Mechanisms of dissolution

In a polyphasic ceramic, the total rate of dissolution (R ot) is the sum total of the rates of individual phases. The rates of individual phases are also a function of fheir areas exposed to the corrosive medium. Thus, we have to consider their weighted surface areas. The weighfed surface area of fhe i-fh phase is given by ifs surface area (SJ divided by fhe fofal surface area of fhe ceramic body (S) under consideration. Therefore, is given by  

During the dissolution of the bulk ceramic, there could be three types of secondary effects. As the amount of major phases will be more in the solution, this solution can modify the dissolution of the minor phases. Secondly, solvents can induce chemical reactions among the phases. The insoluble reaction products formed out of the major phases can cover the minor phases and thereby reduce their dissolution. 

The second mechanism for the corrosion of ceramics is congruent dissolution by chemical reaction with the solvent. Here, the dissolution takes place by the attack of the solvent on the surface of the ceramic. The attack is uniform. Hence, the aftereffect of this kind of dissolution is the same as that of the first—that is, there will be uniform retreat of the surface. 

Incongruent dissolution with the formation of crystalline reaction products is the third mechanism for the corrosion of ceramics. In this mechanism, on reaction between the ceramic and the solvent, a crystalline reaction product is formed. It happens in the dissolution of strontium titanate in an acid. The reaction is shown in Equation 5.2. 

Concentration profile for congruent dissolution by simple dissociation. 

---

The properties of water near ionic salt surfaces are of interest not only for the understanding of the mechanism of dissolution processes but also for the understanding of the chemistry in the atmosphere next to oceans [205]. Experiments in UHV [205-208] indicate that the water-covered NaCl surface is quite stable at low temperatures. An early simulation study by Anastasiou et al. [209] focused on the arrangements and orientations of water molecules in contact with a rigid NaCl crystal. Ohtaki and coworkers investigated the dissolution of very small cubic crystals of NaF, KF, CsF, LiCl, NaCl, and KCl [210] and the nucleation [211] of NaCl and CsF in a... 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Based on an analysis of the initial dissolution rate in different solutions at different temperatures, several very useful conclusions and recommendations were made. It was found that the apparent activation energies for the dissolution of niobium and tantalum in 10 mol/1 HF solution are 56.5 and 65.5 kJ/mol, respectively for columbite, and 42.7 and 61.1, respectively, in the case of tantalite. It was also concluded that the mechanism of dissolution is the same for both columbite and tantalite. In addition, the initial dissolution rate of niobiuth (RNb) from columbite is controlled primarily by the activities of the... 

The properties of SEI electrodes, the growth rate of the SEI, the mechanism of dissolution and deposition, and the effects of various factors on SEI conductivity have been addressed elsewhere [1, 2] space limitations do not permit their repetition here. 

LJ Naylor, V Bakatselou, JB Dressman. Comparison of the mechanism of dissolution of hydrocortisone in simple and mixed micelle systems. Pharm Res 10 865-870, 1993. 

It is much less clear how the adsorption leads to such a dramatic change as a potential decay of several hundred volts, occurring within milliseconds. This short time is difficult to associate with film thinning, as assumed in the adsorption mechanism of pit initiation. It is not only that the mechanism of dissolution changes so much that the current efficiency falls from virtually 100% to virtually zero, but also that the resistance of the oxide decreases by orders of magnitude. The control of the process is, to a great extent, taken over by the events at the O/S interface, judging from the capacitance values measured,115 which approach those typical of the electrochemical double layer (cf. Fig. 22). 

This phenomenon, however, is not difficult to understand in view of the mechanism of dissolution under such conditions. Since the number of active sites increases linearly with current density and these sites are characterized by a film structure (or thickness or both) different from that at the OCP, one could expect corresponding increases in the corrosion rate. However, as was mentioned earlier, the active surface area in the pits increases with time, and hence one should expect the corrosion rate to increase correspondingly. Therefore, since the effect is not time dependent, one... 

The fact that impurities do not affect the active dissolution in chloride solutions at current densities larger than 0.01mA/ cm2 shows that the inhomogeneity resulting in a pitting mechanism of dissolution is unrelated to impurities and is an inherent property of the metal. 

Mechanisms of dissolution kinetics of crystals have been intensively studied in the pharmaceutical domain, because the rate of dissolution affects the bioavailability of drug crystals. Many efforts have been made to describe the crystal dissolution behavior. A variety of empirical or semi-empirical models have been used to describe drug dissolution or release from formulations [1-6]. Noyes and Whitney published the first quantitative study of the dissolution process in 1897 [7]. They found that the dissolution process is diffusion controlled and involves no chemical reaction. The Noyes-Whitney equation simply states that the dissolution rate is directly proportional to the difference between the solubility and the solution concentration ... 

Equations (36) and (38) are generally applicable, independent of the mechanism of dissolution. 

Conditions in the dissolution medium, which, together with the nature of the dissolving solid, determine the dissolution mechanism (see Theories and Mechanisms of Dissolution, pp. 355-358). 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

The dissolution rate and mechanism of dissolution for the soluble pigment particles. Related to that, the characteristics of the porous layer subsequently formed and its influence on the diffusion of species in and out of the paint. 

Wollast, R. (1990), "Rate and Mechanism of Dissolution of Carbonates in the System CaC03-MgC03", in W. Stumm, Ed., Aquatic Chemical Kinetics, Reaction Rates of Processes in Natural Waters, Wiley-Interscience, New York, pp. 431-445. 

Chou, L, R.M. Garrels, and R. Wollast (1989), "Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals", Chemical Geology 78, 269-282. 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

A different pattern of dissolution was seen with a Zn-Sn alloy containing 26% zinc. In this case the stable dissolution situation established after ca. 90 min showed a ratio of EC to CMT measurements of 1 4. As seen in Fig. 3, this remained fairly constant, though the corrosion potential increased by more than 50 mV. Only selective zinc dissolution took place, and analysis by atomic absorption spectroscopy of the amount of dissolved zinc agreed within 10% with the value according to the titration. This pattern is still difficult to understand. The ratio of ca. 1 4 between EC and CMT measurements could be interpreted in terms of formation of the low-valent zinc species ZnJ, which seems unlikely, or in terms of dissolution of divalent zinc ions accompanied by loss of chunks consisting of precisely three zinc atoms, each time a zinc ion is dissolved. The latter alternative seems to require a more discrete mechanism of dissolution than... 

Blesa MA, Marinovich HA, Baumgartner EC, Maroto AJG. 1987. Mechanism of dissolution of magnetite by oxalic acid ferrous iron solutions. Inorganic Chemistry 26 3713-3717. 

Most dissolution studies concentrate on establishing the mechanism of dissolution. There are few studies in which different oxides have been compared to provide... 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

The mechanism of dissolution of the Cu UPD layer on Au(lll) has been studied by Ataka etal. [409]. The monolayer comprised Cu in 2/3 and sulfate in 1/3 proportions. It has been found by applying time-resolved surface-enhanced infrared absorption spectroscopy and chronoam-perometry that dissolution proceeds in two steps. In the first step, 1/3 Cu monolayer and all sulfate anions are removed via the Langmuir-type kinetics. In the second step, the rest of Cu is desorbed via nucleation and growth kinetics. 

When working with capillary columns, the splitless mode is used for very dilute samples. In this mode, the injection is made very slowly, leaving valve no. 2 in the closed position (Fig. 2.5) for approximately 0.5 to 1 min. This allows vaporisation of the compounds and solvent in the first decimetre of the column by a complex mechanism of dissolution in the stationary phase, which is saturated with solvent. Compound discrimination is very weak using this method. The proper use of this injection mode, which demands some experience, requires a temperature program that starts with a colder temperature so that the solvent can precede the analytes in the column. This mode is typically used for trace analyses. The opening of valve no. 2 eliminates, from the injector, compounds which are less volatile and that can interfere with the analyses. 

Naylor et al. (1993) studied the ability of lecithin to modify the rate and mechanism of dissolution of hydrocortisone in the presence of sodium taurocholate (NaTC) solutions. They found that in the presence of lecithin, the CMC of NaTC dropped owing to the more effective solubilization capacity of the mixed micelle. Furthermore, the CMC value dropped more on saturation with hydrocortisone, implying some interaction between hydrocortisone and the NaTC/lecithin micelles. These results indicated that in the NaTC-only system, wetting effects predominated dissolution, whereas in the NaTC/lecithin system, the dissolution rate of hydrocortisone was enhanced mainly through solubilization. 

As a chemical phenomenon, weathering can be viewed as the result of the tendency of the rock-water-mineral system to attain equilibrium. This occurs through the usual chemical mechanisms of dissolution and precipitation, acid-base reactions, complexation, hydrolysis, and oxidation-reduction. 

Wollast R. and Reinhard-Derie D. (1977) Equilibrium and mechanism of dissolution of magnesium calcites. In The Fate of Fossil Fuel CO2 in the Oceans (eds. N.R. Andersen and A. Malahoff), pp. 479-492. Plenum Press. New York. 

Three fluoride ions have dissolved within 12 ps. Some fluoride ions move around the surface of the crystal but are not separated from it. The position of fluoride in the crystal fluctuates more rapidly than that of cesium (Fig. 4b). The mechanism of dissolution of the CsF in water will be discussed in a later section. 

R. Wollast, Rate and mechanism of dissolution of carbonates in the system CaC03-MgC03, Chap. 15 in Stumm, op. cit.3... 

---