Chemical Stability of Glasses

Chemical Reaction Mechanisms with Water, Acid, and Alkaline Solutions 

Chemical stability is understood as the resistance of a glass surface to chemical attack by defined agents here, temperature, exposure time, and the condition of the glass surface play irqtortant roles. 

Every chemical attack on glass involves water or one of its dissociation products, i. e. H+ or OH- ions. For this reason, we differentiate between hydrolytic (water), acid, and alkali resistance. In water or acid attack, small amounts of (mostly monovalent or divalent) cations are leached out. In resistant glasses, a very thin layer of silica gel then forms on the surface, which normally inhibits further attack (Fig. 3.4-8a,b). Hydrofluoric acid, alkaline solutions, and in some cases phosphoric acid, however, gradually destroy the silica framework and thus ablate the glass surface in total (Fig. 3.4-8c). In contrast, water-free (i. e. organic) solutions do not react with glass. 

Chemical reactions are often increased or decreased by the presence of other components. Alkali attack on glass is thus hindered by certain ions, particularly those 

Because acid and alkali attacks on glass are fundamentally different, silica-gel layers produced by acid attack obviously are not necessarily effective against alkali solutions and may be destroyed. Conversely, the presence of ions that inhibit alkali attack does not necessarily represent protection against acids and water. The most severe chemical exposure is therefore an alternating treatment with acids and alkaline solutions. As in all chemical reactions, the intensity of the interaction in- 

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In the modern world, we are accustomed to taking the chemical stability of glass very much for granted - we rely on the durability of glass for so many things, such as windows and (until the widespread availability of plastics) bottles, as well as its use in the chemical laboratory as an extremely inert and unreactive container. In addition to its apparent inertness, glass has a number of other beneficial properties, such as its transparency or the ability to take on virtually any colour as the result of the addition of a small amount of transition metals. 

The chemical stability of glass containers for pharmaceutical use is expressed by the hydrolytic resistance, i.e., the resistance to release of water-soluble mineral substances, evaluated by titrating the released alkalinity. According to the European Pharmacopoeia (2002), aqueous preparations for parenteral use are to be dispensed into glass containers of high hydrolytic resistance, while nonaqueous preparations and powders for parenteral use can be filled into glass containers of moderate hydrolytic resistance. It is obvious that release of alkaline substances may influence photochemical stability by an increase in pH (see Section 14.2.3). 

While acid corrosion in glass fibers is diffusion-controiied and therefore /f-kinetics are expected, the process in aqueous and aikaiine soiutions is considered much more complicated because of the many influencing factors. The reaction kinetics depends on the (local) pH value. It is conventional opinion that, with the switch from a diffusion-controlled corrosion mechanism to an interfacial-controlled mechanism, a rapid shift from ft-to t-kinetics takes place, and the process follows linear t-kinetics except for short exposure times and low temperatures. However, in the literature, dependencies on t are also found, with values for a varying between 0.5 and 1 [819]. The chemical stability of glass fibers under alkaline attack is also significantly influenced by insoluble corrosion or reaction products on the fiber surface. 

The chemical stability of glasses is decisive for microfluidic, medicine and biological applications. It is characterised by the class of resistance against acids, water and lyes. The industrial standards DIN 12116, DIN 12111 and DIN 52322 provide a classification of the glasses. The chemical stability classes of the photostmcturable glass FS21 are shown in Table 1.7. 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

Hornsby, P. R. (1980). Dimensional stability of glass-ionomer cements. Journal of Chemical Technology and Biotechnology, 30, 595-601. 

The chemical stability of an amorphous formulation is usually also a function of its storage temperatme relative to Tg. The enhanced molecular mobility achieved near the glass transition translates into an increase in translational diffusion-dependent degradation pathways, such as aggregation in proteins. It should be noted that the reaction kinetics near the Tg do not obey Arrhenius kinetics, and that extrapolation of the accelerated stability data generated near the Tg to stability at the storage temperature should be viewed with extreme caution. Amorphous materials must be stored well below the glass transition (at least 10°C, and typically 40 to 50°C below Tg) to maintain their physical and chemical stability. 

Guo and coworkers (29) examined the chemical stability of lyophilized quinapril HC1 as a function of initial solution pH. Lyophilization of different quinapril solutions produces mixtures of amorphous quinapril and its neutralized form, with glass transition values between the Tg values of quinapril and neutralized quinapril. As the fraction of quinapril increases the rate of chemical degradation increases relative to that of quinapril HC1 alone. This is most likely caused by the plasticizing effects of neutralized quinapril. 

In these studies, thermodynamic equilibrium conditions were assumed. Constant pH monitoring was used as an estimator of the equilibration times involved, since a stable pH is an overall indicator of the Internal chemical stability of pH dependent processes. In order to allow the assumptions of atmospheric O2 and CO2 partial pressures, water scrubbed compressed air was pumped into the reaction vessels under constant temperature and constant vigorous mixing conditions. The air was introduced by fritted glass bubblers which had been teflon coated while air flowed through them in order to minimize the glass-solution interface. 

The stability of glass-forming systems is also a matter of considerable importance. Early studies have explored the association between the glassy behavior and the chemical stability (80), whereas a number of investigations have examined the recrystallization behavior of glassy drugs and excipients. In essence, the increased molecular mobility above the glass transition temperature renders recrystallization... 

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