Dissolution rates and mechanisms

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. 

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. 

In this chapter, reductive dissolution rates and mechanisms of oxides will be discussed. Additionally, oxidation kinetics of As(III), Cr(III), and Pu(III) by Mn(III/IV) oxides will covered. 

The study of chemical weathering, and more specifically determining mineral dissolution rates and mechanisms, has been a topic of great interest to geochemists for over 100 years. An excellent book which covers the field has been published recently (4). 

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. 

Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is important in determining rates of each of these contributing steps. Limited available evidence concerning rates and mechanism of surface chemical reactions and analogy to similar reactions in homogeneous solution both support this conclusion. 

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

Surfactants are used in a variety of applications, frequently in the form of dilute aqueous solutions. However, it is not cost effective to transport, store, and display in retail outlets surfactant products such as household detergents in this form. Accordingly, it is important to have products that dissolve quickly and to understand what aspects of surfactant composition and structure promote rapid dissolution. The dissolution process is more complex for surfactants than for most other materials because it typically involves formation of one or more concentrated and highly viscous liquid crystalline phases, which are not present initially and which could potentially hinder dissolution. In this article the rates and mechanisms of surfactant dissolution are reviewed and discussed. 

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. 

While particle size continues to be the physical property most frequently determined by NIR spectroscopy, several other parameters including the dissolution rate and the thickness and hardness of the ethylcellulose coating on theophylline tablets have also been determined using NIR, all with good errors of prediction.94 Tablet hardness, which dictates mechanical stability and dissolution rate, has been determined in hydrochlorothiazide tablets.95... 

Dissolution occurring by a surface reaction is often slower than by transport-controlled kinetics because the latter results from more rapid surface detachment. There appears to be a good correlation between the solubility of a mineral and the rate-controlling mechanism for dissolution. Table 7.1 lists dissolution rate-controlling mechanisms for a number of substances. The less soluble minerals all dissolve by surface reaction-controlled kinetics. Silver chloride is an exception, but its dissolution... 

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

Heterogeneous nucleation mechanisms can significantly affect the dissolution of metastable solid phases, because this form of nucleation can occur at low driving forces. While the choice of a metastable solid phase with solubility higher than other crystalline modifications is motivated by the expectation of faster dissolution rates, achievement of faster dissolution rates and higher concentrations in solutions is jeopardized by surface-mediated nucleation. We have reported that the surface of the metastable phase of theophylline promoted the nucleation of the stable monohydrate crystals. The observed oriented growth of monohydrate crystals on the anhydrous surface is consistent with a close lattice match between the b and c... 

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