Crystallinity, sulfur

Amorphous or "plastic" sulfur is obtained by fast cooling of the crystalline form. X-ray studies indicate that amorphous sulfur may have a helical structure with eight atoms per spiral. Crystalline sulfur seems to be made of rings, each containing eight sulfur atoms, which fit together to give a normal X-ray pattern. 

The presence of high-molecular weight p-sulfur with chain structure seemed improbable since the sulfur was not extractable with boiling toluene. The p-sulfur is known to convert to the soluble ring structure (Sg) rather rapidly at 115°. Wibaut (119) thought the formation of a carbon-sulfur complex similar to the surface oxide formed with oxygen very likely. He was not able, however, to analyze definite surface groups. Hofmann and Nobbe (123) established that the sulfur content was dependent on the specific surface area. Enoksson and Wetterholm (124) confirmed by X-ray diffraction that no crystalline sulfur was present in exhaustively extracted charcoal with 13% sulfur content. 

There are several types of crystalline sulfur (S). Sulfur is a major oxidizer component of black powder. Metals react with sulfur to form various types of sulfides such as FeS, FeS2, ZnS, CdS, and LijS. 

Generally, i.r. absorptions of cyclic molecules S are weak since these species are of low or no polarity (rings belonging to the molecular point groups Cj, C, C or C with n 2 may have a small dipole moment). On the other hand, Raman scattering from crystalline sulfur allotropes is very intense and leads to excellent spectra with low noise levels at good resolution conditions. However, since most compounds... 

Figure 7. Typical proportions of dissolved and crystallized sulfur in sulfur-extended asphalt blends (32). Key , dissolved sulfur and , crystalline sulfur.

Unlike covellite, the silver sulfide reaction with Hg(0) vapor is not exclusively a redox process. XRD analysis showed that no silver sulfide remained in samples having consumed 42 wt% Hg(0) and that cinnabar formation was minimal. The products formed are mercurial amalgam, HgAg, and Ag,HgS as reflected in Reaction (2). Reaction (2) does not account for two third of the initial sulfur present, however, no residual crystalline sulfur was observed in the diffraction data. None the less, this reaction reflects partial reduction of the silver(I) and oxidation of Hg(0). 

Bondi105 indicates that, for single-bonded sulfur, X-ray diffraction points to 1.83 A, whereas various physical properties suggest a radius of 1.80 A. The smaller values found for crystalline sulfur and polysulfides may be indicative of some double-bond character of the S-S bonds. 

When a pure substance is liquefied from the solid state or vaporized from the liquid at constant pressure, there is no change in temperature but there is a definite transfer of heat from the surroundings to the substance. These heat effects are commonly called the latent heat of fusion and the latent heat of vaporization. Similarly, there are heats of transition accompanying the change of a substance from one solid state to another for example, the heat absorbed when rhombic crystalline sulfur changes to the monoclinic structure at 95°C and 1 bar is 360 J for each gram-atom. 

Whereas voxygen gas consists of diatomic molecules O2, crystalline sulfur is Sg, a cyclic octomer. Such multiples are usually called oligomers (oligo = a few). The monomer in this — S " ... 

Figure 2 An example of molecular isomers (a), here the bridged and terminal forms of [Ir4(SCH2)3] [26] and a pair of supramolecular isomers (b), the orthorhombic a-Sg and the monoclinic y-Sg polymorphic modifications of crystalline sulfur [27].

The form of the dissolved sulfur has not been characterized properly yet. While stable at ambient temperatures, a substantial amount can be converted to crystalline sulfur at elevated temperatures or by solvent separation. This observation led to the development of a rapid liquid chromatography method to determine elemental sulfur in SA binders. The procedure which has been described previously by Cassidy (17) is based on gel permeation principle and uses a Styragel column and a uv detector. Results showed that 2-14% of the elemental sulfur added reacted chemically with the asphalt. Petrossi (18) and Lee (19), who determined free sulfur by extraction with sodium sulfite followed by titration with iodine, calculated a higher percent of bonded sulfur in sulfur-asphalt compositions. The observed differences are most likely caused by variations in the asphalt composition with regard to polar aromatics and naphthene components as well as by reaction temperature and contact time. 

While asphalt itself consists of a complex colloidal dispersion of resins and asphaltenes in oils, introduction of liquid elemental sulfur, which on cooling congeals into finely dispersed crystalline sulfur particles and in part reacts with the asphalt, necessarily complicates the rheology of such a SA binder. Differences and changes with SA binder preparation, curing time, temperature etc. must be expected and may be demonstrated by viscosity characteristics. 

The SA binder is tested for dispersion and particle size prior to mix production with a microscope. The binder level of the mix is constantly measured with a Troxler model 2226 asphalt content gauge. Hot solvent extraction (ASTM D2172) using tetrachloroethylene solvent can also be used to measure the binder content of a SA mix. The sulfur—asphalt ratio of the binder is monitored in the field with the Troxler or by density measurements. Other methods that can be used to measure SA ratios are x-ray fluorescence of solutions of sulfur-asphalt in tetrachloroethylene, liquid chromatography, and differential scanning calorimetry. X-ray fluorescence measures total sulfur, liquid chromatography determines elemental sulfur, and DSC monitors crystalline sulfur. 

A form of crystalline sulfur with rhombohedral symmetry can be made by extracting an acidified solution of sodium thiosulfate with chloroform and evaporating the chloroform solution. These crystals, which are orange in color, consist of molecules they are unstable, nd change into long chains and then into orthorhombic sulfur (Sg) in a few hours. 

Migdisov A. A., Williams-Jones A. E., Lakshtanov L. Z., and Alekhin Y. V. (2002) Estimates of the second dissociation constant of H2S from the surface sulfidation of crystalline sulfur. Geochim. Cosmochim. Acta 66, 1713-1725. 

Sulfur formation along the coast of eastern India near the village of Kona (Masulipatam, Madras) was described by lya and Sreenivasayi (1944,1945). Clays in certain coastal areas of the Bay of Bengal may be flooded during monsoons for several months at a time so that they become logged with sea water halophilic sulfate-reducers develop abundantly in the lower black clays. Diffusing sulfide ions are oxidised near the surface and produce colloidal and crystalline sulfur to a depth of some 20 cm. 

This is the second crystalline sulfur alkaloid of N. luteum (10,11, 24). Its similarity to neothiobinupharidine is suggested by analytical data (empirical formula C30H42O2N2S, and two methyl groups), and by UV-, IR-, NMR-, and mass spectra. However, the spectral data for this alkaloid (11) are more unequivocal and indicate in addition to a j8-substituted furan and the [Pg.463]

Eary (1992) prepared an amorphous sample of AS2S3 by precipitation of the sulfide from an aqueous solution of NaAs02, buffered at pH 4.0, to which was added an excess of Na2S-9H20. The precipitate was aged 1 to 3 days, and washed with water to remove the buffer. X-ray analysis showed no realgar, orpiment, crystalline sulfur or crystalline arsenic. Eary determined the solubility of this material at 298.15 K, 313.15 K, 333.15 K, and 363.15 K in solutions with the presence and absence of added sulfide. He gave equilibrium constants for the reaction ... 

Above) Yellow crystalline sulfur contains Sg rings. 

Fig. 7 Hexadecane-water partition test. Biologically produced sulfur remains in the lower water phase (1) whereas crystalline sulfur remains in the upper hexadecane-phase (2) [40]...

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