Search Results
Abstract
Synthesis and characterization of N,N,N',N'-tetrabutylsuccinamide (TBSA)was carried out and used for the extraction of U(VI) and Th(IV) from nitricacid solutions. Toluene was found to be the most suitable diluent for TBSAcompared with the other diluents tested. Extraction distribution ratios (D)of U(VI) and Th(IV) have been studied as a function of aqueous HNO 3 concentrations,NO3 – ion concentration, TBSA concentration and temperature.The results obtained indicated that U(VI) and Th(IV) are mainly extractedas UO2 (NO3 ) 2 . 2TBSA and Th(NO3 ) 4 . TBSA, respectively, and the IR spectra of the extractedspecies have been investigated. The values of thermodynamic functions havebeen calculated. Back-extraction of U(VI) and Th(IV) from organic phases wasalso studied.
Abstract
The batch kinetics of Fe(III) adsorption on HTTA-loaded polyurethane (PU) foam have been investigated. The rate of controlling the adsorption is found to be intraparticle diffusion. The reaction rate of adsorption and desorption was also evaluated and found to increase and decrease with temperature, respectively. This indicates an endothermic adsorption behavior of Fe(III) on HTTA loaded PU foam. The activation energy of adsorption (80±10 kJ·mol–1) and of desorption (–45±±2 kJ· mol–1) indicates the chemical adsorption rather than physical adsorption. The isosteric heat of adsorption (
H
ads) was found to be –82.7±5.05 kJ·mol–1. This shows the formation of new chemical bonds among Fe(III)-HTTA-PU foam. The thermodynamic parameters of
G,
H and
S, and equilibrium constantK
c have been calculated. These functions further support that the process of adsorption of Fe(III) on HTTA-loaded PU foam is endothermic and chemisorption, stabilized through thermodynamic functions.
Abstract
Heat capacity measurements between 293 K and 363 K have been carried out in order to elucidate the different states appearing in 2-amino-2-methyl-1,3 propanediol (AMP) plastic crystal. The results allowed one of them to be identified as a glassy crystal. The changes of enthalpy, entropy and Gibbs free energy thermodynamic functions with temperature have been calculated from the experimental heat capacity values.
Heat capacity of α-glycylglycine in a temperature range of 6 to 440 K
Comparison with glycines
Abstract
Heat capacity of α-glycylglycine was measured using adiabatic calorimetry (6 to 304 K) and DSC (264 to 443 K), and then thermodynamic functions were calculated. Heat capacity has no anomalies. The molecular crystal melts at 493 K (enthalpy of melting is about 62 kJ mol–1). The melting is accompanied by decomposition. C P(T) function for glycylglycine is very similar to those of three glycine polymorphs. The ‘universal’ curve consists of two parts: low-temperature Debye-like function (from 0 to about 120 K) and a straight line (up to the melting point). At very low temperatures rigid molecules oscillate as a whole, and the Debye temperature is proportional to the molecular mass to the power of 3/2.
Abstract
Equilibrium adiabatic heat-capacity measurements have been made on zone refined samples of CeB6 and PrB6. Companion measurements made on LaB6, NdB6, and GdB6 have been reported elsewhere. These show cooperative lambda-type anomalies associated with antiferro-magnetic ordering. Except for lanthanum hexaboride, Schottky internal crystal field levels result in significant contributions to the thermodynamic functions. The gross thermodynamic properties at 298.15 K heat capacity (Cp/R), entropy increment (ΔT 0,m S 0/R), and Gibbs energy function are correlated with the nature of the lanthanide. For LaB6, CeB6, PrB6, NdB6, and GdB6 the three properties are, respectively: {11.654, 12.014, 11.997, 11.916, 11.695} Cp/R; {10.001, 11.803, 12.430, 12.558, 13.982} S0/R, and finally {4.379, 5.912, 6.232, 6.451, 7.905}Φ0 m/R.
Abstract
Isoproturon [N'-(p-cumenyl)-N,N-dimethylurea] was synthesized, and the low-temperature heat capacities were measured with a small sample precise automatic adiabatic calorimeter over the temperature range from 78 to 342 K. No thermal anomaly or phase transition was observed in this temperature range. The melting and thermal decomposition behavior of isoproturon was investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The melting point and decomposition temperature of isoproturon were determined to be 152.4 and 239.0C. The molar melting enthalpy, and entropy of isoproturon, ΔH m and ΔS m, were determined to be 21.33 and 50.13 J K-1 mol-1, respectively. The fundamental thermodynamic functions of isoproturon relative to standard reference temperature, 298.15 K, were derived from the heat capacity data.
Abstract
The thermodynamic properties of carbosilane dendrimer of second generation with terminal methoxyundecylene groups were studied between 6 and 340 K by adiabatic vacuum calorimetry: the temperature dependence of the molar heat capacity Cp 0 was measured, the physical transformations were established and their thermodynamic characteristics were obtained. The experimental data were used to calculate the thermodynamic functions Cp 0 (T), H 0(T)-H 0(0), S0(T), G 0(T)-H 0(0) of the compound in the range 0 to 340 K. from the relation Cp 0 (T) the fractal dimension of the dendrimer was experimentally determined. The heat capacity of the dendrimer was compared with the corresponding additive values calculated from the properties of its constituents - a dendritic matrix (carbosilane dendrimer of second generation) and the corresponding amount of moles of methyl ester of 11-(tetramethyldisiloxy)undecanoic acid serving as terminal groups.
Abstract
Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs.
Successive phase transitions of p-methylbenzyl alcohol crystal studied by X-ray and adiabatic calorimetry
Structural disorder in room-temperature phase
Summary Crystal structures of the room-temperature (RT) and low-temperature (LT) phases of p-methylbenzyl alcohol were reexamined by single-crystal X-ray diffraction method while paying special attention to detect structural disorder in the RT phase involved in successive structural phase transitions at 179 and 210 K. In the RT phase at 250 K, positional disorder of oxygen atoms was detected in contrast to the previous structure report. The structure of the LT phase coincided to the previous one. Heat capacities were measured by adiabatic calorimetry below 350 K, which covers the structural phase transitions and fusion at 331.87 K. The structural phase transitions were of first-order and required long time for completion. The combined magnitude of entropies of transition was ca. 5 J K-1 mol-1, a part of which can be ascribed to the positional disorder observed in the structure analysis. Standard thermodynamic functions are tabulated below 350 K.
Abstract
Phases may be smaller than visible to the human eye. In order to characterize a microphase, a phase smaller than 1 μm, one must consider surface area and free energy in addition to the standard thermodynamic functions. As one approaches nanometer sizes, one also needs to know the changing thermodynamic functions within the phases. The Gibbs–Thomson equation can be used to characterize microphases, but not nanophases. For the latter, the glass transition is needed to assess the properties in the interior. In order to classify condensed phases as liquid, solid, mesophase, or crystal, one needs to consider the molecular motion in addition to the molecular structure. Most important are large-amplitude displacements in form of translation, rotation, and conformational motion. An operational definition based on experiments and an updated classification of the phases is given. The surprising result is the observation that crystals, earlier assumed prime examples of solids, can have order–disorder transitions to more mobile mesophases, as well as a glass transition without change in crystal structure, i.e., under certain condition, they cannot be identified as a solid. To these observations, one has to add the fact that large-amplitude motion may start gradually to a more mobile phase without abrupt changes in structure. These observations limit the usefulness of the 80-year-old classification of transitions as being of first or second order. Quantitative thermal analysis is shown to be an important tool to identify the possible total of 57 different condensed states in terms of their macroscopic properties as well as molecular structure and motion.
