The stability of pharmaceutical powders: effect of additives and coating

The decomposition of drugs in the solid state has been studied using aspirin and salsalate as models. The feasibility of using suspension systems for predicting the stability of these drugs in the solid state has been investigated.. It has been found that such systems are inappropriate in defining the effect of excipients on 'the decomposition of the active drug due to chqnges in the degradation pathway. Using a high performance liquid chromatographic method, magnesium stearate was shown to induce the formation of potentlally immunogenic products in aspirin powders. These products which included salicylsalicylic acid .and acetylsalicyclsalicylic acid were not detected in aspirin suspensions which had undergone the same extent of decomposition. By studying the effect of pH and of added excipients on the rate of decomposition of aspirin in suspension systems, it has been shown that excipients such as magnesium stearate containing magnesium oxide, most probably enhance the decomposition of both aspirin and salsalate by alkalinising the aqueous phase. In the solid state, pH effects produced by excipients appear to be relatively unimportant. Evidence is presented to suggest that the critical parameter is a depression in melting point induced by: the added excipient. Microscopical examination in fact showed the formation of clear liquid layers in aspirin samples containing added magnesium stearate but not in control samples. Kinetic equations which take into account both the diffusive barrier presented by the liquid films and the. geometry of the aspirin crystals were developed. Fitting of the .experimental data to these equations showed good agreement. with the postulated theory. Monitorjng of weight issues during the decomposition of aspirin revealed that in the solid systems studied where the bulk of the decomposition product sublimes, it is possible to estimate the extent of degradation from the residual weight, provided the initial weight is known. The corollary is that in such open systems, monitoring of decomposition products is inadequate for assessing the extent of decomposition. In addition to the magnesium stearate-aspirin system, mapyramine maleate-aspirin mixtures were used to model interactive systems. Work carried out in an attempt to stabilise such systems included microencapsulation and film coating. The protection obtained was dependent on the interactive species used. Gelatin for example appeared to stabilise aspirin against the adverse effects of magnesium stearate but increased its decomposition in the presence of mapyramine maleate.

Hypervalent iodine in synthesis 64:Syntheses of diaryl selenides and alkyl aryl selenides by palladium-catalyzed arylation of areneselenyl or alkaneselenyl magnesium bromide with diaryliodonium salt

Areneselenyl or alkaneselenyl magnesium bromide reacts rapidly with diaryliodonium salt to give the corresponding diaryl or alkyl aryl selenide in the presence of catalytic amounts of Pd-(PPh3)4 in good yield.

The influence of calcium and magnesium on the growth of the lichens Parmelia saxatilis and Xanthoria parietina on slate substrates

In South Gwynedd, Wales, U.K., the calcicole lichen Xanthoria parietina occurs not only on alkaline substrates at inland sites but also on siliceous rock at coastal martimie sites while the calcifuge species Parmelia saxatilis occurs only at inland sites and on slate rocks. Samples of maritime and inland slate did not differ significantly in their calcium or magnesium content. Thalli of X. parietina on pieces of slate did not survive when transplanted from maritime rocks to a site inland. Thalli of maritime X. parietina and P. saxatilis on slate were then transplanted to a site inland and were treated at intervals during 1 year either with calcium carbonate applied as a thick paste or a 0.25 mM solution of calcium chloride. Treatment of X. parietina with calcium carbonate enabled the thalli to survive and grow. However, addition of calcium carbonate to P. saxatilis resulted in low growth rates and fragmentation of the centres of the thalli. The calcium chloride solution had no statistically significant effects on the growth of either species. In addition, thalli of both species were treated with calcium or magnesium carbonates or wetted with an alkaline buffer at intervals over 12-14 months. Thalli of X. parietina survived and grew rapidly when treated with either carbonate but the growth of the buffer-treated thalli gradually declined over the experimental period. Thalli of P. saxatilis fragmented and disappeared after 8-10 months after treatment with either carbonate but normal growth occurred in the buffer treatment. Xanthoria parietina may occur on siliceous maritime rocks at the site because of the presence of calcium or magnesium in sea spray combined with the spray’s alkaline pH. By contrast, P. saxatilis may be confined to siliceous rocks inland because the thalli grow poorly in the presence of calcium and magnesium.

Electrodeposition on magnesium alloy diecasting

Magnesium alloy diecasting AZ91CC, AZ61CC', AZ91HC and AZ71HC were electroplated using different pretreatment sequences which incorporated conventional zincate immersion processes. Satisfactory peel adhesion in excess of 7. 7 KNm -1 was achieved on AZ61CC using a sequence which was designated Canning. The comparatively low adhesion achieved on the AZ91HC was due to its poor surface quality as cast. Growth of deposits was monitored using a strip-and-analysis technique and the morphology of the various deposits were studied using scanning electron microscopy. Different pretreatment sequences resulted in different surface responses for the alloys but all alloys behaved in a similar manner in a particular sequence with regard to potential time-curves and the rate of zinc deposition. The role of fluoride in both the second stage solution and zinc immersion stages of the Canning pretreatment sequence was studied using techniques listed above and Auger electron spectroscopy. Complete coverage of the magnesium alloy surface with immersion zinc was achieved when fluoride was absent from the zincating solution. However, a zero adhesion value was indicated in both thermal cycling and peel tests. The presence of fluoride in the immersion zinc solution suppressed the rate of zinc deposition and affected the time taken to reach equilibrium during potential-time determinations. A mechanism is suggested to explain the significance of fluoride additions to the processing solutions. pH and composition of the zincating solution had a significant effect on the time taken to produce the step observed in the potential/time curves and hence equilibrium potential. Immersion zinc deposition occurred rapidly at first but then changed to a lower uniform rate at a point corresponding approximately to the step in the potential/time curve. Although the minimun levels of adhesion, using the Canning sequence, varied from 7.72 KNm-1 for alloy AZ61CC to 1.54 KNm-1 for alloy AZ91HC, all the alloys revealed ductile failure characteristics in the surface layer of the substrate after peel testing. Plated magnesium alloys exhibited good corrosion resistance when appropriately pretreated and overplated with adequate nickel chromium coatings. The immersion zinc layer was not preferentially attacked when pits penetrated to the coating/substrate interface. Hemispherical pits formed and attack on the substrate was severe. Of the pretreatment sequences investigated, the Canning one was the most premising with respect to peel adhesion and corrosion behaviour.

Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

Precipitation of magnesium ammonium phosphate from homogeneous solution

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An examination of the production, heat treatment and mechanical properties of a new wrought magnesium alloy containing 6% Zn. and 1.2% Mn.

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In situ aberration corrected-transmission electron microscopy of magnesium oxide nanocatalysts for biodiesels

Biofuels are promising renewable energy sources and can be derived from vegetable oil feedstocks. Although solid catalysts show great promise in plant oil triglyceride transesterification to biodiesel, the identification of active sites and operating surface nanostructures created during their processing is essential for the development of efficient heterogeneous catalysts. Systematic, direct observations of dynamic MgO nanocatalysts from a magnesium hydroxide-methoxide precursor were performed under controlled calcination conditions using novel in situ aberration corrected-transmission electron microscopy at the 0.1 nm level and quantified with catalytic reactivity and physico-chemical studies. Surface structural modifications and the evolution of extended atomic scale glide defects implicate coplanar anion vacancies in active sites in the transesterification of triglycerides to biodiesel. The linear correlation between surface defect density (and therefore polarisability) and activity affords a simple means to fine tune new, energy efficient nanocatalysts for biofuel synthesis. © 2009 Springer Science+Business Media, LLC.