Kinetics of dissolution of calcium hydroxyapatite powder. III: pH and sample conditioning effects |
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Authors: | J M Thomann J C Voegel Ph Gramain |
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Institution: | 1. C.T.R. Odontologiques, Inserm U-157, CNRS UA-105, 1, place de H?pital, 67000, Strasbourg, France 2. Institut Charles Sadron (C.R.M.-E.A.H.P.), CNRS-ULP, Strasbourg Cedex, France
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Abstract: | Summary The kinetics of dissolution of synthetic hydroxyapatite powder (HAP) were studied at 37°C and constant pH in the pH range
3.7–6.9 by continuously recording proton uptake and calcium release. The effect of sample conditioning was carefully investigated.
The powder previously equilibrated in saturated solutions shows an initial dissolution rate higher than the one obtained when
dry powder directly added to the dissolution solution is used. This effect is interpreted by considering surface state differences.
As previously shown, dry powder contains important amounts of calcium and phosphate ions adsorbed onto apatite surface, ions
which are desorbed during equilibration. It is assumed that the initial presence of these ions slows the dissolution rate
during the first stage of the process by the formation of a permselective layer. Except for these adsorption phenomena which
are less important for human enamel powder (HEP) having a lower specific surface area, it is shown that in spite of structural,
morphological, and purity differences, the general dissolution behavior of HAP is quite similar to that of HEP, previously
studied, and for which a quantitative model has been proposed. The dissolution rates are stirring dependent in a large range
of stirring speeds and are proportional to H+]0.64. Moreover, it is shown that in the whole range of studied pH, a calcium accumulation process occurs at the interface during
the first minutes of the acidic attack. It is concluded that in our experimental conditions, the dissolution process is limited
by the diffusion of calcium and/or phosphate ions in the interface. The calcium-rich interface constitutes a layer of low
permeability in which strong interactions considerably reduce the diffusion of calcium and/or phosphate ions released during
the attack and thus considerably slows the dissolution process. |
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Keywords: | Hydroxyapatite Dissolution Calcium accumulation Diffusion-controlled process Adherent surface layer |
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