New mechanism for the dissolution of sparingly
Ruikang Tang and George H. Nancollas
Department of Chemistry, Natural Sciences Complex,
University at Buffalo, State University of New York, Buffalo, NY 14260,
Abstract: A requirement for the determination of the solubility
of minerals is to ensure that equilibrium has been reached. Recent constant
composition dissolution studies of sparingly soluble calcium phosphates
have revealed an interesting and unusual behavior in that the rates
decreased, eventually resulting in effective reaction suppression, even
though the solutions remained undersaturated. Traditional theories of
dissolution assume a volume diffusion-controlled mechanism with reaction
continuing until true equilibrium has been reached. The new results
for sparingly soluble salts point to the importance not only of particle
size on the dissolution rate but also the participation of critical
phenomena. Although the crystal size decreases during dissolution, when
the reaction is controlled by poly-pitting (the formation and growth
of pits), the edge free energy increases at the very first stage of
reaction owing to the creation of pits and dissolution steps. The constant
composition experimental results demonstrate the development of surface
roughness as the dissolution steps are formed, implying an increase
of the total edge length during the reaction. This is an exactly analogous
mechanism to that of crystal growth, in which the formation of embryos
of critical size plays a key role in the overall mechanism. In contrast
to crystal growth, dissolution is a process of size reduction, and,
when the particle size is sufficiently reduced, critical phenomena become
important so that the influence of size must be taken into consideration.
It is interesting to recognize that these critical phenomena are readily
apparent for sparingly soluble minerals for which the critical conditions
are attained much more readily. The results point to the importance
of understanding the detailed mechanism of dissolution when attempts
are made to measure, experimentally, the solubilities of sparingly soluble
* Lectures presented at the 10th International Symposium
on Solubility Phenomena, Varna, Bulgaria, 22 -26 July 2002. Other
lectures are published in this issue, pp. 1785-1920.
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