Improved Chemical Stability

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Crucial to this class of inhibitors was the discovery of (1 J ,25)-l-amino-2-vinylcyclopropane carboxylic acid (vinyl-ACC A) as a PI residue [99]. Initially, several reports appeared describing chemically stable replacements for the cysteine residue at the PI position of all tra i-cleaved substrates. An aliphatic side chain with two (aminobutanoic acid) or three (nor-valine) carbon atoms seemed to be an acceptable replacement, with only a moderate loss in potency and improved chemical stability [96, 100]. An improved PI... 

The cyclic stability (data are not shown) was also tested. After 2000 cycles, there is only a loss of 7 % in the acidic medium. This supports our view that N-doped carbons possess a significantly improved chemical stability. 

Carbacephems aOO 1 loracarbef ° N v Acl co2h improved chemical stability, longer half-life, better oral bioavailability compared to cefaclor... 

Activate cytotoxic agent Improve chemical stability Decrease presystemic... 

As might be expected from simple kinetic considerations, chromophore decomposition typically involves bimolecular processes and the rate of decomposition usually depends on the ability of species to diffuse in condensed media. The denser the polymer medium (e.g., the more heavily crosslinked), the slower the rate diffusion and the slower the rate of chromophore decomposition. Decomposition also often depends on attack at certain reactive positions of the chromophore consistent with well-known principals of organic chemistry. Ster-ic protection of these reactive sites can dramatically improve chemical stability including at elevated temperatures [183,210-212]. 

As is evident from a consideration of Figs. 7-9, each of these chromophores has exhibited electro-optic activity exceeding that of lithium niobate while at the same time exhibiting auxiliary properties of chemical stability (Td >300 °C) and solubility that permits preparation of device quality materials [183,210-212]. These materials also illustrate another major direction in the preparation of electro-optic materials namely, the development of bridging segments that lead to improved chemical stability, improved solubility in spin-casting solvents, improved compatibility with polymer host materials, and which inhibit unwanted intermolecular electrostatic interactions (we shall discuss such interactions... 

Niosomes In order to circumvent some of the limitations encountered with liposomes, such as their chemical instability, the cost and purity of the natural phospholipids, and oxidative degradation of the phospholipids, niosomes have been developed. Niosomes are nonionic surfactant vesicles which exhibit the same bilay-ered structures as liposomes. Their advantages over liposomes include improved chemical stability and low production costs. Moreover, niosomes are biocompatible, biodegradable, and nonimmunogenic [215], They were also shown to increase the ocular bioavailability of hydrophilic drugs significantly more than liposomes. This is due to the fact that the surfactants in the niosomes act as penetrations enhancers and remove the mucous layer from the ocular surface [209]. 

Cyclodextrins improve chemical stability, increase the drug s bioavailability, and decrease local irritation. However, the improvement of ocular bioavailability seems to be limited by the very slow dissociation of the complexes in the precorneal tear fluid. 

In the next step these particles are incorporated in the transparent silane matrix described above. In this network of silanes, fhe nano scaled Si02 particles on the one hand and the coloring pigments on the other can be dispersed without agglomeration. In Fig. 11 the chemical composition is shown schematically. To further improve chemical stability, the steric stabilization can be combined with an electrostatic stabilization. Depending on the incorporated pigments, these inks prove to be chemically stable up to several weeks. 

The stability and durability of Pt alloys, especially those involving a >d transition metal, are the major hurdles preventing them from commercial fuel cell applications. "" The transition metals in these alloys are not thermodynamically stable and may leach out in the acidic PEM fuel cell environment. Transition metal atoms at the surface of the alloy particles leach out faster than those under the surface of Pt atom layers." The metal cations of the leaching products can replace the protons of ionomers in the membrane and lead to reduced ionic conductivity, which in turn increases the resistance loss and activation overpotential loss. Gasteiger et al. showed that preleached Pt alloys displayed improved chemical stability and reduced ORR overpotential loss (in the mass transport region), but their long-term stability has not been demonstrated. " These alloys experienced rapid activity loss after a few hundred hours of fuel cell tests, which was attributed to changes in their surface composition and structure." ... 

One way to improve chemical stability of a ceramic membrane is to introduce another oxide to the system as mentioned previously. On the other hand, even a small quantity of an ingredient present in the membrane composition may appreciably change the resulting chemical resistance in an undesirable direction. An example is cakia-stabilized zirconia which is likely to offer less resistance to acids than pure zirconia. 

Attempts were therefore made to prepare pro-drug forms of (27) which might improve chemical stability, but retain site-specific delivery. This approach met with limited success [77],... 

Pharmaceutical objectives Improved formulation (e.g., increased hydrosolubility) Improved chemical stability Improved patient acceptance and compliance... 

Cross-linking Improve chemical stability selectivity UF... 

Insoluble salts may be prepared using high molecular weight counterions to reduce the solubility of the drug for formulation of a suspension, to improve chemical stability, provide improvements in solid-state stability, reduce acid lability and permit formulation of tasteless or controlled release products. Erythromycin stearate, which is poorly soluble in acidic media (an enteric salt), is less prone to decomposition in gastric fluids than the more acid-soluble free base. 

An essential property required to obtain a physically stable suspension formulation is that the drag substance has a low solubility in the suspension vehicle. This applies to all types of suspension whether intended for oral, parenteral or inhalation delivery. Since most suspensions are aqueous, a low solubility in water is required to prevent drug dissolution and crystal growth on storage. Some drugs intended for intramuscular use are formulated as suspensions in oil (e.g. penicillin injections are formulated in sesame oil) to improve chemical stability via a reduction in solubility. 

FIGURE 41.4 Improved chemical stability through suppression of an... 

A salt can also provide improved chemical stability compared to the parent drug substance. An example of this was reported for xilobam, whose structure is shown in Fig. 13 (15). In order to protect xilobam from the effects of high temperature and humidities without decreasing the dissolution rate, three arylsul-fonic acid salts (tosylate, 1-napsylate, and 2-napsylate), as well as the saccharate salt, were prepared. The 1-napsylate was found to be the most chemically stable form at 70°C and 74% RH after 7 days. Dissolution data from compressed tablets... 

All conservation treatments should ideally be reversible. The use of aluminium alkoxide is most definitely not. It should be noted also that while treatment with aluminium alkoxide improves chemical stability, there is no improvement in physical characteristics and the use of suitable consolidation... 

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