Gallium cluster "magic melters"

Calorimetry measurements (using a method based on multicollision induced dissociation) have been performed for unsupported gallium clusters, Gan+ (n = 30-50 and 55). Melting transitions have been identified from spikes in the heat capacities recorded as a function of temperature. There are enormous fluctuations in the melting temperatures and the heats of fusion with cluster size. Clusters with n = 31, 33, 37, and 45-47 are "magic melters" with particularly well-defined melting transitions. There is a strong correlation between the heats of fusion, entropies of fusion, and the stabilities of the clusters. However, these quantities are not strongly correlated with the melting temperatures.     

Melting of polymers by non-isothermal, temperature-modulated calorimetry: analysis of various irreversible latent heat contributions to the reversing heat capacity

This paper provides an analysis of contributions to the apparent, reversing heat capacity when measured by temperature-modulated differential scanning analysis (TMDSC) with an underlying heating rate in the temperature range where irreversible transitions with latent heats occur. To deconvolute the data of a TMDSC scan into a total and reversing part, it is common practice to use the sliding averages and the first harmonics of the Fourier series of temperature and heat-flow rate. Under certain conditions, this procedure produces erroneous reversing contributions which are detailed by experiment and simulation. Unless the response to the temperature modulation is linear, the total heat-flow rate is stationary, and the transition is truly reversible and occurs only once during the temperature scan, one cannot expect a true deconvolution of total and reversible effects. In the presence of multiple, irreversible transitions within a modulation period, however, each process involving latent heat can increase the modulation amplitude, as demonstrated by computer-simulation of polymer melting. As a result, the multiple transitions may give erroneously high latent heats when integrating the apparent reversing heat capacity with respect to temperature.     

Side-chain crystallinity. I. Heats of fusion and melting transitions on selected homopolymers having long side chains

Heats of fusion, melting transitions, and the derived entropies of fusion were obtained by differential scanning calorimetry for examples from three homologous series of homopolymers having long side chains. Homopolymers having side-chain lengths between 12 and 22 carbon atoms were chosen from the poly(n-alkyl acrylates), the poly(N-n-alkyl-acrylamides) and the poly(vinyl esters). The data demonstrated that only the outer paraffinic methylene groups were present in the crystal lattice. This was concluded because phase diagrams obtained for mixtures of structurally different monomers and homopolymers, as well as for selected copolymers, showed only isomorphism in the polymeric examples. In addition, scanning curves, reflecting the distribution of crystallite sizes, became narrower as the side chains became longer. The critical chain length required to maintain a stable nucleus in the bulk homopolymers was a constant value for each homologous series. It varied between 9 to 12 carbon atoms. When heats of fusion were determined in the presence of methanol, main-chain restraints were freed, thus permitting more methylene groups to enter the crystal lattice. Hence, the heats of fusion, the crystallinity, and melting points increased above that of the bulk state. The magnitude of the contribution to the heats of fusion by each methylene group indicated that the hexagonal paraffin crystal modification prevailed in these homopolymers, in agreement with x-ray data from the literature.     

Structural phase transitions in pentacosane C25H52

The pentacosane undergoes a series of structural phase transitions in addition to the melting transition according to an x-ray scattering study. A phenomenological theory is developed to describe the structural phase transitions between different phases observed in pentacosane. We present a detailed analysis of the different phases and analyze the temperature anomalies of the elastic constants and heat capacity. The thermodynamic anomalies on the different phase transitions are described by the coupling between the order parameters and the elastic strains. The theoretical predictions are found to be in good qualitative agreement with experimental results.     

Some solid state properties of enantiomers and their racemates

Heats of sublimation and fusion for l and dl-menthol, d and dl-carvoxime and d and dl-dimethyl tartrate were measured. Values of 22.9, 18.8, 21.7, 24.3, 18.5 and 26.9 $\text{kcal}\text{mole}$ and 18.2, 15.7, 24.3, 30.9, 23.3 and 36.2 $\text{cal}\text{g}$ were obtained for heats of sublimation and fusion respectively. Heats of sublimation are believed known to ±5% while heats of fusion are believed qualitatively correct. Generally good agreement was observed with literature values when available. Higher heats of fusion and sublimation are observed for the enantiomer or racemate with the higher fusion temperature. Correlation of these physical properties with the phase diagram for fusion and solubility suggest that differences in the temperature dependence of vapor pressure and solubility might be useful diagnostic tests for discontinuities in molecular packing and phase behavior in the solid state. Prediction of the temperature where racemic mixture to conglomerate transitions occur would be extremely useful for resolving racemates.     

Etude de la cristallinite de derives n-octadecyliques de l'alcool polyvinylique obtenus par greffage chimique

Differential scanning calorimetry has been used to study the heats of fusion and melting transitions of n-octadecyl derivatives of polyvinyl alcohol. These derivatives were obtained by grafting paraffinic chains on polyvinyl alcohol, at various proportions. The melting transitions are independent of the degree of substitution; the heats of fusion are proportional to the fraction of crystallizable units. Using Flory's equilibrium crystallization theory, it is possible to interpret these results assuming, as demonstrated previously, that the chemical grafting is sequential with two unreacted hydroxyl groups between two adjacent paraffinic side-chains.     

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH₃(CH₂) _n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.     

Polymorphic and Thermodynamic Study of Indomethacin

Indomethacin is known to exhibit polymorphism. As a consequence the various forms have different solubilities and may have different bioavailabilities. This study has been carried out with the following techniques: calorimetry, differential scanning calorimetry (DSC), thermogravimetric analysis (TG), X-ray diffraction and thermomicroscopy. Two solid forms have been prepared and studied: their melting temperature and their enthalpy of fusion are determined. The heat capacity and heat content were measured vs. the temperature for these two solid forms and for the liquid phase. This is fundamental for the determination of the stable form. More of this, with a view to study phase diagrams of indomethacin with another compound (solvent or not), the knowledge of the C _p of the various forms is necessary for calculation of the liquidus curve, this allows to minimize the number of experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of a novel type of adiabatic scanning calorimeter for high-resolution thermal data near the melting point of gallium

A novel type of adiabatic scanning calorimeter (ASC) based on Peltier elements (PEs) is used to obtain high-resolution enthalpy and heat capacity data on the melting transition of gallium. The accuracy of the specific heat capacity and specific enthalpy is about 2 %, for a sub-mK temperature resolution. The simultaneously determined equilibrium specific heat capacity and specific enthalpy are used to determine the heat of fusion and the purity. In addition, the use of the PE-based ASC as a classical heat step calorimeter and as a constant rate (DSC-type) calorimeter is discussed. A comparison of the ASC results with literature data and DSC data shows the advantages of ASC for the study of phase transitions.     

Thermodynamic properties of hexacosane

The heat capacity of a purified sample of hexacosane, n-C₂₆H₅₄, has been measured from 13 to 358 K. The enthalpies of transition near 325.5 K and fusion at 329.25 K, and the purity of the sample have been determined; the accuracy of the results is influenced by the closeness in temperature, and overlap, of the crystal-to-crystal and crystal-to-liquid phase transitions. Thermodynamic properties have been calculated from the heat capacities and enthalpies.     

Thermotropic uniaxial and biaxial nematic and smectic phases in bent-core mesogens

Two azo substituted achiral bent-core mesogens have been synthesized. Optical polarizing microscopy and synchrotron X-ray scattering studies of both compounds reveal the existence of the thermotropic uniaxial and biaxial nematic and three smectic phases at different temperatures in these single component small molecule systems. The transition from the uniaxial to biaxial nematic phase is confirmed to be second order. The transitions from the biaxial nematic to the underlying smectic phase and between the smectic phases have barely discernible heat capacity signatures and thus are also second order.     

硝酸肼的量热研究

用精密自动绝热量热计测定了在220 K—370 K温度范围内硝酸肼的热容、熔化热、熔化温度和熔化熵。所得热容数据的精密度以百分偏差的均方根值表示为±0.2％。三次熔化热测定的相对偏差为±0.1％。为验证结果的可靠性, 用该装置测定了冰的熔化热和熔化温度, 其结果与文献值一致; 又用法国SETARAM公司的高温量热计测定了硝酸肼的熔化热和熔化温度, 其结果与我们用量热法测定的结果一致; 从量热结果计算出了该试样的纯度, 该结果亦与化学分析的结果一致。这些均可说明我们所测得的数据是可靠的。     

Thermal characterization of tetrafluoroethylene-perfluoroalkylvinylether copolymers: Melting behavior and low-temperature transitions

Differential scanning calorimetry has been used to study the melting and the low-temperature transitions of tetrafluoroethylene-perfluoroalkylvinylether copolymers (PFA). Melting is similar to that of PTFE in that it is subject to superheating, moreover the melt-crystallized samples show temperatures and heats of fusion lower than the native ones. At temperatures lower than room temperature, crystal-crystal transitions are observed for copolymers containing 2 mol % of perfluoromethyl-, perfluoroethyl-, and perfluoro-n-propylvinylether. In the case of perfluoromethylvinylether as a comonomer, low-temperature transitions are present only up to a content of 4 mol %. The effect on melting and low-temperature transitions of both the comonomer content and of the type of side groups has been investigated. The temperatures and heats of transitions show the expected dependence as a function of the comonomer concentration and have been found to be independent on the chemical nature of the side group. © 1994 John Wiley & Sons, Inc.     

Interactions of surfactants and fatty acids with lipids

The recent studies on the interaction of surfactants and fatty acids with lipids are inspired by the want of knowledge from several research fields of highest activity: identification of membrane rafts; membrane protein crystallization; formation of non-lamellar phases in membranes; membrane fusion. Detailed phase diagrams for lipid–surfactant and lipid–fatty acid mixtures, obtained in the last few years, reveal complicated mesomorphic and polymorphic behavior, including miscibility gaps and compound formation. Surfactant-induced non-lamellar to lamellar transitions in lipids and specific temperature-driven bilayer–micelle transitions represent extensions of the general three-stage model of membrane solubilization. Further research is required to construct phase diagrams as to delineate the principles of lipid–surfactant and lipid–fatty acid interactions for the variety of membrane lipid classes.     

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.     

二元贮冷水合盐熔解热的差示扫描量热法研究

用差示扫描量热法（differential scanning calorimetry)对二元贮冷水合盐的相变温度与熔解热进行了研究，实验结果对认识多元贮冷水合盐的相变过程的相变机理及选配贮冷水合盐材料，具有重要意义。     

Evaluation of heats of transition in scanning calorimetry

A procedure for the evaluation of heats of transition in scanning calorimetry has been developed. A formula for evaluation of the energy of fusion of a compound at its melting point is derived that takes into account the baseline shift that is attributable to the heat capacity change on melting. Several other calorimeter parameters of importance are discussed. These include heat exchange between calorimeter vessel and jacket, the time constant of the instrument, the scanning rate, and the heater placement.     

Overall analysis of first‐order phase transitions in some alkyloxycyanobiphenyl liquid crystals

Measurements of pressure, molar volume and specific heat as functions of temperature in the isotropic (I) phase as well as in the smectic A (SmA) and nematic (N) mesophases of some alkyloxycyanobiphenyl compounds (nOCB, n = 6–10) were carried out using differential thermal analysis under pressure, densitometry, X‐ray powder diffraction and modulated differential scanning calorimetry. Thermodynamic properties, such as latent heats and volume jumps at the different phase transitions, were determined. The coherence of this whole set of data was tested using pressure–temperature data through the slopes associated to their phase transitions, extrapolated at normal pressure in the light of the Clausius–Clapeyron equation.     

Differential scanning calorimetry of chlorinated polyethylenes

The heats of fusion of low-density polyethylene and chlorinated polyethylenes of varying chlorine content have been investigated by differential scanning colorimetry (DSC). Linear correlations are found between the chlorine content and the apparent heat of fusion, crystallinity, and the maximum specific heat measured in the temperature range 90–120°C. The effect of heat treatment on crystallinity is shown to be strongly dependent on the chlorine content of the polymers.     

Theory of phase diagrams described by thermodynamic potentials with T d symmetry

Phase diagrams of crystals induced by irreducible representations with symmetry group \(L = \bar 43m\) (T _d ) are constructed within the phenomenological theory of second-order phase transitions. A model of the Landau thermodynamic potential is studied, state equations of all symmetry-conditioned phases are obtained, and general conditions for their thermodynamic stability are formulated. Equations for the boundaries of phase areas and lines of phase transitions are obtained for the fourth order of expansion of the potential via components of the order parameter. Some types of the collapse of the multicritical point of the phase diagram for the eighth order of potential expansion are studied using computer calculations. The possible existence of phase diagrams that contain one or more triple points and areas of existence of three and four phases is shown for the first time for the potentials with the above symmetry. Examples are given of crystals that undergo phase transitions in the considered symmetry of the order parameter.