Dehydration and Rehydration of Carbonated Fluor- and Hydroxylapatite

Claude Yoder,Zachary Wilt,Jennifer Goldenberg,Jill Pasteris,Kimberly Worcester,Mitchell Sternlieb,Demetra Schermerhorn

doi:10.3390/min2020100

Claude Yoder, Zachary Wilt + Show 5 more

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https://doi.org/10.3390/min2020100

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Abstract

The recent definitive deuterium solid state NMR spectroscopic evidence for structural water in fluor- and hydroxylapatites has prompted our study of the conditions necessary for the removal and reincorporation of this important structural feature of apatites. Thermal gravimetric analysis of 20 synthetic carbonated calcium hydroxylapatite (CCaApOH) samples and nine carbonated calcium fluorapatite (CCaApF) samples has been used to determine the amount of structural and adsorbed water in each sample. No correlation between the weight percent and number of moles of structural water and the weight percent carbonate in CCaApOH and CCaApF has been found. In contrast, there appears to be a relationship between the amount of adsorbed water and the carbonate concentration in the fluorapatites prepared with a two hour digestion time, as well as in the hydroxylapatites prepared with one hour digestion periods, presumably due to the effect of carbonate on crystallite size. Structural water can be removed from the apatite lattice, primarily above 200 °C, but heating to over 550 °C is required for complete removal. This water can be partly reincorporated through an apparently kinetically-controlled process that is enhanced by an increase in time and/or temperature. We speculate that the incorporation of structural water occurs at the beginning of the formation of the apatite structure, approximately coincident with the incorporation of carbonate. We also speculate that water is both removed and reincorporated by proton transfers from water molecules to hydroxide ions.

Highlights

The presence of water in apatite (Ca10(PO4)6(OH)2 = CaApOH) has long been a topic of debate [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
Raman spectroscopic analysis confirmed that the only phosphate phase present is CCaApOH or CCaApF, as appropriate to the given experiment
There is a positive correlation between the concentrations of adsorbed water and carbonate. This correlation most likely is the result of a decrease in crystallite size as carbonate content increases [27,28]. This correlation with adsorbed water is seen in our CCaApF samples prepared with a two-hour digestion time and in CCaApOH samples prepared with a one-hour digestion time

Summary

Introduction

We have proposed that the water molecules are likely located between the monovalent anions that are positioned in the channels created by the Ca(2) ions. Studies are accurately summarized by LeGeros et al [5]. Some of these studies were accomplished on biological apatite and were confounded by the presence of organic matter. The study by LeGeros et al [5] on both dental apatites and synthetically precipitated apatites showed that the adsorbed water was reversibly removed and readsorbed below 200 °C, whereas the water released between 200 °C and 400 °C was irreversibly lost and caused a contraction in the a-axis [5]. The a-axis is larger in aqueously precipitated apatites than in those “mineral or synthetic apatites from nonaqueous systems” [5]

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