Crystal dissolution interface-reaction controlled

The concentration profiles for crystal growth under different controls and their evolution with time are shown in Figure 1-11. Whether crystal growth (or dissolution) is controlled by interface reaction or mass transfer can be determined experimentally using these criteria. Theoretically, when departure from equilibrium (i.e., degree of oversaturation or undercooling) is small (e.g., undercooling... 

Crystal dissolution/melting/growth may be controlled by interface reaction rate (Figure 1-lla), meaning that mass/heat transfer rate is very high and interface reaction rate is low. Examples include dissolution of minerals with low... 

Crystal dissolution and growth may also be controlled by both mass or heat transport and interface reaction (Figure 1-1 Id). In this case, the interface reaction... 

Diffusive crystal dissolution means that crystal dissolution is controlled by diffusion, which requires high interface reaction rate and absence of convection. In nature, diffusive crystal dissolution is rarely encountered, because there is almost always fluid flow, or crystal falling or rising in the fluid. That is, crystal dissolution in nature is often convective dissolution, which is discussed in the next section. One possible case of diffusive crystal dissolution is for crystals on the roof or floor of a magma chamber if melt produced by dissolution does not sink or rise. For these... 

Convective crystal dissolution means that crystal dissolution is controlled by convection, which requires (i) a high interface reaction rate so that crystal dissolution is controlled by mass transport (see previous section), and (ii) that mass transport be controlled by convection. In nature, convective crystal dissolution is common. In aqueous solutions, the dissolution of a falling crystal with high solubility (Figure 1-12) is convective. In a basaltic melt, the dissolution of most minerals is likely convection-controlled. 

When an ionic single crystal is immersed in solution, the surrounding solution becomes saturated with respect to the substrate ions, so, initially the system is at equilibrium and there is no net dissolution or growth. With the UME positioned close to the substrate, the tip potential is stepped from a value where no electrochemical reactions occur to one where the electrolysis of one type of the lattice ion occurs at a diffusion controlled rate. This process creates a local undersaturation at the crystal-solution interface, perturbs the interfacial equilibrium, and provides the driving force for the dissolution reaction. The perturbation mode can be employed to initiate, and quantitatively monitor, dissolution reactions, providing unequivocal information on the kinetics and mechanism of the process. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

Under most of the natural conditions, the rate of dissolution of carbonate minerals is far less than that expected for rate control by diffusion. The chemical reaction at the water-mineral interface is then assumed to be the rate-determining step. This reaction consists in the attachment or interactions of reactants with specific surface sites where the critical crystal bonds are weakened, which, in turn, allows the detachment of anions and cations of the surface into the solution. 

Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH>4 monomers and NP+ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolyinerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 

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