Ruikang Tang

Constant composition kinetics study of carbonated apatite dissolution

Abstract The carbonated apatites (CAP) may be more suitable models for biominerals such as bone and dental hard tissues than is pure hydroxyapatite (HAP) since they have similar chemical compositions. Although they contain only a relatively small amount of carbonate, the solubility and dissolution properties are different. The solubility product of the CAP particles used in this dissolution study, 2.88×10 −112 xa0mol 18 xa0l −18 , was significantly greater than that of HAP, 5.52×10 −118 xa0mol 18 xa0l −18 . The kinetics of dissolution of CAP has been studied using the constant composition (CC) method. At low undersaturations, the dissolution reaction appeared to be controlled mainly by surface diffusion with an effective reaction order of 1.9±0.1 with respect to the relative undersaturation. These results together with those obtained by scanning electron microscopy (SEM) suggest a dissolution model. Based on the surface diffusion theory of Burton, Cabrera and Frank (BCF). The interfacial tension between CAP and the aqueous phase calculated from this dissolution model, 9.0xa0mxa0Jxa0m −2 , was consistent with its relatively low solubility. An abnormal but interesting dissolution behavior is that the CAP dissolution rate was relatively insensitive to changes in calcium and phosphate concentrations at higher undersaturations, suggesting the importance of the carbonate component under these conditions.

Constant composition dissolution of mixed phases: II. Selective dissolution of calcium phosphates

Characterization of the dissolution kinetics of individual synthetic and biological calcium phosphates is of considerable importance since these phases often coexist in biological minerals. The constant composition method has been used to study the dissolution kinetics of a series of synthetic calcium phosphates, brushite (DCPD), beta-tricalcium phosphate (TCP), octacalcium phosphate (OCP), hydroxyapatite (HAP), and carbonated apatite (CAP) in the presence and absence of citric acid, as a function of pH and thermodynamic driving force. While citric acid markedly accelerates the dissolution of TCP, HAP dissolution is significantly inhibited. Moreover, this additive has almost no influence on the dissolution of DCPD, OCP, and CAP. Dual constant composition dissolution studies of mixed calcium phosphates in the presence of citric acid have also been made. Another factor, pH, also plays an important role in the dissolution of these calcium phosphates. In suspensions of calcium phosphate mixtures, specific phases can be selectively dissolved by changing experimental parameters such as pH and the presence of rate modifiers. This result has important applications for the dissolution control of dental hard tissues such as dentin, enamel, and calculus.

Phosphorylation of osteopontin is required for inhibition of calcium oxalate crystallization.

Under near-physiological pH, temperature, and ionic strength, a kinetics constant composition (CC) method was used to examine the roles of phosphorylation of a 14 amino acid segment (DDVDDTDDSHQSDE) corresponding to potential crystal binding domains within the osteopontin (OPN) sequence. The phosphorylated 14-mer OPN peptide segment significantly inhibits both the nucleation and growth of calcium oxalate monohydrate (COM), inhibiting nucleation by markedly increasing induction times and delaying subsequent growth by at least 50% at concentrations less than 44 nM. Molecular modeling predicts that the doubly phosphorylated peptide binds much more strongly to both (-101) and (010) faces of COM. The estimated binding energies are, in part, consistent with the CC experimental observations. Circular dichroism spectroscopy indicates that phosphorylation does not result in conformational changes in the secondary peptide structure, suggesting that the local binding of negatively charged phosphate side chains to crystal faces controls growth inhibition. These in vitro results reveal that the interactions between phosphorylated peptide and COM crystal faces are predominantly electrostatic, further supporting the importance of macromolecules rich in anionic side chains in the inhibition of kidney stone formation. In addition, the phosphorylation-deficient form of this segment fails to inhibit COM crystal growth up to concentrations of 1450 nM. However, at sufficiently high concentrations, this nonphosphorylated segment promotes COM nucleation. Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) results confirm that aggregation of the nonphosphorylated peptide segment takes place in solution above 900 nM when the aggregated peptide particles may exceed a well-defined minimum size to be effective crystallization promoters.

Size-effects in the dissolution of hydroxyapatite: an understanding of biological demineralization

A comprehensive dissolution theory that integrates size effects and surface energy control into the overall reaction rate has been developed. This model is successfully applied to the dissolution of hydroxyapatite (HAP) at different undersaturations, using the highly reproducible Constant Composition (CC) method. It is found that the commonly used rate law is not suitable for the dissolution of nanoscale particles where the sizes of the crystallites must be taken into account. Dissolution is induced by the formation of pits and continues with the spreading of their stepwaves. However, only the larger pits (of size greater than a critical value, r*) are active, with stepwaves contributing to dissolution and the spreading velocities are also dependent on the pit sizes, decreasing with decreasing pit size. Size effects dominate the reaction when the crystallite sizes are of the same order as r*, resulting in self-inhibition of dissolution and even reaction suppression. A metastable zone of undersaturation manifests itself when particle and pit critical sizes approach each other. These findings are also applicable for in vivo demineralization of biological materials such as tooth enamel. As the biomaterials select nanosized particles as their basic building blocks, the size-effects are magnified and the materials are insensitive to demineralization. These effects confer on biominerals the ability to be stabilized against dissolution in the undersaturated biological milieux.

New mechanism for the dissolution of sparingly soluble minerals

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 minerals.

An Understanding of Renal Stone Development in a Mixed Oxalate–Phosphate System

The in vivo formation of calcium oxalate concretions having calcium phosphate nidi is simulated in an in vitro (37 degrees C, pH 6.0) dual constant composition (DCC) system undersaturated (sigma DCPD = -0.330) with respect to brushite (DCPD, CaHPO 4 . 2H 2O) and slightly supersaturated (sigma COM = 0.328) with respect to calcium oxalate monohydrate (COM, CaC2O4 . H2O). The brushite dissolution provides calcium ions that raise the COM supersaturation, which is heterogeneously nucleated either on or near the surface of the dissolving calcium phosphate crystals. The COM crystallites may then aggregate, simulating kidney stone formation. Interestingly, two intermediate phases, anhydrous dicalcium phosphate (monetite, CaHPO4) and calcium oxalate trihydrate (COT), are also detected by X-ray diffraction during this brushite-COM transformation. In support of clinical observations, the results of these studies demonstrate the participation of calcium phosphate phases in COM crystallization providing a possible physical chemical mechanism for kidney stone formation.