Calcium hydroxyapatite surface structure

Vega, E. D., Pedregosa, J. C. Narda, G. E. 1999. Interaction of oxyvanadium (IV) with crystalline calcium hydroxyapatite surface mechanism with no structural modification. Journal of Physics and Chemistry of Solids, 60, 759-766. 

Wilson, Prosser Powis (1983) studied the adsorption of polyacrylate on hydroxyapatite using infrared and chemical methods. They observed an exchange of ions and concluded that polyacrylate displaced surface phosphate and calcium, and entered the hydroxyapatite structure itself (Figure 5.2). They postulated that an intermediate layer of calcium and aluminium phosphates and polyacrylates must be formed at the cement-... 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

Jager, C., Maltsev, S., and Karrasch, A. (2006) Progress of structural investigation of amorphous calcium phosphate (ACP) and hydroxyapatite (HAp) disorder and surfaces as seen by solid state NMR. Key Eng. Mater., 309-311, 69-72. 

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