Thermodynamic Stability in Aqueous Solutions

Silicon, with a standard potential of -0.857 Vshe. is a rather active element and is readily oxidized in water. The stability of silicon in water and aqueous solutions in the absence of complex formation at 25 °C is determined by the equilibrium conditions listed in Table 2.5. °  

TABLE 2.4. Calculated Bond Density (xlO Vcm ) and Surface Free Energy (eV) of Various Silicon Crystal Planes  

Plane Surface In-plane Total Surface energy 

TABLE 2.5. Reactions of Silicon in Aqueous Solution at Equilibrium Conditions  

The occurrence of two different SiFeVSiOa boundaries as shown in Fig. 2.4 is due to the change of the predominant fluoride species from the un-ionized HF molecule to the 

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The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

Powell MF, Fleitman J, Sanders LM, Si VC. Peptide liquid crystals inverse correlation of kinetic formation and thermodynamic stability in aqueous solution. Pharm Res 1994 11 1352-1354. 

Many attempts have been made to synthesize CaP ceramics with optimal properties for bone reconstruction. An overview of these different CaP compounds and their Ca/P ratios are given in Table 3.1. The thermodynamic stabilities in aqueous solution as a function of pFI are also cited. 

Higuchi and Lachman [122] pioneered the approach of improving drug stability by complexation. They showed that aromatic esters can be stabilized in aqueous solutions in the presence of xanthines such as caffeine. Thus, the half-lives of benzocaine, procaine hydrochloride, and tetracaine are increased by approximately two- to fivefold in the presence of 2.5% caffeine. This increase in stability is attributed to the formation of a less reactive complex between caffeine and the aromatic ester. Professor K. A. Connors has written a comprehensive textbook that describes methods for the measurement of binding constants for complex formation in solution—along with discussions of pertinent thermodynamics, modeling statistics,... 

There is a difference between the thermodynamic terms stable and unstable and the kinetic terms labile and inert. Furthermore, the differences between the terms stable and unstable, and labile and inert are relative. Thus, Ni(CN)4 and Cr(CN)6 are both thermodynamically stable in aqueous solution, yet kinetically the rate of exchange of radiocarbon-labeled cyanide is quite different. The half-life for exchange is about 30 sec for the nickel complex and 1 month for the chromium complex. Taube has suggested that those complexes that react completely within about 60 sec at 25°C be considered labile, while those that take a longer time be called inert. This rule of thumb is often given in texts, but is not in general use in the literature. Actual rates and conditions are superior tools for the evaluation of the kinetic and thermodynamic stability of complexes. 

The perhalic acids and their anions are strong oxidising agents, especially Br04-, which is not thermodynamically stable in aqueous solution. They do, however have considerable kinetic stability. Perchlorates of organic or organometallic cations are very dangerous as they may appear stable, but can explode unpredictably with extreme force. 

The single-electron reduced state, the semiquinone, is thermodynamically unstable in aqueous solution, with only 5% present in equimolar mixtures of fully oxidized and fully reduced flavins. However, protein interactions alter the stability of the semiquinone markedly - some stabilizing it, others suppressing it. The... 

Apart from a few exceptions, polyelectrolytes such as proteins and nucleic acids need a definite pH range and the presence of counterions for stability in aqueous solution. Usually this condition is reaUzed by use of dilute salt or buffer solutions. In this case only weak interactions between macromolecules and low-molecular solutes occur, thus reflecting only small contributions from thermodynamic nonideality. 

The factors leading to the high resistance of the noble metals to chemical attack, i.e. their thermodynamic stability over a wide range of conditions and the possibility of the formation of very thin protective films under oxidising conditions, have already been mentioned. A factor tending to reduce corrosion resistance in aqueous solutions is the tendency of these metals to form complexes with some anions. 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

As demonstrated in Section 3.1.1, the Au SR clusters formed in reaction (1) correspond to trapped intermediates of the growing clusters and are thus not always thermodynamically stable. The stabilities of the Au SG clusters (1-9) are acutely dependent on the core sizes. The Aui8(SG)i4 Au25(SG)i8, and Au39(SG)24 clusters were found to be stable when allowed to stand in aqueous solution while other Au SG clusters were degraded into smaller clusters (Figure 7a) [16]. 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

From the thermodynamic behavior of metals pertinent to gold leaching in cyanide solutions it may be observed that Au(CN) shows a large region of stability over the whole pH range, whilst metallic gold, which is usually stable in aqueous solutions under all condi-... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

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