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PubMed1998

Phase Transition in Langmuir Films of Octadecylmalonic Acid.

The phase transitions observed in the surface pressure-molecular area (pi-A) isotherms of monolayers of the dicarboxylic compound octadecylmalonic acid (OMA) at the air/water interface were investigated using molecular dynamics simulation in the molecular area ranging from 20 to 50 A2/molecule in a triangular lattice. Potential energy, torsion angles, and hydration of the head groups under periodic boundary conditions have been used for the study. Hydration was done by a stacked array of water molecules relaxing under these boundary conditions. Twelve simulations were performed at T = 300 K to study the conformation of the packed molecular system. Bonded and nonbonded interaction potentials were taken from Biosym force fields. The transitions seen in the pi-A curves are explained from the breaks in the plots of potential energy at various concentrations, related to a liquid expanded-to-liquid condensed (LE/LC) transition, and classified under the first order type. Electron diffraction patterns of these films transferred onto transmission electron microscopy (TEM) grids were studied by high-resolution imaging and showed a clear transition from the LE to the LC phase. Copyright 1998 Academic Press.

Journal Article↗
PubMed1980

Lateral and transversal diffusion and phase transitions in erythrocyte membranes. An excimer fluorescence study.

The lateral diffusion of the excimer-forming probe pyrene decanoic acid has been determined in erythrocyte membranes and in vesicles of the lipid extracts. The random walk of the probe molecules is characterized by their jump frequency, nu j, within the lipid matrix. At T = 35 degrees C a value of nu i = 1.6 . 10(8) s-1 is found in erythrocyte membranes. A somewhat slower mobility is determined in vesicles prepared from lipid extracts of the erythrocyte membrane. Depending on structure and change of the lipids we obtain jump frequencies between 0.8 . 10(8) s-1 and 1.5 . 10(8) s-1 at T - 35 degrees C. The results are compared with jump frequencies yielded in model membranes. The mobility of molecules perpendicular to the membrane surface (transversal diffusion) is investigated. Erythrocyte ghosts doped with pyrene phosphatidylcholine were mixed with undoped ghosts in order to study the exchange kinetics of the probe molecule. A fast transfer between the outer layers of the ghost cells tau 1/2 = 1.6 min at T = 37 degrees C) is found. The exchange process between the inner and the outer layer of one erythrocyte ghost (flip-flop process) following this fast transfer occurs with a half-life time value of t 1/2 = 100 min at T = 37 C. The application of excimer-forming probes presumes a fluid state of the membrane. Therefore we investigated the phase transition behaviour using the excimer technique. Beside a thermotropic phase transition at T = 38 degrees C and T = 33 degrees C we observed an additional fluidity change at T = 38 degrees C in erythrocyte ghost. This transition is attached to a separation of the boundary lipid layer from the intrinsic proteins. No lipid phase transition is observed in liposomes from isolated extracts of the erythrocyte membrane with our methods.

Decanoic Acids↗
PubMed2006

Density functional theory study on the structure and capillary phase transition of a polymer melt in a slitlike pore: effect of attraction.

A density functional theory is proposed to investigate the effects of polymer monomer-monomer and monomer-wall attractions on the density profile, chain configuration, and equilibrium capillary phase transition of a freely jointed multi-Yukawa fluid confined in a slitlike pore. The excess Helmholtz energy functional is constructed by using the modified fundamental measure theory, Wertheim's first-order thermodynamic perturbation theory, and Rosenfeld's perturbative method, in which the bulk radial distribution function and direct correlation function of hard-core multi-Yukawa monomers are obtained from the first-order mean spherical approximation. Comparisons of density profiles and bond orientation correlation functions of inhomogeneous chain fluids predicted from the present theory with the simulation data show that the present theory is very accurate, superior to the previous theory. The present theory predicts that the polymer monomer-monomer attraction lowers the strength of oscillations for density profiles and bond orientation correlation functions and makes the excess adsorption more negative. It is interesting to find that the equilibrium capillary phase transition of the polymeric fluid in the hard slitlike pore occurs at a higher chemical potential than in bulk condition, but as the attraction of the pore wall is increased sufficiently, the chemical potential for equilibrium capillary phase transition becomes lower than that for bulk vapor-liquid equilibrium.

Journal Article↗
PubMed1999

Effect of Polyvinyl Pyrrolidone on the Thermal Phase Transition of 1,2 Dipalmitoyl-sn-glycero-3-phosphocholine Bilayer.

The purpose of this study was to investigate the interaction of polyvinyl pyrrolidone (PVP) with phospholipid bilayers in an effort to add a new dimension to our understanding of polymer interaction with lipids. For the preparation of lipid-polymer dispersions, measured amounts of DPPC (1,2 dipalmitoyl-sn-glycero-3-phosphocholine) and PVP were mixed in chloroform. After complete elimination of organic solvent, the dry mixtures were hydrated at 50-55 degrees C. Interactions between DPPC and PVP were assessed by differential scanning calorimetry (DSC) and photon correlation spectroscopy (PS). Separation of liposomes and micelles was performed by centrifugation. Liquid scintillation counting and a UV spectrophotometer were used for their analysis. PVP added as dry powder or added as aqueous solution to dry lipid or preformed liposomes failed to interact. Only PVP previously dissolved in chloroform interacted with DPPC. The DPPC main phase transition moved to lower temperatures with increasing PVP concentrations. This reduction of the phase transition temperature was accompanied by an increase of the DPPC phase transition enthalpy. Analysis of solubilization indicated that the amount of PVP present in the bilayer is dependent on the PVP bulk concentration. The data suggest interaction of PVP previously dissolved in chloroform with the acyl chains of the phospholipid deep into the bilayer. Copyright 1999 Academic Press.

Journal Article↗
PubMed2004

Cooperativity and kinetics of phase transitions in nanopore-confined bilayers studied by differential scanning calorimetry.

The first-order nature of the gel-to-liquid crystal phase transition of phospholipid bilayers requires very slow temperature rates in differential scanning calorimetry (DSC) experiments to minimize any rate-dependent distortions. Proportionality of the DSC signal to the rate poses a problem for studies of substrate-supported bilayers that contain very small volumes of the lipid phase. Recently, we described lipid bilayers self-assembled inside nanoporous substrates. The high density of the nanochannels in these structures provides at least a 500-fold increase in the bilayer surface area for the same size of the planar substrate chips. The increased surface area enables the DSC studies. The rate-dependent DSC curves were modeled as a convolution of a conventional van't Hoff shape and a first-order decay curve of the lipid rearrangement. This analysis shows that although confinement of bilayers to the nanopores of approximately 177 nm in diameter results in a more than threefold longer characteristic time of the lipid rearrangement and a decrease in the cooperative unit number, the phase transition temperature is unaffected.

Calorimetry, Differential Scanning↗
PubMed1986

Destabilization of phosphatidylethanolamine liposomes at the hexagonal phase transition temperature.

We have examined whether there is a relationship between the lamellar-hexagonal phase transition temperature, TH, and the initial kinetics of H+- and Ca2+-induced destabilization of phosphatidylethanolamine (PE) liposomes. The liposomes were composed of dioleoylphosphatidylethanolamine, egg phosphatidylethanolamine (EPE), or phosphatidylethanolamine prepared from egg phosphatidylcholine by transesterification (TPE). These lipids have well-spaced lamellar-hexagonal phase transition temperatures (approximately 12, approximately 45, and approximately 57 degrees C) in a temperature range that allows us to measure the initial kinetics of bilayer destabilization, both below and above TH. The liposomes were prepared at pH 9.5. The TH of EPE and TPE was measured by using differential scanning calorimetry, and it was found that the TH was essentially the same at low pH or at high pH in the presence of 20 mM Ca2+. At temperatures well below TH, either at pH 4.5 or at pH 9.5 in the presence of Ca2+, the liposomes aggregate, leak, and undergo lipid mixing and mixing of contents. We show that liposome/liposome contact is involved in the destabilization of the PE liposomes. The temperature dependence of leakage, lipid mixing, and mixing of contents shows that there is a massive enhancement in the rate of leakage when the temperature approaches the TH of the particular PE and that lipid mixing appears to be enhanced. However, the fusion (mixing of aqueous contents) is diminished or even abolished at temperatures above TH. At and above the TH, a new mechanism of liposome destabilization arises, evidently dependent upon the ability of the PE molecules to adapt new morphological structures at these temperatures. We propose that this destabilization demarks the first step in the pathway to the eventual formation of the HII phase. Thus, the polymorphism accessible to PE is a powerful agent for membrane destabilization, but additional factors are required for fusion.

Calcium↗
PubMed2001

Surface soft phonon and the sqrt[3] x sqrt[3] <--> 3x3 phase transition in Sn/Ge(111) and Sn/Si(111).

Density functional theory calculations show that the reversible Sn/Ge(111) sqrt[3]xsqrt[3]<-->3x3 phase transition can be described in terms of a surface soft phonon. The isovalent Sn/Si(111) case does not display this transition since the sqrt[3]xsqrt[3] phase is the stable structure at low temperature, although it presents a partial softening of the 3x3 surface phonon. The rather flat energy surfaces for the atomic motion associated with this phonon mode in both cases explain the experimental similarities found at room temperature between these systems. The driving force underlying the sqrt[3]xsqrt[3]<-->3x3 phase transition is shown to be associated with the electronic energy gain due to the Sn dangling bond rehybridization.

Journal Article↗
PubMed2007

Phenomenological theory of the nematic to lamellar phase transition in lyotropic liquid crystals.

A phenomenological theory is presented to describe the nematic to lamellar phase transition in lyotropic liquid crystals. The problem of the first or second order transition is explored by means of the variation of the surfactant concentration. The possibility of the tricritical point at the nematic to lamellar phase transition is discussed in a phenomenological way. The influence of the electrolyte on this transition is also discussed by varying the coupling between the electrolyte concentration variables and the order parameters. The theoretical prediction is found to be in good qualitative agreement with experimental results.

Journal Article↗
PubMed2004

Effect of high-pressure-induced ice I-to-ice III phase transitions on inactivation of Listeria innocua in frozen suspension.

The inactivation of Listeria innocua BGA 3532 at subzero temperatures and pressures up to 400 MPa in buffer solution was studied to examine the impact of high-pressure treatments on bacteria in frozen matrices. The state of aggregation of water was taken into account. The inactivation was progressing rapidly during pressure holding under liquid conditions, whereas in the ice phases, extended pressure holding times had comparatively little effect. The transient phase change of ice I to other ice polymorphs (ice II or ice III) during pressure cycles above 200 MPa resulted in an inactivation of about 3 log cycles, probably due to the mechanical stress associated with the phase transition. This effect was independent of the applied pressure holding time. Flow cytometric analyses supported the assumption of different mechanisms of inactivation of L. innocua in the liquid phase and ice I (large fraction of sublethally damaged cells due to pressure inactivation) in contrast to cells subjected to ice I-to-ice III phase transitions (complete inactivation due to cell rupture). Possible applications of high-pressure-induced phase transitions include cell disintegration for the recovery of intracellular components and inactivation of microorganisms in frozen food.

Flow Cytometry↗
PubMed1996

Continuous phase transitions in the perception of multistable visual patterns.

The phenomenon of stroboscopic alternative motion exhibits five different percepts that are seen with an increase in the frequency of presentation: (a) succession, (b) fluttering motion, (c) reversible clockwise and counter-clockwise turning motion, (d) oppositional motion and (e) simultaneity. From a synergetic point of view the increase in frequency is a control parameter and the different percepts are order parameters with phase transitions in between. The neural network model of Carmesin and Arndt is applied to receive predictions about hysteresis and phase transitions between these order parameters. Empirical data show the different motion percepts (b), (c) and (e) have lognormal distributions. Following the theoretical model, it is argued that there are three different phases, (a), (c) and (e), with two continuous phase transitions, (b) and (d), between them. The experimental data substantially match the theoretical assumptions.

Animals↗
PubMed2004

Direct evidence of the charge ordered phase transition of indium nanowires on Si111.

A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.

Journal Article↗
PubMed2005

Phase transition of nanostructure zirconias doped with rare earth elements.

Nanostructure zirconias doped with rare earth elements were obtained by coprecipitation. The morphology, structure, and phase transition of the as-prepared samples were studied. According to transmission electron microscopy, the particles appear in near spherical shape and have a mean particle sizes of about 150 nm without change with the rare earth elements. From laser Raman and X-ray diffraction results, it is known that the tetragonal phase of zirconia is stabilized by a certain concentration of the rare earth dopants, and the cubic phase is obtained if enough dopants are added. The crystal lattice increases with the increase of dopant concentration. Moreover, the heavy rare earths are better than the light ones in stabilizing effect. According to our study, laser Raman is more sensitive than X-ray diffraction in monitoring the phase transition.

Crystallization↗
PubMed1986

Thermotropic phase transitions in the plasma membrane of ram spermatozoa.

A steady-state fluorescence polarization technique, using the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH), showed that separately detectable transitions occurred in the regions of 17, 26 and 36 degrees C in isolated preparations of ram sperm plasma membrane. An independent technique based on the temperature-related behaviour of calcium- and magnesium-activated ATPase detected a single phase transition in the region of 24 degrees C. Modulation of ATPase by neighbouring lipid composition was inferred from findings that phospholipase A2 caused significant stimulation of the enzyme. Cholesterol-rich liposomes caused an upward shift of the phase-transition temperature from 24 degrees C to 30 degrees C, but the reasons for this are unclear. It is considered that these phase transitions may have profound effects on sperm survival and physiology, both during normal fertilization processes and in response to cryostorage.

Adenosine Triphosphatases↗
PubMed1996

Solubility behavior, phase transition, and structure-based nucleation inhibition of etanidazole in aqueous solutions.

PURPOSE: The solubility behavior, phase transition and inhibition of the nucleation process of etanidazole were characterized. METHODS: Solubility measurements as a function of time permitted characterization of the solubility behavior and phase transition. The precipitate from saturated solutions was isolated and characterized by differential scanning calorimetry, polarized light microscopy, x-ray powder diffraction and coulometric analysis. The physical stability of metastable systems was examined in the presence of various structure-based nucleation inhibitors. RESULTS: Etanidazole is soluble in water with an equilibrium solubility of 68.1 mg/mL, pH 6.5 with changes in pH having virtually no effect on the solubility. Etanidazole reaches concentrations in excess of 150 mg/mL within one hour. Etanidazole solutions prepared at 150 mg/ mL contained crystals after rotating for 24 hours. The crystals were isolated and characterized as etanidazole monohydrate. the solubility of etanidazole monohydrate in water increased with time reaching an equilibrium solubility of 68 mg/mL after 24 hours. Therefore, the solubility studies were actually determining the solubility of the more stable monohydrate from of etanidazole. Etanidazole solutions at concentrations of 50, 100 and 150 mg/mL were stabilized to varying degrees with structure-based nucleation inhibitors (imidazole, ethanolamine or diethanolamine). CONCLUSIONS: Anhydrous etanidazole undergoes a transition in aqueous solutions to the more stable monohydrate when the solubility of the monohydrate is exceeded. The physical stability of etanidazole solutions at 4 degrees C is improved following autoclaving. The addition of structure-based nucleation inhibitors effectively stabilized the metastable systems.

Calorimetry, Differential Scanning↗
PubMed2005

General rules for predicting phase transitions in perovskites due to octahedral tilting.

Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that dTc/dP<0, contrary to a general rule previously proposed for such zone-boundary transitions. We show that the negative slope of the phase boundary is a consequence of the octahedra in these perovskites being more compressible than the extra-framework cation sites. Conversely, when the octahedra are stiffer than the extra-framework cation sites, the phase transition temperatures increase with increasing pressure, dTc/dP>0.

Journal Article↗
PubMed2000

Universality class of absorbing phase transitions with a conserved field

We investigate the critical behavior of systems exhibiting a continuous absorbing phase transition in the presence of a conserved field coupled to the order parameter. The results obtained point out the existence of a new universality class of nonequilibrium phase transitions that characterizes a vast set of systems including conserved threshold transfer processes and stochastic sandpile models.

Journal Article↗
PubMed2001

Quantum effects after decoherence in a quenched phase transition.

We study a quantum mechanical toy model that mimics some features of a quenched phase transition. Both by virtue of a time-dependent Hamiltonian or by changing the temperature of the bath we are able to show that even after classicalization has been reached, the system may display quantum behavior again. We explain this behavior in terms of simple nonlinear analysis and estimate relevant time scales that match the results of numerical simulations of the master equation. This opens new possibilities both in the study of quantum effects in nonequilibrium phase transitions and in general time-dependent problems where quantum effects may be relevant even after decoherence has been completed.

Journal Article↗
PubMedDate not supplied

[Electric capacitance of bilayer lipid membranes from hydrogenated egg lecithin during phase transition from liquid crystalline state to gel].

The electrical capacity of planar bilayer lipid membranes (BLM) from natural hydrogenated egg lecithin (HEL) in n-decane at a temperature of phase transition was measured. The temperature of phase transition was determined calorimetrically to be 51 degrees C. The data obtained revealed a phase separation of HEL in BLM into two fractions, one freezing at 42-44 degrees C and one that is converted to a liquid-crystal state at 51-59 degrees C. It was assumed that the first fraction is rich in dipalmitoyl lecithin, and the second fraction is rich in distearoyl lecithin. Freezing and the transition to the liquid-crystal state were accompanied by an increase and decrease in membrane thickness, respectively, in part due to a displacement of the solvent from the torus to the planar part of the bilayer. The displacement of the solvent is explained by changes in the disjoining pressure in BLM, which arises across the lipid bilayer due to van der Waals forces of attraction between water layers on both sides of the BLM.

Animals↗