“…Since high temperatures are attainable during milling, we suppose that the proton transfer mechanism was in operation for our samples. It was probably supported by physical diffusion of water molecules to fill voids in the c -axis channels (Kaflak and Kolodziejski 2008, 2011; Wilson et al 2006). …”
Section: Resultsmentioning
confidence: 99%
“…They can be coordinated to surface Ca 2+ -sites or can form hydrogen bonds with surface P–OH and P=O sites, and all of them can participate in hydrogen bonding with neighboring water molecules (Bertinetti et al 2007; Bolis et al 2012; Ishikawa et al 1989; Wilson et al 2006). The surface species are involved in a proton exchange via hydrogen bonds (Kaflak-Hachulska et al 2003; Kaflak and Kolodziejski 2007, 2011; Kolodziejski 2005), which process controls the shape and chemical shift of the proton line at 5.4 ppm.Fig. 3The exemplary solid-state NMR spectra of the apatite powders, shown for HA24: ( a ) 1 H NMR under MAS at 12 kHz; ( b )–( g ) 31 P NMR spectra under MAS at 3.5 kHz.…”
Section: Resultsmentioning
confidence: 99%
“…Indeed, water molecules from the crystal surface can enter intracrystalline c -axis channels used by structural hydroxide ions (Kaflak and Kolodziejski 2008, 2011; Wilson et al 2006; Yoder et al 2012a). In the former works from our group (Kaflak-Hachulska 2000; Kaflak and Kolodziejski 2008) it has been postulated that there is probably a proton exchange involving hydroxide ions and water in those channels.…”
A series of nanocrystalline calcium hydroxyapatites was prepared by dry milling and characterized using proton and 31P MAS NMR, TEM, and TGA methods. The samples contained stubby rod-shaped crystals, which length and width varied in the 130–30 and 95–20 nm ranges, respectively. It was confirmed that concentration of structural hydroxyl groups in nanocrystalline apatites decreases with the decreasing crystal size. In the series of the studied apatites, the decrease was from 86 to ca. 50 % in reference to stoichiometric apatite. Water was found in the surface hydrated layer and in the c-axis channels, in which compartments existed as adsorbed and structural, respectively. Molecules of the adsorbed water were capable of moving from the crystal surface into the lattice c-axis channels of apatite. This process introduced considerable structural disorder within and around those channels and reduced the content of the structural hydroxyl groups, particularly in the region underneath the apatite crystal surface.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-013-1868-y) contains supplementary material, which is available to authorized users.
“…Since high temperatures are attainable during milling, we suppose that the proton transfer mechanism was in operation for our samples. It was probably supported by physical diffusion of water molecules to fill voids in the c -axis channels (Kaflak and Kolodziejski 2008, 2011; Wilson et al 2006). …”
Section: Resultsmentioning
confidence: 99%
“…They can be coordinated to surface Ca 2+ -sites or can form hydrogen bonds with surface P–OH and P=O sites, and all of them can participate in hydrogen bonding with neighboring water molecules (Bertinetti et al 2007; Bolis et al 2012; Ishikawa et al 1989; Wilson et al 2006). The surface species are involved in a proton exchange via hydrogen bonds (Kaflak-Hachulska et al 2003; Kaflak and Kolodziejski 2007, 2011; Kolodziejski 2005), which process controls the shape and chemical shift of the proton line at 5.4 ppm.Fig. 3The exemplary solid-state NMR spectra of the apatite powders, shown for HA24: ( a ) 1 H NMR under MAS at 12 kHz; ( b )–( g ) 31 P NMR spectra under MAS at 3.5 kHz.…”
Section: Resultsmentioning
confidence: 99%
“…Indeed, water molecules from the crystal surface can enter intracrystalline c -axis channels used by structural hydroxide ions (Kaflak and Kolodziejski 2008, 2011; Wilson et al 2006; Yoder et al 2012a). In the former works from our group (Kaflak-Hachulska 2000; Kaflak and Kolodziejski 2008) it has been postulated that there is probably a proton exchange involving hydroxide ions and water in those channels.…”
A series of nanocrystalline calcium hydroxyapatites was prepared by dry milling and characterized using proton and 31P MAS NMR, TEM, and TGA methods. The samples contained stubby rod-shaped crystals, which length and width varied in the 130–30 and 95–20 nm ranges, respectively. It was confirmed that concentration of structural hydroxyl groups in nanocrystalline apatites decreases with the decreasing crystal size. In the series of the studied apatites, the decrease was from 86 to ca. 50 % in reference to stoichiometric apatite. Water was found in the surface hydrated layer and in the c-axis channels, in which compartments existed as adsorbed and structural, respectively. Molecules of the adsorbed water were capable of moving from the crystal surface into the lattice c-axis channels of apatite. This process introduced considerable structural disorder within and around those channels and reduced the content of the structural hydroxyl groups, particularly in the region underneath the apatite crystal surface.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-013-1868-y) contains supplementary material, which is available to authorized users.
“…The latter situation has been reported in the heating of some synthetic apatites and some actual bone (Holcomb and Young 1980;Mkukuma et al 2004;Zyman et al 2009). A proposed alternative mechanism for rehydroxylation of apatite (LeGeros et al 1978;Kaflak and Kolodziejski 2011), i.e., the reaction of 2(HPO 4 ) 2-+ (CO 3 ) 2-→ (P 2 O 7 ) 4-+ CO 2 + 2(OH) -, would not affect the amount of water detected by TGA.…”
Section: Possible Underestimation Of the Amount Of Water In Carbonatementioning
Despite numerous analytical studies, the exact nature of the mineral component of bone is not yet totally defined, even though it is recognized as a type of carbonated hydroxylapatite. The present study addresses the hydration state of bone mineral through Raman spectroscopic and thermogravimetric analysis of 56 samples of carbonated apatite containing from 1 to 17 wt% CO 3 , synthesized in H 2 O or D 2 O. Focus is on the relation between the concentration of molecular water (as distinguished from hydroxyl ions) and the concentration of carbonate in the apatite. Raman spectra confirm the presence of molecular water as part of the crystalline structure in all the aqueously precipitated carbonated apatites. TGA results quantitatively document that, regardless of the concentration of carbonate in the structure, all hydroxylapatites contain ~3 wt% of structurally incorporated water in addition to multiple wt% adsorbed water. We spectroscopically confirmed that natural bone mineral also contains structurally incorporated molecular H 2 O based on independent analyses of bone by means of spectral stripping (subtracting the spectrum of collagen from that of bone) and chemical stripping (chemically removing the collagen content of bone prior to analysis). Taken together, the above data support a model in which water molecules densely populate the apatite channels regardless of the abundance of hydroxyl vacancies. We hypothesize that water molecules keep the apatite channels stable even when 80% of the hydroxyl sites are vacant (typical in bone), hinder carbonate ions from substituting for hydroxyl ions in the channels, and help regulate chemical access to the channels (e.g., ion exchange, entry of small molecules). Our results show that bone apatite is not a "flawed hydroxylapatite," but instead a definable mineralogical entity, a combined hydrated-hydroxylated calcium phosphate phase of the form Ca 10-x [(PO 4 ) 6-x (CO 3 ) x ](OH) 2-x ·nH 2 O, where n ~ 1.5. Water is therefore not an accidental, but rather an essential, component of bone mineral and other natural and synthetic low-temperature carbonated apatite phases.
Bone minerals possess two primary hydrogen sources: hydroxide ions in the nanocrystalline core and structural water in the amorphous surface layer. In order to accurately measure their concentrations using hydrogen to phosphorus cross polarization NMR spectroscopy, it is necessary to analyze the dependence of signal intensities on serial contact times, namely, cross polarization kinetics. A reliable protocol is developed to iteratively decompose the severely overlapped spectra and to analyze the cross-polarization kinetics, leading to measurement of hydroxyl and structural water concentrations. Structural water concentration is used to estimate mineral specific surface area and nanocrystal thickness for intact bone.
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