Thermodynamic assessment of phase diagram and concentration–temperature dependences of properties of solid solutions of the GaS–GaSe system

Journal of Thermal Analysis and Calorimetry - Before 100,000 years ago, during the Middle Stone Age (MSA) of South Africa, silica varieties of minerals and rocks were sometimes heated...     

Molar conductivities of concentrated lithium chloride–glycerol solutions at low and high temperatures: application of a quasi-lattice model

Conductivities of concentrated solutions of lithium chloride in glycerol were measured for concentrations ranging from 0.005 to 1.5 mol.dm^?3. The conductivity dependencies were analysed successively using the Debye–Huckel–Onsager limiting law (DHO) at very low concentrations, the Fuoss equation of 1978 up to 0.1 mol.dm^?3, the Casteel–Amis empirical equation and the quasi-lattice model (QLM) at moderate and higher concentrations. The molar conductivities at infinite dilution, obtained using DHO and QLM were quite different from each other, because the salt forms contact pairs which were underestimated in the Λ = f(C^1/3) in QLM, as it may well be proved by Raman spectroscopy. Besides, the value of Madelung constant suggests that LiCl crystallises face centred cubic (FCC) at higher concentrations. On the basis of Raman spectroscopy analysis of previous lithium salts, we assume that the dissociation coefficient varies slightly with concentration and fraction of paired ion constant, the QLM equation is applied successfully in the concentration range used in this study. The temperature dependency of conductivity was also described using the Vogel–Tamman–Fulcher (VTF) empirical equation where the Arrhenius type was found. The results also suggest that as NaCl, LiCl can be considered as a structure maker electrolyte.     

Argon–water system at low temperatures

The molecular dynamics method is used to simulate argon solutions in water and a thin water film–argon system at low temperatures. The correlation in motions of two closely spaced argon atoms is of another nature than the correlation of two neon atoms in a neon solid solution in ice II. The structure of hydrate shells of argon atoms contains five-membered rings composed of water molecules. The solubility of argon in a water film at low temperatures is noticeably higher than at room temperature. If a water film is first cooled to the glassy state and then argon atoms are added to it, then approximately as many argon atoms are absorbed on the film surface as they are present in a cooled film in equilibrium with the argon atmosphere. Argon atoms migrate from one pit to another on the rough surface of a solid water film.     

The heat capacity and thermodynamic functions of pyromorphite over the temperature range 5–320 K

The heat capacity of natural mineral, pyromorphite Pb₅(PO₄)₃Cl, was measured over the temperature range 4.2–320 K using low-temperature adiabatic calorimetry. An anomalous temperature dependence of heat capacity with a maximum at 273.24 K was observed between 250 and 290 K. The heat capacity, entropy, enthalpy, and reduced thermodynamic potential of pyromorphite were calculated and tabulated over the temperature range 5–320 K. The standard thermodynamic functions of the mineral are C _p298.15^o = 414.98 ± 0.44 J/(mol K), S _298.15^o = 585.31 ± 0.99 J/(mol K), H _298.15^o − H ₀^o = 80.90 ± 0.08 kJ/mol, and Φ_298.15^o = 313.97 ± 0.84 J/(mol K).     

Electron transfer reaction of meso-phenyl-tetrabenzo-porphyrinato-zinc and dicyanobenzene at low temperature

The behavior of photon-gated spectral hole burning with meso-phenyl-tetrabenzoporphyrinato-zinc as electron donor and dicyanobenzene as electron acceptor dispersed in polymethylmethacrylate was investigated.The data of absorption spectrum of the photoproduct acquired through photon-gated hole burning process by high power density and long hole burning time at 20 K were given The mechanism of photomduced donor-acceptor electron transfer for the iarget system in photon-gated spectral hole burning was demonstrated.     

Formation of solid solutions in the CdSe–PbSe system under the action of high pressures and temperatures

A method was proposed for producing solid solutions in the CdSe–PbSe systems, which is based on heat and high pressure treatment. X-ray powder diffraction analysis showed the formation of substitutional solid solutions Cd_xPb_1–xSe with the NaCl structure, which contained 20, 40, 60, and 80 mol % cadmium selenide. The solid solutions were characterized by scanning electron microscopy, impedance spectroscopy, gas pycnometry, and Raman spectroscopy.     

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.     

Low-temperature heat capacity of β-glycine and a phase transition at 252 K

Low-temperature heat capacity of unstable -glycine was measured in a temperature range 5.5 to 295 K, and thermodynamic functions were calculated. At very low temperatures, heat capacity fits a sum of cubic (Debye) and linear terms: C_p=aT+bT ³. The linear contribution increases with temperature and disappears at the second-order phase transition near 252 K which was observed for the first time.     

Low temperature heat capacity of bulk and nanophase ZnO and Zn1−xCoxO wurtzite phases

The low temperature heat capacity of the ZnO–CoO solid solution system was measured from 2 to 300 K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). The thermodynamic functions in this temperature range were derived by curve fitting. The standard entropies of bulk ZnO and bulk ZnO–CoO (wurtzite, 18 mol% CoO) at T = 298.15 K were calculated to be (43.1 ± 0.4) J · mol⁻¹ · K⁻¹ and (45.2 ± 0.5) J · mol⁻¹ · K⁻¹, respectively. The surface entropy of ZnO was evaluated to be (0.02 ± 0.01) mJ · K⁻¹ · m⁻², which is essentially zero. No sharp magnetic transitions were observed in the solid solution samples. The nanophase solid solution, 12 mol% CoO, appears to bind H₂O on its surface more strongly than ZnO.     

Towards an accurate and precise determination of the solid–solid transition temperature of enantiotropic systems

This paper presents two distinct methods for the determination of the solid–solid transition temperature (T_tr) separating the temperature ranges of stability of two crystallographic forms, hereafter called morphs, of a same substance. The first method, based on thermodynamic calculations, consists in determining T_tr as the temperature at which the Gibbs free energies of the two morphs are equal to each other. For this purpose, some thermodynamic characteristics of both morphs are required, such as the specific heat capacities, the melting temperatures and the melting enthalpies. These are obtained using the Differential Scanning Calorimetry (DSC). In the second method, T_tr is determined directly by an experimental study of the temperature ranges of stability of each morph. The three main originalities of the method developed are (i) to prepare samples composed by an isomassic mixture of crystals of both morphs, (ii) to set them in a thermostated and agitated suspension, and (iii) to use an in situ Raman spectroscopic probe for the determination of the crystallographic form of the crystals in suspension at equilibrium. Both methods are applied to determine the solid–solid transition temperature of the enantiotropic system of Etiracetam, and both of its two crystallographic forms so far identified, named morph I and morph II. The first method is shown to be very sensitive to the experimental data obtained by DSC while the second experimental method is a more accurate, precise, time- and effort-friendly method for the determination of T_tr. The solid–solid transition temperature of the Etiracetam system, determined with the second method, using three different solvents, is found to be equal to 303.65 K ± 0.5 K.     

Kinetics of direct and water-mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well-known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H-O=C-N- $\rightleftharpoons$ O=C-N-H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5-aza-uracil (1,3,5-triazine-2,4(1H,3H)-dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions.     

π-dimerization of pleiadiene radical cations at low temperatures revealed by UV–vis spectroelectrochemistry and quantum theory

One-electron oxidation of the non-alternant polycyclic aromatic hydrocarbon pleiadiene and related cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene in THF produces corresponding radical cations detectable in the temperature range of 293–263 K only on the subsecond time scale of cyclic voltammetry. Although the EPR-active red-coloured pleiadiene radical cation is stable according to the literature in concentrated sulfuric acid, spectroelectrochemical measurements reported in this study provide convincing evidence for its facile conversion into the green-coloured, formally closed shell and, hence, EPR-silent π-bound dimer dication stable in THF at 253 K. The unexpected formation of the thermally unstable dimeric product featuring a characteristic intense low-energy absorption band at 673 nm (1.84 eV; logε _max = 4.0) is substantiated by ab initio calculations on the parent pleiadiene molecule and the PF₆⁻ salts of the corresponding radical cation and dimer dication. The latter is stabilized with respect to the radical cation by 14.40 kcal mol⁻¹ (DFT B3LYP) [37.64 kcal mol⁻¹ (CASPT2/DFT B3LYP)]. An excellent match has been obtained between the experimental and TD-DFT-calculated UV–vis spectra of the PF₆⁻ salt of the pleiadiene dimer dication, considering solvent (THF) effects.     

Self-association and reverse hemimicelle formation at solid–water interfaces in dilute surfactant solutions

Using monodisperse anatase and hematite particles, such classical techniques as colloid stability, ζ potential and wettability measurements along with fluorescence spectroscopy show that anionic alkyl sulfate and sulfonate surfactants first associate into hemimicelles at the interface and cause the particles to begin to coagulate and to become hydrophobic. Maximum coagulation and hydrophobicity occur under conditions where the ζ potential is zero. At higher surfactant concentrations, the particles redisperse and regain their hydrophilicity. Upon reversal of the ζ potential, the adsorbed surfactant ions appear to form hemimicelles with reverse orientation or bilayers, at surfactant concentrations significantly lower than the critical micelle concentration. The results of these experiments correlate well with spectroscopic and adsorption measurements.     

Preparation of titanium carbide powders by sol–gel and microwave carbothermal reduction methods at low temperature

Titanium carbide ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction. The influences of reaction temperature and molar ratio of Ti to C on the synthesis of titanium carbide were studied. The results show that excess amount of carbon plays a positive effect on the carbothermal reduction of TiO₂ at low temperature. The inceptive carbothermal reduction temperature of TiO₂ and formation of titanium oxycarbide was below 900 °C, and pure TiC can be prepared at 1,200 °C, which was considerably lower compared to that by conventional carbothermal reduction using a mixture of TiO₂ and carbon powders as raw materials. The morphology and particle size of synthesized TiC powder were examined by field emission-scanning electron microscopy (FE-SEM) and the quantities of the phases of the powders were analyzed by Rietveld refinement method, the particle sizes of the TiC powders synthesized at 1,300 °C distribute over 0.1–0.5 μm.     

Apparent molar volumes and apparent molar heat capacities of aqueous solutions ofα- andβ-cyclodextrins at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Apparent molar heat capacities C_{p, φ}and apparent molar volumesV_φ were determined for aqueous solutions of α - and β -cyclodextrins at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa. The molalities investigated ranged from 0.008 mol · kg ^− 1to 0.12 mol · kg ^− 1forα -cyclodextrin and from 0.004 mol · kg ^− 1to 0.014 mol · kg ^− 1for β -cyclodextrin. We used a vibrating-tube densimeter (DMA 512P, Anton PAAR, Austria) to determine the densities and volumetric properties. Heat capacities were obtained using a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, USA). Equations were fit by regression to our experimental (V_φ, T, m) and (C_{p, φ},T , m) results. Infinite dilution partial molar volumes V₂^oand heat capacities C_p,2^owere obtained over the range of temperatures by extrapolation of these surfaces to m =  0.