Nucleation products

Keywords PCM phase change latent heat melting heat storage cold storage corrosion phase separation incongruent melting subcooling nucleator products. 

As for submicron particles (AP) consisting, as has been mentioned above, of transformed minor gaseous components nucleation products, it is assumed that their lifetime does not exceed 2-3 days and they are locally formed over ice caps at a rate of about 4 X 10 g/cm s. 

Seeded batch crystallization process Three problems with two or three objectives from (1) maximization of the weight mean size of the crystal size disuibution, (2) minimization of the nucleated product, (3) minimization of total time of operation, and (4) minimization of coefficient of variation. NSGA-n Dynamic optimization problems were solved to find the optimal temperature profile. Sarkar et al. (2006)... 

Arguably, one of the major needs in modeling sorption on soils and natural materials is to include surface precipitation and other non-adsorption phenomena, based on molecular level data, as part of the model description and prediction. This is particularly important as recent research, based on in-situ spectroscopic analyses, indicates that metal-nucleation products form on an array of natural surfaces at low surface coverages and at relatively rapid time scales (10-17). 

Bruemmer et al. (55) studied Ni, Zn, and Cd sorption on goethite, a porous iron oxide known to have defects within the structure in which metals can be incorporated to satisfy charge imbalances. At pH 6, as reaction time increased from 2 hours to 42 days (at 293K), sorbed Ni increased from 12 to 70% of Ni removed from solution, and total increases in Zn and Cd sorption over this period increased 33 and 21%, respectively. The kinetics of Cd, Zn, and Ni were described well with a solution to Pick s second law (a linear relation with the square root of time). Bruemmer et al. (55) proposed that the uptake of the metal follows three-steps (i) adsorption of metals on external surfaces (ii) solid-state diffusion of metals from external to internal sites and (iii) metal binding and fixation at positions inside the goethite particle. They suggest that the second step is the rate-limiting step. However, they did not conduct microscopic level experiments to confirm the proposed mechanism. In view of more recent studies, it is likely that the formation of metal-nucleation products could have caused the slow metal sorption reactions observed by Bruemmer et al. (55). 

Nucleation products of Co, Cr(III), Cu, Ni and Pb on oxides and aluminosilicates have been observed (10-14,16,17,61-68). Such products have been observed at metal surface loadings far below a theoretical monolayer coverage, and in a pH range below the pH where the formation of metal hydroxide precipitates would be expected according to the thermodynamic solubility product (10-12,14,16,17). 

Three different types of nucleation products have been proposed formation or sorption of polymers (dimers, trimers, etc.) on the surface (polynuclear surface complexes) a solid solution or coprecipitate that involves co-ions dissolved from the adsorbent and a precipitate formed on the surface composed of ions from the bulk solution, or their hydrolysis products (5,15,33,62,69). The two latter products are examples of surface precipitates. 

In case the nucleation product is associated with the surface (polynuclear surface complexes), the process leads to the saturation of sites, whereas precipitation mechanisms create new reactive surface area. 

The formation of metal nucleation products could be a significant cause of slow metal sorption on mineral surfaces. For example, XAFS results for Co sorption on rutile (TiOj) showed an increase in the number of backscattering Co atoms for residence times of one day to 11 days, suggesting an increase in the size of nucleation products formed (di). However, similar results were not seen for Co aging on quartz (a-Si02). Data analysis of the Co/quartz system showed that the reaction time had no effect on the Co coordination environment and revealed the presence of a Co hydroxide-like surface precipitate even at a low surface loading. 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

The data analysis reveals 2.8 (montmorillonite) to 5.0 (gibbsite) Ni second-neighbor (N) atoms, indicative of the presence of Ni-nucleation products (see Table II). No correlation between the Ni surface sorption densities, Ts, and... 

Nickel sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite at pH 7.5 results in formation of Ni-nucleation products from solutions which are undersaturated with respect to the thermodynamic solubility product of Ni(OH)2(s). An important finding of the study of Scheidegger et al. (16) is that the structural environment of Ni in all Ni sorption samples is similar. There is also an obvious similarity among the spectra of the Ni sorption samples and the spectrum of takovite, suggesting the presence of Ni phases of similar structure (Table II). 

Ni, and Zn on montmorillonite, Al-montmorillonite and Ali3-montmorillonite. Addition of Al enhanced metal sorption of Ni and Zn and sorption increased with time, while Pb and Cd sorption were not affected by addition of Al. This finding agrees with the results of a recent XAFS study with Pb in our laboratory. No nucleation products seem to occur with Pb at surface loadings on clay minerals and y-Al-oxide where nuclation products have been observed with smaller metals such as Co, Cu and Ni (Strawn, D. G., The University of Delaware, unpublished data). This sorption behaviour of Pb appears to be related to the mismatch in size between Pb (1.19A), and AP (0.54A) that is contained in the structure of the clay minerals and Al-oxide. The Pb ion is too large to fit into the mineral structure, while ions such as Ni (0.69A) and Co (0.75A) can fit into the structure (Table I). 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

An increase in the time required to form a visible precipitate under conditions of low RSS is a consequence of both a slow rate of nucleation and a steady decrease in RSS as the precipitate forms. One solution to the latter problem is to chemically generate the precipitant in solution as the product of a slow chemical reaction. This maintains the RSS at an effectively constant level. The precipitate initially forms under conditions of low RSS, leading to the nucleation of a limited number of particles. As additional precipitant is created, nucleation is eventually superseded by particle growth. This process is called homogeneous precipitation. ... 

The development of the principles of nucleation and growth eady in the twentieth century (2) ultimately led to the discovery that certain nucleating agents can induce a glass to crystallize with a fine-grained, highly uniform microstmcture that offers unique physical properties (3). The first commercial glass-ceramic products were missile nose cones and cookware. 

The consumption of 4-/ f2 -butylphenol in the production of phenohc resins represents an appHcation in a mature market and Htfle growth is projected. Its use in end-capping polycarbonates, in the production of glycidyl ethers, and in the production of nucleation agents for polypropylene is expected to grow at a rate above the growth of the GNP (see Table 3). 

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