R. A. Young

Thermal decomposition of human tooth enamel

SummaryFurther insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO32−, OH−, structurally incorporated H2O, Cl−, and, indirectly, HPO42− after TE had been heated in N2 or vacuum in the range 25–1000°C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO32− components begins at much lower temperature (e.g., 100°C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO32− in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200° to a maximum at ∼800°C (>10% of the possible A sites filled), where it is responsible for an increase ina lattice parameter. A substantial fraction of the CO32− in B sites moves to A sites before being evolved, apparently via a CO2 intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl− was observed at temperatures below 700–800°C. Structural OH− content increases ∼70% to a maximum near 400°C. Structurally incorporated water is lost continuously up to ∼800°C with a sharp loss at 250–300°C. The “sudden”a lattice parameter contraction, ∼0.014Å, occurs at a kinetics-dependent temperature in the 250–300°C range and is accompanied by reordering and the “sharp” loss of ∼1/3 of the structurally incorporated H2O. The hypothesis that structurally incorporated H2O is the principal cause of the enlargement of thea lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42Å) is thus allowed by these experimental results.

Infrared determination of the degree of substitution of hydroxyl by carbonate ions in human dental enamel

SummaryAbout 11±1% of the carbonate ions in a human tooth enamel specimen (dense fraction, sp. g.>2.95) were found to be in the A-sites, replacing hydroxyl ions. The determination was made with a difference infrared spectrometry technique utilizing both tooth enamel and a reconstituted biological apatite with a known amount of replacement of hydroxyl by carbonate ions.

Influence of preparation conditions on the composition of type B carbonated hydroxyapatite and on the localization of the carbonate ions.

SummaryIt is shown how certain aspects of the composition and structure of carbonated apatites depend strictly on preparation conditions, for example, excess of phosphate or calcium ions in the reaction medium, CO32− concentration, pH, ammonia added or not. Depending on those conditions, either one or the other of the two proposed mechanisms of introduction of carbonate ions into the B sites is dominant. The mechanisms are (1) replacement of a phosphate ion by a carbonate ion with the formation of three vacancies, one in a phosphate oxygen site and one each in the neigh-boring Ca2+ and OH− sites; and (2) replacement of a phosphate ion by a carbonate accompanied by a hydroxyl ion. Whether mechanism (1) is observed to dominate over mechanism (2), or vice versa, is accounted for by the relative concentrations of the various ions in the reaction medium. The number of vacancies is decreased by the presence of either, or both, excess calcium ions or ammonia in the reaction medium. A structural-chemical mechanism is advanced for the view that, with the smallest CO32− content, the A sites are favored but with increasing carbonate content the B sites become favored and the A-site content becomes less than it is when the total carbonate content is less.

Relationships among carbonated apatite solubility, crystallite size, and microstrain parameters.

Abstract. The use of the metastable equilibrium solubility (MES) concept to describe the solubility properties of carbonated apatites (CAPs) and human dental enamel (HE) has been well established in previous studies using a range of CAPs with varying carbonate contents and crystallinities. It was shown in these studies that the mean value of the CAP MES is directly related to the broadening parameter full width at half maximum (FWHM) of the 002 reflection of the X-ray diffraction profile. The apparent solubility of the CAPs increased monotonically with an increase in the broadening of the diffraction peaks, and when this peak broadening was taken into account, carbonate had no additional effect upon the MES. The broadening of the diffraction peaks has been used as an indicator of crystallinity, and is generally influenced by both crystallite size and microstrain. The purpose of the present study was to extract the crystallite size and microstrain parameters separately from the X-ray diffraction peaks and then to determine their relationships to the corresponding MES values. The samples studied were CAPs synthesized by precipitation from Ca(NO3)2 and NaH2PO4 solutions in carbonate containing media at temperatures of 95, 80, and 70°C, and powdered HE. The crystallite size and microstrain parameters were determined simultaneously with the refinement of the structural parameters with the Rietveld method of whole-pattern-fitting structure-refinement. A modified pseudo-Voigt function was used to model the observed peak profiles. The MES distributions for the CAPs and HE were determined by a previously described method. The results of this study showed that the CAPs possessed an MES distribution and therefore provided further support that MES distribution is a common phenomenon, regardless of the method of CAP synthesis. The crystallite size decreased and the microstrain increased with increasing carbonate content and decreasing temperature of synthesis of the CAPs. A plot of the mean of the MES distribution versus the microstrain parameter showed that the apparent solubility of the CAPs and HE correlated very well with the microstrain parameter. On the other hand, a plot of the mean of the MES distribution versus the crystallite size parameter showed a poor correlation between MES and crystallite size. These findings support a view that microstrain, rather than crystallite size, is the dominant factor governing the effective solubility of the CAPs and dental enamel.

Occurrence of nitrogenous species in precipitated B-type carbonated hydroxyapatites

SummaryB-type carbonated hydroxyapatites, prepared in aqueous media free of alkali ions, fix ammonium ions present in the reaction medium. A small portion of the carbonate ions introduced into the apatite structure enter by the substitution mechanism (CO32−, NH4+)→(PO43−, Ca2+). With these results for the structural incorporation of ammonium ions, differences in lattice parameters observed among specimens with the same degree of carbonation were attributed to some substitution of NH4+ for Ca2+. The fixed ammonium ions were shown to be the source of the cyanamide and cyanate ions that develop on heating. Above 500°C these apatites lost both the carbonate and the cyanate and cyanamide ions.

OH− dipole reorientability in hydroxyapatites: Effect of tunnel size

Abstract A study of calcium phosphate hydroxyapatite with the thermally stimulated currents methods (TSC) has led to the conclusion that the reorientable dipoles responsible for the dielectric properties measured are the structural OH − ions in the “tunnels”. The co-operative motions along chains of these dipoles have been followed by the identification and determination of compensation phenomena at T c = 212°C and 356°C. These phenomena correspond to physical events characterized by co-operative motions, such as the monoclinic-to-hexagonal phase transition observed in several apatites. Even in a material in which the higher temperature phase is frozen in at a lower temperature (e.g. by defects or by quenching), the TSC method can reveal the onset of those co-operative dipole reorientations which would otherwise produce a phase transition. An effect of the “tunnel” size and environment of OH − dipole reorientation movements has been investigated with two additional hydroxyapatites, strontium phosphate hydroxyapatite, Sr 10 (PO 4 ) 6 OH 2 , and calcium arsenate hydroxyapatite, Ca 10 (AsO 4 ) 6 OH 2 . “Tunnel size” is here defined as the distance from the center (6, axis) to the ion center less the ion radius. The larger tunnel sizes led to lower compensation temperatures.

Role of acid phosphate in hydroxyapatite lattice expansion

SummaryQuestions remain about which subcomponents of human tooth enamel (TE) are responsible for its crystallographica axis being nearly 0.02Å longer than that of pure hydroxyapatite (OHAp) and contracting to that of OHAp on heating. From infrared spectroscopic and X-ray diffraction studies of a synthetic OHAp containing HPO4 and “structural” H2O, it has been concluded that HPO4 expands thea axis at the rate of ∼ 0.0015Å/ wt % but that this accounts for substantially less than one-half of the total observable contraction. The remaining, more than one-half of thea axis change, may be only partially ascribable to “structural” H2O and partially to P2O7 (formed from the HPO4), coming out of solid solution in the apatite. Some 90% of the HPO4 observed with infrared is lost in the 160–240° temperature range and more than one-half of the P2O7 observed as a separate phase is developed above that temperature and continues to increase all the way up to the 500°C, the limit of the experiments. The loss of HPO4 is accompanied by reduction of disorder or variety in the structural OH ion sites, consistent with the view that initially some of the PO4 groups neighboring the OH ions were actually HPO4 groups.

Reorientable Electric Dipoles and Cooperative Phenomena in Human Tooth Enamel

SummaryA preliminary investigation of electric dipole reorientability in human tooth enamel (TE) in comparison to that in hydroxyapatite (OHAp) has been made with the fractional-polarization form of the thermally stimulated currents (TSC) method. The reorientable dipoles are the structural OH− ions. The OHAp exhibited compensation phenomena at 211.5°C and at 356°C which are associated here with the hexagonal form becoming quasi-statically stabilized and dynamically stabilized, respectively, against the monoclinic form. TE specimens were pretreated at various temperatures. All showed the onset of cooperative motions that could quasi-statically stabilize the hexagonal form at the same temperature, approximately 212°C, as did OHAp, even though the TE was already statically stabilized in the hexagonal form. Parts of the TSC spectra that did not conform to the 212°C compensation changed progressively with pretreatment temperature. Loss of incorporated H2O is identified as the most probable cause of most of these changes. This work shows considerable promise for TSC as a tool for further quantitative investigation of TE.

TSC study of electric dipole relaxations in chlorapatite

Abstract Electric dipole relaxations in chlorapatite, Ca 5 (PO 4 ) 3 Cl, have been studied with the fractional polarization mode of the thermally stimulated currents (TSC) method. Fifty-one of the fifty-seven sets of data obtained in the range 10–443°K fell naturally into four groups yielding compensation temperatures T C of T C 1 , = 202° C , T C 2 : = 202° C , T C 3 = 420° C and T C 4 = 644° C , with estimated error τ C of τ C 1 = 1.3 × 10 −7 s , τ C 2 = 3.2 × 10 −6 s , τ C 3 = 8.8 × 10 −5 s and τ C 4 = 2.3 × 10 −4 s . Atomic-scale physical models involving Cl − ion motion are offered for the 202°C compensation at the temperature of the reported monoclinic-to-hexagonal phase transition and for the 420°C compensation, at which temperature the Cl − ions individually are thought to have enough thermal energy to maintain the hexagonal form dynamically.

Oh− reorientability in hydroxyapatites: Effect of F− and Cl−☆

Abstract The effects of partially substituted F − and Cl − in hydroxyapatite on the co-operative reorientation motions in the OH-dipole chains associated with the monoclinic-hexagonal phase transition have been examined with the fractional-polarization thermally stimulated current (TSC) method. The motions can be sensed with the TSC method even though various chain segments may be oppositely oriented and thus mandate the hexagonal phase. Substitution of one in four OH − ions by F − causes the OH − dipole chains to be too short, and too bound at the ends, to display this co-operative reorientation. With 16% F substitution, there are enough OH − chains of sufficient length to show the co-operative motion, identifiable by the occurrence of a compensation phenomenon. The occurrence of three compensations with essentially the same temperature (209–216°C) but different characteristic relaxation times is ascribed to the existence of different OH − chain lengths between interrupting F − ions. Chlorhydroxyapatite (Cl 0.5 OH 0.5 ) shows no compensation below 800°C even though both end-members do, at the monoclinic-hexagonal phase transition temperature. This is ascribed to short chain lengths and to direct OH − −Cl − interactions. Fluorapatite shows no compensation below 1200°C, and no phase transition in that temperature range is known.