Precipitation electrolyte role

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

Minor differences between the three electrolyte solutions are also observed. First, electrolyte number 3 only shows a peak maximum in the current-potential curves at potentials higher than 8 V. However, this is very clear because its pH value is smaller, indicating that this electrolyte solution possesses a higher buffer capacity against consumption of hydrogen ions in the vicinity of the fibre surface, avoiding hydrogen gas formation and Ni(OH)2 precipitation. Secondly, at a potential of 4V, no deposition occurred in electrolyte solution number 3, indicated by the absence of an increase in the measured electrical current and confirmed by XPS data. Additionally in this case, the lower pH plays an important role because of the lower pH value, the applied potential difference does not overlap with the potential window in which the reduction of Ni(II) occurs. Therefore no deposition is observed. 

Salinity Salinity plays at least two important roles, namely it maintains the integrity of the reservoir and it balances the physicochemical environment so that surfactant formulation stays close to optimal. Thus, ultra-low interfacial tension and oil solubilisation are very sensitive to salinity. Mixing of the surfactant slug with connate water may alter the surfactant formulation mainly due to dilution and to the incorporation of new electrolytes to the formula. Adsorption and desorption of electrolytes, particularly divalent cations, onto or from solid materials such as clay, will also change the salinity of the aqueous phases to some extent and may cause surfactant precipitation, which is however not always an adverse effect [151]. In order to attenuate the undesirable salinity effects on formulation, surfactants able to tolerate salinity changes [109], high salinity [150] and the presence of divalent ions [112] maybe used. 

Other reports (27-29) have focused on the role of citric acid, as a source of carboxylate anions, during precipitation of calcium phosphates from electrolyte solutions. It has been found that citrate anions inhibit the ciystal growth of calcium phosphates and hinder their transformation into hydroxyapatites. This was attributed to the adsorption of citrate anions into the crystals and the displacement of an equivalent amount of phosphate anions. Interestingly, Rhee and Tanaka (30) found that the presence of a collagen membrane in the medium changed the behavior of citrate anions from being an inhibitor to becoming a promoter of calcification, provided that the molar ratios of calcium to citric acid were between 2 and 12. 

In general, the precipitation of proteins requires the presence of alcohol, tannins or heating. Furthermore, it only takes place in the presence of electrolytes. The role of alcohol, tannin or heating is to denature the protein. The protein, a hydrophilic colloid, becomes a hydrophobic colloid that can be flocculated by... 

This approach involves considerations of surface catalysis which occurs prior to thin-film deposition. In this model the induction period for formation of a catalytic surface for thin-film growth tc is the probe of the surface catalysis. From Equation 5.74, it can be seen that there is no film deposition when no catalytic surface is created on the substrate surface, that is, tc and facile thin-film formation when a catalytic surface is provided, tc 0. On the other hand, when y 0, no precipitate formation occurs in the electrolyte and a catalytic surface is provided on the substrate. The model is based on only the effect of the concentration of metal ions and not on each of the components (anion concentrations, supporting electrolyte, pH adjustment, etc.), all of which play an important role in the growth rate and film thickness as indicated elsewhere [23, 25, 65]. 

We need to know which way reactions will go when solutions are involved, such as minerals dissolving/precipitating, or electrolytes dissociating. We have a thermodynamic potential (the Gibbs energy) which fills this role for pure phases, so we just need to know how to determine it for dissolved substances. In addition, we need to know how it changes with T, P, and composition of the solution, which involves knowing how to determine the temperature, pressure, and compositional derivatives of this quantity. 

Many texts and monographs have been written on the subject of solution-precipitation phenomena. The formation of colloid phases and the rheological properties of suspensions are outside the scope of this chapter. The electrical properties of the solid surface and the associated layer of electrolyte or solvent phase play major roles in the transport and flocculation of the solid phase. 

Fiiredi-Milhofer (Israel), in Chapter 11, provides a broad overview of the role of thermal analysis techniques in basic and applied studies of the formation and transformation of crystalline dispersions. Crystalline disperisons are formed by a succession of some of the following precipitation processes nucleation, crystal growth, flocculation, Ostwald ripening, and/or phase tfansformation. After a brief elaboration of the theories underlying these processes, a review is given of experimental studies on the formation and transformation of ionic precipitates from bulk electrolyte solutions. 

NOW Nowicka, G. and Nowicki, W., On the role of electrolytes in precipitation of polyacrylarrride from aqueous solution by addition of methanol (Russ.), Vysokomol. Soedin., Ser. A, 44, 2030, 2002. 

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