Precipitation nuclei

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

After the addition of precipitating chemicals, the precipitation reaction commences to form very small particles called precipitation nuclei. The flocculating agents allow these particles to agglomerate. 

Aerosol particles of the size likely to contain the nuclear weapon debris radionuclides and to come from stratospheric air masses are also more likely to serve efficiently as precipitation nuclei (13). Either of these two precipitation removal processes could result in the actual fractionation of potentially fractionated nuclear weapon debris. 

Lyophobic colloids (sols) may be prepared by grinding crystalline materials or running an electric arc between metallic electrodes, both in the dispersing medium. More commonly, they are prepared by precipitating the solid from a strongly supersaturated solution, which produces a large number of precipitation nuclei. Because there is little attractive interaction between the particles and the medium, attractive forces between the particles would soon lead to their aggregation flocculation). This tendency, however, is counterbalanced by repulsive electrical forces between the particles. 

With prolonged exposure, the concentration of ion pairs in the aqueous phase gradually increases. If supersaturation is reached, the ion pairs precipitate into a solid phase. This transformation is complex and the ion pairs may pass through the colloidal state before they reach the solid state. Nucleation of precipitated species is facilitated by the heterogeneous nature of the substrate surface and the overall formation rate of the precipitate seems more often to be limited by its growth rate rather than by its nucleation rate. Evidence of this lies in the frequent observation of many small precipitated nuclei rather than a few larger ones, see Fig. 5 [11]. 

Coalescence of Corrosion Products The number of precipitated nuclei, and also their size, increases with exposure to the atmosphere. The nuclei will soon coalesce and will completely cover the metal surface. These precipitates are frequently referred to as corrosion products and play a crucial role for the metal in any given... 

Oolitic limestone is composed of small rounded grains of CaCO, precipitated in concentric laminates around a nucleus of CaCO or siUca. It is frequently very pure but may be impure. 

It seems that deep-seated cleavage of the dioxin nucleus must accompany dechlorination in methanol. When pure dibenzo-p-dioxin (II) was irradiated in cyclohexane solution in a quartz cuvette, it darkened in color, and a precipitate of intractable dark brown material was collected and was insoluble in the common solvents except for methanol. 

At a high degree of supersaturation, the nucleation rate is so high that the precipitate formed consists mostly of extremely small crystallites. Incipiently formed crystallites might be of a different polymorphous form than the final crystals. If the nucleus is smaller than a one-unit cell, the growing crystallite produced initially is most likely to be amorphous substances with a large unit cell tend to precipitate initially as an amorphous phase ("gels"). 

When the attachment of the substrate to the precipitate to be formed is strong, the clusters tend to spread themselves out on the substrate and form thin surface islands. A special limiting case is the formation of a surface nucleus on a seed crystal of the same mineral (as in surface nucleation crystal growth). As the cohesive bonding within the cluster becomes stronger relative to the bonding between the cluster and the substrate, the cluster will tend to grow three-dimensionally (Steefel and Van Cappellen, 1990). 

In many tissues cholesterol and other sterols exist as a mixture of the free alchohol and its long chain fatty acid ester (esterified at position 3 of the steroid nucleus). The determination of the cholesterol content of a sample may involve the measurement of either of these two fractions individually or the total cholesterol. It is possible to precipitate free cholesterol by adding an equal volume of digitonin (1 gl-1 in 95% ethanol), a naturally occurring glu-coside, to form a complex that is insoluble in most solvents, including water. 

Opioids also interact with excitatory amino acid neurotransmitters. At lower micromolar concentrations, p agonists (e.g., DAMGO) enhance NMDA activity in the nucleus accumbens, but inhibit non-NMDA activity (Martin et al. 1997). At higher concentrations (5 pM), NMDA currents are reduced. Conversely, central administration of glutamate can precipitate a withdrawal syndrome in morphine-dependent animals, similar to the opioid antagonist naloxone. NMDA mechanisms also appear to be involved in the development of morphine tolerance. Competitive and noncompetitive NMDA antagonists and inhibitors of nitric oxide synthase reduce or eliminate tolerance to morphine (Elliott et al. 1995 Bilsky et al. 1996). However, this does not occur for tolerance to k opioids. Pharmacokinetics... 

Hildebrand BE, Nomikos GG, Hertel P, SchEstrom B, Svensson TH. 1998. Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome. Brain Res 779(1—2) 214-225. 

Panagis G, Hildebrand BE, Svensson TH, Nomikos GG. 2000. Selective c-fos induction and decreased dopamine release in the central nucleus of amygdala in rats displaying a meca-mylamine-precipitated nicotine withdrawal syndrome. 

Heimer L, Zahm DS, ChurchUl L, Kalivas PW, Wohltman C (1991) Specificity in the projection patterns of accumbal core and medial shell in the rat. Neuroscience 41 89-125 Hemby SE, No C, Koves TR, Smith JE, Dworkin SI (1997) Differences in extracellular dopamine concentration in the nucleus accumbens during response-dependent and response-independent cocaine administration in the rat. Psychopharmacology 133 7-16 Hildebrand BE, Nomikos GG, Hertel P, Sclrilstrom B, Svensson TH (1998) Reduced dopamine output in the nucleus accumbens but not the prefrontal cortex in rats displaying mecamylamine-precipitated nicotine withdrawal syndrome. Brain Res 779 214-225 Ikemoto S (2003) Involvement of the olfactory tubercle in cocaine reward intracranial selfadministration studies. J Neurosd 23 9305-9311... 

Firing rate of substantia nigra DA neurons increased Recovery from depressed firing rate of VTA DA neurons D 1-like antagonist in amygdala doesn t precipitate withdrawal Reduced amygdala central nucleus DA... 

Jonkman S, Markou A (2006) Blockade of nicotinic acetylcholine or dopamine D 1-fike receptors in the central nucleus of the amygdala or the bed nucleus of the stria terminads does not precipitate nicotine withdrawal in nicotine-dependent rats, Neurosci Lett 400 140-145 Katsura M, Shuto K, Mohri Y, Tsujimura A, Ohkuma S (2001) Withdrawal from nicotine facilitates diazepam binding inhibitor mRNA expression in mouse cerebral cortex. Brain Res Mol Brain Res 97 194-218... 

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

The pH is thus slowly raised in the solution by the gradual release of ammonia basic carbonates are precipitated. Although very uniform spherical particles are obtained (Figure 2), they do not have a homogenous cation distribution since upon the slow increase of pH, Cu is precipitated first as the nucleus of these particles, then Y (32). Ba can be precipitated only if large amounts of urea are used (31). Another procedure is to first precipitate Y Cu particles then disperse them in a solution containing Ba(NOs)2 and urea in order to, by the... 

---