Hydration glass formation

Petit, J.-C., Df.lla Mea, G., Dran, J.-C., Magonthier, M.-C., Mando, P. A. Paccagnella, A. 1990. Hydrated layer formation during dissolution of complex silicate glasses and minerals. Geochimica et Cosmochimica Acta, 54, 1941-1955. 

Citric and Succinic Acids Citric acid was used in solid dispersion preparations because of its high water solubility and its capability of glass formation. The monohydrate melts at approximately 100°C, whereas the anhydrous form melts at approximately 153°C. Drug degradation is quite probable at the higher temperature because the release of the water of hydration from the monohydrate form can affect moisture-sensitive drugs. 

Figure 3 Hydrate crystal formations in the free fluid phase between glass beads (light balls)...

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Ethjl Silicate-Bonded Investments. These investments are mixtures of powder and Uquid. The powder consists of refractory particles of sUica glass, crystobahte, and other metal oxides plus magnesium oxide. The Uquid is a hydrated sUica, tetrasUicic acid [10193-36-9] Si [OH], that is suppUed in a stabUized form it can be developed by mixing ethyl sUicate [78-10 ] denatured ethyl alcohol [64-17-5] and hydrochloric acid [7647-01 -OJ. The binding of the powder is accompUshed by the formation of a sUica gel according to the reaction ... 

Sodium borate (decahydrate, hydrated borax) [1303-96-4] M 381.2, m 75 (loses 5H2O at 60 ), d 1.73. Crystd from water (3.3mL/g) keeping below 55° to avoid formation of the pentahydrate. Filtered at the pump, washed with water and equilibrated for several days in a desiccator containing an aqueous solution saturated with respect to sucrose and NaCl. Borax can be prepared more quickly (but its water content is somewhat variable) by washing the recrystd material at the pump with water, followed by 95% EtOH, then Et20, and air dried at room temperature for 12-18h on a clock glass. 

Water is also a component of set AB cements. In glass-ionomer cements, for example, it may serve to coordinate to certain sites around the metal ions. It also hydrates the siliceous hydrogel that is formed from the glass after add attack has liberated the various metal ions (Wilson McLean, 1988). Such reactions continue long after the initial hardening of the cement is complete, and for this reason water must be retained as far as possible during the first hours and days after formation of the cement. If water is lost from the cement and desiccation occurs, these post-hardening... 

The formation of niclosamide hydrates, and the effect of relative humidity on the solvatomorphs obtained from acetone and ethyl acetate has been studied [79], The acetone and ethyl acetate solvatomorphs could be desolvated, and exposure to elevated humidity resulted in the formation of two hydrate structures. Each hydrate could be dehydrated into a different anhydrate phase, but only the hydrate formed from the acetone desolvate could be rehydrated to form a hydrate phase. Dynamic vapor sorption has been used to develop a method for determining the onset relative humidity of a glass transition and associated crystallization process [80]. 

A glass membrane in an electrolyte solution cannot be taken to be a homogeneous system in the direction perpendicular to the surface. When the membrane is in contact with the solution, water molecules can enter it and form a 5-100 nm thick hydrated layer [319]. The formation of this hydrated layer is actually a condition for good functioning of the glass electrode. The basic characteristics of the glass structure probably do not change in the hydrated layer, but the cation mobility increases considerably compared with the compact membrane interior... 

Further work by Cornish and Watt ( ) and Silvernail and Goet-zinger ( ) established the active role played by the presence of water, and these authors concluded that a chemical-mechanical hypothesis would fit the observed data. In the case of ceric oxide polishing of glass, Cornish and Watt suggest the formation of a "CeO-Si" activated complex which permits the rupture of the O-Si-0 bonds by hydrolysis. The complex "CeO-Si" then breaks apart, the hydrated silica is swept away along with alkalis released from the glass surface, and the process repeats. 

Traces of water or acidic impurities can lead to the formation of insoluble, hydrated lanthanide adducts, or of paramagnetic lanthanum oxide80, adversely affecting spectral resolution. Careful purification of the solvents and substrate is advised, and filtration of the solution through glass-wool or cotton plugs may be useful. 

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