Kinetics amorphous aqueous solutions

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

Fig. 10 Examples of the kinetics of secondary drying. Triangles = mannitol (crystalline) squares = poly (vinylpyrrolidone) circles = moxalactam di-sodium (amorphous). All solids were prepared by freeze-drying a 5% aqueous solution from a 1-cm fill depth, followed by hydration to a uniform moisture level of 7%. The quantity, F, is the fractional attainment of equilibrium, which corresponds to near zero water content. The secondary drying conditions were product temperature = 18°C chamber pressure = 200mTorr. (From Ref °l)...

In the case of Al(OH)3 freshly precipitated from solution, there is a slow recrystallization from the amorphous to the crystalline form, with the result that the ion activity product, (aAl3+) (aoH-) of the aqueous solution decreases over time, approaching the value for the crystalline mineral. Here, as in many chemical processes, activities of ions and molecules in solution are subject to control by reaction rates (kinetics) as well as equilibrium constants. The important role of kinetics in chemical reactions will be discussed later in this chapter. 

Heggie Ml, Jones R, Latham CD, Maynard SCP, Tole P (1992) Molectrlar diffusion of oxygen and water in crystalline and amorphous silica. Philosophical Magazine B 65 463-471 Helgeson HC, Mtrrphy WM, Aagaard P (1984) Thermodynamic and kinetic corrstraints on reaction rates among mineral and aqueous solutions. II. Rate constants, effective strrface area, and hydrolysis of feldspar. Geochim Cosmochim Acta 48 2405-2432... 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

The apparent aqueous solubility of amorphous materials is much higher than that of their crystalline counterparts (Fig. 1). This is a kinetic phenomenon and, eventually, the solute in the supersaturated solution that is formed will begin to crystallize and the equilibrium solubility of the crystalline phase will be attained. The transient increase in solubility is often significant (>10x) and can be exploited to give markedly improved biopharma-ceutical performance. ... 

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