Solid-solid Transition Research Articles

CONFORMATION AND PHASE TRANSITION OF POLY-GLUTAMINE

In order to provide insights into the misfolding mechanism and the subsequent aggregate formation which cause what are known as the neurodegenerative polyglutamine diseases, we have simulated a 10-residue polyglutamine (poly-Q) chain in vacuum and in solvent by multicanonical method, which enabled us to study the system in a wide temperature range and discuss thermodynamic properties. It is understood that the system in vacuum shows two phase transitions, first of them occur at high temperature that is the well-known helix-coil transition and the second one is a solid-solid transition. However, the poly-Q chain in solvent is in a random coil state at higher temperatures, goes through a conformational change at T = 200 K and assumes predominantly a mixture of anti-parallel β-sheet and α-helix structures at low temperatures. One-residue glutamine dipeptide is also simulated and low-energy stable conformations are identified.

Adiabatic calorimetry and thermal analysis on acetaminophen

Molar heat capacities of acetaminophen were precisely measured with a small sample precision automated adiabatic calorimeter over the temperature range from 80 to 330 K. A solid-solid transition at 149.96 K was found from the Cp,m-T curve. The polynomial functions of Cp,.m(J K-1 mol-1) vs. T were established on the heat capacity measurements by means of the least square fitting method.

Simulations of Spinodal Nucleation in Systems with Elastic Interactions

Systems with long-range interactions quenched into a metastable state near the pseudospinodal exhibit nucleation that is qualitatively different from classical nucleation near the coexistence curve. We observe nucleation droplets in Langevin simulations of a two-dimensional model of martensitic transformations and determine that the structure of the nucleating droplet differs from the stable martensite structure. Our results, together with experimental measurements of the phonon dispersion curve, allow us to predict the nature of the droplet. The results have implications for nucleation in many solid-solid transitions and the structure of the final state.

Self‐Assembly and Crystallization Behavior of Mesoporous, Crystalline HfO2 Thin Films: A Model System for the Generation of Mesostructured Transition‐Metal Oxides

Mesoporous thin films of crystalline hafnium oxide were fabricated by evaporation-induced self-assembly in combination with sol-gel processing, followed by a suitable post heat-treatment procedure to initiate the crystallization. A novel type of block-copolymer template was used as structure-directing agent, which generated a distorted cubic arrangement of spherical mesopores, the size of which could be quantified by suitable techniques, such as ellipsometry-porosimetry, small-angle X-ray scattering, and atomic force microscopy. Detailed insights into the nature of the crystallization process of mesostructured hafnium oxide were obtained by temperature-dependent, in situ X-ray scattering experiments. These investigations revealed that crystallization takes place, within the confinement of the mesostructure, as a solid-solid transition from a dehydrated, amorphous form of hafnium oxide. The study suggests that one main benefit of the novel template results from the ability of the polymer to stabilize the mesostructure of amorphous hafnium oxide up to 400-450 degrees C.

A Quantitative Model to Evaluate Solubility Relationship of Polymorphs from Their Thermal Properties

The objective of the study is to develop a model to estimate the solubility ratio of two polymorphic forms based on the calculation of the free energy difference of two forms at any temperature. This model can be used for compounds with low solubility (a few mole percent) in which infinite dilution can be approximated. The model is derived using the melting temperature and heat of fusion for apparent monotropic systems, and the solid-solid transition temperature and heat of transition for apparent enantiotropic systems. A rigorous derivation also requires heat capacity (Cp) measurement of liquid and two solid forms. This model is validated by collecting thermal properties of polymorphs for several drugs using conventional or modulated differential scanning calorimetry. From these properties the solubility ratio of two polymorphs is evaluated using the model and compared with the experimental value at different temperatures. The predicted values using the full model agree well with the experimental ones. For the purpose of easy measurement, working equations without Cp terms are also applied. Ignoring Cp may result in an error of 10% or less, suggesting that the working equation is applicable in practice. Additional error may be generated for the apparent enantiotropic systems due to the inconsistency between the observed solid-solid transition temperature and the true thermodynamic transition temperature. This inconsistency allows the predicted solubility ratios (low melt/high melt) to be smaller. Therefore, a correction factor of 1.1 is recommended to reduce the error when the working equation is used to estimate the solubility ratio of an enantiotropic system. The study of the free energy changes of two crystalline forms of a drug allows for the development of a model that successfully predicts the solubility ratio at any temperature from their thermal properties. This model provides a thermodynamic foundation as to how the free energy difference of two polymorphs is reflected by their equilibrium solubilities. It also provides a quick and practical way of evaluating the relative solubility of two polymorphs from single differential scanning calorimetry runs.

Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

57Fe-Labeled Octamethylferrocenium Tetrafluoroborate. X-ray Crystal Structures of Conformational Isomers, Hyperfine Interactions, and Spin−Lattice Relaxation by Moessbauer Spectroscopy

X-ray structure determinations of two different single crystals of octamethylferrocenium tetrafluoroborate (OMFc(+)BF(4)(-)) revealed conformational polymorphism with ligand twist angles of 180 degrees and 108 degrees , respectively. Their concomitant occurrence could be explained by the small lattice energy difference of 3.2 kJ mol(-1). Temperature-dependent Moessbauer spectroscopy of (57)Fe-labeled OMFc(+)BF(4)(-) over the range 90 < T < 370 K did not show the anomalous sudden increase in the motion of the metal atom as observed in neutral OMFc. Broadened absorption curves characteristic of relaxation spectra were obtained with an isomer shift of 0.466(6) mm s(-1) at 90 K. The temperature dependence of the isomer shift corresponded to an effective vibrating mass of 79 +/- 10 Da and, in conjunction with the temperature dependence of the recoil-free fraction, to a Moessbauer lattice temperature of 89 K. The spin relaxation rate could be better described by an Orbach rather than a Raman process. At 400 K, a reversible solid-solid transition to a plastic crystalline mesophase was noted.

Chain length dependence of the thermodynamic properties of linear and cyclic alkanes and polymers

The specific heat capacity was measured with step-scan differential scanning calorimetry for linear alkanes from pentane (C(5)H(12)) to nonadecane (C(19)H(40)), for several cyclic alkanes, for linear and cyclic polyethylenes, and for a linear and a cyclic polystyrene. For the linear alkanes, the specific heat capacity in the equilibrium liquid state decreases as chain length increases; above a carbon number N of 10 (decane) the specific heat asymptotes to a constant value. For the cyclic alkanes, the heat capacity in the equilibrium liquid state is lower than that of the corresponding linear chains and increases with increasing chain length. At high enough molecular weights, the heat capacities of cyclic and linear molecules are expected to be equal, and this is found to be the case for the polyethylenes and polystyrenes studied. In addition, the thermal properties of the solid-liquid and the solid-solid transitions are examined for the linear and cyclic alkanes; solid-solid transitions are observed only in the odd-numbered alkanes. The thermal expansion coefficients and the specific volumes of the linear and cyclic alkanes are also calculated from literature data and compared with the trends in the specific heats.

Supercooling and giant relaxation of the disordered vortex state in a dopedCeRu2alloy

Giant relaxation in magnetization of metastable (supercooled) states in $\mathrm{Ce}{({\mathrm{Ru}}_{0.95}{\mathrm{Nd}}_{0.05})}_{2}$ alloy in the vortex solid-solid transition regime is reported. The metastable states were prepared by cooling the alloy sample from a temperature well above the ${T}_{C}$, in the presence of different magnetic fields. This was followed by an isothermal field change to set up a critical state in the entire bulk of the sample. Relaxation in magnetization was measured isothermally in this critical state. Through such a choice of experimental protocol, the relaxation in magnetization contributed by supercooled states is clearly distinguished. Variation of the relaxation rates with respect to the fields applied for producing the field-cooled states is explained in terms of supercooling of the disordered vortex phase.

Unique global solvability of the Fried-Gurtin model for phase transitions in solids

The paper is concerned with the existence and uniqueness of solutions to the Allen-Cahn equation coupled with elasticity. The system represents a particular, simple version of the Fried-Gurtin model for solid-solid transitions with phase characterized by an order parameter. The system is studied with the help of the Leray-Schauder fixed point theorem. The main tool applied in the existence proof is the solvability theory of parabolic problems in anisotropic Sobolev spaces with mixed time-space norms.

On structural phase transitions in 4-aminopyridinium fluoroborate, [4-NH 2C 5H 5N][BF 4]: differential scanning calorimetry, dielectric and infrared studies

Crystal structure of the 4-aminopyridinium tetrafluoroborate, [4-NH 2C 5H 5N][BF 4], has been determined at 293 K by means of X-ray diffraction as monoclinic space group, C2. The crystal shows a reach sequence of phase transition, four solid-solid transitions: at 250, 281, 388 and 485 K on heating. Most of the phase transitions, except that at 388 K, are clearly of first order and classified as an ‘order-disorder’ type. The rotational disorder both of cations and anions was studied by differential scanning calorimetry, dielectric and infrared spectroscopy. The low temperature phase transitions are associated with plastic crystal behavior.

Dielectric response of acetonitrile condensed into mesoporous glass

Electrical Engineering Department, Institute for Computer Science, Technical University of Czestochowa, Al. Armii Krajowej 17,PL-42200 Czestochowa, Poland~Received 24 July 2003; revised manuscript received 18 December 2003; published 30 April 2004!Acetonitrile has been condensed into porous Vycor glass. The dielectric response in a frequency range from0.1 Hz to 1 MHz and temperatures between 40 K and 220 K, has been studied for a complete filling and apartial filling of 20%. Thereby the response of the adsorbed layer on the pore walls could be separated from thequasibulk material in the pore center. The quasibulk component shows a conventional melting transition, but ata reduced temperature and with hysteresis. The solid-solid transition of bulk acetonitrile is replaced by arelaxor behavior. The adsorbed layer does not support the melting transition.DOI: 10.1103/PhysRevB.69.134207 PACS number~s!: 61.43.Gt, 64.70.Nd, 77.22.2dI. INTRODUCTION

Optical Transmission Measurements on Phase Transitions of O2and CO in Mesoporous Glass

The optical transmission of O2 and CO condensates embedded in porous Vycor glass has been studied as function of the filling fraction and of the thermal history of the samples. The freezing transition as well as the solid-solid transitions (β-α of CO and γ-β of O2) induce a coarsening of the separation into empty and filled regions which results from the hysteretic behavior of the transitions in the presence of a pore size distribution. In the birefringent β phase of O2 domains with a fixed crystallographic orientation extend over distances much larger than the pore diameter (10 nm).

Physico-chemical characterization of an active pharmaceutical ingredient

The physico-chemical properties and polymorphism of a new active pharmaceutical ingredient entity has been analyzed and the gain of knowledge during the chemical development of the substance is described. Initial crystallization revealed an anhydrous crystal form with good crystallinity and a single, sharp DSC melting peak at 171°C and a straightforward development of this crystal form seemed possible. However, during polymorphism screening, new crystalline forms were detected that were often analyzed as mixtures of crystal forms. The process of characterization and identification of the different crystalline forms and its thermodynamical relationship has been supported by a combination of experimental and computational work including determination of the three-dimensional structures of the crystal forms. The crystal structure of one polymorphic form was solved by single crystal X-ray structure analysis. Unfortunately, Mod B resisted in formation of suitable single crystals, but its structure could be solved by high resolution powder diffraction data analysis using synchrotron radiation. Calculation of the theoretical X-ray powder diffraction pattern from three dimensional crystal coordinates allowed an unambiguous identification of the different crystalline forms. Two polymorphic crystal forms of the API-CG3, named Mod A and Mod B, are enantiotropic whereas Mod B is the most stable polymorph at room temperature up to about 50°C and Mod A at temperatures above 50°C. The mechanism of the solid-solid transition can be explained by analyzing the molecular packing information gained from the single crystal structures. A third crystalline form with the highest melting peak turned out to be not a polymorphic or pseudopolymorphic crystal modification of our API-CG3 but a chemically different substance.

Chain-Length-Dependent Conformational Transformation and Melting Behavior of Alkyl/Oligo(oxyethylene)/Alkyl Triblock Compounds: α-Hexyl-ω-hexyloxyoligo(oxyethylene)s

The chain-length-dependent conformational transformation and melting behavior of triblock compounds α-hexyl-ω-hexyloxyoligo(oxyethylene)s, H(CH 2 ) 6 (OCH 2 CH 2 ) m O(CH 2 ) 6 H (abbreviated as C 6 E m C 6 s) (m = 1-7), have been studied by differential scanning calorimetry and infrared spectroscopy. With an increase in the number of oxyethylene units (m), the molecular form of the triblock compounds in the solid state changes from an all-trans planar form (y form) to a planar/helical/planar triblock form (β form) at m = 5. The melting points of the y form C 6 E m C 6 s are much lower than the melting points of n-alkanes with similar molecular masses. This result is interpreted as due to the higher Gibbs energy of crystal for C 6 E m C 6 s. The observed thermodynamic quantities indicate that the planar structure of the oligo(oxyethylene) chain is stabilized by the force of the magnitude that maintains the rotator phase of n-alkanes. The /3-form C 6 E m C 6 s melt stepwise through a solid-solid transition, at which the melting of only the alkyl blocks of the molecule occurs. The alkyl chains in the β-form C 6 E m C 6 s are loosely packed in the crystal even below the solid-solid transition temperature, while the crystallinity of the oligo(oxyethylene) block is significantly high. The molecular form of the alkyl/oligo(oxyethylene)/alkyl triblock compounds in the solid state is determined by the balance of the intramolecular conformational restoring force in the central oligo(oxyethylene) block and the intermolecular packing force in the end alkyl blocks. The conformational restoring force results from a tendency of the oligo(oxyethylene) chain to resume its intrinsic helical structure.

Characterization and Thermal Stability of Polymorphic Forms of Synthesized Tristearin

ABSTRACT: The polymorphic characteristics and thermal stability of α, β′, and β forms of synthesized tristearin were studied using X‐ray diffractometry (XRD), differential scanning calorimetry (DSC), polarized light microscopy (PLM), and scanning electron microscopy (SEM). Polymorphic forms of α, β′, and β were induced from tristearin melts with increasing temperatures and times. Solid‐solid and melt‐mediated polymorphic transitions of tristearin occurred predominantly during slow heating rates and rapid heating rates in the DSC, respectively. The stability of α and β forms of tristearin were dependent on storage temperatures. As storage temperatures increased, the rate of polymorphic transitions from α to β and β′ to β forms of tristearin increased.

Phase behavior of hard spheres with a short-range Yukawa attraction.

The phase diagram of a system consisting of hard spheres with an attractive Yukawa interaction is computed by Monte Carlo simulations. Upon decreasing the range of attraction, we find the following scenarios: (a) a stable fluid-fluid and fluid-solid transition, a triple point with fluid-fluid-solid coexistence, and a metastable isostructural solid-solid transition (b) a fluid-fluid and isostructural solid-solid transition, which are both metastable with respect to a broad fluid-solid transition, (c) a metastable fluid-fluid transition, a stable isostructural solid-solid and fluid-solid transition and a triple point with fluid-solid-solid coexistence. Our results show that a large part of the fluid-solid coexistence region is occupied by metastable fluid-fluid and/or solid-solid coexistence regions, which has repercussions on the kinetics and on the nonequilibrium behavior of the system, such as nucleation, vitrification, and gelation.

Platinum nanoclusters on graphite substrates: a molecular dynamics study

Molecular dynamics simulations are used to investigate the shape and structure evolution of single platinum clusters of cubic and spherical shape containing 256 and 260 atoms, respectively, deposited on a static graphite substrate. The evolution is monitored at variable temperature, and as a function of metal-substrate interactions at constant temperature. The Pt-Pt interactions are modelled with the many-body Sutton-Chen potential, whereas a Lennard-Jones potential is used to describe the Pt-C interactions. Heating and cooling curves calculated between 200 K and 1800 K are used to determine solid-solid and solid-liquid transitions. Structural changes are detected through analyses of density profiles and diffusion coefficients. A clear analogy is observed between temperature-induced wetting phenomena and those resulting from enhancement of the metal-substrate interactions.

Phase transitions of n-C 32H 66 measured by means of high resolution and super-sensitive DSC

A high resolution and super-sensitive differential scanning calorimetry (DSC) has been constructed by using two kinds of semi-conducting thermoelectric modules; one is used as a high-sensitive heat flow sensor and the other is for a main heater or cooler using the Peltier effect. The baseline fluctuation of the calorimeter was within ±25 nW. Thermal analysis of n-C 32H 66 was conducted using this DSC and two large peaks due to the solid-solid transition at 339.1 K from the crystalline phase to the rotator phase and the melting transition at 341.9 K were observed at the heating rate of 0.4 mK s −1. In addition, a small peak was detected first by DSC at 338.1 K, indicating a high sensitivity of this DSC. In the solidifying process from the melting state at the cooling rate of 40 μK s −1, several small exothermic sub-peaks were observed in addition to the main solidifying peak.

Validity of the perturbation theory for hard particle systems with very-short-range attraction.

Motivated by recent studies of colloidal systems at the effective one-component level, the validity of the first-order perturbation theory of classical systems of hard particles interacting via short-range potentials is investigated. The influence of the physical parameters on the accuracy of the perturbation theory is examined. It is shown that this simple method is intrinsically appropriate to describe the fluid-solid transition. Concerning the fluid-fluid one, the first-order perturbation theory provides acceptable results when the interaction range is not too small. For very-short-range potentials it systematically leads to an unphysical fluid-fluid transition. In the case of the depletion interaction between hard sphere solutes such a transition is found even at moderate size asymmetry. It is finally shown that the perturbation theory is not more appropriate for an extended solid than for a liquid with the same density, thus making difficult a quantitative description of the isostructural solid-solid transition.