A differential scanning calorimetric study of phase transitions in Na2SO4

The results of a differential scanning calorimetric study of the transitions involving the various polymorphs of Na₂SO₄ are described. They are discussed in the light of their crystal structures and also in conjunction with more recently published data regarding these transitions.

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Zusammenfassung Die Ergebnisse einer mit Differentialabtastkalorimetrie durchgeführten Untersuchung der Übergänge verschiedener Polymorphe von Na₂SO₄ werden beschrieben. Sie werden im Hinblick ihrer Kristallstrukturen und im Zusammenhang mit verschiedenen unlängst veröffentlichten Daten bezüglich dieser Übergänge diskutiert.

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Résumé On décrit les résultats de l'étude, effectuée par analyse calorimétrique différentielle, des transitions mettant en jeu les diverses formes polymorphes de Na₂SO₄. Les résultats sont discutés en tenant compte des données récemment publiées sur ces transitions.

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A differential scanning calorimetric study on phase transformations of HgI2

A quantitative study of the different factors that influence the- transformation of mercuric iodide was carried out with differential scanning calorimetry. The transformation temperature and the peak width during the heating of the material were followed. An attempt was made to explain the experimental results.

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Zusammenfassung Mittels Differential-Scanning-Kalorimetrie wurde der Einfluss verschiedener Faktoren auf die- Umwandlung von Quecksilberjodid quantitativ untersucht. Dabei wurde die Umwandlungstemperatur und die Signalbreite beim Erhitzen der Substanz beobachtet. Es wurde ein Versuch unternommen, die experimentellen Ergebnisse zu erklären.

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A differential scanning calorimetric study on phase transformations of HgI2

The influence of stoichiometry on the mercuric iodide to transformation temperature was studied in a limited region around the exact stoichiometry. It is concluded that although this influence is small, the transformation temperature has its highest value at the exact stoichiometry composition.

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Zusammenfassung In einem begrenzten Intervall in der Nähe der exakten Stöchiometrie von Quecksilberjodid wurde der Einfluß der Stöchiometrie auf die/ß Umwandlungstemperatur untersucht. Es konnte festgestellt werden, daß dieser Einfluß zwar gering ist, die Umwandlungstemperatur ihren größten Wert jedoch bei der exakten Stöchiometrie erreicht.

     

A differential scanning calorimetric study of some phenol derivatives

Differential scanning calorimetry has been applied to derive the fusion enthalpies and entropies of series of mono and dimethylphenols, mono and dichlorophenols, and mono and dinitrophenols.

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Zusammenfassung DSC wurde verwendet zur Ermittlung der Schmelzenthalpie und Entropie der Reihen von Mono- und Dimethylphenolen, Mono- und Dichlorphenolen, Mono- und Dinitrophenolen.

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A differential scanning calorimetric study of cysteine hydrochloride monohydrate

An irreversible phase change, ΔH (transition) = 5.44 ± 0.11 kcal mol^?1 at 337.1 ± 0.3 K has been confirmed in L-(+)-cysteine hydrochloride monohydrate. Phase transitions resulting from thermal pretreatment are also reported and discussed.     

AC calorimetric study of phase transitions in phosphatidylcholine-cholesterol systems

Using an ac calorimetric method, detailed behaviour of the heat capacity in dipalmitoyl-phosphatidylcholine-cholesterol system was studied in the cholesterol concentration less than 5 mol%. It was revealed that the heat capacity near the main transition was composed of at least four anomalies, i.e., multipeak took place in the heat capacity. This fact indicates that a simple theory explaining coexistence of two phases in two component systems does not work in the multipeak region. Then, relation between the multipeak heat capacity and the change of the ripple structure with the cholesterol concentration should be taken into account, when we consider thermodynamical behaviour of the systems.

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Zusammenfassung Mittels AC-Kalorimetrie wurde bei Cholesterol-Konzentrationen von weniger als 5 mol% das Verhalten der Wärmekapazität im System Dipalmitoylphosphatidylcholin-Cholesterol untersucht. Es wurde gezeigt, daß sich die Wärmekapazität in der Nähe der Hauptumwandlung aus mindestens vier Anomalien zusammensetzt, d.h. bei der Wärmekapazität kann ein Multipeak beobachtet werden. Diese Tatsache zeigt, daß eine einfache Theorie, welche die Koexistenz zweier Phasen in einem Zweikomponenten-System erklärt, für die Multipeakregion nicht geeignet ist. Weiterhin sollte bei Überlegungen zum thermodynamischen Verhalten von Systemen eine Beziehung zwischen der Multipeak-Wärmekapazität bzw. der Welligkeitsstruktur und der Cholesterol-Konzentration berücksichtigt werden.

     

Interface influenced phase transitions of alkylcyanobiphenyl liquid crystals A calorimetric study

Abstract

Using an AC-calorimetry technique, we report measurements of the heat capacity and the phase shift between the applied heating power and the resulting thermal oscillations on the thermotropic liquid crystal series of alkylcyanobiphenyl (nCB) above the tricritical point. Specifically, we studied the first order phase transition smectic A to isotropic as a function of substrate, under atmospheric pressure. Different combinations of sapphire, Kapton type 200H polyimide film and air, are used to vary systematically the interfaces encountered by a liquid crystal. The calorimeter uses a sapphire disk, 10 mm in diameter and 0·13 mm thick, above which the samples are placed. A second sapphire disk, or a disk-shaped Kapton film, are used to sandwich the liquid crystal. Air interface samples are droplets allowed to spread naturally on the chosen substrate. The chosen geometry is such that interface effects appear to be maximized. Striking features found in the heat capacity and the phase shift studies with 10CB and 12CB will be presented for the different interfaces.     

A differential scanning calorimetric study of the radical polymerization of N-vinylpyrrolidone in dimethylformamide

Polymerization of N-vinylpyrrolidone (NVP) in dimethylformamide was studied by differential scanning calorimetry (DSC). The concentration of NVP was 0.75 M, that of the radical initiator (azobisisobutyronitrile) 0.01 or 0.02 M. For proper evaluation of scanning thermograms, it was necessary to correct for incomplete monomer conversion due to premature initiator exhaustion and to consider the instantaneous initiator concentration. If these points were allowed for in the computations, activation parameters agree well with those obtained from isothermal thermograms. The values for the overall activation energy were E_a = 82 and 84 kJ/mol, and those for the pre-exponential factor $\text{Z = 6 × 10}{}^{\mathrm{?9}}\text{and}\text{13 × 10}{}^{\mathrm{?9}}\text{(}\text{mol/l}\text{)}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{.}\text{sec}{}^{\mathrm{?1}}$, obtained from the scanning (dynamic) and the isothermal method, respectively.     

A differential scanning calorimetric study on the irreversible thermal unfolding of concanavalin A

A differential scanning calorimetry study on the thermal denaturation of concanavalin A at pH 5.2 where it exists in the dimeric form was carried out. The calorimetric transitions were observed to be irreversible and the transition temperature of the protein increased with increasing scan rate, indicating that the thermal denaturation process is under kinetic control. The thermal unfolding, and its scan rate dependence could be explained according to the kinetic scheme with k as first-order kinetic constant whose change with temperature is given by the Arrhenius equation. Using this model, rate constant as a function of temperature and activation energy of the process have been calculated. The average activation energy of the kinetic process using different approaches is 129±10 kJ mol⁻¹. The differential scanning calorimetric results on transition temperatures and calorimetric enthalpies supported by intrinsic fluorescence indicate that the irreversibility in the calorimetric transitions of concanavalin A includes a combination of post-transition aggregation, chain separation and loss of cofactor.     

The calorimetric calibration of differential scanning calorimetry cells

In this paper it is shown that in many cases enthalpy determinations can be carried out with a precision <1%. The influences of various sample and instrumental properties are described. The enthalpies of 24 compounds with 30 phase changes (polymorphic transitions or melting points) were redetermined. Twelve of the compounds with 15 transitions in the temperature range 0?670°C are selected and recommended for calorimetric DSC calibration. The linearization of the calibration curve as stated by the manufacturer of the instrument employed was fully confirmed.     

Effects of exposure of chloroquine picrate to the sun: A differential scanning calorimetric study

In a continuing study of the effect of exposure of chloroquine and its salt derivatives to the sun, the picrate was investigated. The picrate gives a sharp endothermic effect of melting at 209.2 °C, before it decomposes, to give an exothermic peak at 255.6 °C. The endothermic peak of melting at 209.2 °C was found suitable for the quantitative determination of chloroquine picrate in sun-exposed samples.

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Zusammenfassung In Fortführung einer Untersuchung über Veränderungen von Chloroquin und dessen Derivaten, wenn diese der Sonne ausgesetzt werden, wurden die Pikrate untersucht. Bei den Pikraten werden ein scharfer, dem Schmelzvorgang zuzuschreibender Effekt bei 209.2 °C und ein mit der Zersetzung einhergehender exothermer Peak bei 255.6 °C beobachtet. Der endotherme Peak bei 209.2 °C erwies sich als geeignet zur quantitativen Bestimmung von Chloroquinpikrat in der Sonne ausgesetzten Proben.

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, . 209,2 ° 255,6 ° . , 209,2 ° , .

     

Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

A novel differential scanning calorimetric method to study the complexation of β-cyclodextrin with steroid in solid state

This communication demonstrates that, by simply minotiring the evaporation of water molecules included in β-cyclodextrin cavity, complexation of β-cyclodextrin with steroid in solid state can be studied. The reported method is simple, rapid and novel.     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Positron annihilation lifetime and differential scanning calorimetric study of immiscible NBR/PE blends

The temperature dependence of the mean size of nanoscale free‐volume holes, 〈V_h〉, in polymer blend system consisting of polar and nonpolar polymers has been investigated. The positron lifetime spectra were measured for a series of polymer blends between polyethylene (PE) and nitrile butadiene rubber (NBR) as a function of temperature from 100 to 300 K. The glass transition temperatures (T_g) for blends were determined from the ortho‐positronium (o‐Ps) lifetime τ₃ and the mean size of free‐volume holes 〈V_h〉 versus temperature as a function of wt % of NBR. The T_gs estimated from the PALS data agree very well with those estimated from DSC in view of different time scales involved in the two measurements. Both DSC and PALS results for the blends showed two clear T_gs of a two‐phase system. Furthermore, from the variation of thermal expansivity of the nanoscale free‐volume holes, the thermal expansion coefficients of glass and amorphous phases were estimated. Variations of the o‐Ps formation probability I₃ versus temperature for pure PE and blends with low wt % of NBR were interpreted on the basis of the spur reaction model of Ps formation with reference to the effects of localized electrons and trapping centers produced by positron irradiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 227–238, 2009     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Investigations of solid phase transitions of low molecular organic compounds by differential scanning calorimetry

For the purpose of finding temperature standards for DTA at the temperature below 150°, some solid transitions of low molecular organic compounds were measured by differential scanning calorimetry. Among fourteen materials, we have concluded to select transitions for hexachloroethane (71°) and hexamethylbenzene (110°) as candidates for such standards.

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Zusammenfassung Zur Temperaturnormung unter 150° die DTA Methode wurden verschiedene organische Verbindungen von niedrigem Molekulargewicht und geeigneten Umwandlungen in fester Phase durch Differential Scanning Kalorimetrie geprüft. Von 14 verschiedenen Substanzen wurden Hexachloräthan (71°) und Hexamethylbenzol (110°) zum gesuchten Zweck geeignet gefunden.

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Résumé Pour le but de trouver des étalons de température au-dessous de 150°C pour l'ATD, on a examiné les transitions en phase solide de différents composés organiques de bas poids moléculaire à l'aide d'un analyseur enthalpique différentiel. Sur les 14 échantillons étudiés, on propose l'hexachloroéthane (71°) et l'hexaméthylbenzène (110°) comme étalons.

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150° . , 14 (71°) (110°), .

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The authors wish to thank Dr. H. G. McAdie of the Ontario Research Foundation, Canada, for his kind suggestions on this work.     

A study of the phase behaviour of metal-free phthalocyanine in a differential scanning calorimeter

The phase behaviour of metal-free phthalocyanine (H₂Pc) has been studied in a differential scanning calorimeter. The effect of the thermal history of samples on the DSC curves was investigated to obtain data concerning the phase transition which appeared on α as well as β forms of H₂Pc over the termperature range from 250 to 340 K. To characterize the α metal-free phthalocyanine at low temperatures, the capacitance of an Ag(H₂Pc)Al sandwich was measured as a function of the temperature. The specific heats of the α and β forms of H₂Pc and the heat of the α → β polymorphic transition were measured. Kinetic parameters of the α → β polymorphic transition have been derived from the calorimetric results.     

Thermogravimetric, differential scanning calorimetric and experimental thermal transport study of MWCNT/NBR nanocomposites

Pristine multiwalled carbon nanotubes (MWCNTs) were impregnated in acrylonitrile butadiene rubber (NBR) using internal dispersion kneader and two roller mixing mill to investigate the effects of various nanotubes concentrations on the thermal transport/stability of rubber nanocomposites. Thermal conductivity (λ _N) and thermal impedance (R) measurement experimental setups were established according to ASTM E1225-99 and D5470-03. The 1 mass % addition of MWCNTs in the polymer matrix has enhanced R up to 44 % and reduced λ _N of the rubber nanocomposite up to 40 % compare to the base composite formulation. Thermal decomposition and differential thermal analyses of the fabricated composite specimens simulate that the thermal stability and endothermic capability are augmented with increasing the nanotubes contents in the host matrix. The progressive incorporations of carbon nanotubes into the rubber matrix have efficiently influenced the composite specimens regarding glass transition, crystallization, and melting temperatures including their specific enthalpies. Scanning electron microscopy along with the energy dispersive spectroscopy was used to analyze MWCNTs dispersion in NBR matrix, compositional analysis of the nanocomposite, and impregnated nanotubes.