Melting Entropy Research Articles

Heat capacity and thermodynamic properties of benzyl disulfide (C 14H 14S 2)

Heat capacities of benzyl disulfide have been measured with a high-precision automatic adiabatic calorimeter over the temperature range from 80 to 377 K. The melting point, molar enthalpy and entropy of fusion were determined to be 341.72 ± 0.07 K, 44965 ± 10 J mol −1 and 130.78 ± 0.11 J K −1 mol −1, respectively. The thermal stability and the kinetics of thermal decomposition of the compound were investigated in air by means of thermogravimetry (TG) and differential thermal analysis (DTA). TG/DTA curves showed that the decomposition proceeded through one step. The activation energy and the reaction order for one-step decomposition was calculated to be 110.7 ± 12.3 kJ mol −1 and 1.2 ± 0.2 through Kissinger method.

The enthalpy of ErSi and ErSi1.67 in the temperature range from 400 to 2300 K

The enthalpy of erbium monosilicide and disilicide is measured for the first time by the method of mixtures in a wide temperature range. Experimental data are used to obtain the temperature dependences of heat capacity, entropy, and reduced Gibbs energy of the compounds. The enthalpies and entropies of polymorphous transformation and melting of the investigated compounds are determined.

Galena stability to 26 KBAR

The melting point of galena was measured by differential thermal analysis to 25 kbar. The melting point is 1191 °C at 5.9 kbar and increases to ∼1315 °C by 25 kbar along a concave-down trajectory. The form of the PbS melting curve resembles those of similarly structured compounds that show a range of initial slopes and curvatures. Initial liquidus slopes of several B1-structured compounds including PbS correlate well with melting volume change. However, the roughly 25 J/mol*K entropy of melting required for PbS by the volume of melting, initial liquidus slope, and Clapeyron equation is approximately double literature estimates. Similar discrepancies exist for the other B1-structured compounds. Liquidus curvature correlates well with the initial ratio of liquid to solid compressibility. PbS liquid compressibility of ∼12*10-12 cm2/ dyne is estimated from galena compressibility and initial melting slope. The transformation of galena from cubic to orthorhombic structure with pressure was determined by electrical resistance measurements up to 1000 °C and occurs at about 26 kbar with little temperature variation. Galena9s maximum stability is at the inferred triple point among galena, liquid, and orthorhombic-structured PbS at ∼26 kbar and ∼1315 °C.

On the Definition of a Monte Carlo Model for Binary Crystal Growth

We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for binary crystal growth with the thermodynamic properties of a system does not guarantee the MC simulations near equilibrium to be in agreement with the thermodynamic equilibrium phase diagram for that system. The deviations remain small for systems with small bond energies, but they can increase significantly for systems with large melting entropy, typical for molecular systems. These deviations are attributed to the surface kinetics, which is responsible for a metastable zone below the liquidus line where no growth occurs, even in the absence of a 2D nucleation barrier. Here we propose an extension of the MC model that introduces a freedom of choice in the transition probabilities while staying within the thermodynamic constraints. This freedom can be used to eliminate the discrepancy between the MC simulations and the thermodynamic equilibrium phase diagram. Agreement is achieved for that choice of the transition probabilities yielding the fastest decrease of the free energy (i.e., largest growth rate) of the system at a temperature slightly below the equilibrium temperature. An analytical model is developed, which reproduces quite well the MC results, enabling a straightforward determination of the optimal set of transition probabilities. Application of both the MC and analytical model to conditions well away from equilibrium, giving rise to kinetic phase diagrams, shows that the effect of kinetics on segregation is even stronger than that predicted by previous models.

Solvent Release upon Ion Association from Entropy Data. II

When a cation and an anion associate, the charge on the product is lower than that on the individual ions and solvent is released from their solvation shells to the bulk solvent. This release occurs when the associate is a solvent-shared or contact ion pair or an inner-type complex. The measurable molar entropy change involved is considered to be made up of four contributions: translational, rotational, electrostatic, and desolvation entropies. The former three can be calculated from the properties of the ions and solvents involved; hence, the fourth is obtained by difference. The release of solvent molecules from the crystalline frozen solvent to the liquid on melting is analogous to the solvent release from translational immobilization in the solvation shells of the ions. The molar entropy of melting of the solvent is used to estimate the amount of solvent released in the association process.

Negative heat capacity of Ar 55 cluster

The solid–liquid phase transitions of Ar 55 cluster was simulated by the microcanonical molecular dynamics and microcanonical parallel tempering methods using Lennard–Jones potential, and thermodynamic quantities were calculated. The caloric curve of cluster has S-bend. To understand this behaviour, configurational and total entropies were evaluated, and the dents on the entropy curves were noticed as the sign of negative heat capacity. The heat capacities were evaluated by using configurational entropy data. The potential energy distributions have bimodal behaviour in the given range at the melting temperature. At the same time by using configurational entropy canonical caloric curve and canonical heat capacity were calculated. To obtain entropy change upon melting, total entropy were calculated from the caloric curve. The microcanonical results melting temperature, latent heat and entropy change upon melting values were reported and compared with the values reported in the literature and the values calculated from the thermodynamic relations offered for bulk matter, consistent values were found.

Estimation of Melting Points of Organic Compounds-II

A model for calculation of melting points of organic compounds from structure is described. The model utilizes additive, constitutive and nonadditive, constitutive molecular properties to calculate the enthalpy of melting and the entropy of melting, respectively. Application of the model to over 2200 compounds, including a number of drugs with complex structures, gives an average absolute error of 30.1°.

The vitrification of alkali borate and alkali silicate melts

Glasses and crystals of compositions corresponding to the congruently melting compounds M2O·2SiO2 (M = Na. Rb, and Cs) and M2O·4SiO2 (M = K, Rb, and Cs) were studied by differential scanning calorimetry. The structure temperatures (T f) and excess entropies at T f of glasses were measured depending on the rate of cooling of the corresponding melts. The activation energies of glass formation (ΔE) and scale of cooperative motion in the transition region (ξa) were estimated. The totality of the data obtained were used to compare the thermodynamic (the ratio between the excess (with respect to the corresponding crystals) entropy of glass at T f and the entropy of crystal melting), kinetic (fragility m = f(ΔE, T f)), and microscopic (ξa) parameters of the vitrification of alkali silicate melts. The behaviors of alkali silicate and alkali borate melts were shown to be similar.

Equilibrium Melting Temperature of Aliphatic Polyesters Using Model Compounds

A series of oligomers for use as a biodegradable aliphatic polyester model compounds such as poly(dodecamethylene dodecanedioate) (C12-n) and poly(tetradecamethylene tetradecanedioate) (C14-n) were synthesized. C12-n and C14-n were separated by a silica gel column, and pure n-mers were obtained. The melting enthalpies and entropies of n-mers were observed to follow a linear equation in terms of n. The equilibrium melting temperatures of C12-n and C14-n obtained from ΔH/ΔS were 104 °C and 108 °C, respectively. We can easily calculate the approximate equilibrium melting temperature of aliphatic polyesters (Cm) using Tm0=411(m−2.48)/(m−1.59).

Effects on the structure and properties of membranes formed by blending polydimethylsiloxane polyurethane into different soft‐segment waterborne polyurethanes

AbstractPolydimethylsiloxane polyurethane (PDMS‐PU), which was synthesized from PDMS as the soft segment, was blended into a variety of ester‐ or ether‐based soft‐segment waterborne polyurethanes with different concentrations to investigate the crystallization, thermal, and physical properties of the membrane formations. According to X‐ray analysis, the ether‐based PUs, synthesized from soft segments of poly(propylene glycol) (PPG1000) or poly(ethylene glycol) (PEG2000), were found to have maximum crystallinity at a 5% blending ratio of PDMS‐PU, but the ester‐based PU, synthesized from soft segments of polycaprolactone (PCL1250), had decreased crystallinity at a 5% blending ratio. Differential scanning calorimetric analysis revealed that the Tg,s values of PUs were highest when the blending ratio of PDMS‐PU was 5%–10%, except for PU from PCL1250. Moreover, ether‐based PUs showed maximum Tm,h values, but the Tm,h of the ester‐based PU was greatly reduced when PU with PCL1250 was blended with PDMS‐PU. In addition, the PU from PEG2000 had the highest melting entropy. Mechanical property analysis showed that the stress of ether‐based PUs would be increased when PUs were blended with a small amount of PDMS‐PU and that the stress of PU from poly(tetramethylene glycol) (PTMG1000) increased to its greatest value (20–30 MPa). On the other hand, the ester‐based PU, from PCL1250 blended with PDMS‐PU, would have reduced stress. On the whole, the stress and strain of PU from PEG1000 had excellent balance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 210–221, 2006

Contribution of solid-state properties to the aqueous solubility of drugs

This study investigates the influence of the solid-state properties melting point (Tm), enthalpy of melting (ΔHm) and entropy of melting (ΔSm) of a drug on its intrinsic solubility (S0). For this purpose, 26 chemically and structurally diverse drugs covering the oral drug space were selected and the S0, Tm, ΔHm and ΔSm were determined experimentally. The influence of Tm, ΔHm and ΔSm on S0 was studied using regression analysis. The overall improvement of the predictions were 0.3 log units, however, five compounds (astemizole, glyburide, fenbufen, gliclazide and griseofulvin) were improved by more than one log unit. Tm and ΔHm had a larger effect than ΔSm on the solubility predictions. The well-known general solubility equation (GSE) and the Dannenfelser semi-empirical equation for the calculation of ΔSm were evaluated using our data set. While predictions of drug solubility obtained using the GSE were acceptable, the use of the experimental ΔSm values instead of the constant 56.5Jmol−1K−1 improved the accuracy of the prediction. The Dannenfelser equation underestimated the ΔSm for most compounds with on average 15Jmol−1K−1. Our results show that solid-state properties should be considered for improved performance of future models for prediction of drug solubility. In addition our study provides accurate experimental data on intrinsic solubility for 26 compounds, supplying a useful external data set for validation of drug solubility models.

The thermal capacity and enthalpy of gadolinium sesquiselenide over a wide temperature range

Measurements have been made on the thermal capacity of γ-Gd2Se3 at 58.88–298.34 K. Values have been obtained for the thermal capacity, entropy, reduced Gibbs energy, and enthalpy under standard conditions: C°p = 125.87 ± 0.5 J· mole−1 · K−1; S°(298.15 K) = 196.5 · 1.6 J · mole−1 · K−1; Φ°(298.15 K) = 103.6 ± 1.6 J · mole−1 · K−1; H°(298.15 K)-H°(0) = 27681 ± 138 J · mole−1. The enthalpy of Gd2Se3 has been measured and the major thermodynamic functions have been calculated for the solid and liquid states over the temperature range 450–2300 K. The temperature dependence of the enthalpy in the ranges 300–1800 K and 2000–2300 K are represented: H°(T)-H°(298.15 K) = = 1.1949 · 10−2 · T2 + 122.38 · T + 347402 · T−1 − 38716 and H°(T)-H°(298.15 K) = 262.81 · T-− 196047, respectively. The calculated temperature, enthalpy, and entropy of melting for Gd2Se3 are: Tm = 1925 ± 40 K, ΔmH° (Gd2Se3) = 68.5 kJ · mole-1, ΔmS°(Gd2Se3) = 35.6 J · mole−1 · K−1.

Amorphous Drug Nanosuspensions. 1. Inhibition of Ostwald Ripening

Amorphous drug nanosuspensions are prone to particle growth due to Ostwald ripening. By incorporating a second component of extremely low aqueous solubility, Ostwald ripening can be inhibited. These studies indicate that to inhibit ripening, the drug/inhibitor mixture (in the particles) must form a single phase. The drug/inhibitor mixture can be characterized by the interaction parameter chi using the Bragg-Williams theory, in which single phase mixtures are obtained for chi < 2. The chi parameter can be calculated from the (crystalline) solubility of the drug in the inhibitor, provided the inhibitor is a liquid, and the melting entropy and temperature of the drug.

A survey of energies in materials science

A table is presented that compares energies that govern a variety of phenomena in materials science: thermal, structural and chemical energies; energies associated with the formation of vacancies, dislocations and interfaces; energies associated with the application of an external load, strain, magnetic field, or supersaturation. A simple argument, based on the polytetrahedral nature of the structure of liquids, is presented to explain why the configurational part of the entropy of melting for many elements is close to 1k.

Thermal stability of indium nanocrystals: A theoretical study

A simplified model is introduced to predict the size-dependent undercooling, superheating and melting entropy of indium nanocrystal. The conditions of melting thermodynamics increasing or decreasing with the reduced size have been discussed. The minimum critical size and the lowest melting temperature of free indium nanoparticles are calculated, and the differences of the degressive slope of melting temperature between embedded indium nanocrystals with incoherent interface and free indium nanocrystals are discussed as well. It is found that the model predictions agree well with the experiments of indium nanocrystals.

Two New Parameters for Predicting the Entropy of Melting: Eccentricity (ε) and Spirality (μ)

The prediction of the entropy of fusion of organic compounds is studied. Rotational symmetry (σ) and flexibility (φ) have already been shown to contribute to the orientational and conformational entropy of fusion, respectively. Eccentricity (e) and spirality (μ) are two new parameters that influence translational and configurational entropy. e is defined as the ratio of the volume of a box around a rigid molecule to the cubed radius of a sphere containing the same molecular van der Waals volume. μ is defined as the number of benzo[c]phenanthrene regions present in the molecule. This moiety results in repulsion and out of plane twisting to maximize the distance between hydrogens. Enthalpy of melting, melting point, and geometrical data for 117 compounds were gathered from the literature. The database consists of alkanes, polycyclic aromatic hydrocarbons, alkyl benzenes, and other alkyl aromatics. The addition of e and μ to σ and φ as predictors for the entropy of melting revealed a best fit melting point p...

Heat capacities and thermodynamic properties of 2-benzoylpyridine (C12H9NO)

The heat capacities of 2-benzoylpyridine were measured with an automated adiabatic calorimeter over the temperature range from 80 to 340 K. The melting point, molar enthalpy, ΔfusHm, and entropy, ΔfusSm, of fusion of this compound were determined to be 316.49±0.04 K, 20.91±0.03 kJ mol–1 and 66.07±0.05 J mol–1 K–1, respectively. The purity of the compound was calculated to be 99.60 mol% by using the fractional melting technique. The thermodynamic functions (HT–H298.15) and (ST–S298.15) were calculated based on the heat capacity measurements in the temperature range of 80–340 K with an interval of 5 K. The thermal properties of the compound were further investigated by differential scanning calorimetry (DSC). From the DSC curve, the temperature corresponding to the maximum evaporation rate, the molar enthalpy and entropy of evaporation were determined to be 556.3±0.1 K, 51.3±0.2 kJ mol–1 and 92.2±0.4 J K–1 mol–1, respectively, under the experimental conditions.

Thermodynamic−Kinetic Correlations in Supercooled Liquids: A Critical Survey of Experimental Data and Predictions of the Random First-Order Transition Theory of Glasses

Thermodynamics and kinetics are thought to be linked in glass transitions. The quantitative predictions of alpha-relaxation activation barriers provided by the theory of glasses based on random first-order transitions are compared with the experimental results for 44 substances. The agreement found between the predicted activation energies near T(g) and experiment is excellent. These predictions depend on the configurational heat capacity change on vitrification and the entropy of melting the crystal which are experimental inputs.

Competing Thermal Activation Mechanisms in the Meltinglike Transition of NaN (N = 135−147) Clusters

The meltinglike transition in unsupported icosahedral Na(N)() clusters, with N = 135-147, has been studied by isokinetic molecular dynamics simulations based on an orbital-free version of density functional theory. A maximum in the melting temperature, T(m), is obtained for Na141, while the latent heat, deltaE, and entropy of melting, deltaS, are maximal for Na147. These observations are in close agreement with calorimetric experiments on N clusters. The size evolution of deltaS is rationalized by the emergence of important premelting effects associated with the diffusive motion of atomic vacancies at the cluster surface. The precise location of the maximum in T(m) is explained in terms of two different thermally activated structural instability mechanisms which trigger the meltinglike transition in the size ranges N = 135-141 and N = 141-147, respectively.

The Hg—Ra (Mercury—Radium) System

AbstractFor Abstract see ChemInform Abstract in Full Text.