Dissolution kinetics mechanism

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 ... 

Wehrli, B., E. Wieland, and G. Furrer (1990), "Chemical Mechanisms in the Dissolution Kinetics of Minerals the Aspect of Active Sites , Aquatic Sciences 52, 1-114. 

Most of the data in this chapter was obtained from laboratory experiments in which the dissolution kinetics were followed by monitoring the change in the level of iron released into solution. The dissolution rate and mechanism are often established on the basis of data corresponding to the first few percent of the reaction, (e.g. Stumm et ak, 1985). To insure that the initial stages are in fact representative of the behaviour of the bulk oxide ( and not an impurity, for example), a complete dissolution curve should be obtained in any investigation. 

Under hydrothermal conditions (150-180 °C) maghemite transforms to hematite via solution probably by a dissolution/reprecipitation mechanism (Swaddle Olt-mann, 1980 Blesa Matijevic, 1989). In water, the small, cubic crystals of maghemite were replaced by much larger hematite rhombohedra (up to 0.3 Lim across). Large hematite plates up to 5 Lim across were produced in KOH. The reaction conditions influenced both the extent of nucleation and crystal morphology. The transformation curve was sigmoidal and the kinetic data in water and in KOH fitted a first order, random nucleation model (Avrami-Erofejev), i.e. 

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

Ks do not match with each other. This is partly the result of the effects of the specific surface which was different in the two methods. However, the mechanisms of the dissolution kinetics seem to be identical. The reaction rate of the acid with carbonate mineral would be controlled by diffusion of the reactant into and the products out of the pores. Therefore, the availability of only the contact surface is not adequate. The type of surface in terms of relevant diffusion model and the closest theory to that model, such as film, penetration, or any other, should also be specified. 

Rate-Limiting Steps in Mineral Dissolution 146 Feldspar, Amphibole, and Pyroxene Dissolution Kinetics 148 Parabolic Kinetics 149 Dissolution Mechanism 155 Dissolution Rates of Oxides and Hydroxides 156 Supplementary Reading 161... 

Chou L., Garrels R.M. and Wollast R. (1989) Comparative study of the dissolution kinetics and mechanisms of carbonates in aqueous solutions. Chem. Geol. 78, 269-282. 

Stumm, W., and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981. Chapter 5 of this standard textbook gives many examples of the concepts discussed in the present chapter. A broad conceptual picture of mineral dissolution kinetics and mechanisms is developed in the celebrated three-part paper The Coordination Chemistry of Weathering ... 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Sjoberg, E.L. A fundamental equation for calcite dissolution kinetics. Geochim. Cosmochim. Acta 40, 441-447 (1976). Weiss, R.F. Carbon dioxide in water and sea water the solubility of a non-ideal gas. Mar. Chem. 2, 203-215 (1974). Lyman, J. Buffer mechanism of sea water. Ph. D. Thesis,... 

Recent measurements of calcium and alkalinity in the ocean above the calcite saturation horizon (Milliman et ai, 1999 Chen, 2002) suggest dissolution in supersaturated waters. The proposed mechanisms are variations of the organic matter driven CaC03 dissolution mechanism. In these cases the authors suggest that microenvironments in falling particulate material (Milliman et al., 1999) or anerobic dissolution in sediments of the continental shelves and marginal seas (Chen, 2002) are locations of CaC03 dissolution. As the details and accuracy of measurements improve, thermodynamic and kinetic mechanisms required to interpret the results become more and more complex. 

Other Approaches to the Investigation of Anodic Dissolution Kinetics and Mechanisms... 

In the study to characterize the dissolution kinetics of saccharin as a sweetener excipient, it was found that as dissolution continues, cracks appeared on the surface resulting in faster dissolution. Therefore Dr of Brownian and non-Brownian particles cannot be restricted to the range of 1-3. Using Dr as a parameter to distinguish dissolution mechanism is not appropriate because Dr depends on variety of parameters and not only on the type and size of the particles. 

Kinetic mechanisms of the dissolution of higher valence hydrous oxides by organic reductants have been extensively investigated (Hering and Stumm, 1990 Stone et al., 1994 Stumm, 1992) (Table 11.7) and are discussed in Section 13.3. 

Dissolution kinetics of 5-AI2O3 and BeO. Geochim. Cosmochim. Acta 50,1847-60. Gardiner, W. C. (1972) Rates and Mechanisms of Chemical Reactions. W.A. Benjamin. 

Petrovich, R. 1981. Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates - I. Deformation and dissolution of quartz under laboratory conditions. Geochim. Cosmochim. Aclu 45 1665-1674. 

The objectives of this chapter are (1) to illustrate that the surface structure is important in characterizing surface reactivity and that kinetic mechanisms depend on the coordinative environment of the surface groups, (2) to derive a general rate law for the surface-controlled dissolution of oxide and silicate minerals and illustrate that such rate laws are conveniently written in terms of surface species, and (3) to illustrate a few geochemical implications of the kinetics nf oxide dissolution. 

Dissolution kinetics of a simple component close to saturation and the mechanism of the backward precipitation reaction are still subject to controversy. Minerals such as calcite and aragonite are known to reach rapidly a dissolution equilibrium when placed in closed aqueous systems. According to simple and classical thermodynamical concepts, this requires that each forward... 

Fig. 4.12 Micelle kinetics mechanisms 1- formation-dissolution, 2 - rearrangement, 3 aggregation-disintegration...

In the discussion of the adsorption kinetics of micellar solutions, different micelle kinetics mechanisms are taken into account, such as formation/dissolution or stepwise aggregation/disaggregation (Dushkin Ivanov 1991). It is clear that the presence of micelles in the solution influences the adsorption rate remarkably. Under certain conditions, the aggregation number, micelle concentration, and the rate constant of micelle kinetics become the rate controlling parameters of the whole adsorption process. Models, which consider solubilisation effects in surfactant systems, do not yet exist. 

This mechanism is similar to the one occurring during liquid-phase sintering, where the dissolution of crystalline material into the glassy phase occurs at the interfaces loaded in compression and their reprecipitation on interfaces loaded in tension. The rate-limiting step in this case can be either the dissolution kinetics or transport through the boundary phase, whichever is slower. This topic was discussed in some detail in Chap. 10, and will not be repeated here. 

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