We expect the nature of the transition in this three-spin model to be the same as the transition in the Edwards-Anderson model in a magnetic field, with the advantage that the strong crossover effects present in the latter case are absent.

Journal Article↗
PubMed2005

Mechanism of thermal phase transition of a ferroelectric liquid crystal with monotropic transition temperature studied by infrared spectroscopy combined with principal component analysis and sample-sample two-dimensional correlation spectroscopy.

Infrared (IR) spectra of FLC-154 (FLC: ferroelectric liquid crystal) with monotropic phase transition under a nonalignment state with a sample layer thickness of 24.5 microm were measured for heating process from 55 to 90 degrees C and a cooling process from 90 to 55 degrees C in increments of 1 degrees C. The thermal dynamics of FLC-154 were investigated by use of IR spectroscopy combined with principal component analysis (PCA) and sample-sample two-dimensional (2D) correlation spectroscopy. During the cooling, the FLC-154 molecule passes through the monotropic smectic-C* (Sm-C*) phase, which is transformed from the Sm-A phase. The results from PCA suggest that during the heating process, the thermal dynamics of the alkyl chains, core moiety, and C=O groups are similar to each other. Furthermore, PCA and sample-sample 2D correlation spectroscopy indicate that the alkyl chains and C=O groups in the chiral and core moieties are responsible for the emergence of the Sm-C* phase. This conclusion is very important because the IR data have given more evident cause for the emergence of the Sm-C* phase than the theoretical models such as the molecular-statistical theory of ferroelectric ordering and the indigenous polarization theory. Moreover, it has been found that some of the trans conformations of the alkyl chains of FLC-154 change partly to the gauche conformation when the phase transition from the crystalline phase to the Sm-A phase occurs. It has also been found that the intermolecular interactions of the C=O group in the core moiety in the Sm-A phase are weaker than those in the crystalline phase and that the conformational change occurs on the C-O-C bonds in the core moiety upon going from the crystalline to the Sm-A phase.

Algorithms↗
PubMed1978

Phase transitions in phospholipid model membranes of different curvature.

1. Nuclear magnetic resonance, light scattering and freeze fracturing electron microscopic techniques were used to characterize the size of unilamellar phospholipid vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. 2. Differential scanning calorimetric and light scattering analyses showed that very small unilamellar vesicles obtained by the sonication method exhibit a downward shifted, largely broadened phase transition with a slightly decreased enthalpy change when compared with multilayered liposomes. 3. The phase transition of vesicles with variable diameter as obtained by injection methods resembled the pattern of multilayered liposomes the more the diameter was increased. 4. Repeated cycling through the lipid phase transition was shown to have a progressive effect on a fusion process. This effect was strongly increased when the osmolarity of the medium was enhanced (e.g. by the addition of cryoprotectors). Furthermore it was shown that ice-water of the systems caused abrupt fusion of the lipid structures. 5. Controversial results in the literature on the thermotropic behavior of vesicles could be explained in terms of these fusion processes.

Calorimetry, Differential Scanning↗
PubMed2000

X-ray diffraction study of the phase transitions of (CH3)4NCdCl3 between 293 and 80 K: a quantitative analysis of the ferroelastic domains distribution below 118 K

X-ray diffraction patterns of [N(CH3)4][CdCl3], tetramethylammonium trichlorocadmate(II), have been investigated in the temperature range 80-293 K, which includes two phase transitions at 118 and 104 K, respectively. The main interest in this compound is to establish the mechanism of the structural phase transitions common to other members of the isostructural family [(CH3)4N][MX3]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origin of the order-disorder mechanism would be the orientationally disordered distribution of the tetramethylammonium tetrahedra at room temperature. Maximum Entropy Methods suggest that the most probable distribution of the organic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with the crystallographic threefold axis of the structure. Below 118 K the reflections are split. However, the splitting cannot be fully explained by the ferroelastic domains expected to appear after the phase transitions. Recent NMR results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465-2476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.

Journal Article↗
PubMed2003

Universal finite-size scaling behavior and universal dynamical scaling behavior of absorbing phase transitions with a conserved field.

We analyze numerically three different models exhibiting an absorbing phase transition. We focus on the finite-size scaling as well as the dynamical scaling behavior. An accurate determination of several critical exponents allows one to validate certain hyperscaling relations. Using these hyperscaling relations it is possible to express the avalanche exponents of a self-organized critical system in terms of the ordinary exponents of a continuous absorbing phase transition.

Journal Article↗
PubMed1980

The effect of surface curvature on the head-group structure and phase transition properties of phospholipid bilayer vesicles.

Proton nuclear magnetic resonance spectra at 360 MHz of small sonicated distearoyl phosphatidylcholine vesicles show easily distinguishable resonances due to choline N-methyl head-group protons located in the inner and outer bilayer halves. A study of the chemical shift of these resonances as a function of temperature reveals that the splitting between them increases below the phase transition. This occurs as a result of an upfield shift of the inner layer resonance at the phase transition. Consideration of the possible causes of this effect results in the conclusion that, at the phase transition, there is a change in the organization of the inner layer head-groups which does not occur for the outer layer head-groups.

Choline↗
PubMed1980

The incorporation of cholesterol into inner mitochondrial membranes and its effect on lipid phase transition.

Incubations of rat liver inner mitochondrial membranes with liposomes prepared from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol resulted in a considerable enrichment of the cholesterol composition of these membranes. This enrichment is not accompanied by an alteration in the membrane phospholipid content or fatty acid composition. The exogenous cholesterol appears to be integrated into the membrane structure because it has effects consistent with the known properties of this sterol in other natural and artificial membrane systems. Differential scanning calorimetry on both intact membranes and extracted lipids showed that as the ratio of cholesterol to phospholipid was increased, the endotherm corresponding to the lipid phase transition was reduced. Freeze-fracture electron microscopy of the native membranes showed that intramembranous particles are randomly distributed above the phase transition temperature. Below this temperature large smooth areas, believed to correspond to lipid in the gel state from which proteins have been excluded, can be observed. In the presence of high concentrations of cholesterol the fracture faces observed below the lipid transition temperature show no regions of phase segregation, and observation consistent with previous studies using pure lipids where cholesterol was observed to prevent the lipid undergoing a cooperative phase transition. The results are discussed in terms of the observed low concentrations of cholesterol in normal liver inner mitochondrial membranes and the distribution of cholesterol within the liver cells.

Animals↗
PubMed1977

Phase transition in charged lipid membranes.

Experimental results on the effect of electrostatics on bilayer phase transitions are compared with corresponding data for monolayers and the predictions of electrical double layer theory. The two substantial conclusions which emerge are that: (i) double layer theory based on a continuous surface charge distribution cannot explain all the relevant data, a situation which may be improved by taking into account the discrete nature of the surface charge distribution; (ii) the crystal - liquid crystal phase transition of charged bilayer membranes is always a continuous one which takes place through an intermediate state consisting of both fluid and frozen domains.

Electricity↗
PubMed1998

Chain Mobility in Polymer Systems: On the Borderline between Solid and Melt. 2. Crystal Size Influence in Phase Transition and Sintering of Ultrahigh Molecular Weight Polyethylene via the Mobile Hexagonal Phase.

Polymorphism is a well-established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our study concerning the processability of polymers, we came across an unusual observation related to polymorphism induced by pressure. The experimental observation is that polyethylene crystals transform from the stable orthorhombic crystal into a transient hexagonal phase. The occurrence of a transient hexagonal phase is shown to be dependent on the polymer crystal size; smaller crystals transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point Qo, which is located at P = 3.6 kbar and T = 230 degreesC. The crystal size dependence in the phase transition was investigated by in situ X-ray studies in the unirradiated and irradiated solution-crystallized films. Since the chain mobility is rather high in the hexagonal phase, sintering has been attempted via this transient phase using ultrahigh molecular weight polyethylene (UHMW-PE) as a model system. UHMW-PE is an intractable polymer due to its high molar mass but possesses excellent abrasion resistance properties. For this reason it is used as an inlay in demanding applications such as artificial hip and knee joints. The service life of UHMW-PE in these artificial joints, however, is limited due to the poor processing characteristics notably during sintering, and often a second operation is needed to replace the UHMW-PE interface. Sintering via the transient hexagonal phase could provide a solution for this important problem which concerns an increasing number of people.

Journal Article↗
PubMed2006

Phosphoinositides are involved in control of the glucose-dependent growth resumption that follows the transition phase in Streptomyces lividans.

The interruption of the sblA gene of Streptomyces lividans was previously shown to lead to relief of glucose repression of the normally strongly glucose-repressed alpha-amylase gene. In addition to this relief, an early entry into stationary phase was observed when cells were grown in a minimal medium containing glucose as the main carbon source. In this study, we established that this mutant does not resume growth after the transition phase when cultured in the complex glucose-rich liquid medium R2YE and sporulates much earlier than the wild-type strain when plated on solid R2YE. These phenotypic differences, which were abolished when glucose was omitted from the R2YE medium, correlated with a reduced glucose uptake ability of the sblA mutant strain. sblA was shown to encode a bifunctional enzyme possessing phospholipase C-like and phosphoinositide phosphatase activities. The cleavage of phosphoinositides by SblA seems necessary to trigger the glucose-dependent renewed growth that follows the transition phase. The transient expression of sblA that takes place just before the transition phase is consistent with a regulatory role for this gene during the late stages of growth. The tight temporal control of sblA expression was shown to depend on two operator sites. One, located just upstream of the -35 promoter region, likely constitutes a repressor binding site. The other, located 170 bp downstream of the GTG sblA translational start codon, may be involved in the regulation of the degradation of the sblA transcript. This study suggests that phosphoinositides constitute important regulatory molecules in Streptomyces, as they do in eukaryotes.

Bacterial Proteins↗
PubMed2003

Cation effects on sol-gel and gel-sol phase transitions of kappa-carrageenan-water system.

Sol-gel and gel-sol phase transitions of kappa-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I(tr), was monitored against temperature to determine the sol-gel and gel-sol temperatures (T(sg) and T(gs)) and activation energies (deltaH(sg) and deltaH(gs)). It was observed that T(gs) was notably higher than T(sg) due to the hysteresis on the phase transition loops. T(gs) and deltaH(gs) values were also higher for gels containing KCl than for those without KCl. The increase in carrageenan content caused an increase in both critical temperatures and activation energies for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T(gs) and T(sg). Similarly deltaH(sg) was elevated by the increase in cation content of the gel. These results were interpreted as the formation of stronger gels in the presence of KCl in water.

Carrageenan↗
PubMed2003

Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films.

'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.

Calcium Compounds↗
PubMed2004

Structures and phase transitions of trigonal ZrMo2O8 and HfMo2O8.

This paper describes the structures, thermal-expansion properties and phase transitions of the trigonal forms of ZrMo(2)O(8) and HfMo(2)O(8). Both phases adopt a P(-)3m structure at room temperature and show positive thermal expansion. Both phases also undergo a displacive phase transition at high temperature (ZrMo(2)O(8) at 487 K and HfMo(2)O(8) at 463 K) to a higher-symmetry structure that has lower thermal expansion. The structure of the high-temperature alpha'-AMo(2)O(8) form (A = Zr and Hf) has been refined from powder diffraction data in space group P(-)3m1.

Journal Article↗
PubMed2001

Solvent isotope effects on the phase-transition properties of lipid bilayers.

Highly sensitive differential scanning microcalorimetry (DSC) has been used to investigate the phase transition properties of lipid vesicles prepared from 1,2-distearoyl-L-3-glyceryl-phosphatidylcholine (DSPC) in H(2)O and D(2)O. The data show that the response of pre-transition properties to D(2)O-->H(2)O substitution is stronger than the main transition properties. We find that there is a small increase in the phase transition temperature (DeltaT approximately 0.5 K) and in the co-operative unit in the main transition. The increase in enthalpy (DeltaH congruent with1 kJ(.)mol(-1)) and in transition temperature (DeltaT congruent with2 K) observed in the pre-transition is comparable with that observed in quite different processes and systems, i.e. melting of nucleic acids and proteins and gel formation. It is suggested that D(2)O-->H(2)O substitution affects the thermal transition in these systems in such a way that the contributions of enthalpy and entropy to structural reorganization of water in these processes is modified.

Journal Article↗
PubMed1991

Verapamil prevents the effects of daunomycin on the thermotropic phase transition of model lipid bilayers.

High-sensitivity differential scanning calorimetry and fluorescence-depolarization techniques were used to study how the presence of daunomycin and/or verapamil affect the thermotropic behaviour of dipalmitoyl phosphatidylcholine (DPPC) vesicles. Daunomycin, a potent anti-cancer agent, perturbs the thermodynamic parameters associated with the lipid phase transition: it decreases the enthalpy change, lowers the transition temperature and reduces the co-operative behavior of the phospholipid molecules. Verapamil, on the other hand, produces smaller alterations in the lipid phase transition. However, when daunomycin and verapamil are present simultaneously in the DPPC vesicles, it is observed that verapamil prevents, in a concentration-dependent manner, the alteration in the phospholipid phase transition expected from the presence of daunomycin in the bilayer. Furthermore, drug-binding studies suggest that the observed interference of verapamil in the daunomycin/phospholipid interaction occurs without a decrease in the amount of daunomycin bound to the lipid bilayer and without the formation of a daunomycin-verapamil complex. Because of the importance of drug-membrane interactions in anthracycline cytotoxicity, we speculate that the lipid bilayer of biological membranes may provide appropriate sites at which the presence of verapamil influences the activity of daunomycin.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2004

Shear-induced phase transitions in sucrose ester surfactant.

The behavior of a commercial sucrose stearate blend has been examined by means of various experimental techniques (differential scanning calorimetry, light polarization and electron microscopy, and rotational rheometry). A partial phase diagram in water has been established. It shows that the binary system forms a lamellar lyotropic mesophase and that the melting behavior is characterized by a lamellar gel-lamellar liquid crystalline phase transition. The identification of the liquid crystalline phase has been carried out from textural observation using polarization microscopy and freeze-fracture electron microscopy. At low surfactant concentrations, the phase transition has been followed through rheological experiments. Furthermore, a shear-induced transition, from the lamellar phase (sheets of surfactant bilayers including a few large multilamellar vesicles) to an onion phase, has been observed above a critical temperature of 43 degrees C. The vesicles so obtained did not relax over more than 3 weeks. The presence of a small ratio of distearate in the sugar ester blend seems to be the key to vesicle formation at low surface-active material concentration.

Journal Article↗
PubMed1994

Trehalose lowers membrane phase transitions in dry yeast cells.

Recent work has clearly demonstrated a direct correlation between the amount of trehalose present in the yeast Saccharomyces cerevisiae and its ability to tolerate dehydration, but has failed to elucidate the specific role played by trehalose. By using Fourier transform infrared spectroscopy we measured the transition temperature of phospholipids in both intact S. cerevisiae and isolated plasma membranes dried in the presence and absence of trehalose. Our results show that trehalose lowers the temperature of the dry gel to liquid crystal phase transition in yeast from around 60 degrees C to about 40 degrees C, thus allowing yeast rehydrated above 40 degrees C to avoid the damaging effects of passing through a phase transition. These results explain both the need for trehalose and the observation that yeast must be rehydrated with warm water if they are to remain viable. Only when trehalose is present is the dry transition within a physiologically tolerable range and only when the cells are rehydrated above 40 degrees C will they avoid passing through a phase transition.

Cell Membrane↗
PubMed2006

Four-dimensional ultrafast electron microscopy of phase transitions.

Reported here is direct imaging (and diffraction) by using 4D ultrafast electron microscopy (UEM) with combined spatial and temporal resolutions. In the first phase of UEM, it was possible to obtain snapshot images by using timed, single-electron packets; each packet is free of space-charge effects. Here, we demonstrate the ability to obtain sequences of snapshots ("movies") with atomic-scale spatial resolution and ultrashort temporal resolution. Specifically, it is shown that ultrafast metal-insulator phase transitions can be studied with these achieved spatial and temporal resolutions. The diffraction (atomic scale) and images (nanometer scale) we obtained manifest the structural phase transition with its characteristic hysteresis, and the time scale involved (100 fs) is now studied by directly monitoring coordinates of the atoms themselves.

Crystallization↗