Solubility gradient

Because these solvents are used in the open, evaporation can be a problem. Continued use of a mobile phase solvent mixture will probably result in preferential evaporation of the most volatile component, thus changing the composition. Demixing can also occur on the bed if the solvents do not have a high degree of mutual solubility. Gradient techniques can be used in the mobile phase and in the stationary phase.19... 

The addition of alloying elements can also indirectly affect the oxygen solubility gradient across the thin metallic surface layer by modifying the activity of an element that interacts with Al. For example, Fe forms very stable intermetallics with silicon, while the Fe-Al and Fe-Mg intermetallics are less stable. Additions of Fe to an Al-Si-Mg alloy, therefore, will decrease the Si activity, and thereby increase the Mg activity. This will lead to a lower oxygen potential gradient and a lower growth rate. 

When the rate of quartz cementation becomes significant at temperatures exceeding 80-100°C, the porosity loss is mainly due to quartz cementation. Calculations show that the advective transport of silica in the pore water can be ignored as the low flow rates due to compaction are so low and because of the low solubility and solubility gradients of silica. The rate of import or export of silica is a function of the water flux (F), the solubility of quartz as a function of temperature (at) and the geothermal gradient (VT)... 

Fig. 11 Schematic plot of the Gas phase solubility gradient and transport processes within the melt film. At the melt/gas interface, the oxide precipitates.

The reaction Eqs. (63 and 65) are producing oxide ions at the melt/scale interface, whereas at the melt/gas interface, oxide ions (in form of C03 ) are consumed by the dissolution reactions involving O2 in the melt. This gives rise to a gradient in 0 concentration toward the melt film and a solubility gradient, as described in Sect. 6.1.4.1. 

Barium thiocyanate is a white solid that is very soluble in water but has a very steep temperature-solubility gradient. This solid is also soluble in acetone, methanol, ethanol, methylamine and ethylamine, moderately soluble in isopropylamine and dimethylamine, but insoluble in trimethylamine. The anhydrous salt is very hygroscopic. Crystallization from water yields the 3-hydrate as well-formed, needle-shaped crystals. Double salts are formed with the thiocyanates of the alkali and other alkaline earth metals. ... 

When a negative solubility gradient exists from the metal surface to the outside of the salt layer, dissolution of the NiO film covering the metal is followed by reprecipitation of NiO at some distance away from the surface. The oxide precipitate is highly porous and therefore non-protective [23]. Figure 9.40 schematically shows such a situation. 

For ionomeric systems in which the strong interactions between ionic sites and the penetrant (water) result in concentration dependent diffusion coefficients, the Fickian diffusion model, which assumes the solubility coefficient is independent of the concentration, is not valid. The commonly used Fickian diffusion constants actually contain mobility and solubility gradient contributions [19], In addition, the concentration dependent solubilities lead to nonlinear concentration profiles during steady state diffusion. Therefore, mobility measurements which generate average diffusion coefficients are generally not satisfactory. 

Rapp and Goto pointed out that in order to sustain the dissolution reaction a solubility gradient must be present in the molten salt layer. Thus oxide can dissolve at the oxide/melt interface, migrate down a concentration gradient to a site of lower solubility where precipitation occurs this is... 

Rapp and Goto (1981) proposed that, if the gradient in solubility of the protective oxide with distance into the salt layer was negative at the oxide/ salt interface, accelerated attack could be sustained. This provides a quantitative expression for the earlier concepts of solubility gradients and oxide precipitation, that is, when... 

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