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PubMed2002

Deceptive signals of phase transitions in small magnetic clusters.

We present an analysis of the thermodynamic properties of small transition-metal clusters and show how the commonly used indicators of phase transitions such as peaks in the specific heat or magnetic susceptibility can lead to deceptive interpretations of the underlying physics. The analysis of the distribution of zeros of the canonical partition function in the whole complex temperature plane reveals the nature of the transition. We show that signals in the magnetic susceptibility at positive temperatures have their origin at zeros lying at negative temperatures.

Journal Article↗

PubMed2001

Toward a theory of the general-anesthetic-induced phase transition of the cerebral cortex. I. A thermodynamics analogy.

In a recent paper the authors developed a stochastic model for the response of the cerebral cortex to a general anesthetic agent. The model predicted that there would be an anesthetic-induced phase change at the point of transition into unconsciousness, manifested as a divergence in the electroencephalogram spectral power, and a change in spectral energy distribution from being relatively broadband in the conscious state to being strongly biased towards much lower frequencies in the unconscious state. Both predictions have been verified in recent clinical measurements. In the present paper we extend the model by calculating the equilibrium distribution function for the cortex, allowing us to establish a correspondence between the cortical phase transition and the more familiar thermodynamic phase transitions. This correspondence is achieved by first identifying a cortical free energy function, then by postulating that there exists an inverse relationship between an anesthetic effect and a quantity we define as cortical excitability, which plays a role analogous to temperature in thermodynamic phase transitions. We follow standard thermodynamic theory to compute a cortical entropy and a cortical "heat capacity," and we investigate how these will vary with anesthetic concentration. The significant result is the prediction that the entropy will decrease discontinuously at the moment of induction into unconsciousness, concomitant with a release of "latent heat" which should manifest as a divergence in the analogous heat capacity. There is clear clinical evidence of heat capacity divergence in historical anesthetic-effect measurements performed in 1977 by Stullken et al. [Anesthesiology 46, 28 (1977)]. The discontinuous step change in cortical entropy suggests that the cortical phase transition is analogous to a first-order thermodynamic transition in which the comatose-quiescent state is strongly ordered, while the active cortical state is relatively disordered.

Anesthesia, General↗

PubMed1997

Novel thermal phase transition behavior of phosphatidylcholine analogs containing 1,2,4-butanetriol as their backbone.

The sn-glycerol moiety in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was replaced by the rac-1,2,4-butanetriol residue, and the aqueous dispersions of the resulting DPPC analogs, viz. 1,2-dihexadecanoyloxy-rac-but-4-yl-[2-(trimethyl-ammonium)ethyl]ph osphate (1,2-bPC) and 1,3-dihexadecanoyloxy-rac-but-4-yl-[2-(trimethylammonium)ethyl]pho sphate (1,3-bPC), were characterized by differential scanning calorimetry (DSC) and fluorescence polarization technique. Also, the corresponding (3R)-isomer of 1,3-bPC (1,3-(3R)bPC) was prepared and characterized by DSC. While the thermal phase transition properties of 1.2-bPC were similar to that of racemic DPPC, 1.3-bPC in identical conditions showed an abnormal property by exhibiting a metastable phase behavior at about 15 degrees C. This abnormal property was associated only with the racemic mixture and was completely absent in its 3R-isomer. 1,3-(3R)bPC, unlike DPPC, showed only two high enthalpy transitions at about 29.7 degrees C (delta H, 7.45 kcal/mol) and 35.3 degrees C (delta H, 6.93 kcal/mol). These results clearly demonstrate that an insertion of one additional methylene residue between the glycerol C1 and C2 carbons in DPPC markedly alters its thermal phase transitional properties, whereas these properties remain virtually unchanged if a similar chemical change is introduced between the glycerol C2 and C3 carbon atoms.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2005

Lipid phase transition in saccharide-coated cholate-containing liposomes: coupling to the surrounding matrix.

We performed FTIR measurements on cholate-containing liposomes (CCL) embedded in saccharide (trehalose or sucrose) matrixes with different contents of residual water. We obtained information on the CCL phase transition following the thermal evolution (310-70 K) of the IR spectrum of the carbonyl moieties of phospholipids in the frequency range 4225-4550 cm(-1). Furthermore, we simultaneously followed the thermal evolution of the water association band, which gave information on the behavior of the surrounding water-saccharide matrix. The analysis revealed a small sub-band of the water association band present in CCL but not in cholate-free liposomes, the thermal evolution of which is tightly coupled to that of the spectrum of the carbonyl moieties of phospholipids. We suggest that this band arises from water molecules, which are inserted within the lipidic structure, in the region located at the border between the hydrophilic and the hydrophobic moieties of phospholipids in the presence of cholic acid. Such water molecules could be responsible for the peculiar flexibility and hydrophilicity of CCL. Following Giuffrida et al. (J. Phys. Chem. B 2003, 107, 13211-13217), we also performed a Spectra Distance analysis, which enabled us to detect an overall liposomes-matrix structural coupling.

Cholates↗

PubMed1994

The mechanism of lamellar-to-inverted hexagonal phase transitions: a study using temperature-jump cryo-electron microscopy.

The lamellar/inverted hexagonal (L alpha/HII) phase transition can be very fast, despite the drastic change in the topology of the lipid/water interfaces. The first structures to form in this transition may be similar to those that mediate membrane fusion in many lipid systems. To study the transition mechanism and other dynamic phenomena in membrane dispersions, we constructed an apparatus to rapidly trigger the transition and then vitrify the specimens to preserve the structure of transient intermediates. The apparatus applies millisecond-long temperature jumps of variable size to aqueous dispersions of lipids on electron microscope grids at times 9-16 ms before specimen vitrification. The vitrified specimens are then examined by cryo-transmission electron microscopy. Dispersions of egg phosphatidylethanolamine completed the transition within 9 ms when superheated by 20 K. Similar transition times have been observed in dioleoylphosphatidylethanolamine via time-resolved x-ray diffraction. N-monomethylated dioleoylphosphatidylethanolamine dispersions superheated to lesser extent exhibited slower transitions and more complex morphology. The structure of the first intermediates to form in the transition process could not be determined, probably because the intermediates are labile on the time scale of sample cooling and vitrification (< 1 ms) and because of the poor contrast developed by some of these small structures. However, the results are more compatible with a transition mechanism based on "stalk" intermediates than a mechanism involving inverted micellar intermediates. Temperature-jump cryo-transmission electron microscopy should be useful in studying dynamic phenomena in biomembranes, large protein complexes, and other colloidal dispersions. It should be especially helpful in studying the mechanism of protein-induced membrane fusion.

Biophysical Phenomena↗

PubMed2000

Phase transition in dioctadecyldimethylammonium bromide and chloride vesicles prepared by different methods.

The gel to liquid crystalline phase transition of the double-chained cationic dioctadecyldimethylammonium chloride and bromide (DODAX, X = Cl- or Br-) in aqueous vesicle dispersions prepared by non-sonication. sonication and extrusion has been investigated using high-sensitivity differential scanning calorimetry (DSC). The transition temperature (Tm) is a function of the preparation method, amphiphile concentration, vesicle curvature and nature of the counterion. DSC thermograms for DODAB and DODAC non-sonicated vesicle dispersions exhibit a single endothermic peak at Tm roughly independent of concentration up to 10 mM. Extrusion broadens the transition peak and shifts Tm downwards. Sonication, however, broadens slightly the transition peak and tends to shift Tm upwards suggesting that extrusion and sonication form vesicles with different characteristics. DODAC always exhibits higher Tm than DODAB irrespective of the preparation method. Tm changes as follows: Tm (sonicated) > or = Tm (non-sonicated) > Tm (extruded). Hysteresis of about 7 degrees C was observed for DODAB vesicle dispersions.

Bromine↗

PubMed2001

Effect of pressure on the photoinduced nematic-isotropic phase transition.

We report the results of high-pressure studies on the photoinduced nematic-isotropic phase transition. It is observed that pressure has a significant influence on the shift in the transition temperature caused by uv radiation. We propose a possible mechanism to explain the observed results.

Journal Article↗

PubMed2001

First-order irreversible phase transitions in a nonequilibrium system: mean-field analysis and simulation results.

First-order irreversible phase transitions (IPT's) between an active regime and an absorbing state are studied in a single-component, two-dimensional interacting particle system by means of both simulations and a mean-field analysis. Several features obtained using the mean-field approximation such as the presence of a first-order IPT and hysteresis effects, are in excellent agreement with simulation results. In addition, extensive epidemic simulations show that the dynamical critical behavior of the system is by no means scale invariant.

Journal Article↗

PubMed2002

Phase transition in complex /psi/(4) theory.

Motivated by recent claims for rather unconventional first-order phase transitions in the two- and three-dimensional complex /psi/(4) theory in certain parameter ranges we performed Monte Carlo simulation studies of this model. From our results in two and three dimensions we can unambiguously conclude that there is no evidence for a first-order transition, provided the measure of field fluctuations is treated properly. The origin of the discrepancy is traced by comparative simulations reproducing the erroneous results and by a transfer-matrix study of the one-dimensional case.

Journal Article↗

PubMed2002

Phase transition to hyperon matter in neutron stars.

Recent progress in the understanding of the high density phase of neutron stars advances the view that a substantial fraction of the matter consists of hyperons. The possible impacts of a highly attractive interaction between hyperons on the properties of compact stars are investigated. We find that a hadronic equation of state with hyperons allows for a first order phase transition to hyperonic matter. The corresponding hyperon stars can have rather small radii of R approximately equal 8 km.

Journal Article↗

PubMed2005

Nonperturbative fixed point in a nonequilibrium phase transition.

We apply the nonperturbative renormalization group method to a class of out-of-equilibrium phase transitions (usually called "parity-conserving" or, more properly, "generalized voter" class) which is out of the reach of perturbative approaches. We show the existence of a genuinely nonperturbative fixed point, i.e., a critical point that does not seem to be Gaussian in any dimension.

Journal Article↗

PubMed2005

Nonequilibrium quantum phase transition in itinerant electron systems.

We study the effect of the voltage bias on the ferromagnetic phase transition in a one-dimensional itinerant electron system. The applied voltage drives the system into a nonequilibrium steady state with a nonzero electric current. The bias changes the universality class of the second order ferromagnetic transition. While the equilibrium transition belongs to the universality class of the uniaxial ferroelectric, we find the mean-field behavior near the nonequilibrium critical point.

Journal Article↗

PubMed1983

Phase transition and crystal structure of the 37 degrees C form of cholesterol.

Crystalline cholesterol undergoes a phase transition a few degrees below human body temperature. The high-temperature form has an unusually complex structure with 16 independent molecules. In the transition two molecules change side chain conformation, four reorient about their long axes, and ten remain unchanged. The transition mechanism implies relatively nonspecific intermolecular interactions, qualitatively consistent with the behavior of cholesterol in biomembranes. The transition preserves a remarkably closely obeyed pseudosymmetry present in the structure.

Body Temperature↗

PubMed2001

Room-temperature monoclinic and low-temperature triclinic phase-transition structures of meso-octamethylcalix[4]pyrrole-dimethyl sulfoxide (1/1).

Crystals of the title complex, C28H36N4*C2H6OS, undergo a phase transition between room temperature and 198 K, as determined by X-ray diffraction techniques. A monoclinic form is observed at room temperature, while a triclinic modification is found at 198 K, with Z' changing from 1 to 2. Differential scanning calorimetry (DSC) of the calixpyrrole-dimethyl sulfoxide complex revealed a series of phase changes between 273 and 243 K. The transition from the room-temperature monoclinic form to the low-temperature triclinic form is reversible, as determined by changes in the cell dimensions from remeasuring selected reflections at room temperature and at temperatures below 223 K. The uncomplexed calix[4]pyrrole molecule shows no phase changes occurring between room temperature and 233 K, the low-temperature limit of the DSC.

Calorimetry, Differential Scanning↗

PubMed2005

Neat DODAB vesicles: Effect of sonication time on the phase transition thermodynamic parameters and its relation with incomplete chain freezing.

Vesicle solutions formed by the cationic lipid dioctadecyldimethylammonium bromide have been extensively used in the past 20 years as model membrane systems. Often, discrepancies are found in the literature for the thermodynamic parameters of the gel-to-liquid crystal (gel-lc) phase transition of these vesicles. In this work we present a systematic DSC investigation on the influence of the sonication method on the main temperature and enthalpy of the transition for DODAB vesicles, prepared both in normal and deuterated water. It is shown that as the sonication time increases, the T(m) and enthalpy values decrease until a plateau value is reached. This effect is associated with incomplete chain crystallization in the highly curved vesicles (produced by sonication) when they are cooled below the transition temperature. The effect of aging on the sonicated solutions has also been monitored by DSC.

Calorimetry, Differential Scanning↗

PubMed2002

Phase transitions in liquid crystals under negative pressures.

We report the first measurements of orientational order parameters and phase transition temperatures in nematic and smectic A liquid crystals under negative pressures generated by an isochoric cooling of small droplets embedded in a glass former. Comparison of isobaric and isochoric measurements allows us to estimate the coefficients coupling the order parameter and density of an extended Landau--de Gennes model of the nematic phase.

Journal Article↗

PubMed2005

Mid-infrared fiber-optic attenuated total reflection spectroscopy of the solid-liquid phase transition of water.

Measurements of mid-infrared (MIR) absorption spectra of water and heavy water were carried out by fiber-optic evanescent wave spectroscopy, using silver halide (AgClBr) infrared fibers. Such measurements were performed for the first time on one sample, during the solid-liquid phase transition. From the variation of the spectra with temperature we found a new isosbestic point (at 3280 cm(-1) for H(2)O or at 2475 cm(-1) for D(2)O) and we identified five components of the O-H (O-D) stretch band. These phenomena have provided new information about the molecular structure of water.

Computer Simulation↗

PubMed2004

Structural phase transitions in heavy alkali metals under pressure.

We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs III and Rb III phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.

Cesium↗