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PubMed2003

Phase transitions in angle variables.

Phase transitions in angle variables are studied. An example of angular phase transition, an axially to triaxially deformed "shape" transition in nuclei, is discussed. Spectroscopic signatures for the occurrence of these transitions are suggested. Preliminary experimental evidence is presented.

Journal Article↗
PubMed1978

Direct analysis for cholesterol in gallstones by use of differential scanning calorimetry transition phase.

A cholesterol transition phase at 37.5 degrees C has been shown by use of a Differential Scanning Calorimeter. The transition, reversible and reproducible, is useful for analytical purpose. We propose an analytical method for cholesterol in gallstones based on this transition. The method requires no intermediate reactions and cholesterol can be assayed directly in the powdered stone, with no other pretreatment or reagents. The analysis is highly specific, requires about 10 min, and is quite precise (CV about 1.5%) and accurate (about 2%).

Calorimetry, Differential Scanning↗
PubMed2006

Phase transitions and criticality in small systems: vapor-liquid transition in nanoscale spherical cavities.

Phase transformations in fluids confined to nanoscale pores, which demonstrate characteristic signatures of first-order phase transitions, have been extensively documented in experiments and molecular simulations. They are characterized by a pronounced hysteresis, which disappears above a certain temperature. A rigorous interpretation of these observations represents a fundamental problem from the point of view of statistical mechanics. Nanoscale systems are essentially small, finite volume systems, in which the concept of the thermodynamic limit is no longer valid, and the statistical ensembles are not equivalent. Here, we present a rigorous approach to the description and molecular simulations of phase transitions and criticality in small confined systems, as illustrated by the example of vapor-liquid transition (capillary condensation) in spherical cavities. The method is based on the analysis of the canonical ensemble isotherms, which can be generated by the gauge cell Monte Carlo simulation method. The method allows one to define the critical temperature of phase transition, conditions of phase equilibrium, limits of stability of metastable states, and nucleation barriers, which determine hysteretic phase transformations.

Journal Article↗
PubMed2005

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant.

We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate (n-TDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure (pi)-time (t) adsorption isotherm is found to occur from 2 to 20 degrees C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 degrees C and it is considered as a gas (G)-liquid condensed (LC) phase transition. Above 6.5 degrees C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the pi-t curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 degrees C, the former transition is classified as a first-order G-liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE-LC phase transition. The critical surface pressure (pi(c)) necessary for the G-LC and G-LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE-LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 degrees C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air-water interface.

Adsorption↗
PubMed2005

Phase diagrams in the lattice restricted primitive model: from order-disorder to gas-liquid phase transition.

The phase behavior of the lattice restricted primitive model (RPM) for ionic systems with additional short-range nearest neighbor (NN) repulsive interactions has been studied by grand canonical Monte Carlo simulations. We obtain a rich phase behavior as the NN strength is varied. In particular, the phase diagram is very similar to that of the continuum RPM for high NN strength. Specifically, we have found both gas-liquid phase separation, with associated Ising critical point, and a first-order liquid-solid transition. We discuss how the line of continuous order-disorder transitions present for the low NN strength changes into continuum-space behavior as one increases the NN strength and compare our findings with recent theoretical results by Ciach and Stell [Phys. Rev. Lett. 91, 060601 (2003)].

Journal Article↗
PubMed1985

Diacylglycerols, lysolecithin, or hydrocarbons markedly alter the bilayer to hexagonal phase transition temperature of phosphatidylethanolamines.

The bilayer to hexagonal phase transition temperatures of dielaidoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylethanolamine are 65.6 and 71.4 degrees C, respectively. Using high-sensitivity differential scanning calorimetry, I have shown that these transition temperatures are extremely sensitive to the presence of small amounts of other lipid components. For example, at a mole fraction of only 0.01, dilinolenin lowers the bilayer to hexagonal phase transition temperature of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine by 8.5 degrees C. Other diacylglycerols have similar effects on this transition temperature, although the degree of unsaturation of the acyl chains has some effect, with distearin being less potent. In comparison, the 20-carbon alkane eicosane lowers this transition temperature by 5 degrees C, while palmitoyl-lysolecithin raises it by 2.5 degrees C. Similar effects of these additives on the bilayer to to hexagonal phase transition temperature are observed with dielaidoylphosphatidylethanolamine. At these concentrations of additive, there is no effect on the gel-state to liquid-crystalline-state transition temperature. The observed shifts in the temperature of the bilayer to the hexagonal phase transition can be qualitatively interpreted in terms of the effects of these additives on the hydrophilic surface area and on the hydrophobic volume. Substances expanding the hydrophobic domain promote hexagonal phase formation and lower the bilayer to hexagonal phase transition temperature. The sensitivity of the bilayer to hexagonal phase transition temperature to the presence of additives is at least as great as that which has been observed for any other lipid phase transition.

Calorimetry, Differential Scanning↗
PubMed2001

Phase transitions in Euler fluids.

Phase transitions in two-dimensional (2D) Euler fluids are studied using mean field theory (MFT) solutions and Monte Carlo simulations. The MFT solutions show the possibility of first and second order phase transitions and the critical pointlike behavior. The simulations of the dynamics of 2D vortex patches agree with the MFT solutions over a wide range of parameters except at high energies where there are deviations between the two.

Journal Article↗
PubMed2004

Phase transitions of a polymer threading a membrane coupled to coil-globule transitions.

We theoretically study phase transitions of a polymer threading through a pore imbedded in a membrane. We focus on the coupling between a partition of the polymer segments through the membrane and a coil-globule transition of the single polymer chain. Based on the Flory model for collapse transitions of a polymer chain, we calculate the fraction of polymer segments and the expansion factor of a polymer coil on each side of the membrane. We predict a first-order phase transition of a polymer threading a membrane; polymer segments in one side are discontinuously translocated into the other side, depending on solvent conditions and molecular weight of the polymer. We also discuss the equilibrium conformation of the polymer chain on each side of the membrane.

Journal Article↗
PubMed2001

Random Ginzburg-Landau model revisited: reentrant phase transitions.

We analyze the phase diagram of the random Ginzburg-Landau model, where a quenched dichotomous noise affects the control parameter. We show that the system exhibits two types of counterintuitive reentrant second-order phase transitions. In the first case, increasing the coupling drives the system from a disordered to an ordered state and then back to a disordered state. In the second case, increasing the intensity of the quenched noise, the system goes from an ordered phase to a disordered phase and back to an ordered state. We discuss the general mechanism that produces these reentrant phase transitions, showing that it may appear in other physical systems, such as a modification of the spin-1 Blume-Capel model proposed to describe the critical behavior of helium mixtures in a random medium.

Journal Article↗
PubMed1981

Calorimetric and fluorescence depolarization studies on the lipid phase transition of bacteriorhodopsin--dimyristoylphosphatidylcholine vesicles.

The thermotropic lipid phase transition of dimyristoylphosphatidylcholine vesicles reconstituted with bacteriorhodopsin was investigated as a function of the lipid to protein ratio by means of differential scanning calorimetry and fluorescence depolarization of the embedded probe 1,6-diphenyl-1,3,5-hexatriene. Two attractive features of this system are that the lipid phase transition induces lipid-protein segregation and that the state of aggregation of the protein is known. Above the lipid phase transition and above molar lipid to protein ratios of about 100, bacteriorhodopsin is monomeric. Well below the phase transition, bacteriorhodopsin is aggregated in a hexagonal protein lattice. With increasing amounts of incorporated bacteriorhodopsin, the calorimetric transition broadens, and a second component develops at a temperature which is lower than that of the unperturbed transition. The latter transition was assigned to the disaggregation of the bacteriorhodpsin lattice which occurs 6-7 degrees C below the phase transition of the protein-free lipids according to previous measurements. The van't Hoff enthalpy of the phase transition, as determined from the fluorescence depolarization of diphenylhexatriene, is in surprisingly good agreement with that obtained from differential scanning calorimetry over a wide range of lipid to protein ratios. The differential scanning calorimetry data can be simulated on the basis of a model which takes the protein segregation and crystallization specifically into account. The essential feature of this model is that calorimetrically detectable lipid melting occurs in the temperature region of the protein crystallization, since for the disaggregation of the close packed bacteriorhodopsin lattice additional lipids of an intermediate chain conformation are required.

Bacteriorhodopsins↗
PubMed2005

Effects of selection for the timing of vegetative phase transition on corn borer (Lepidoptera: Noctuidae and Crambidae) damage.

In maize, Zea mays L., the timing of vegetative phase transition from juvenile to adult vegetative phases can be modified through selection. A reduction in the juvenile vegetative phase has been associated with resistance to diseases and pests. The major maize pest in temperate areas is Ostrinia nubilalis (Hübner) and in Europe Sesamia nonagrioides Lefebvre. The objective of our study was to determine the effects of divergent selection for the timing of vegetative phase transition in maize on resistance to corn borers. Three cycles of divergent selection for early and late phase transition in a field corn synthetic and in a sweet corn population were evaluated separately under S. nonagrioides and O. nubilalis artificial infestation. For the field corn experiment, yield and moisture improved with selection for phase transition in both directions, but improvement was due to artifacts of selection, rather than to the change in phase transition. There were no correlated responses for corn borer damage, yield, or grain moisture due to selection for the timing of vegetative phase transition. In the sweet corn experiment, selection for the timing of vegetative phase transition had no significant effects on corn borer damage in sweet corn harvested at the fresh stage. Our results do not support the use of phase transition as an indirect criterion for improving resistance to corn borers in maize. The relationship between phase transition and pest resistance reported by other studies could depend on the genotypes or could be too weak to be detected in a selection program with wild-type maize.

Animals↗
PubMed2003

Revisiting phase transition during flowering in Arabidopsis.

Single-phase transition during flowering has been suggested by Hempel and Feldman (1994) [Planta 192: 276]. When early flowering ecotypes of Arabidopsis were microscopically observed, a long day signal simultaneously induced the acropetal (bottom to top) production of flower primordia and the basipetal (top to bottom) differentiation of paraclades (axillary flowering shoots) from the axils of pre-existing leaf primordia. However, this model could not account for the production of an extra number of secondary shoots in the TERMINAL FLOWER 1 overexpressor line or AGL20 overexpressor line in Columbia background with a functional allele of FRIGIDA. We report here that Columbia with a functional allele of FRIGIDA under long days and Columbia under short days show an inflorescence-producing phase between the vegetative and the flower-producing phases, supporting two-step phase transition during flowering. In addition, a late-flowering mutant, fwa shows an inflorescence phase but fca, fy and fve follow a single-phase transition, suggesting flowering time mutations have different effects on phase transition during flowering.

Arabidopsis↗
PubMed1976

[Transitional phases in human development (author's transl)].

Transitional phases in the course of Human Development which divide the course of development into different steps, have been searched far from the old. These transitional phases are very often seen to be at least initiated by physical resp. biological changes and accompanied by psychological or mental changes. The results of the Life-span-Psychology and especially the findings of the biographical studies of our Psychological Institue, which were started in the Fifties, shows that there are Transitional phases which neither related to biological change nor to role change nor to the change from the one decade of life to the other one. Much more important are very personal experiences which very often lead to a reorientation. - These findings are in agreement with cognitive theory of personality, according to which human behavior is dependent more on the situation as perceived, than on the stimulus-pattern contigent with this situation. Therefore the study of these situational concepts of individual is relevant for the Life-span-Psychology and specially for the analysis of transitional phases.

Aging↗