Search results for "Anhydrous"
showing 10 items of 48 documents
Structural properties of magnesium stearate pseudopolymorphs: effect of temperature.
2003
A thorough review of the relevant literature reveals that the interaction between water vapour and magnesium stearate, in contrast to many other metal soaps, is not properly understood. The structural modifications associated with the up-take or loss of water of vegetable-derived commercial magnesium stearate powders exposed to humid air or vacuum at room temperature are investigated using standard powder X-ray diffractometry. It is found that in such conditions magnesium stearate reacts reversibly with the vapour phase with structural consequences very similar to the high temperature transition between the crystalline and rotator phases of other anhydrous metal soaps. When temperature is i…
Semiempirical correlation between the optical band gap of oxides and hydroxides and the electronegativity of their constituents
2000
AbstractOn the basis of new experimental results a previous proposed correlation between the optical band gap of oxides and the difference of electronegativity of their constituents is extended to mixed crystalline and amorphous TiO2-Fe2O3 (d,d-metal oxides) as well as to amorphous passive films grown on Al-Ta, Al-Ti, Al-Nb and Al-W alloys (sp,d-metal oxides). Moreover in analogy with previous results on anhydrous oxides, a correlation is proposed between the optical band gap of hydroxides and the electronegativities of their constituents after substituting the electronegativity of hydroxilic group to that of the oxygen. Like in the case of anhydrous oxides, two different interpolation line…
Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resulting…
2001
Abstract Endothermic decomposition reactions (solid 1→solid 2+gas) generally affect only part of the inter-atomic bonds of the solid 1 structure. In consequence, the morphology and external dimensions of particles remain unchanged (the particles of solid 2 are referred to as pseudomorphs of those of solid 1). The gas release normally leads to a decrease of the molar volume of the precursor solid, resulting in cracking and formation of intra-particle porosity. This work deals with such a reaction: the dehydration of gypsum into sub-hydrated and anhydrous phases. Ultrathin (010) gypsum cleaved plates have been dehydrated under controlled conditions and studied by electron microdiffraction. Al…
Structural, Thermal, and Magnetic Study of Solvation Processes in Spin-Crossover [Fe(bpp)2][Cr(L)(ox)2]2·nH2O Complexes
2007
The influence of lattice water in the magnetic properties of spin-crossover [Fe(bpp)2]X2.nH2O salts [bpp = 2,6-bis(pyrazol-3-yl)pyridine] is well-documented. In most cases, it stabilizes the low-spin state compared to the anhydrous compound. In other cases, it is rather the contrary. Unraveling this mystery implies the study of the microscopic changes that accompany the loss of water. This might be difficult from an experimental point of view. Our strategy is to focus on some salts that undergo a nonreversible dehydration-hydration process without loss of crystallinity. By comparison of the structural and magnetic properties of original and rehydrated samples, several rules concerning the r…
A Simple “Plastic Bag” Technique for Visible Spectra of Solid Metal Complexes
1979
The techniques commonly used to study the visible spectra of transition metal complexes in the solid state involve diffuse reflectance from the powdered sample or transmission through a mull of the compound in mineral oil on filter paper. Pressed KBr (for anhydrous compounds) and AgCl (for compounds containing water or other highly polar ligands) disk techniques have been used even for low temperature spectra. All of the transmission techniques mentioned are based on mixing the sample with another compound which may induce changes in the compound under study via chemical reactions. Air- and water-sensitive complexes also need special care. These interferences may be reduced by the use of th…
Retrograde melt-residue interaction and the formation of near-anhydrous leucosomes in migmatites
2010
Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P–T conditions change, gradients in chemical potential are established between the melt-rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H2O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with mel…
Structural, Thermoanalytical and Molecular Modeling Studies on N-(3-hydroxypropyl) 3a,12a-Dihydroxy-5b-cholan-24-amide and Its Monohydrates
2007
The synthetic method for preparing N-(3-hydroxypropyl) 3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-amide can lead to formation of at least three different crystal forms - an anhydrous compound and two monohydrates. The structural and thermal properties of these forms have been characterized by 13C-CP/MAS-NMR and IR spectroscopy, thermo- gravimetry, differential scanning calorimetry and by powder and single crystal x-ray crystallography. In addition, theoretical 13C-NMR chemical shift calculations were also performed for the anhydrous compound and for the first monohydrate, starting from single crystal structures and the structures of these species have now been verified. The first monohydra…
Dehydration of mildronate dihydrate: a study of structural transformations and kinetics
2014
The dehydration of mildronate dihydrate (3-(1,1,1-trimethylhydrazin-1-ium-2-yl)propionate dihydrate) was investigated by powder X-ray diffraction, thermal analysis, hot-stage microscopy, water sorption–desorption studies and dehydration kinetic studies. It was determined that mildronate dihydrate dehydrated in a single step, directly transforming into the anhydrous form. In order to understand the reasons for a one step dehydration mechanism, crystal structures of dihydrate, monohydrate and anhydrous forms were compared, proving the similarity of the dihydrate and anhydrous forms. In order to understand the reasons for molecule reorganization during dehydration, the energy of the anhydrous …
Recent advances in photocurrent spectroscopy of passive films
2003
Abstract The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-me…
Microwave-assisted synthesis of anhydrous CdS nanoparticles in a water-oil microemulsion.
2006
Abstract Microwave irradiation at a frequency of 2.45 GHz and a power ranging between 22 and 30 W was used, in a water–oil microemulsion at 35 ± 2 ° C , to obtain stable, small, crystalline, anhydrous CdS nanoparticles exhibiting enhanced luminescence properties. The process of nanoparticles growth at different irradiation times was followed by UV–vis spectroscopy. It was observed that irradiated nanoparticles grew faster and their size reached a constant value. The final mean nanoparticle diameter was 2.7 nm, smaller than that observed in a non-irradiated sample, in which particle dimensions slowly increased even after 10 h. This finding was confirmed by high resolution transmission electr…