Apparent Phase Transition Research Articles

Interfacial behavior of block polyelectrolytes. 1. Evidence for novel surface micelle formation

Monolayers of a polystyrene/poly-4-vinylpyridine AB diblock ionomer, (PS) 260 (4-PVP) 240 , fully quaternized with decyl iodide have been studied at the air/water interface with the Langmuir film balance technique. Surface pressure/area isotherms exhibit an abrupt onset and an apparent first-order phase transition at high pressures. Transmission electron micrographs of LB films deposited on carbon-coated copper grids provide direct evidence of self-assembly of the diblock copolymers into circular surface micelles

Influence of lattice imperfections on the transition temperatures of structural phase transitions: The plateau effect

Abstract The influence of defects on the apparent phase transition temperature has been studied over a large range of defect concentrations. An interval was found for low concentrations (around 1 mol%) in which the transition temperature depends only weakly on the number of defects and where defect tails occur. This interval is called the plateau. For higher defect concentrations we find that chemical mixing leads to strong variations of the transition temperature which are well described in the approximation of a homogeneous crystal.

Rotational Lattice Vibrations of Tetramethyl- and Tetramethyl-d12-pyrazine Crystals

Abstract The low-frequency Raman bands of the tetramethyl- and tetramethyl-d12-pyrazine crystals and their hydra ted crystals were studied at various temperatures between 4.2 and 298 K. The isotopic factors and the temperature effects on the frequencies of the Raman bands indicate that the low-frequency Raman bands observed in the crystal are to be ascribed to the rotational lattice vibrations. A smooth curve of the plot of the Raman frequency versus temperature suggests that no apparent temperature-induced phase transition takes place in the tetramethylpyrazine crystal. Classification of the rotational lattice vibrations into symmetry species was made based on the polarization behavior of the Raman bands. It was also observed that tetramethylpyrazine hydrates quite easily with a trace of moisture in the air.

Synthesis, structure and properties of (BMDT-TTF) 2Au(CN) 2

A novel 2:1 salt, di[bis(methylenedithio)-tetrathiafulvalene] dicyanoaurate(I) (BMDT-TTF) 2Au(CN) 2, has been prepared by electrochemical oxidation. The structure was monoclinic, P2 1/a, with a = 10.824(7), b = 8.420(5), c = 15.614(8) A ̊ and β = 102.46(4)°, V = 1390 A ̊ 3 , and Z = 2, having an unusual packing motif. The electrical conductivity is metallic down to 76 K at which point an apparent structural phase transition to a semimetallic state occurs. The spin susceptibility is relatively large and independent of temperature above 70 K as expected for a metal. Below 70 K the susceptibility is Curie-like suggesting 0.1 localized spins per unit cell.

Microwaves and the Cell Membrane: II. Temperature, Plasma, and Oxygen Mediate Microwave-Induced Membrane Permeability in the Erythrocyte

Microwaves (2450 MHz) are shown to increase 22Na permeability of rabbit erythrocytes for exposures only within the narrow temperature range of 17.7 to 19.5 degrees C (Tc) which coincides with a nonlinearity in the Arrhenius plot reflecting an apparent membrane phase transition. Significantly, this response is not observed for cholesterol-loaded erythrocyte membranes which exhibit a linear Arrhenius plot and no apparent phase transition at Tc. The permeability increase at Tc is a nonlinear function of absorbed power but is a linear function of the internal electric field strength of the sample and saturates at approximately 400 mW/g and 600 V/m, respectively. The permeability increase was found to be reversible and transient in that immediately following termination of exposure sodium influx is significantly reduced but returns to normal within 60 min. Extracellular factors exert a significant influence on the microwave effect. The presence of plasma markedly potentiates the increase in 22Na permeability at Tc. Oxygen also modulates the microwave effect with relative hypoxia (5 mm Hg) and hyperoxia (760 mm Hg) enhancing the permeability increase. In contrast, the presence of two antioxidants, ascorbic acid or mercaptoethanol, inhibits the effect. These findings raise important questions about the physical and chemical nature of microwave interactions with cell membranes and also shed light on earlier studies reporting either positive or negative effects on membrane permeability.

Explosive loading of lead hemispherical liners

This paper documents results of a combined analytical/experimental program to study the behavior of lead hemispheres of various thicknesses subjected to explosive shock loading. Previous studies of collapse and jetting of lead structures indicated that, due to the low melting temperature of lead, significant amounts of vapor were present in the jet. However, its high density, ready availability and low cost make lead an attractive material for industrial and military applications. Hemispherical lead liners of three different thicknesses were fabricated into aluminum confined cavity charges using OCTOL high explosive to drive the liners. Analysis of flash radiographs of the formation and jetting process indicated a gradual transition from an apparent liquid-vapor jet for the smallest wall thickness to a liquid-solid jet for the largest thickness. This apparent phase transition of the lead liners through a region of decreasing vapor lead to an analysis of the phenomena with the two-dimensional HULL code, a finite-difference Eulerian code for computations of hydrodynamic and elastic-plastic phenomena. Good correlation (8%) was obtained between experimentally determined and computed jet characteristics, thus permitting inferences to be drawn regarding the effects of material properties and strain rates on the response of lead to shock loading. Additional computations were made with HULL for copper hemispherical liners (for which more extensive experimental data was available) and these were compared with results from HELP, another Eulerian code. The EPIC code was also used to predict the geometric characteristics and velocities of the jets from the lead liners. On the whole, good agreement with experiments was obtained from all codes indicating that existing two- and three-dimensional codes may be valuable tools for the study of explosive forming problems. In addition, the temperature profiles of the jets from the lead liners are in general agreement with the quality of the flash radiographs (i.e. appearance of vapor) and with experimental measurements of the jet surface temperature.

Differences in lipid composition and fluidity of cardiac sacrolemma prepared from newborn and adult rabbits

As the lipid composition of sarcolemma isolated from the rabbit heart changes during ontogeny, the object of this study is to determine if concomitant changes in sarcolemmal fluidity occur. Highly purified sarcolemmal fractions of newborn and adult myocardium were prepared and lipids of those fractions were analyzed. The fluorescence anisotropy of sarcolemmal fractions of both groups was also measured on a fluorometer using 1,6-diphenylhexatriene (DPH) as a probe. Total phospholipid, cholesterol, total fatty acid, and saturated fatty acid contents in the sarcolemmal fraction of the newborn were significantly higher than those of the adult. Total fluorescence intensity at saturating levels of DPH was significantly higher in the sarcolemmal fraction isolated from the newborn compared to the adult, but anisotropy was not significantly different between the newborn and the adult. An Arrhenius plot demonstrated phase transition points at nearly 23°C in both myocardium, but an apparent phase transition at breaking point was not observed in the newborn sarcolemmal fraction. The slight difference in the value of the fusion activation energy above and below the phase transition point was observed between the newborn and the adult. Thus, it is apparent that the changes of sarcolemmal fluidity with age do occur but are smaller than expected, based on the difference in the sarcolemmal lipid composition.

Recombination reactions on Ni(111)

Abstract The dissociative adsorption and recombination of CO, N2, SO, and O2 on Ni(111) has been studied by LEED, AES and TPD. The kinetic parameters were determined for O + O, N + N, S + O, Ccarbide + O recombination. All four reactions displayed second order kinetics with pre-exponential factors of 10 −2 ± 1 cm 2 atom·s . The activation energies were E o+o = 440 kJ mol , E N + N = 210 kJ mol , E s+o = 310 kJ mol and E c carbide +o = 175 kJ mol . The kinetics of the Cgraphite + O recombination were found to be first order indicating a condensed graphite phase with reaction at the phase boundary. These kinetics were further complicated due to an apparent phase transition of the adsorbed graphite at 970 K. The TPD results were used to estimate heats of adsorption. These estimates correlated with the heats of formation of bulk compounds.

Recombination reactions on Ni(111)

The dissociative adsorption and recombination of CO, N 2, SO, and O 2 on Ni(111) has been studied by LEED, AES and TPD. The kinetic parameters were determined for O + O, N + N, S + O, C carbide + O recombination. All four reactions displayed second order kinetics with pre-exponential factors of 10 −2 ± 1 cm 2 atom·s . The activation energies were E o+o = 440 kJ mol , E N + N = 210 kJ mol , E s+o = 310 kJ mol and E c carbide+o = 175 kJ mol . The kinetics of the C graphite + O recombination were found to be first order indicating a condensed graphite phase with reaction at the phase boundary. These kinetics were further complicated due to an apparent phase transition of the adsorbed graphite at 970 K. The TPD results were used to estimate heats of adsorption. These estimates correlated with the heats of formation of bulk compounds.

Irreversibility in Ising and Heisenberg spin glasses (invited)

Spin glasses are rather unique systems which show simultaneously apparent phase transitions as well as metastable or glassy behavior. The existence of irreversible behavior appears to be intimately connected with the phase transition. Here, we will review experiments which show irreversible phenomena and will deduce from the data a simple heuristic picture of the free energy surface F[mi], where mi is the thermally averaged spin at site i. The picture that follows from this analysis is then made more rigorous within a calculational scheme, in which, for large-size systems, we numerically compute the evolution of minima of mean field models for F[mi] as the magnetic field H and temperature T are changed. For Ising spins, magnetic hysteresis, field-cooled, zero-field-cooled, and remanent magnetizations are computed and found to be in good qualitative agreement with experiment. For Heisenberg spins, we find no irreversibility unless anisotropy is present. We discuss the re-entrant ferromagnet–spin glass transition as well as the effects of various kinds of anisotropy, on vector spin glasses. The overall good qualitative agreement between theory and experiment lends support to our hypothesis that, on intermediate time scales, the behavior of spin glasses reflects the properties of the free energy surface: that irreversibility occurs when minima of F are destroyed with changing H or T.

Sulphate ion dynamics in α-alums studied by esr at high pressures

The variation of zero-field splitting and linewidth of Cr 3+ ion in KCr and KAI alums with hydrostatic pressure and with temperature is investigated. A model for the apparent phase transition is proposed on the basis of the reorientational motion of the SO 2− 4 groups.

Irreversible and Reversible Behavior of Spin Glasses: Broken Ergodicity

Over the past decade, a great deal of effort has gone into understanding the properties of spin glasses [1,2]. However, because these are rather unique systems which show simultaneously apparent phase transition as well as metastable or glassy behavior, progress has been slow. Though it was initially believed that spin glasses could be treated as if they had a true equilibrium phase transition, we now recognize that this cannot be the whole story. Recently, it has become clear that spin glasses are very complex systems, in which irreversible and time dependent effects play an important role. We now know that one must go beyond the regime of validity of equilibrium thermodynamics. In this paper, we will discuss the mountinq evidence, both experimental and theoretical, for why nonequilibrium approaches are essential in order to understand spin glasses.

A Monte Carlo study of the U(2) chiral model

We investigate via the Monte Carlo method the non-linear U(2) matrix sigma model. The average link and specific heat measurements indicate that the apparent phase transition is higher than second order. The U(1) susceptibility and determinantal correlation functions turn out to be order parameters and yield 0.41 as the approximate critical coupling. The role played by topological excitations particularly in the critical region is studied. We also use our work as a stepping stone to obtain a glimpse of the matrix large- N problem. The results all support the conclusions of Green and Samuel. Finally, the implications for gauge theories are discussed.

Reassembled model lipoproteins. Lipid dynamics in recombinants of human apolipoprotein A-II and dimyristoylphosphatidylcholine.

Dimyristoylphosphatidylcholine (DMPC) and apolipoprotein A-II (apoA-II) combine to form three isolatable complexes of molar stoichiometry of 240:1 75:1, and 45:1, respectively. Steady state fluorescence depolarization studies with diphenylhexatriene (DPH) and parinaric acid incorporated into the DMPC/apoA-II complexes reveal that increasing the protein content reduces fluidity and increases the apparent lipid phase transition (Tc). Time-resolved studies with DPH reveal hindered rotational motion and a loss of fluidity with increasing protein content, arising from an increase in the order of the lipid assay. The parinaric acid probes could detect no lateral phase separation of lipid ensembles in the DMPC/apoA-II complexes, suggesting that the DMPC adjacent to the protein is as mobile as bulk lipid. Measurements of the excimer-forming properties of a pyrene lecithin analog show that the increasing protein content in the DMPC/apoA-II complexes reduces lateral diffusion of the lipid.

The rippled structure in bilayer membranes of phosphatidylcholine and binary mixtures of phosphatidylcholine and cholesterol

Freeze-fracture electron microscopy is used to study the rippled texture in pure dimyristoyl and dipalmitoyl phosphatidylcholine membranes and in mixtures of dimyristoyl phosphatidylcholine and cholesterol. Evidence is presented that the apparent phase transition properties of multilamellar liposomes may be dependent on the manner in which liposomes are prepared. Under certain conditions the ripple structures as visualized by freeze-fracture electron microscopy for the pure phosphatidylcholines are observed to be temperature dependent in the vicinity of the pretransition. Thus the transition can sometimes appear to be a gradual transition rather than a sharp, first-order phase transition. In mixtures of dimyristoyl phosphatidylcholine and cholesterol, the ripple repeat distance is found to increase as the cholesterol concentration is increased between 0 and 20 mol%. Above 20 mol%, no rippling is observed. A simple theory is presented for the dependence of ripple repeat spacing on cholesterol concentration in the range 0–20 mol%. This theory accounts for the otherwise inexplicable abrupt increase in the lateral diffusion coefficients of fluorescent lipids in binary mixtures of phosphatidylcholine and cholesterol when the cholesterol concentration is increased above 20 mol%.

Electrical transport properties of low-stage AsF5-intercalated graphite

The in-plane resistivity of stage 1 and stage 2 AsF5-graphite intercalation compounds was measured using a contactless r.f. eddy current technique from 1.6 to 290 K. The magnetoresistance of a stage 1 compound was similarly measured from 4.2 K to 290 K. The low temperature stage 2 resistivity data show a well-defined intermediate∞ ∝T2 region in addition to the usual∞ ∝T high temperature region, in qualitative agreement with the Kukkonen theory and indicative of a small, elongated cylindrical Fermi surface. Stage 2 resistivity data also show, for the first time in a graphite-acceptor compound, an apparent low temperature phase transition at ≈ 21 K. Magnetoresistance data were used to determine a stage 1 carrier concentration of ≈ 9×1020 holes cm−3. Resistive anomalies were observed at ≈ 200 K and ≈ 220 K for stage 1 and stage 2 compounds, respectively.

An investigation of the freezing of water associated with wool keratin by NMR methods

This paper describes nuclear magnetic resonance experiments used to detect the occurrence of a diffuse freezing of some of the water sorbed by wool keratin and to estimate the saturation water content of the system. The data are interpreted using a distribution of molecular correlation times model. It is assumed that on cooling, it is the short correlation time molecules which freeze, while molecules at the long correlation time end of the distribution gradually undergo an “apparent phase transition” Resing, H. A., J. Chem. Phys. 43, 669 (1965). The effect of these truncations of the distribution on the observed proton magnetic resonance is discussed.

The desorption kinetics of water and formic acid from Ni(110) following low-temperature adsorption

The flash desorption of water and of formic acid and the flash decomposition of formic acid isotopes were studied on a clean Ni(110) surface for adsorption temperatures below −55 °C. Most of the adsorbed water was found to be weakly bound to the surface and to desorb with first-order kinetics. An apparent surface phase transition was observed at high coverages which indicated interactions between adsorbed water molecules at high coverage. A fraction of the formic acid adsorbed at −60 °C was very weakly bound to the surface and exhibited first-order desorption with a coverage-dependent activation energy. This coverage dependence was attributed to strong adsorbate-adsorbate interactions. The decomposition products for adsorption of a formic acid isotope, HCOOD, were H 2, CO 2, CO, and D 2O. The water product observed leaving the surface was formed from the acid hydrogens and was the result of a decomposition-limited reaction step. The kinetics to form water, which were zero order at high coverage and first order at low coverage, were explained in terms of an island mechanism in which formic acid adsorbed in a condensed phase. The decomposition to form CO 2 and H 2 proceeded at a faster rate than that observed from adsorption at 37 °C. Also, the CO 2 and H 2 did not exhibit the narrow autocatalytic flash desorption peaks observed following 37 °C adsorption. The same kinetic mechanism was operating at both adsorption temperatures but for low temperature adsorption the bare metal sites needed for reaction were created by desorption of the water product and thus the rate was faster at the lower adsorption temperature. For −60 °C adsorption, all the CO product was desorption-limited.

Pressure-induced phase transition in GaAs under shock compression

Shock compression experiments for GaAs are performed up to about 400 kbar. Hugoniot data are obtained both by the argon flash gap technique and by the inclined mirror technique. Inclined mirror experiments reveal the three-wave structure, yielding the Hugoniot elastic limit (HEL) and apparent phase transition pressure to be 84±8 kbar and 203±11 kbar, respectively. A correction to remove the effect of HEL on the phase transition pressure reduce the latter value to 162±11 kbar, which corresponds to the static phase transition pressure. Preliminary argon flash gap experiments for GaP show the phase transition at about 260 kbar.

Commentary on the NMR apparent phase transition effect in natrolite, fluor-montmorillionite and other systems: Generalizations when the Arrhenius law applies

In situations where the NMR apparent phase transition effect occurs and the Arrhenius rate law is applicable, it has been shown that the mean activation enthalpy is roughly 37 times the temperature at which the mobile fraction is one-half. This and further generalizations have been applied to proton NMR line-shape analysis of water in natrolite and other systems; the mean activation enthalpy for water molecule diffusion in natrolite is estimated to be 18 kcal/mol.