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PubMed2006

Simulation of the pressure-driven wurtzite to rock salt phase transition in nanocrystals.

Nanocrystals in the size range 12-21 nm of a model binary ionic material in the wurtzite (B4) structure were constructed with morphologies which minimize the surface energy. These were then embedded in a pressurization medium, consisting of a binary Lennard-Jones-type fluid and progressively pressurized in "constant pressure" molecular dynamics simulation runs. Phase transitions to the rocksalt (B1) phase were confirmed by examining calculated powder diffraction patterns, which show the same changes in features as seen for experimental systems. By directly observing the atomic trajectories throughout the duration of the transition the local mechanism has been identified. The transition proceeds via a trigonal bipyramidal intermediate, denoted as the h-MgO structure. It is initiated by a single nucleation event at a [1120]B4 surface with subsequent growth of the B1 region throughout the remainder of the nanocrystal. The consequences of this mechanism for the particle shape of the product phase are detailed and contrasted with those previously found for initially zincblende (B3) structured nanoparticles, using the same interaction potential. The observed transition pressures are elevated relative to the thermodynamically predicted pressure for the bulk, but there is no observable system size effect on the transition pressure across the size range of nanocrystals investigated.

Computer Simulation↗

PubMed2004

Phase transition of a single sheet of sashlike polydiacetylene atomic sash on a solid surface.

The phase transition of sashlike polydiacetylene (PD) atomic sash in an extrathin (0.4 nm) vapor-deposited monolayer on a graphite (0001) surface has been observed by in-situ scanning tunneling microscopy (STM). The conformation of the atomic sash composed of a row of lying alkyl chains stitched up with a PD chain is different around the crossing points of the alkyl and PD chains in the two phases, and hence the contrast of the STM image is different in the middle of the sashes. The difference is associated with the color transition of bulk PDs.

Journal Article↗

PubMed2001

Counterion phase transitions in dilute polyelectrolyte solutions.

In dilute solutions of rodlike polyelectrolytes some counterions are distributed far from polyions while others are located in their vicinity in the regions of cylindrical symmetry of the electrostatic potential. For these cylindrical regions around rodlike polyelectrolytes we find an exact solution of the nonlinear Poisson-Boltzmann equation for the case of nonzero net charge in these regions. This exact solution implies three qualitatively different phases of counterion distribution around the polyions with second order phase transitions between these phases.

Journal Article↗

PubMed2001

Direct observation of amorphous to amorphous apparently first-order phase transition in fused quartz.

We report for the first time the direct observation of an apparently first-order phase transition in amorphous silica from the low density amorphous phase to the high density amorphous phase with a discontinuous volume change of about 20% from the dilatometric measurements using the piston cylinder apparatus at 36 kbar and 680 degrees C. Our Raman spectroscopy measurements on the retrieved samples quenched from high pressure and high temperature experiments do not show any Raman peaks indicating a large modification in the intermediate range order in the structure of amorphous silica.

Journal Article↗

PubMed2001

Two fluoradene derivatives: pseudosymmetry, eccentric ellipsoids and a phase transition.

Structures of two derivatives of the curved fluoradene ring system (C(19)H(12)) have been determined. Both have phases that are highly pseudosymmetric. At room temperature crystals of 7b-triisopropylsilylfluoradene (C(28)H(32)Si) have a P1 cell that contains two independent molecules (Z' = 2) and that is almost centered. Crystals of 7b-(2,4-dinitrophenyl)fluoradene (C(25)H(14)N(2)O(4)) have both a P2(1)/c cell with Z' = 1 and a P2(1)/c cell with Z' = 2. The molecular volumes in these two P2(1)/c structures differ by 0.7%, but the structures are otherwise virtually the same; the two independent molecules in the larger cell are related by a pseudotranslation. Some of the atomic ellipsoids in the P2(1)/c, Z' = 1 structure are very large and eccentric, and there are some hints in the diffraction pattern of an incipient phase transition, but the Z' = 1 and Z' = 2 phases are clearly different. The P2(1)/c, Z' = 2 crystal at 295 K probably contains some volume fraction of the Z' = 1 phase; when the temperature is lowered to 273 K this fraction is decreased markedly. The pronounced pseudosymmetry in the P1 and P2(1)/c structures that have Z' = 2 has been investigated by analysing the atomic coordinates, by performing refinements in the smaller pseudocells and by making separate Wilson plots for the classes of reflections which are systematically strong and systematically weak. All three approaches are informative, but they reveal different information. Least-squares fits of coordinates of corresponding atoms measure the similarity of the molecular conformations. The Wilson plots allow a quantitative comparison of the intensities of the strong and weak reflections and thus an assessment of the deviations of the true structure from the smaller pseudocell structure. Comparison of the atomic displacements obtained in the full and pseudocell refinements shows where the structural distortions are largest and provides an indication of their directions.

Journal Article↗

PubMed1996

On the bilayer phase transition temperatures for monoenoic phosphatidylcholines and phosphatidylethanolamines and the interconversion between them.

The dependence of the phase transition temperature, Tm, on the structural parameters of the lipid molecule has been firmly established for lipid bilayers composed of saturated phosphatidylcholines (PC) or phosphatidylethanolamines (PE). It is from such a dependence that conversions of Tm values from saturated PC to PE and vice versa have been successfully demonstrated. In this communication, similar chain-melting-structure relationships are sought for monoenoic PC and PE in which the long hydrocarbons linked at the sn-1 and sn-2 positions are saturated and cis-mono-unsaturated chains, respectively. Specifically, various molecular species of monoenoic PC and PE were first semisynthesized and their Tm values were subsequently determined by high-resolution differential scanning calorimetry. Based on the crankshaft-like kink motif model, the synthesized monoenoic PC and PE can each be divided into two groups, each group being characterized by three structural parameters. For each group of monoenoic PC or PE, a specific equation correlating the changes in Tm with variations in the structural parameters was derived using the multiple regression analysis. Furthermore, within each group of lipids including both PC and PE, an explicit equation describing the interconversion of Tm values between PC and PE with the same fatty acyl chain composition was established. The present work not only extends the earlier work relating the chain-melting temperature of saturated PC and PE to their structural parameters, but also indicates that the basic topology of the sn-2 mono-unsaturated acyl chain of PC and PE in the gel-state bilayer can be approximated by the crankshaft-like kink motif with the longer segment being packed much more orderly than the shorter segment.

Calorimetry, Differential Scanning↗

PubMed1996

Is vitrification involved in depression of the phase transition temperature in dry phospholipids?

Recent literature has suggested that the depression of the phase transition temperature (Tm) in dry phospholipids by sugars may be ascribed to vitrification of the stabilizing solute, rather than by the direct interaction between sugar and phospholipid we have proposed. Koster et al. ((1994) Biochim. Biophys. Acta 1193, 143-150) claim that the only necessity is that the glass transition (Tg) for the sugar exceed Tm for the lipid. Evidence is presented in the present paper that this is not sufficient. Based on the vitrification hypothesis of Koster et al., the predicted order of effectiveness in depressing Tm in dry dipalmitoylphosphatidylcholine (DPPC) is dextran > or = hydroxyethyl starch > stachyose > raffinose > trehalose > sucrose > glucose. In fact, the opposite order was seen. The effect of raffinose, sucrose, or trehalose on Tm in dry DPPC depends on the thermal history of the sample, as we have reported previously. When DPPC dried with trehalose is heated for the first time, Tm is about 55 degrees C, but on the second and subsequent heating scans Tm falls to about 25 degrees C. Koster et al. suggest that this effect is due to heating the sample above Tg rather than to melting the hydrocarbon chains. We present evidence here that all that is required is for the chains to be melted. Further, we show that retention of residual water by DPPC dried with trehalose depends on the drying temperature, but is independent of drying temperature with glucose, a finding that is consistent with direct interaction. We conclude that vitrification is not in itself sufficient to depress Tm in dry phospholipids.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2006

Spectroscopic study of laser-induced phase transition of gold nanoparticles on nanosecond time scales and longer.

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 micros, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.

Journal Article↗

PubMed1999

Electro-optic response of surface-induced nematic order above the nematic-isotropic phase transition temperature.

The optical retardation of a liquid crystal above the nematic-isotropic phase transition temperature T(NI) and subjected to planar alignment conditions at the substrate was investigated in the presence of an electric field applied normal to the substrates. The response was found to exhibit "S"-shaped behavior with electric field, and was larger near T(NI) than well above T(NI). The results were examined in the context of a model that permits both biaxiality and a field-induced tilt of the molecular director. The results suggest that the primary effect of the electric field is to induce biaxiality, and in consequence suppress the order induced by the surface. No clear indication of a Freedericksz-like transition, either experimental or theoretical, was observed.

Journal Article↗

PubMed2001

Phase transitions in systems with 1/r(alpha) attractive interactions.

Collapse, or a gravitational-like phase transition is studied in a microcanonical ensemble of particles with an attractive 1/r(alpha) potential. A mean-field continuous integral equation is used to determine a saddle-point density profile that extremizes the entropy functional. For all 0 < alpha < 3, a critical energy is determined below which the entropy of the system exhibits a discontinuous jump. If an effective short-range cutoff is applied, the entropy jump is finite; if not, the entropy diverges to +infinity. A stable integral equation solution represents a state with maximal entropy; the reverse is always true only for a modified integral equation introduced here.

Journal Article↗

PubMed2002

Numerical comparison of two approaches for the study of phase transitions in small systems.

We compare two recently proposed methods for the characterization of phase transitions in small systems. The validity and usefulness of these approaches are studied for the cases of the q=4 and q=5 Potts model, i.e., systems where a thermodynamic limit and exact results exist. Guided by this analysis we then discuss the helix-coil transition in polyalanine, an example of structural transitions in biological molecules.

Journal Article↗

PubMed2001

QCD phase transition in the inhomogeneous universe.

We investigate a new mechanism for the cosmological QCD phase transition: inhomogeneous nucleation. The primordial temperature fluctuations, measured to be deltaT/T approximately 10(-5), are larger than the tiny temperature interval in which bubbles would form in the standard picture of homogeneous nucleation. Thus the bubbles nucleate at cold spots. We find the typical distance between bubble centers to be a few meters. This exceeds the estimates from homogeneous nucleation by 2 orders of magnitude. The resulting baryon inhomogeneities may affect primordial nucleosynthesis.

Journal Article↗

PubMed2004

Cascade of quantum phase transitions in tunnel-coupled edge states.

We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields B(c) is a function of a single scaling argument /B-B(c)/T(-kappa), where the exponent kappa=0.42. This puzzling resemblance to a quantum Hall-insulator transition points to the importance of interedge correlation between the coupled edge states.

Journal Article↗

PubMed2004

Entanglement and the phase transition in single-mode superradiance.

We consider the entanglement properties of the quantum phase transition in the single-mode superradiance model, involving the interaction of a boson mode and an ensemble of atoms. For an infinite size system, the atom-field entanglement diverges logarithmically with the correlation length exponent. Using a continuous variable representation, we compare this to the divergence of the entropy in conformal field theories and derive an exact expression for the scaled concurrence and the cusplike nonanalyticity of the momentum squeezing.

Journal Article↗

PubMed2004

Quantum phase transitions and bipartite entanglement.

We develop a general theory of the relation between quantum phase transitions (QPTs) characterized by nonanalyticities in the energy and bipartite entanglement. We derive a functional relation between the matrix elements of two-particle reduced density matrices and the eigenvalues of general two-body Hamiltonians of d-level systems. The ground state energy eigenvalue and its derivatives, whose nonanalyticity characterizes a QPT, are directly tied to bipartite entanglement measures. We show that first-order QPTs are signaled by density matrix elements themselves and second-order QPTs by the first derivative of density matrix elements. Our general conclusions are illustrated via several quantum spin models.

Journal Article↗

PubMed1986

Phase transitions of calcium oxalate trihydrate and epitaxy in the weddellite-whewellite system.

The phase changes calcium oxalate trihydrate-weddellite, weddellite-calcium oxalate monohydrate and calcium oxalate trihydrate-whewellite are individually examined at the atomic level from a theoretical point of view; concomitantly the topological requirements necessary for phase stability are clarified for each structure type. In solution a sequential series of phase transitions according to the steps calcium oxalate trihydrate-weddellite-whewellite is not likely to be energetically favoured; direct conversion of calcium oxalate trihydrate to whewellite should be, instead, ordinarily expected. It is formally demonstrated that along two axial directions a set of atoms is in essentially identical positions in both weddellite and whewellite. This notwithstanding, it is concluded that epitactic catalysis cannot and should not be considered a common mechanism for the formation of whewellite from weddellite (and vice versa) or of kidney stones in general.

Calcium Oxalate↗

PubMedDate not supplied

[Effect of bivalent cations on the temperature of phase transition in lipid membranes].

A theory is developed which explains the temperature shift of phase transition of bilayer delta Tc resulting from the interaction of charged polar heads of lipid molecules. It has been shown that adsorption of monovalent cations on the strongly charged membrane increases the transition temperature by about 15 degrees, this value being independent of the ionic strength and dielectric permeability epsilon of the medium. When divalent cations are adsorbed the temperature shift of transition is larger, the value delta Tc depending on medium epsilon, where the polar heads are located. If the membrane is in the electrolyte solution delta Tc approximately equal to 19 degrees; when a dehydrated contact with low dielectric permeability is initiated between the membranes, the transition temperature in this zone is to increase by about 160 degrees, which well agrees with the experimental data.

Calcium↗

PubMed2006

Thermodynamics of activated phase transitions of 8CB: DSC and MC calorimetry.

The present paper reports the heating rate effect on the phase transitions of a pure liquid crystal octylcyanobiphenyl (8CB) with use of Differential Scanning Calorimetry (DSC) and Modulation Calorimetry (MC) techniques. The DSC runs were taken at various temperature ramp rates from 20 to 0.5 K/min for heating and cooling scans. Well-defined endothermic/exothermic peaks were found at the melting/crystallization, smectic-A to nematic (SmA-N), and nematic to isotropic (N-I) transitions on heating/cooling scans, respectively. All transitions shift in temperature significantly with different ramp rates. The temperature shift of C(p) peaks between heating and cooling scans indicates the order of the transitions. In addition, all transitions follow an Arrhenius behavior. The activation energy of a transition increases as the total energy involved in the transition decreases. The respective enthalpy and entropy change of each transition provides information on the Gibbs free energy. The significance of the results is discussed in terms of the order of transitions. A comparative analysis of MC and DSC techniques highlights the significance of the two techniques. MC is a practicable tool for observing the phase dynamics whereas DSC is a good tool for studying the rate kinematics of the transitions.

Journal Article↗