Surface modification of monetite in water at 37 °C: characterisation by XPS

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Abstract
Hydrolysis of dicalcium phosphate CaHPO 4 (DCPA) was studied as a function of time, in deionised water at 37 °C, with a high liquid/solid ratio (2500 ml g –1 ) in order to avoid solubility limits being reached. Analyses of the liquid showed that DCPA dissolution is not congruent. Although the solutions were not saturated as regards the solubility of monetite, dissolution stopped after 4 days and a very thin film, a few tens of Å thick, was formed. Usual bulk analysis methods did not allow the composition of the film to be determined but X-ray photoelectron spectroscopy (XPS) showed that the film was composed of a calcium phosphate, probably apatitic, with an atomic Ca/P ratio close to 1.50. The change of Ca/P with time in solution indicates that two processes occur. The first is DCPA dissolution while the second corresponds to incorporation of part of the Ca ions released by dissolution, at the monetite surface, leading to an apatitic film through a topotactic reaction. Formation of this film, probably in heteroepitaxy with DCPA, explains the low dissolution observed in deionised water.

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