Chemical stability dehydration

Dehydration or Chemical Stabilization. The removal of surface silanol (Si—OH) bonds from the pore network results in a chemically stable ultraporous soHd (step F, Fig. 1). Porous gel—siHca made in this manner by method 3 is optically transparent, having both interconnected porosity and sufficient strength to be used as unique optical components when impregnated with optically active polymers, such as fiuors, wavelength shifters, dyes, or nonlinear polymers (3,23). 

The pathways for hexose and pentose are differentiated mainly through the relative chemical stability of the homologues, glucosepane and pentosinane. The former is a proper AGE under physiological conditions, but the latter is smoothly oxidised to an intermediate, which is subsequently dehydrated to the advanced glycoxidation product, pentosidine. At this stage, it is not known whether oxidation or dehydration is rate determining the role of metal ions needs to be clarified as well. 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

Talc. Thanks to its useful properties (thermal and chemical stability, lubricant ability, etc.), talc is one of the most important magnesium silicates. After dehydration it turns into enstatite which is widely used in ceramic industry. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

Chemical Stability Chemical degradation of the drug includes reactions such as hydrolysis, dehydration, oxidation, photochemical degradation, or reaction with excipients. The constant presence of water and oxygen in our environment means that exposure to moisture or oxygen can affect the chemical stability of a compound. Chemical stability is very important, not only because a sufficient amount of the dmg is needed at the time of administration for therapeutic purposes, but also because chemical degradation products may adversely affect the properties of the formulated product and may even be toxic. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, and organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of polyvinylalco-hol (PVA) have been widely employed for dehydration and separation of organic mixtures however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixture is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures, they usually present a low selectivity. 

Membranes that have high selectivity and high flux are not commonly available. Polymeric membranes have been limited to dehydration of solvents due to insufficiency of their thermal, mechanical, and chemical stabilities. The development of zeolite membranes has made it possible to overcome this limitation. 

Before initiating the design of a formulation and method of preparation, additional physico-chemical properties of the active substance should be defined such as particle size and particle size distribution, salt, polymorphism, aqueous solubility in dependence on pH, hygroscopicity, melting point, sublimation behaviour, water of crystallisation, dehydration temperature. The properties of the raw material are also relevant to the physical and chemical stability and the compatibility with the excipients and packaging. Furthermore, compatibility with other active substances has to be investigated when the new substance is to be administered through the same infusion line. 

Titania gels and aerogels are promising catalysts owing to their high chemical stability [64-67] and intrinsic catalytic activity for a number of reactions, such as phenol amination [68], olefin epoxidation [69], cumene dealkylation [70], 2-propanol dehydration [69], and the decomposition of 1,2-dichloroethane [71]. More... 

THF was obtained from Kanto Chemical Co., Inc. as dehydrated stabilizer-free grade. 

The selective separation of water from aqueous solutions of isopropanol or the dehydration of isopropanol can be carried out with different membranes, which contain polar groups, either in the backbone or as pendent moieties. For the dehydration of such a mixture, poly(vinyl alcohol) (PVA) and PVA-based membranes have been used extensively. PVA is the primary material from which the commercial membranes are fabricated and has been studied intensively for pervaporation because of its excellent film forming, high hydrophilicity due to -OH groups as pendant moieties, and chemical-resistant properties. On the contrary, PVA has poor stability at higher water concentrations, and hence selectivity decreases remarkably. 

---