Mechanism of precipitation

The thickening mechanism of precipitated silicas is similar to that of the fumed silicas. Precipitated silicas have more silanol groups than fumed silicas, but they have a lower concentration of silica (precipitated silicas generally contain some sodium sulphate). 

In the second kind of explanation, it was postulated that particles of certain sizes are particularly abundant in the size distribution The occurrence of magic agglomeration numbers was explainai by the following mechanism of precipitation ... 

A better insight into the mechanisms of the individual steps in the formation of crystals would be of great help in explaining the creation and transformation of sedimentary deposits and biological precipitates. Valuable reviews are available on the principles of nucleation of crystals and the kinetics of precipitation and crystal growth (Zhang and Nancollas, 1990 Steefel and Van Cappellen, 1990 Van Cappellen, 1991). Only a few important considerations are summarized here to illustrate the wide scope of questions to be answered in order to predict rates and mechanisms of precipitation in natural systems. 

D.H. Atha, K.C. Ingham, Mechanism of precipitation of proteins by polyethylene glycols analysis in terms of excluded volume, J. Biol. Chem. 256(1981) I2I08-I2II7. 

The mechanism of precipitation-dissolution-reprecipitation is not yet well understood. Li (2007) provided some explanations. When the petroleum sulfonate concentration is below its CMC, the single surfactant molecules in the solution increase as the surfactant concentration is increased. The concentrations of Na+ and RSO3 also increase linearly with the surfactant concentration. Their concentrations reach their maximums at the CMC. Meanwhile, the concentrations of Ca + and Cl are constant. Ca + reacts with RSOs to generate Ca(RS03)2 precipitation then Ca + concentration is reduced. Ca(RS03)2 is suspended in the solution, resulting in lower light transmittance. 

Mechanism of Precipitation. It has been proposed (JO) that hydroxy-aluminum polymers are composed of two closely packed planes... 

H. L, Recht and M. Ghassemi, "Kinetics and Mechanism of Precipitation and Nature of the Precipitate Obtained in Phosphate Removal from Wastewater Using Aluminum(III) and Iron(III) Salts," Report No. 17010EKI 04/70, U.S. Dept, of Interior, Federal Water Quality Adm., Cincinnati, 1970. 

Many of the synthesization methods to produce nanoparticles are based on coprecipitation steps, or nucleation and growth in reactional media [98,108]. Precipitation reactions involve the simultaneous occurrence of these steps, as well as coarsening and agglomeration processes [122,123]. Because of the difficulties in isolating each process for independent study, the fundamental mechanisms of precipitation are still not entirely understood. However, a good understanding of the nucleation step is fundamental for grasping the nature of nanosize particles. 

If instead of a nonsolvent liquid, the nonsolvent is used in vapor state, the mechanism of precipitation is called phase inversion induced by nonsolvent vapor. Vapor nonsolvent is drawn into the polymer solution due to the affinity between solvent and nonsolvent. ... 

Gunn, D.J. and Murthy, M.S. (1972) Kinetics and mechanisms of precipitations. Chemical Engineering Science, 27, 1293-1313. 

The kinetics and mechanism of precipitation have been well studied and widely available in the open literature [8, 22, 23, 102-107]. Precipitation from solution involves two basic steps (i) nucleation of fine particles and (ii) growth of the nuclei. The control of the powder characteristics is realized by controlling the reaction conditions for both nucleation and growth, as well as the extent of coupling between the two processes. 

The powder systems S, A, and B, in the shape of full spheres are obtained, and the dominant mechanism of precipitation of these powders was volume precipitation. It is conditioned by relatively high concentration of given systems (expressed in percentage) with Si02 — 30%, with PWg026 — 20%, and with calciumhydroxysiapatite — 6.5%. 

Radioactive tracers have been extensively and successfully applied to the smdies of mechanism of precipitation and also aging or ripening of precipitates. Studies of corrosion and erosion using radioactive tracers have greatly contributed to improvement of industrial processes. 

The kinetics and mechanism of precipitation are well covered in a textbook (40). Several review articles on the precipitation from solution are available, covaing... 

At the suggestion of the author. Flicker and Ross [11] allowed a uniform Phl2 sol to age to see if a symmetry breaking instability would occur - see also [12,13] The system did indeed pattern, displaying phenomenon (vi). To explain this phenomenon a new mechanism of precipitate pattern formation, the "competitive particle growth" (CPG) model was introduced [4,5,12, 14] and found strong experimental support [4,5,12-15] ... 

The correction factor Pend in Eq. (65) is dimensionless and accounts for noncontinuum effects. The mobility of solute molecules in the metastable gas phase is expressed by the gas-phase diffusion coefficient 2 g, and the driving force for condensation is the difference in mole fraction between the metastable phase (yj) and a gas phase that is in equilibrium with the precipitated phase (y ). Nucleation and condensation are mechanisms of precipitation that compete with each other. Condensation depends on the availability of the outer surface area of particles that have been generated by nucleation. Both processes lower the actual solute mole fraction yj and therefore diminish the driving force for either process. 

I 0. Make a schematic plot of room-temperature strength (or hardness) versus the logarithm of time for a precipitation heat treatment at constant temperature. Explain the shape of this curve in terms of the mechanism of precipitation hardening. 

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