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

Abstract

Hydrolysis of dicalcium phosphate CaHPO 4 (DCPA) was studied as a function of time, in deionised water at 37 ³C, with a high liquid/solid ratio (2500 ml g 21 ) 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 ®lm, a few tens of A Ê thick, was formed. Usual bulk analysis methods did not allow the composition of the ®lm to be determined but X-ray photoelectron spectroscopy (XPS) showed that the ®lm 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 ®rst 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 ®lm through a topotactic reaction. Formation of this ®lm, probably in heteroepitaxy with DCPA, explains the low dissolution observed in deionised water.

Monetite (CaHPO4) Synthesis in Ethanol at Room Temperature

“…The Ca/P atomic ratio in water, in which these dissolution experiments were performed, also decreased with time and was 0.75 after 15 min of dissolution; it finally (after 4 days) reached a value of 0.62. The dissolution of DCPA was found to be incongruent 12 . This incongruency was explained by the formation of a thin apatitic (apatite‐like) calcium phosphate ( Ap‐CaP ) layer on the surfaces of CaHPO 4 particles through a topotactic reaction, which hindered further dissolution 12–16 .…”

Monetite promoting effect of citric acid on brushite cement setting kinetics

“…10 This reduction in monetite dissolution is a function of time and is not expected to reduce the calcium concentration of samples prepared for ICP analysis due to the short period of analysis time. Despite their lower surface area, monetite crystals seem to accommodate more citrate ions and still release calcium ions, but brushite dissolution is confined to the step edges on the dominant (010) faces where calcium and phosphate ions are exposed to the solution.…”

“…9 Its stacked sheet-like, compact structure is free from hydration; however, development of a thin apatitic layer in aqueous solutions in 1 week is reported. 10 A schematic representation of the crystal unit cells of both phases based on the crystallography studies by Catti et al 11 and Schofield et al 12 was adapted in Fig. 1 from a mineralogy database.…”

CONVERSION OF INDUCTION HEATING DEPOSITED MONETITE COATING TO HYDROXYAPATITE COATING ON HT–C/C COMPOSITES BY HYDROTHERMAL TREATMENT IN KOH AQUEOUS/AMMONIA SOLUTION

“…These changes are related to the mechanism of the conversion of monetite to HA. According to the suggestions of Lebugle et al, 14 the conversion of monetite to produce HA coatings involves the following two steps:…”