Heat capacity measurements of MnxFe3−xO4

Heat capacities of Mn_xFe_3?xO₄ with the composition x = 1.0, 1.5, and 2.0 were measured from 200 to 740 K. λ-type heat capacity anomalies due to the ferri-paramagnetic transition were observed for all the compositions. The transition temperatures were 577, 471, and 385 K for the composition x = 1.0, 1.5 and 2.0, respectively, which are in good agreement with the results of magnetic measurements. The difference in heat capacities between the different samples was small except for the temperature range of the transition. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity can be expressed by the sum of the lattice heat capacity C_v (l), the dilation contribution C(d), and the magnetic contribution C(m). Entropy changes due to the transition were obtained from C(m) as 55.5, 50.7 and 49.2 J K^?1 mole^?1 for the composition x = 1.0, 1.5, and 2.0, respectively. The entropy changes were also calculated by assuming the randomization of unpaired electron spins on each ion, but they were from 6 to 10 J K^?1 mole^?1 smaller than the observed ones. The difference between the experimental and the calculated values is roughly explained by taking into account the cation exchange reaction between the tetrahedral and the octahedral sites in the spinel structure.     

An additivity scheme for the estimation of heats of formations,entropies, and heat capacities of silanes,polysilanes, and their alkyl derivatives

Heat of formation data available for silanes and alkylsilanes have been evaluated using the Benson-Luria electrostatic energy corrected bond additivity method for a priori calculations of heats of formation of hydrocarbons. It is concluded that the calculational method is applicable to silanes and alkylsilanes, and that the recent combustion measurements employing HF and O₂ are reliable. Group additivity enthalpies based on these data are presented. Results of a large number of statistical thermodynamic calculations of entropies and heat capacities are also given, and values of the group additivities derivable from these results are presented. Internal consistencies of estimated thermodynamic properties (i.e., estimated reaction enthalpy, entropy, and heat capacity changes) are thought to be reliable to within ±1.5 kcal and ±1.0 e.u., respectively. Group additivity estimates for individual compounds could be significantly less accurate due to the limited accuracy and extent of the ΔH⁰_f data base, and to the uncertainties in assigned frequencies and internal rotational barriers employed in calculating entropies and heat capacities.     

Improved conditions for measurement of the specific heat capacities of pure triglycerides by differential scanning calorimetry

Reproducible specific heat capacities (C _p) of triglycerides can be obtained by using heat-flux DSC under improved operating conditions. The improved operating parameters, such as the scanning rate, the sample mass and the atmosphere within the DSC chamber, were established via statistical analysis of the experimental data with trilaurin as a sample. The specific heat capacity results on trilaurin were compared with the values calculated by using estimation methods. The precision of the specific heat capacity measured for trilaurin under these conditions was within ±1%.     

Heat Capacity of Micellization of Lithium Perfluoroalkanoates in Aqueous Solution

Specific heats of aqueous solutions of lithium perfluoroalkanoates, from C₆ to C₉, were determined at 298.15 K at concentrations below and above the critical micelle concentration. Infinite dilution apparent molar heat capacities are compared with literature data for corresponding salts with different counterions. Heat capacities of micellization of these surfactants in water were calculated from the specific heat data and also by measurements of the heat of micellization at two temperatures, 298.15 K and 308.15 K. The data were treated under the assumption of the pseudo-phase separation model. The two series of data agree in the case of perfluorononanoate but diverge for perfluorosurfactants with shorter hydrophobic chains. The results are interpreted in terms of the extent of the applicability of the adopted chemical model. Heat capacities of the micellization process obtained from experimental specific heats compare well with literature values relative to the sodium salts of the examined anions.     

Heat capacity of vanadium-oxygen alloy

Heat capacities of vanadium-oxygen alloys with various compositions, VO_0.0834, VO_0.1127, VO_0.1245, and VO_0.1296, were measured from 320 to 920K by adiabatic scanning calorimetry. A heat capacity anomaly due to order-disorder rearrangement of oxygen atoms was observed for all the compositions. The transition temperatures from α′ to β phase were found to be 780, 791, 786, and 768K for VO_0.0839, VO_0.1127, VO_0.1245, and VO_0.1296, respectively. The transition temperatures from β′ to β were also observed to be 665 and 660K for VO_0.1245 and VO_0.1296, respectively, but they shifted to lower temperatures in repeated measurements. The excess heat capacity due to order-disorder transition was obtained by assuming that the heat capacity can be expressed as the sum of a harmonic term of lattice vibration, a dilational term, an electronic term, and an anharmonic term of lattice vibration. The entropy changes due to the transition for VO_0.0834, VO_0.1127, VO_0.1245, VO_0.1296 were determined from the excess heat capacities to be 1.90, 2.88, 2.82, and 2.88 J K^?1 mole^?1, respectively, values which were explained by calculating the entropy changes due to the order-disorder rearrangement of oxygen atoms in the superstructures of VO_x alloys. From the O/V dependence of the transition temperature and entropy change, the most stable composition of the α′ phase was thought to be V₄₈O₅.     

Heat capacity measurements on BaTa<Subscript>2</Subscript>O<Subscript>6</Subscript> and derivation of its thermodynamic functions

Heat capacity measurements of barium tantalate (BaTa₂O₆) were carried out by using a differential scanning calorimeter at temperatures between 323 and 1323 K. From the heat capacity values of BaTa₂O₆, other thermodynamic functions (enthalpy and entropy increments) were derived between 298.15 and 1323 K. The C _p,m (298.15) value of BaTa₂O₆ was computed as 184.857 J mol^?1 K^?1. Moreover, fitted heat capacities exhibited good agreement with Neumann–Kopp rule at the temperatures between 298.15 and 1300 K.     

Thermal analysis of spider silk inspired di-block copolymers in the glass transition region by TMDSC

We used advanced thermal analysis methods to characterize a new family of A-B di-block copolymers based on the amino acid sequences of Nephila clavipes major ampulate dragline spider silk. Using temperature modulated differential scanning calorimetry with a thermal cycling method and thermogravimetry, we captured the effect of bound water acting as a plasticizer for spider silk-like biopolymer films which had been cast from water solution and then dried. A low glass transition because of bound water removal was observed in the first heating cycle, after which, a shift of glass transition was observed in A-block film due to crystallization and annealing, and in BA film due to annealing. No shift of glass transition after bound water removal was observed in B-block film. The reversing heat capacities, C _p, for temperatures below and above the glass transition were measured and compared to the calculated values. The solid state heat capacity was modeled below T _g, based on the vibrational motions of the constituent poly(amino acid)s, heat capacities of which are known from the ATHAS Data Bank. Excellent agreement was found between the measured and calculated values of the heat capacity, showing that this model can serve as a standard method to predict the solid state C _p for other biologically inspired block-copolymers. We also calculated the liquid state heat capacities of the 100% amorphous biopolymer at T _g, and this predicted value can be use to determined the crystallinity of protein-based materials.     

Formation of metastable crystals of [C4mim][NTf2] and [C6mim][NTf2]

Heat capacities of crystalline 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₄mim][NTf₂] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₆mim][NTf₂] in the range 80 K-T_fus were measured in an adiabatic calorimeter. Anomalies in the heat-capacity curves for the both compounds occurred near 240 K. Positions of the anomalies depended on thermal history of the samples. More stable crystals had higher heat capacities in the range 220-260 K. Below 200 K heat capacities of all the crystals of the same compound were indistinguishable.     

Heat capacity of a polydiacetylene single crystal from 3 to 300 K

Heat capacities C_p of a polydiacetylene-bis(toluene sulfonate) single crystal and its monomer have been measured in the temperature range from 3 to 300 K. The temperature dependence of C_p for both monomer and polymer crystals differs from that for monoatomic solids. By applying a chain lattice model for a polymer crystal, the temperature dependence of the heat capacity can be described assuming a phonon density of states given by bending and stretching modes of the polymer backbone. With a combination of one-dimensional and three-dimensional elastic continuum approximations, the heat capacity has been calculated and a good fit to the data has been obtained. A small peak in C_p was detected at 161 K for the monomer and at 198 K for the polymer. This may be ascribed to a lower-temperature phase transition in the polydiacetylene crystals evidenced by previous x-ray and spectroscopic measurements.     

The thermodynamic functions of 4f metal dichlorides in the condensed state

The temperature dependences of heat capacity were obtained for solid 4f metal dichlorides LnCl₂ (Ln = La, …, Lu) in the quasi-harmonic approximation over the temperature range from 0 K to the melting point T _m . The correction for systematic underestimation of the lattice heat capacity component in this approximation was determined from high-temperature EuCl₂ heat capacity measurements. The literature data were analyzed to select the temperatures and enthalpies of phase transitions and estimate the heat capacities of the substances in the liquid state. The thermodynamic functions of LnCl₂ in the condensed state were calculated over the temperature range 298.15–2000 K. The calculations were performed taking into account excited electronic states whose energies did not exceed 10000 cm^?1.     

The apparent molar heat capacities and volumes of aqueous NaCl from 0.01 to 3 mol kg−1 in the temperature range 274.65 to 318.15 K

The heat capacities per unit volume of aqueous solutions of NaCl were measured with a flow microcalorimeter. The molality and temperature range covered were 0.01 to 3 mol kg^?1 and 274.65 to 318.15 K. The derived apparent molar heat capacities C₂, φ, when extrapolated to infinite dilution, give standard partial molar heat capacities C₂^o which are in excellent agreement with those of Criss and Cobble. The excess apparent molar heat capacities (C₂, φ - C₂^o) can be used to predict the temperature dependence of (H₂, φ - H₂^o), the excess apparent molar enthalpy. The calculated values of ΔH₂, φ agree within experimental uncertainty with the integral enthalpies of dilution of Ensor and Anderson and of Messikomer and Wood up to 323.15 K. Above this temperature significant differences are observed. The densities of the solutions were also remeasured in the same range of temperature and molality with a flow densimeter, and the derived apparent molar volumes agree with the literature values.     

Specific heats of guayule and natural rubbers above the glass transition temperature

Heat capacities of guayule and natural rubbers were measured between 228 and 333 K using a DuPont 990 Differential Scanning Calorimeter. Data obtained were fitted to a straight line. We obtained the following equations where C_p is given in cal g^?1 K^?1. For guayule rubber, C_p = 22.6152 × 10^?4T + 0.7731 (correlation factor = 0.99). For natural rubber. C_p = 16.9195 × 10^?4T + 0.9209 (correlation factor = 0.98). Furthermore, some theoretical considerations and instrumental conditions were analyzed so that the determinations of heat capacities could be improved.     

Determination des capacites thermiques d'exces par calorimetrie de reaction directe

Accurate determinations of excess heat capacities,C _p ^E , of liquid and solid phases with respect to composition and temperature are shown to be possible by direct reaction calorimetry. The results are compared with those obtained by heat capacity measurements and departure from the additivity rule. In the case of solutions, the knowledge ofC _p ^E with respect to concentration permits a pertinent analysis of the short-range order. Some results concerning binary alloys, such as In-Te, Cu-Sb and Ag-Te, are given.     

DSC investigation of the polystyrene films filled with fullerene

Polystyrene composite films with different content of C₆₀?+?C₇₀ fullerene mix have been obtained from o-xylene solutions. The mass fraction of fullerene was varied from 0.01 to 0.1 mass%. The glass transition temperatures and specific heat capacities in range of 293?C423?K have been determined for the films by DSC method. The plasticization of the polymer is observed in thermal properties of the films under influence of small fullerene additions. The values of T _g and C _P decrease and thermal coefficient of heat capacity b increase as fullerene content increases up to 0.02 mass%. The effect of interaction between polymer and fullerene molecules on thermal properties becomes evident at higher fullerene content in range from 0.02 to 0.1 mass%. At this the values of T _g and C _P increase and b coefficient decrease with increasing content of fullerene. Concentration dependence of C _P and b values is less steep for polymer composite films in elastic state at temperatures above T _g. Molecular interactions in the composites are discussed in view of our-self and literature data.     

Low temperature heat capacity of the system “silica gel–calcium chloride–water”

Heat capacity was measured for two composite systems based on silica gel KSK and calcium chloride confined to its pores. One corresponds to an anhydrous state, while another contains water bound with the salt to give the composition of CaCl₂·2.04H₂O. The measurements were performed in the temperature range of 6–300 K with a vacuum adiabatic calorimeter. The smoothed experimental curves C _p (T) were used for calculating the calorimetric entropy and the enthalpy increment for both studied systems as well as the effective heat capacity associated only with water in the hydrated composite. The heat capacities C _p (298.15 K) of both composites were compared with those calculated as a linear addition of the heat capacities of silica gel and bulk calcium chloride (or its dihydrate) with appropriate weight coefficients.     

Interpretation of the heat capacity of polymer melts

Heat capacity values for melts of polystyrene, poly(methyl methacrylate), polypropylene, and polyethylene are calculated on the assumption that the total constant-volume heat capacity C_v is made up of two parts: one associated with molecular vibrations, and one, with holes. Numerical values of both components are given for a wide range of temperatures and compared with experimental data. For poly-1-butene insufficient data for complete evaluation are available, so that only the vibrational contribution could be discussed.     

Standard enthalpy and heat capacity of solid tin telluride

The published data on the heat capacity of tin telluride were analyzed. The C _p values were demonstrated to be consistent only at temperatures below 56 K. Some data on the heat capacity of SnTe within 80–453 K were found to differ significantly. The heat capacity C _p was measured on a DSM-2M calorimeter within a temperature range of 350–600 K and other thermodynamic functions of tin telluride were calculated.     

Heat capacities and electrical conductivities of 1-ethyl-3-methylimidazolium-based ionic liquids

We present the heat capacities and electrical conductivities of five [Emim] 1-ethyl-3-methylimidazolium-based ionic liquids: [Emim][BF₄] (tetrafluoroborate), [Emim][CF₃SO₃] (trifluoromethanesulfonate), [Emim][C₂N₃] (dicyanamide), [Emim][C₂H₅SO₄] (ethylsulfate), and [Emim][MDEGSO₄] (2-(2-methoxyethoxy) ethylsulfate). The heat capacities were measured using a differential scanning calorimeter (DSC) over the temperature ranging from (303.2 to 358.2) K. The electrical conductivities were measured over the temperature ranging from (293.2 to 353.2) K using a commercial conductivity meter. The estimated uncertainties of heat capacity C_p and electrical conductivity σ measurements were ±0.015 kJ · kg^?1 · K^?1 and ±0.001 mS · cm^?1, respectively. The measured C_p and σ are presented as a function of temperature. The temperature dependency of the C_P value was correlated using an empirical equation. A modified version of VTF-type (Vogel–Tamman–Fulcher) equation was used to describe the temperature dependency of σ values. The correlations give satisfactory results. Also, the results of this study are in good agreement with the available literature data. The heat capacities and electrical conductivities presented in this work are in good agreement with the available literature data. The results of this study can be applied to numerous chemical processes, since C_p and σ data are essential information for rational design.     

Excess heat capacities and excess volumes of binary liquid mixtures of 1,1,1,-trichloroethane with cyclic ethers at 298.15 K

Measurements of volumetric heat capacities at constant pressure, C_p/V (V being the molar volume), at 298.15 K, of the binary liquid mixtures 1,1,1-trichloroethane + oxolane, +1,3-dioxolane, +oxane, +1,3-dioxane, and +1,4-dioxane were carried out in a Picker-type flow microcalorimeter. Molar heat capacities at constant pressure. C_p, and molar excess heat capacities, C^E_p, were calculated from these results as a function of the mole fraction. C^E_p values for these systems are positive and the magnitude depends on the size of the cycle and on the relative position of the oxygen atoms in the cyclic diethers. The precision and accuracy for C^E_p are estimated as better than 2%. Molar excess volumes, V^E, for the same systems, at 298.15 K, have been determined from density measurements with a high-precision digital flow densimeter. The experimental results of V^E and C^E_p, are interpreted in terms of molecular interactions.     

Heat capacities of a series of terminal linear alkyldiamides determined by DSC

Molar heat capacities of twelve linear alkane-α,ω-diamides H₂NOC-(CH₂)_(n-2)-CONH₂, (n=2 to 12 and n=14) were measured by differential scanning calorimetry at T=183 to 323 K. Heat flow rate calibration of the Mettler DSC 30 calorimeter was carried out by using benzoic acid as reference material. The calibration was checked by determining the molar heat capacity of urea in the same temperature range as that of measurements. The molar heat capacities of alkane-α,ω-diamides increased in function of temperature and fitted into linear equations. Smoothed values of C _p,m at 298.15 K displayed a linear increase with the number of carbon atoms. The C _p,m contribution of CH₂ group was (22.6±0.4) J K⁻¹ mol⁻¹, in agreement with our previous results concerning linear alkane-a,ω-diols and primary alkylamides as well as the literature data on various series of linear alkyl compounds. On leave from the Faculty of Chemistry, University of Craiova, Calea Bucureşti 165, Craiova 1100, Romania