Polyaromatic layers

Disordered carbons usually exhibit a multiscale organization (structure, microtexture, texture)4. Structurally, they are made of more or less distorted polyaromatic layers, nanometric in size. The spatial association or the layers, from the nanometric to the micrometric scales, gives rise to different microtextures (lamellar, porous, concentric, fibrous, etc.) forming the carbons skeleton4. The multiscale organization is the fingerprint of the kind of precursor and of the formation conditions (temperature, pressure, strains, time, etc.) met either in laboratory experiments or in Nature, and is directly related with numerous properties. 

Usually, at least several hundreds of fringes are measured and characterized. Taking the classical example of X-ray diffraction, coherent domains are defined by the stacks of polyaromatic layers (Figure 2). The coherent domains are distinguished from the single layers and their relative... 

The basicity of activated carbon has been discussed extensively by Montes-Moran and co-workers [90]. Using experimental and theoretical resnlts, they snggest that oxygen functional gronps, and the existence of pyrone-type structnres on the edges of the polyaromatic layers, are the main contributors to overall surface basicity. On the other hand, the basicity of the k-electron density of the basal planes is considered to be weak. 

On the other hand, the maximum area covered by coke on the external surfaces of the zeolite crystals may be estimated from the amount of accumulated carbon (see Table 1), the diameters of aromatic rings in polyaromatic layers (5.2 A), and an average number of three carbon atoms per aromatic ring. Using these assumptions, we obtain the results in Table 2. 

Thus, comparing the value of the external surface area of the ZSM-5 erystals derived from IRS data with those obtained from a model of coking by flat, polyaromatic layers, it is possible to assert that monolayer coverage of the external surfaces of the ZSM-5 crystals by coke is not achieved up to a coke content of 4.2 wt%, even under conditions of condensed coke formation. 

In the majority of polymer systems designed for an SiC synthesis, the atom ratio C/Si is significantly >1. Therefore, in the crystallization model described by Monthioux and Delverdier [147] the excess carbon is considered to play a very important role. By means of HRTEM it was observed that more or less in all the systems investigated the formation of the so-called Basic Structure Units (BSU) of the free carbon - i.e. the formation of small stacks of only a few polyaromatic layers - seems to be the first step for the following nucleation mechanism of SiC. Such BSUs (graphenes) have lateral extensions in the order of 1 nm, are laterally saturated by hydrogen atoms and piled up in a... 

In fig. 5 the X-ray diffraction dagram of glasslike carbon is compared with that of a single crystal of natural graphite. The prismatic interferences 100 and 110 and the pyramidic ones 101 and 112 appear very weakly as so-called 2-dimensional reflexes 10 and 11 only. Also very low stacking heights of the polyaromatic layers can be seen from the line broadening of the 002 reflexes. 

Preferred orientation of the polymer carbon can be achieved by preorientation of the chain molecules in the polymer precursor fibre. Polyacrylonitrile has been proved as the most suitable and economic precursor. This thermoplastic polymer, however, needs a cross-linkage treatment before carbonization to preserve the fibre morphology. Simultaneously, cyclization of the side chains and dehydrogenation occur resulting in nuclei for the formation of the planar polyaromatic layers... 

Above description >/amorphous carbon is not applicable to carbon materials with two-dimensional structural elements present in all pyrolysis residues of carbon compounds as polyaromatic layers with a nearly ideal inter-atomic distance of a = 142 pm and an extension >1000pm. 

Electrophoretic and isotachoelectrophoretic techniques are gaining in popularity in soil analysis with applications to polyaromatic hydrocarbons, polychlorobiphenyls, tetrahydrothiophene and triazine herbicides, Paraquat and Diquat and growth regulators. Other lesser-used techniques include spectrophotometric methods (five determinants), spectrofluorimetric methods (two determinants), luminescence methods (one determinant), titration methods (one determinant), thin-layer chromatography (five applications), NHR spectroscopy (two applications) and enzymic immunoassays (one determinant). 

Fowlie and Bulman [43] have carried out a detailed study of the extraction of anthracene and benzo[tf]pyrene from soil. They carried out a replicated [24] factorial experiment using Soxhlet extraction and Polytron techniques. Soxhlet extraction followed by thin layer chromatography gave higher recoveries of the two polyaromatic hydrocarbons. 

Dunn and Stich [78] and Dunn [79] have described a monitoring procedure for polyaromatic hydrocarbons, particularly benzo[a]pyrene in marine sediments. The procedures involve extraction and purification of hydrocarbon fractions from the sediments and determination of compounds by thin layer chromatography and fluorometry, or gas chromatography. In this procedure, the sediment was refluxed with ethanolic potassium hydroxide, then filtered and the filtrate extracted with isooctane. The isooctane extract was cleaned up on a florisil column, then the polyaromatic hydrocarbons were extracted from the isoactive extract with pure dimethyl sulphoxide. The latter phase was contacted with water, then extracted with isooctane to recover polyaromatic hydrocarbons. The overall recovery of polyaromatic hydrocarbons in this extract by fluorescence spectroscopy was 50-70%. 

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