Clathrate formation and phase equilibria in the thiourea-bromoform system

Phase equilibria in the thiourea (host)-bromoform (guest) binary system were studied by physicochemical analysis methods over the temperature range 270–455 K. The stoichiometry and stability region were determined for the channel-type compound CHBr₃ · 2.40(2)(NH₂)₂CS; the compound was observed for the first time. When heated, the clathrate incongruently decomposed at 424.0 ± 0.8 K to rhombic thiourea and the guest component. The solubility isotherm of the thiourea-bromoform-acetic acid system was studied to find that the compound was thermodynamically stable at 293 K over the range of guest component concentrations 100–35 wt %. A decrease in its content in an equilibrium mother liquor resulted in the appearance of X-ray diffraction reflections of the initial host α polymorph. Rhombohedral cell parameters were determined (space group R-3c, a = 15.89(1) Å, c = 12.40(1) Å, V = 2711(6) ų, d _calcd = 2.000 g/cm³, and d _expt = 1.98(2) g/cm³). The mode of packing of bromoform molecules was compared with the organization of the guest subsystem in inclusion compounds formed by the substances studied.     

Some features of phase formation in the system magnetite-potassium carbonate

The solid-phase interaction in the system Fe₃O₄-K₂CO₃ was studied.     

Clathrate formation and phase equilibria in the pyridine-cadmium nitrate system

Preparative, thermal (DTA, TGA), solubility, strain and spectral (Raman) techniques were used to study clathrate and complex formation in the pyridine (Py)-cadmium nitrate system. Three compounds have been isolated and studied: the clathrate compound [CdPy₄(NO₃)₂] · 2Py (I), the complex [CdPy₃(NO₃)₂] (II) and a compound of composition Cd(NO₃)₂·7/4Py (III), of unknown nature. The phase diagram of the system has been determined for the concentration and temperature range 0–66 mass-% Cd(NO₃)₂ and –100 to +200 °C, respectively. ClathrateI undergoes polymorphous conversion at –51.8(4) °C and melts incongruently at 106.0(5) °C, forming complexII. CompoundsII andIII melt congruently at 165.5(4) and 191(1) °C, respectively. The complexes [CdPy₄(NO₃)₂] (the host phase) and [CdPy₂(NO₃)₂] are not observed in the system. The nature and thermodynamic parameters of the dissociation of clathrate I have been determined. For the process 1/13[CdPy₄)NO₃)₂] · 2Py_solid = 1/3[CdPy₃(NO₃)₂]_solid + Py_gas in the range 290–360K H ^o = 54.9(3) kj/mole, S ₂₉₈ ^o = 142(1) J/(mole K), G ₂₉₈ ^o = 12.5(5) kJ/mole.     

Effect of oxygen pressure on phase equilibria in the Eu-Mn-O system

Phase equilibria in the Eu-Mn-O system are studied using the static method in combination with X-ray phase analysis. Compound EuMnO₃ was studied by means of DSC and high-temperature roentgenography. The transformation of the orbitally ordered O′ phase to the orbitally disordered O phase was confirmed for the EuMnO₃ compound. Thermodynamic characteristics of the formation of EuMn₂O₅ and EuMnO₃ compound from their elements were calculated.     

Effect of PEG additive on membrane formation by phase inversion

This study investigates the effect of PEG additive as a pore-former on the structure formation of membranes and their permeation properties connected with the changes of thermodynamic and kinetic properties in phase inversion process. The membranes were prepared by using polysulfone (PSf)/N-methyl-2-pyrrolidone (NMP)/poly(ethylene glycol) (PEG) casting solution and water coagulant. The resulting membranes prepared by changing the molecular weight of PEG additive and the ratio of PEG to NMP were characterized by scanning electron microscope observations, measurements of water flux and PEG rejection. The thermodynamic and kinetic properties of membrane-forming system were studied through coagulation value, light transmittance and viscosity. The correlations between the final membrane structure/permeation properties and thermodynamic/kinetic properties of membrane forming system are discussed extensively.     

Effect of temperature on the surface phase behavior and micelle formation of a mixed system of nonionic/anionic surfactants

The adsorption and micellar behavior of diethylene glycol mono-n-tetradecyl ether (C14E2), sodium 3,6,9,12-tetraoxaoctacosanoate (TOOCNa), and their mixture at a 1:1 molar ratio have been studied by film balance, Brewster angle microscopy (BAM), and surface tensiometry at different temperatures. The monolayers of pure C14E2 and its mixture with TOOCNa show a first-order phase transition with a conspicuous cusp point in their respective adsorption isotherms. This is further confirmed by the observation of bright two-dimensional condensed phase domains visualized by BAM just after the appearance of the phase transition. It is interesting to note here that for C14E2, condensed domains are observed up to 19 degrees C, while in the mixed system, they are observed up to 22 degrees C. To understand why in the mixed system the domains are observed at higher temperatures than for pure C14E2, we have measured the temperature dependency of the equilibrium surface tension at > or = cmc (gammacmc) values of both the pure and the mixed systems. The gammacmc values of pure C14E2 remain almost constant, while those of pure TOOCNa and its mixture with C14E2 decrease appreciably with increasing temperature. It is concluded that higher degree of dehydration of the ethylene oxide (EO) chain reduces the head-group size of TOOCNa, which outweighs the combined effect of the repulsive interactions between the head-groups and the thermal motion of the adsorbed molecules. Furthermore, C14E2 being inserted into the TOOCNa monolayer reduces the electrostatic repulsions between the charged heads, and consequently, the adsorbed monolayers attain closer molecular packing. As a result, the gammacmc values of both pure TOOCNa and its mixture with C14E2 decrease with increasing temperature. This facilitates the formation of condensed domains in the mixed system at higher temperatures, whereas none of the individual members can show any indicative feature of phase transition under the same experimental conditions.     

Effect of shape of mesogenic group on the formation of liquid crystalline phase in polymers

Mesomorphic behaviour of four polymethacrylates with mesogenic side groups has been studied. In contrast to monomers which are nematogenic, mesogenic groups in the polymers form layer structures of the smectic type. A thermodynamic stable mesophase is supposed to be realized in polymers which consist of macromolecules of secondary structure, considering a macromolecule as the smallest structural element of the polymer system.     

The formation and photolysis of tantalum sulfide cluster ions

Tantalum sulfide cluster ions were produced by direct laser ablation, and were studied with a tandem time-of-flight mass spectrometer. The main dissociation channel of the UV-photolysis (248 nm) of tantalum sulfide cluster ions is sequent S₂ loss. Structures with Ta₃S₄ and Ta₄S₆ as frameworks were suggested for the large tantalum sulfide cluster ions.The work was supported by the National Natural Science Fouldation of China.     

Effect of water on the formation of oligosulfide radical-anions in the sulfur-sodium hydroxide-hexamethylphosphorotriamide system

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, p. 222, January, 1990.     

Complex formation of polymeric niobium and tantalum pentafluorides in non-aqueous solvents

¹⁹F NMR and i.r. spectroscopy were employed to study complex formation of niobium and tantalum pentafluorides with triphenylphosphine oxide, ethyl diphenylphosphinate and S-ethyl diphenylthiophosphinate in methylene chloride and in acetonitrile. In methylene chloride solution MF₅L, M₂F_IOL, MF₄L₂⁺ and M₂F_II^? complexes were found (M=Nb, Ta;L- phosphoryl-containing Ligand). I.r. frequencies are assigned. Solvent properties (electric conductivity and donor capacity) affect the relative stability of complexes. The dependence of ¹⁹F NMR parameters on the donor capacity of the ligands is discussed.     

Thermodynamic study of the liquid Fe-S system by use of a drop calorimeter

Heat contents of the Fe-S system were measured with a drop calorimeter in the sulfur composition range X_s = 0.380–0.500 (FeS) and in the temperature range 942–1506 K to construct an (H_T - H_298.15)-temperature-composition ternary diagram. By use of a thermodynamic analysis method, the mixing free energy, enthalpy and entropy of the liquid Fe-S mixtures were determined at 1473 and 1523 K, based on the measured heat contents. The partial molar free energies (activity of iron and partial pressure of diatomic sulfur) agreed well with the literature values, suggesting the applicability of the thermodynamic analysis method for liquid mixtures with semi-metals or chalcogen.     

Effect of citrate to nitrate ratio on the decomposition characteristics and phase formation of alumina

This paper reports the effect of the variation of citrate to nitrate ratio on the thermal decomposition characteristics of alumina precursor and the properties of nanocrystalline alumina synthesized using this auto-ignition process. The technique involves the auto-ignition of a citrate-nitrate gel occurring between Al(NO₃)₃ and citric acid to yield an ash powder that upon calcination at 1373 K produced α-alumina. The auto-ignition was restricted to a particular range of citrate to nitrate ratio in the gel. The resulting powder exhibited large surface area (40–50 m² gm^?1) and fine crystallite size. It was established from various characterization techniques that the alumina powder prepared with a C/N ratio of 0.3 has got the optimum powder characteristics compared to the rest of the batches, thus establishing the importance of maintaining a stoichiometric or near stoichiometric C/N ratio. The process has a higher degree of reproducibility and a good potential for large-scale production of alumina.     

Ab initio Studies on the Fe-S Protein Model Complexes

In this paper, ab initio calculations of the iron-sulfur protein model complexes have been completed on Fe_2S_2(SH)_2~(n-) and Fe_4S_4(SH)_4~(n-)(n=2, 3). The results indicate that the occupied terminal sulfur characteristic orbitals are found in the front orbital site and the energy levels of the occupied Fe 3d-like orbitals appear internally below the S-H bonding orbitals in the valence band. Although the energy sequence is different from what was reported in literature, our results are in agreement with the relevant experimental facts. We have discussed the reason that variations of the active sites are produced by various oxidation levels. The action mechanism of the Fe-S proteins as electron carriers in the biological processes is also explored preliminarily.     

Features of the phase formation in the nanocomposites

Thermodynamic conditions of formation of critical nuclei in the composites that are the phase forming medium, a matrix with inclusions of nanoparticles of the other phase, are considered. It is shown that depending on the nature of the spatial distribution of nanoparticles in the matrix, critical nuclei of different shapes may appear at the phase formation. In some cases the formation and stable existence of metastable phases, including amorphous ones is possible. The conditions of formation in the nanocomposites of the critical nuclei of heterogeneous structure are determined. The experimental data on the phase formation in the nanocomposites ZrO₂-Al(OH)₃ and ZrO₂-SiO₂ were analyzed and the earlier described feature of formation of the new phase nuclei is confirmed theoretically.     

Effect of yttrium oxide on the formation of the phase composition and porous structure of titanium dioxide

The formation of the structure of TiO₂ (anatase) doped with 1–5 mol % Y₂O₃ is reported. The dopant changes the anatase structure from regular to nanocrystalline. The nanocruystalline structure consists of incoherently intergrown 5- to 7-nm anatase crystallites (500°C) separated by interblock boundaries accommodating yttrium ions. The formation of the nanocrystalline anatase structure stabilizes small anatase crystallites and raises the anatase-to-rutile phase transition temperature above 900°C. Owing to this structure, the developed specific surface area and fine porous texture of yttrium oxide-doped titanium dioxide survive up to higher temperatures than those of undoped titanium dioxide.     

Features of phase formation and state of iron in the quasibinary system FeTe1.65-TiTe1.65

A series of tellurides Fe_xTi_1?x Te_1.65 (x = 0, 0.1, 0.2 …, 1.0) synthesized at 850°C were studied by X-ray phase and X-ray fluorescent analysis and Møssbauer spectroscopy on ⁵⁷Fe. In this series two solid solutions are formed: phases I and II with homogeneity regions within the limits of 0 ≤ x ≤ 0.3 and 0.9 ≤ x ≤ 1.0, respectively. Iron in this series exists in two various states: Fe²⁺ _sym and Fe²⁺ _asym as differentiated from the series of tellurides Fe_xTi_1?x Te_1.45 where three states of iron Fe²⁺ _sym, Fe²⁺ _asym, and Fe⁰ were found. On passing from Fe_xTi_1?x Te_1.45 to Fe_xTi_1?x Te_1.65 the number of formed phases decreases, and phase relations become simpler. The absence of Fe⁰ from phase I of tellurides Fe_xTi_1?x Te_1.65 can point to the fact that TiTe_1.45 and TiTe_1.65 belong to different homogeneity regions.