Surface microcrystallinity

Enantioselective packings Polar or nonpolar Packings with enantioselective cages or enantioselective surfaces, microcrystalline cellulose triacetate, cellulose ester or cellulose, carbamate/sil-ica composites, optically active poly(acrylamide)/silica composites, chemically modified silicas (Pirkle phases), cydodextrine modified silicas Operated either with normal phase or reversed phase mobile phases... 

Waxes are one of the two general classes of commercial antiozonants. Waxes are derived from petroleum and are of two common types, paraffin and microcrystalline (20—23). Typical carbon numbers are n = 20 50 for paraffin waxes and n = 30 70 for microcrystalline materials. If a wax is present ia a vulcanizate at a concentration exceeding its solubiUty, some of it will migrate to the mbber surface where it can form a physical barrier to prevent the penetration of ozone. Waxes, of course, are essentially unreactive towards ozone so that there is no appreciable chemical protection. Commercial waxes are... 

Bacterial Cellulose. Development of a new strain of Acetobacter may lead to economical production of another novel ceUulose. CeUulon fiber has a very fine fiber diameter and therefore a much larger surface area, which makes it physicaUy distinct from wood ceUulose. Its physical properties mote closely resemble those of the microcrystalline ceUuloses thus it feels smooth ia the mouth, has a high water-binding capacity, and provides viscous aqueous dispersions at low concentration. It iateracts synergisticaUy with xanthan and CMC for enhanced viscosity and stabUity. 

Cellulose layers are produced from native, fibrous or microcrystalline cellulose (Avicel ). The separation behaviors of these naturally vary, because particle size (fiber length), surface, degree of polycondensation and, hence, swelling behavior are all different. 

Metal foils used as catalysts in the experiments described above turned out to be ill-fitted to these investigations. The electrolytic transformation of alloy foils into alloy hydrides did not guarantee a sufficient purity of the samples. Copper rich alloys should be excluded from the experiments because they could not be hydrogen treated in the same manner as the other alloys, and consequently no active microcrystalline layer was developed on their surface. 

Both inorganic and organic sorbent materials are suitable for apphcation in partition PLC. The relevant material in this connection is cellulose. Celluloses are natural products with the universal chemical formula (CgHioOj),. These native celluloses have a fibrous structure and they need to be groimd and purified before use in PLC. Besides native cellulose, microcrystalline cellulose can also be used in partition PLC. In this case the cellulose has been recrystaUized and is rod-shaped. The specific surface area of celluloses is in the range of about 2 m /g. 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

Celluloses (native or microcrystalline) are organic sorbents. They have a low specific surface area and are applied mainly in partition chromatography, especially for the separation of relatively polar compounds. Works on the topic include those by Whitton and coworkers [8], who examined biosynthetic pathways for the formation of taurine in vertebrates. Taurine and its precursor amino acids were extracted from tissues, and the purified supernatant was spotted onto cellulose plates. The... 

According to the characterizations by TEM and XRD, the sample prepared from a CH4/H2 plasma was composed of nanocrystalline diamond and disordered microcrystalline graphite. Then nondiamond carbon was effectively removed with an increase in [CO]. It is therefore concluded that the VDOS of the nanocrystalline diamond and DEC films extracted from the HREELS data is in qualitative agreement with the characterizations of TEM and XRD. Although the HREELS probes only the region near the surface, the agreement suggests that the surface dynamics do not differ dramatically from those of the bulk. 

The industrial application of Plasma Induced Chemical Vapour Deposition (PICVD) of amorphous and microcrystalline silicon films has led to extensive studies of gas phase and surface processes connected with the deposition process. We are investigating the time response of the concentration of species involved in the deposition process, namely SiH4, Si2H6, and H2 by relaxation mass spectroscopy and SiH2 by laser induced fluorescence. 

Wall painting in the Messer Filippo cell, Tower of Spilamberto (Modena, Italy). The painted surface is in an advanced state of deterioration the detachment of the pictorial matter from the plaster is evident, efflorescence is present, and the pictorial coating is almost completely covered by a grey-whitish film formed by microcrystalline gypsum, as proved by micro-Raman investigations. The FA profile, characterized by an A/P ratio of... 

Although the true waxes, such as beeswax, are esters of fatty acids with alcohols, the term is often applied to certain solid substances which melt at fairly low temperatures, e.g., paraffin wax. Paraffin waxes (crystalline and microcrystalline) are obtained from the refining of petroleum, and are used in rubber compounding as protective agents. They have the property of blooming to the surface of the product, forming a thin film which replenishes itself if removed in service. 

These waxes have branched structures of higher molecular weight (40-70 carbon atoms) than paraffin waxes and form a quite different crystalline structure on the surface of the rubber when emerging from solution from within the vulcanised rubber. Microcrystalline waxes form smaller crystals, which pack tighter together to form a more coherent, much more flexible film on the rubber surface which is more resistant to ozone penetration. 

Measurements of particle porosity are a valuable supplement to studies of specific surface area, and such data are particularly useful in the evaluation of materials used in direct compression processes. For example, both micromeritic properties were measured for several different types of cellulosic-type excipients [53]. Surface areas by the B.E.T. method were used to evaluate all types of pore structures, while the method of mercury intrusion porosimetry used could not detect pores smaller than 10 nm. The data permitted a ready differentiation between the intraparticle pore structure of microcrystalline and agglomerated cellulose powders. 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

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