Preferential dissolution

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

Formation of PS is due to preferential dissolution of a silicon surface the rate is larger at some areas of the surface relative to others. Such relative rates with respect to the spatial position of the areas, on which these processes occur, are determined more by the relative nature and less by the absolute nature of the physical and chemical dimensions and events. Specifically, the relative nature of the following aspects is important in determining the morphology of PS ... 

Relative contribution of the preferential dissolution along direction of carrier source versus that along <100> direction... 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

The explanation for this behavior is similar to that given in the preceding section for nonionic surfactant mixtures. Adding a hydrophihc anionic surfactant raises the temperature at the cloud point and other phase transitions above those for pure Ci2(EO)4. If the amount of anionic added exceeds only slightly that needed for complete solubility, the final stages of the dissolution process are slow because preferential dissolution of the anionic causes the remaining drop to rise above its cloud point and nucleate small droplets of surfactant-rich liquid. But if the amount added is sufficiently large, drop composition remains below the cloud point in spite of preferential dissolution, with the result that dissolution is fast as with pure nonionic surfactants below their cloud points. 

Fig. 26 A Model of the preferential dissolution process of PA at low SDS concentration. B Model for the existence of vesicle and mixed micelles at high SDS concentration...

Earlier work (7) had identified similar aqueous PCB residues and attributed their presence at the study site to the preferential dissolution of the lower chlorinated congeners [primarily 2-monochlorobiphenyl (MCB),... 

Wet chemical methods of analyses of formulations contgaining TNT, as cited above, usually involve the application of preferential dissolution for resolving the mixt into its component parts (Ref 53). Titrimetric methods for the quant determination of the resolved components are also presented (Refs 23,26,11, 66 98)... 

Modifications of the chemical nature of the catalyst under cathodic load are also possible. Sulphides can be reductively dissolved with liberation of H2S [139]. Oxides can be progressively reduced with loss of the specific activity [140]. In the latter case, an additive can be used to diminish the rate of reduction. Intermetallic compounds or alloys may exhibit preferential dissolution of one of the components during cathodic performances in concentrated alkali [141],... 

Similarly, a number of amorphous alloys based on Fe-Zr, Ni-Zr, Co-Zr, Ni-Nb, have not shown any increase in activity over that expected for the mechanical mixture of the crystalline components [571]. For Ni-Nb the overpotential has even increased. Only Cu-Ti alloys have shown apparent synergetic effects, but the results of Machida et al. [89] (cf. Fig. 32) should also be taken into account. Jorge et al. [152] have observed higher activity for the amorphous form of Cu-Ti alloys, but they have attributed it to the preferential dissolution of Ti in the amorphous sample under cathodic load, with formation of a relatively porous Cu layer. The same effect was obtained more rapidly by means of HF etching [89,152]. 

The special construction of the spectrometer permits not only a safe specimen transfer without chemical changes, but also a well-defined specimen pre-treatment by sputtering previous to the electrochemical preparation. This is very important in the case of alloys because active dissolution or etching and transpassive corrosion or electropolishing may change the surface by preferential dissolution of one component. The altered surface composition may have an effect on the kinetics of passivation and on the composition of the passive layer, formed subsequently as has been... 

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