Supersaturated solution stabilization

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

In accordance with these data, ionic associates (lA) can be precipitated at phosphate concentrations more than 10 M. Below this concentration stabile supersaturated solutions of lA are formed. Colour of lA appeal s immediately after mixing of the solutions and remains constant during several hours. There is a new band in spectmm at 570-590 nm. Appearance of color is caused by formation of stable solid phase in the solution. 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

The rate of nucleation is slow at low levels of supersaturation and in highly supersaturated solutions owing to the high viscosity of the solution. The stability of a lactose glass is due to the low probability of nuclei forming at very high concentrations. 

Studies have been reported of the complex equilibria present in electrolytically produced supersaturated solutions of Zn2+ in aqueous KOH. Light-scattering and NMR techniques indicate the excess zinc to be present as a solute, rather than a colloid, and the predominant species appears to be the [Zn(OH)4]2 ion.606 However, Raman and potentiometric studies indicate that initially, quasi-colloidal particles, based on Zn(OH)2 and molecules of solvation, are present.607 These particles undergo a first-order decay to yield a solution containing the species [Zn(0H)2(H20)2], [Zn(0H)3H20] and [Zn(OH)4]2-, the actual constitution depending on the concentration. The non-colloidal zinc species are tetrahedral, rather than planar or octahedral. Stability constants for the ions [Zn(OH)n](" 2) ( = 1-3) have been reported.608... 

The formulation development strategies are similarto thatifoivo PK screening. It is common to use supersaturated solutions in the early phase studies. However, the physical and chemical stabilities of the supersaturation systems should be well understood as the information provided will help formulation scientists to decide if the formulation needs to be prepared daily, weekly or monthly. 

In general, a nucleation inhibitor should be considered to improve the stability of the supersaturated solutions. The pharmaceutical polymers, complexing agents, and surfactants have been used widely forthis purpose (Brewster et al., 2007). 

STABILIZATION OF SUPERSATURATED SOLUTION BY USING CO-EXISTING SALT... 

Even for such a quantity of crystallization, separating the crystals while at the same preventing the crystallization of the opposite enantiomer is often difficult because the solution is supersaturated with the opposite enantiomer. In this case, the stability of the supersaturated solution can be improved remarkably by dissolving a readily soluble salt derivative of the racemic modification (Fig. 8). 

Bitartrate Stabilization. Potassium and tartaric acid are natural constituents of the grape. Wine content of these constituents depends on a number of variables, not all well understood variety, vintage, and weather pattern degree of skin contact alcohol level bitartrate holding capacity of phenolic compounds and potassium binding capacity of the wine (30, 35). Most wines after fermentation are supersaturated solutions of potassium bitartrate. This compound is less soluble at lower temperatures, and, thus, lower temperatures will cause precipitation of bitartrate crystals. This lowering of temperature and subsequent removal of crystals by filtration is called cold stabilization. 

As the pH is increased (see figure 1), other calcium phosphate phases may be stabilized as precursors to the formation of apatite and the growth of HAP seed crystals in stable supersaturated solutions is much more complex (, 1, 49). ... 

Cryst ization needs a supersaturated solution in order to allow nucleation and crystal growth. The supersaturation, however, should take dace be between limits [67]. Indeed, according to dieir structure and composition, microporous solids have different thermodynamic stabilities and, therefore, different solubilities. Thus, the supersaturation domains for the possible phases are generally not completely superimposed. (Figure 22.).The art of the synthesis consists then to reach the supersaturation domain corresponding to the phase required, by using, for instance, an amoiphous gel, with a solubility level in the former domain, as a source of the polycondensable species. 

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