Abstract
The kinetics of the dissolution of salts and minerals remains a field of active research because these reactions are important to many fields, such as geochemistry, extractive metallurgy, corrosion, biomaterials, dentistry, and dietary uptake. A novel model, referred to as the surface-vacancy model, has been proposed by the author as a general mechanism for the primary events in dissolution. This paper expands on the underlying physical model while serving as an update on current progress with the application of the model. This underlying physical model envisages that cations and anions depart separately from the surface leaving a surface vacancy of charge opposite to that of the departing ion on the surface. This results in an excess surface charge, which in turn affects the rate of departing ions. Thus, a feedback mechanism is established in which the departing of ions creates excess surface charge, and this net surface charge, in turn, affects the rate of departure. This model accounts for the orders of reaction, the equilibrium conditions, the acceleration or deceleration of rate in the initial phase and the surface charge. The surface-vacancy model can also account for the effect of impurities in the solution, while it predicts phenomena, such as ‘partial equilibrium’, that are not contemplated by other models. The underlying physical model can be independently verified, for example, by measurements of the surface charge. This underlying physical model has implications for fields beyond dissolution studies.
Highlights
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The effects of H+ and OH− on the rate of dissolution expressed in terms of the order of reaction with respect to these reactants; The approach to equilibrium, including the effects of constituent ions in solution on the rate of dissolution, for example, Na+ on the rate of dissolution of NaCl; The change in rate of dissolution, either acceleration or deceleration, during the initial stage of reaction; The non-stoichiometric dissolution of more complex solids, such as felspars
In the sections that follow this foundational material, it is shown that the surface-vacancy model describes the orders of reaction with respect to H+ and OH−, that it describes the approach to equilibrium, that it describes the change in initial rate of dissolution, and that it describes the non-stoichiometric phases of dissolution
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Kinetic models proposed for dissolution reactions have generally been unsuccessful in three major areas: (i) the orders of reaction with respect to the concentrations of reactant in solution, mostly importantly H+ and OH− ions; (ii) the acceleration or deceleration in the initial rate of reaction; (iii) the development of surface charge during dissolution or crystallization The first of these principal difficulties in understanding the mechanism of dissolution is that the kinetics exhibit fractional orders of reaction. Rate expressions based on degree of saturation have been derived from crystal growth theories of the terrace-edge-kink type [30] These models are concerned mainly with the approach to equilibrium, and do not address any of the three challenges of dissolution reactions that are the subject of this paper, namely, fractional orders of reaction, accelerating or decelerating initial rates, and surface charge. The implications of the model are discussed, and suggestions for independent testing are made
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