Surface-induced precipitation

Additionally, the pellicle modulates the process of mineral precipitation at the enamel surface. Saliva is supersaturated with respect to calcium and phosphate ions and provides the potential for remineralisation of the demineralised enamel surface. In the absence of the pellicle, calcium phosphate salts would continuously precipitate on the enamel surface [100]. Acidic PRPs reveal a high affinity for hydroxyapatite and are obligate components of the in vivo-formed pellicle layer [9, 10]. These pellicle proteins can act as inhibitors for surface-induced precipitation of calcium salts onto the enamel [101, 102], thus providing mineral homeostasis at the enamel-saliva interface. 

O Day et al., 1994). When the precipitate consists of chemical species derived from both the aqueous solution and dissolution of the mineral, it is referred to as a coprecipitate. The composition of the coprecipitate varies between that of the original solid and a pure precipitate of the sorbing metal. The ionic radius of the sorbing metal and sorbent ions must be similar for coprecipitates to form. Thus, Co(II), Mn(II), Ni(II), and Zn(ll) form coprecipitates on sorbents containing Al(III) and Si(lV) but not Pb(ll), which is considerably larger (1.20 A). Coprecipitate formation is most limited by the rate of mineral dissolution ratlier than by the lack of thermodynamic favorability (McBride, 1994 Scheidegger et al., 1998). If the formation of a precipitate occurs under solution conditions that would in the absence of a sorbent be undersaturated with respect to any known solid phase, this is referred to as surface-induced precipitation (Towle et al., 1997 Sparks, 2002). 

Xia, K. et al.. X-ray absorption and electron paramagnetic resonance studies of Cu(II) sorbed to silica Surface-induced precipitation at low surface coverages, J. Colloid Interf. Sci., 185, 252, 1997. 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the sod—water or sod—air interface, resulting in molecular layers on the surface of sod particles. Experimentally, sorption is characterized by the loss of pesticide from the sod solution, making it almost impossible to distinguish between sorption in which molecular layers form on sod particle surfaces, precipitation in which either a separate soHd phase forms on soHd surfaces, covalent bonding with the sod particle surface, or absorption into sod particles or organisms. Sorption is generally considered a reversible equdibrium process. 

Also, the figures contain nephelometric curves for the product t (that is, for p + s, in the proportions formed in the synthesis) and the fraction o, which was also a thermally precipitating product and deposited onto the walls of the reaction vessel in the course of the copolymerization of NVC1 with NVIAz at their initial molar ratio of 85 15 (Table 1). One can see that the precipitation behaviour of the total product t differs, although the amount of the s-fraction is almost the same at 31-33%. Obviously, this depends on the properties of the s-fraction. For instance, the heat-induced precipitation of the sample t formed from the feed with a comonomer molar ratio of 90 10 (Fig. 3b) is suppressed by the presence of its own s-fraction to a markedly lesser extent when compared to the product t obtained at the comonomer molar ratio of 85 15 (Fig. 3a). Most likely, such differences reflect the divergent influence of the s-fractions on the coagulation processes in the thermo-precipitating fractions of the total product t. These differences, for example different surface... 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

If no crystals appear on cooling, you will have formed a supersaturated solution and, to induce precipitation of the solute, you must provide sites for nucleation and crystal growth. This can be achieved by either seeding the solution by adding a few crystals ( dust ) of the crude compound or scratching the inside of the flask at the surface of the liquid, using a Pyrex glass rod (Fig. 13.2). 

However, the mechanism of PEG induced precipitation is almost unknown. The interactions between the protein (component 2), PEG (3) and water (1) as well as the local properties of PEG+water mixed solvent near the protein surface are of particular interest from this point of view. 

If the protective current induces precipitation of an inorganic scale on the cathode surface, such as in hard waters or in seawater, the total required current, as described earlier, falls off as the scale builds up. However, at exposed areas of metal, the current density remains the same as before scale formation only the total current density per apparent unit area is less. 

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