Adsorption from solutions on synthetic hydroxyapatite: nonaqueous vs. aqueous solvents

It is useful in restorative dentistry and etiology of bone diseases to know that certain general rules may be derived from a priori considerations of the adsorption of chemicals to synthetic hydroxyapatite (which is the structural prototype of tooth and bone mineral). This article systematically synthesizes the extensive studies on the subject by the author, and rationalizes and extends these rules. Though not a review article, the work of some researchers may not have found an adequate representation here, because they did not determine, in the case of nonaqueous solvents, the reversibility of adsorption; or, in the case of aqueous solvents, the concentration of all constituent ions of the system, thus missing the occurrence of processes other than adsorption. The adsorption of solutes from nonaqueous solvents on hydroxyapatite is mainly regulated by the interplay of hydrogen bonding between adsorbate, adsorbent, and solvent. It does not involve any significant role of ionic nature of the apatite surface, because it is masked by the chemisorbed and physisorbed water. This interplay of hydrogen bonding in conjunction with chemical and structural characteristics of the adsorbed molecules controls their reversibility and the orientation of the adsorbates on the surface. In general, the process may be considered as true adsorption, because no other material exchange process is involved. The "adsorption" of solutes from aqueous solutions generally involves ion-exchange with the apatite surface, and may be affected by the concentrations of potential determining ions (Ca(2+), phosphates, H(+), etc.) in the solution. The solute in aqueous solution may be removed by two other mechanisms, either by formation of a surface complex or by chemically reacting with calcium or phosphate ions to form a new phase that precipitates out of solution. Therefore, the concentration of calcium and phosphate ions in solution should also be monitored to elucidate the mechanism of the process. Adsorption of polymers seems to be determined primarily by their multiple hydrogen bonding, their relative molar mass, and their ability to self-associate. The hydrogen ion concentration plays a decisive role in each one of the above mechanisms.