Precipitate formation, mechanism

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

The different growth modes discussed above have been exemplified also from structural studies. Froment and Lincot [247] used structural characterization methods, such as TEM and HRTEM, to determine the formation mechanisms and habits of chemically deposited CdS, ZnS, and CdSe thin film at the atomic level. These authors formulated reaction schemes for the different deposition mechanisms and considered that these should be distinguished to (a) atom-by-atom process, providing autoregulation in normal systems (b) aggregation of colloids (precipitation) ... 

The observation that nodules grow at widely varying rates provides further support for the existence of multiple formation mechanisms. The nodules that accrete most slowly (1 mm per million years) appear to have formed primarily by the process of hydrogenous precipitation. This accretion rate is equivalent to the annual deposition of a layer that is only one atom deep. These slow rates cause a significant amount of metal-rich seawater to become occluded between the Fe-Mn oxide layers. 

Unfortunately, it is nontrivial to distinguish reliably between the complex-decomposition and sulphide-formation mechanisms. For example, in the study of PbS (as a precipitate) formation from thiourea [47], the two main results used to support complex decomposition were (a) very little sulphide was formed in alkaline solutions of thiourea and (b) addition of PbS powder catalyzed the reaction, seen by the disappearance of the induction time for precipitation and more rapid PbS formation when PbS was added at the start of the reaction. However, these results would also be obtained in a free-anion mechanism, for the following reasons ... 

Coprecipitation — The -> precipitation of a normally soluble substance that is carried down during the precipitate formation of the desired substance. The coprecipitation of a substance arises from processes such as - adsorption, -r mixed-crystal formation, - occlusion and/or mechanical entrapment [i]. 

Under natural conditions, the thermodynamic conditions are obviously unknown, so the lead enrichment formation mechanism is also unknown. However, the formation of nano- and microparticles containing lead ions under environmental conditions is especially intriguing. At least a portion of lead ions in nature will form similar enrichments. The process, all types of precipitation, can decrease the migration rate of lead ions. If the precipitation of any substance occurs, its migration rate can decrease. 

Hydrothermal/solvethermal s)mthesis of RMO3 is also extensively adopted, by virtue of the low reaction temperature and well-crystallized products. Vazquez-Vazquez and Lopez-Quintela (2006) reported the solvothermal synthesis of Lai j-A MnOs (A = Ca, Sr, Ba) NPs in benzyl alcohol and acetophenone. The obtained precipitate was annealed to form crystalline products and acetophenone was found to be more suited to obtain clean perovskite phase. Zhu et al. (2008a) prepared single-crystalline YbMnOs and LuMnOa nanoplates via hydrothermal method. The products were found to be hexagonal phases. A possible formation mechanism was proposed, which involves the formation of ROOH phase as intermediate. 

The characteristic of the lead-acid battery is that both electrodes are based on the chemistry of lead. The discharge-charge process is known as the double sulfate reaction, with the positive and negative electrodes being the seats of a dissolving-precipitating (and not some kind of solid-state ion transport or film formation) mechanism of the lead sulfate. The cell, the electrode reactions and the cell reaction are ... 

The equations for dN/dt and dRj/dt, as well as for dV dt [Eq. (39)] are solved by numerical integration for the polymerization stem MMA-K2S20e water, with rate constants obtained from the literature. The initiator efficiency was set equal to unity. Particle numbers between 10 and 10 were drained for initiator concentrations of 10 -10 mol/dm. The calculations showed that N should be almost independent of the chosen value offor values between 5 and 70 (in strong contrast to our calculations). The reason for this is probably that aqueous-phase termination with subsequent precipitation is the dominant particle-formation mechanism in Aral s model, even more so with increasing initiator concentration. The theoretical particle-formation time was on the order of 2 sec, a veiy low value compared to the experimental results of Fitch and Tsai. Aral et at. found that their calculated particle numbers were approximately in accordance with the experimental results of Yamazaki et al. (1968) for emulsifier-free polymerizations. Aral s model does not inclnde any coagulation mechanisms. It will therefore have the same shortcomings as most other models, namely that the strongly increased particle number in... 

Reuvers AJ and Smolders CA. Formation of membranes by means of immersion precipitation The mechanism of formation of membranes prepared from the system cellulose acetate-acetone-water. J. Membr. Sci. 1987 34 67-86. 

Scientists have studied precipitate formation for many years, but the mechanism of the process is still not fully understood. It is certain, however, that the particle size of a precipitate is influenced by such experimental variables as precipitate solubility, temperature, reactant concentrations, and rate at which reactants are mixed. The net effect of these variables can be accounted for, at least qualitatively, by assuming that the particle size is related to a single property of the system called the relative supersaturation, where... 

Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low—that is, under conditions of low supersaturation. In addition, digestion is often remarkably helpful in reducing these types of coprecipitation. Undoubtedly, the rapid solution and reprecipitation that go on at the elevated temperature of digestion open up the pockets and allow the impurities to escape into the solution. 

Adsorption-structural properties of co-precipitated specimens of the Zn(OH)2-Cu(OH)2 and Ni(OH)2-Co(OH)2 and other systems, whose components meet the indicated requirements, can serve a good confirmation to the aforesaid. In this case the structure-formation mechanism does not involve exhibition of any unpredictable changes leading to a maximum or minimum of the Vs - composition curve. 

All binary mixtures represented by hydroxides with different pH of initial and complete precipitation are such systems. These systems differ from those discussed above only in the fact that, as mentioned earlier, depending on the order of precipitation of components, the height and position of the maximum in the Vs -composition curve will be determined by the properties of the solution. However, the mechanism of porosity formation and factors that cause departure of the sorption capacity of the samples produced from additivity are the same. Undoubtedly, each of the cases mentioned [12] has its specificities introduced by components to the structure - formation mechanism and a common characteristic of passing the Vs -composition curves through a maximum. 

Adsorption characteristics of co-precipitated samples with the composition Fe(0H)3 -Cd(OH)2 and Fe(OH)3 - Cr(OH)3 confirm this statement and generality of the regularity found follows from the structural formation mechanism considered. This regularity inherent in all two- and multicomponent systems with different pH of initial and complete precipitation of components opens a broad perspective for scientifically justified and purposeful selection of conditions for the synthesis of adsorbents with the given structure and catalysts with the component precipitated first being a carrier and the other, an active phase. 

Analysis of the structure formation mechanism of the studied systems and others with maxima in the Vj -composition curves has revealed their similarity, which suggests uniformity of processes involved in porosity formation of co-precipitated adsorbents. Analogy is also seen in the determination of sample composition in maxima in the Vs -composition curves in terms of sorption capacities of components [2] ... 

Thus, apart from pH of initial and complete precipitation of hydrogels responsible for growing of particles, the formation of the structure of specimens is also affected by sorption capacities of individual components, which do not affect the gel structure formation mechanism, but distort the shape of the Vg -composition curve and to some extent, create illusive impression of effective growth of the particle size and Vg of the sample in the process of co-precipitation of hydroxides. 

When two components with the same pH of initial and complete precipitation of hydroxides are present in the mixture, the situation changes essentially and in the presence of three such components with the same pH, a four-component mixture becomes similar to a binary system in the structure formation mechanism. 

The surface area and chemical nature of the surface of adsorbents and catalysts are their most important characteristics. However, the behaviour of the specific surface as a function of composition of samples has not been studied inadequately so far. There are only fragmentary data on the effect of some factors on the specific surface area of binary systems, whose structure-formation mechanism differs essentially from that of individual porous materials. It is impossible to extend the regularities of porosity formation and specific surface area of individual hydroxides to co-precipitated systems. 

He has contributed to research on the interface between soil chemistry and mineralogy and soil biology. His special areas of research include the formation mechanisms of aluminum hydroxides and oxyhydroxides, the surface chemistry and reactivities of short-range-ordered precipitation products of Al and Fe, the influence of biomolecules on the sorption and desorption of nutrients and xenobiotics on and from variable charge minerals and soils, the factors that influence the sorption and residual activity of enzymes on phyllosilicates, variable charge minerals, organomineral complexes, and soils and the chemistry of arsenic in soil environments. 

The formation mechanism of the material was also studied by fluorescence techniques, and more particularly by time-resolved fluorescence [24]. In these experiments, the synthesis temperature was lowered down to 60 °C in order to slow down the kinetics of precipitation. The value of the aggregation number (N = 104) at the beginning of the experiment indicates that the micelles in the precursor solution have a quasi-spherical shape (axial ratio equal to 1.8) and interact with urea molecules. The pyrene fluorescence lifetime decreases when aluminum nitrate is added to a solution containing SDS and urea. Consequently, nitrate anions, which are fluorescence quenchers, are at the micelle surface. 

Role of partially water soluble additive solvents. There have been published many studies on the membrane formation mechanism and the effects of solvents, additives (swelling agents or poreformers) and precipitants. Membrane performance and morphology are well correlated to polymer precipitation rate in nascent membrane (. Low precipitation rate generally produces membranes of finely pored sponge substrate structure with low solute permeation. Remarks on solvent-precipitant interaction by Frommer et al. (3) is helpful to speculation on membrane formation. In the following paragraphs is discussed the role of partially water soluble solvent as a plasticizer of nascent membrane matrix. 

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