Ralph A. Beebe

The surface chemistry of bone mineral and related calcium phosphates

A review of the surface chemistry of bone mineral, hydroxyapatite and amorphous calcium phosphate is presented. Small-angle x-ray scattering and low-temperature nitrogen adsorption measurements show the magnitude of bone mineral surface to range from 100-200 m-2/g; the synthetic hydroxyapatite surface can vary from 25-200 m-2/g, while synthetic amorphous calcium phosphate ranges in surface from 20-60 m-2/g, according to the respective preparation conditions. The magnitude of heats of adsorption of certain small molecules (CO, Ar, N2, H2O, CH3OH) on bone mineral and hydroxyapatite show that these are polarizing surfaces that form strong bonds with polar or polarizable molecules; water is hydrogen-bonded to these surfaces with energies ranging from 23 kcal/mole for low coverage to 11 kcal/mole after two full monolayers; concomitantly, methanol ranges from 24 kcal/mole to 9 kcal/mole after the adsorption of one and a half monolayers. Stearic acid will close-pack perpendicularly on bone apatite surfaces when adsorbed from cyclohexane solution in a way reminiscent of the adsorption of this long, straight-chain molecule on water surface. It is believed that these molecules are hydrogen-bonded to electronegative ions on the apatite surface. Synthetic hydroxyapatite has long been used in chromatographic adsorption columns because of the specific bonding capacity the surfaces have for certain proteins and polynucleotides. The metabolic interrelationship of bone mineral and the body fluids is in great part dependent upon the nature and magnitude of mineral surface. From the surface studies described herein it was suggested that a chemical linkage could exist in bone between the mineral surface and certain free polar groups of collagen.

Temperature-programmed dehydration of hydroxyapatite

Abstract The temperature-programmed dehydration of several near-stoichiometric hydroxyapatites from aqueous medium has been investigated over the range from 20 to 700°C at constant heating rate. In most of this work the sample was subjected to a vacuum of the order of 10−6 to 10−7 Torr and the effluent water vapor was detected by a mass spectrometer. We shall designate this as the MTA procedure. We find two major peaks in the MTA spectra for all these samples from aqueous medium. The first, at 90–100°C, is attributed to desorption of reversibly adsorbed water on the external surfaces; and the second, at 225–260°C, we attribute to the irreversible removal of water from ultrafine pores in the solid. The MTA technique has been employed to demonstrate that hydroxyapatite in the hydrated state undergoes H2O2O exchange when soaked in heavy water. This applies to both the 90–100°C peaks and the 225–260°C peaks. It is suggested that the considerable variation in the peak temperatures observed over the higher range from 221 to 257°C may be attributed to a difference in pore diameters, and that we must attain a higher temperature to empty a narrower pore.

Temperature programmed dehydration of amorphous calcium phosphate

Abstract The kinetics of dehydration of amorphous calcium phosphate (ACP) has been studied by the temperature programmed desorption technique (TPD) of Cvetanovic and Amenomiya. Each water desorption spectrum for ACP exhibited a single, but fairly narrow, peak. Peak temperatures varied from 65–100°C for sample heating rates ranging from 3–32°C/min. Narrowness of the desorption spectra lends some support to its earlier characterization as an agglomerate of hydrated Ca 2+ and PO 4 3− ions. In contrast, a water desorption spectrum for crystalline hydroxyapatite (HA) showed several broad overlapping peaks with maxima at 120, 250, and 305°C which is typical of a heterogeneous surface. The desorption spectra for ACP when compared to those for HA indicate very different states of bound water in these two substances. Plotted kinetic data for ACP revealed two linear segments with different activation energies, namely 10.5 and 20.0 kcal/mole. A plausible explanation for this finding is that there are two types of bound water in ACP: (i) loosely held water and (ii) tightly bound hydrate water held inside the amorphous particles. The possibility that hydrate water loss is involved in the rate determining step in the in vitro conversion of ACP to HA is tentatively suggested.

Surface areas of synthetic calcium phosphates and bone mineral.

The surface or interface of bone mineral is an important parameter in bone metabolism. In particular, this surface must play a role in the interchange of ions between the body fluids and bone mineral. This is a report on a study of the specific surfaces obtained on a series of synthetic hydroxyapatites. synthetic amorphous calcium phosphates, and bone samples, treated in different ways. The surface measurements have been made with gas adsorption and small angle X-ray scattering techniques. A comparison of the results obtained by these contrasting methods adds to our knowledge of the interface between bone mineral and the remainder of bone which is essentially water and collagen. In addition, this study gives a comparison of surface measurements made by these fundamentally different methods. Materials and Methods. 1. Synthetics. Large batches of a series of samples of poorly crystallized hydroxyapatite and amorphous calcium phosphate were prepared by the method described by Eanes et al. (1). Surface area measurments were obtained using low temperature adsorption of N2 and the application of the BET method (2). Concomitant surface measurements by small angle X-ray scattering were performed. It was ascertained by wide angle X-ray diffraction (3), that the hydroxyapatites were 100% crystalline (i.e., contained no amorphous fraction) and the amorphous calcium phosphates were free of any crystalline apatite. This assay was necessary because the amorphous phase is a precursor in the chemical precipitation of hydroxyapatite (1) and one may contaminate the other. The precipitated samples were separated from the solution by freeze-drying and then stored in stoppered vessels at room temperature. Prior to the adsorption measurements the samples were evacuated to 10-6 Torr at 300° for periods of 18–36 hr. Weight losses on these samples due to loss of water varied from 10–20% as determined by weighing the samples prior to and after evacuation.

Pseudo-phase changes in cupric sulfate pentahydrate during dehydration

Abstract A study of the dehydration kinetics of a model system, cupric sulfate pentahydrate, was made using the temperature programmed desorption technique. The three decomposition steps from the pentahydrate to the anhydrous salt were clearly resolved into distinct spectral peaks even at sample heating rates in excess of 10 K. min −1 . A linear relationship between spectral peak temperature and square root of heating rate was observed for each dehydration step. The kinetic data revealed different activation enthalpies and entropies for each dehydration sequence for heating rates above and below approximately 8 K min −1 . These latter findings are interpreted in terms of crystalline to amorphous pseudo-phase changes in the solid hydrate during decomposition which become apparent only at fast sample heating rates. Enthalpy and entropy changes associated with these structural alterations are evaluated. The results also help to clarify earlier work on the dehydration mechanism in calcium phosphates.

Heats of adsorption of carbon monoxide on bone mineral and on thorium oxide by gas-solid chromatography

Isosteric heats of adsorption qst have been measured by both pulse and continuous flow chromatography for carbon monoxide on bone mineral and on thorium oxide. Pretreatment of the solids consisted of vacuum outgassing at 500°C. Previous work has shown that the first small increments of water (θ < 0.10) are bound to the pretreated surface with a heat of adsorption of 20 kcals/mole for the bone mineral but 30–40 kcals/mole for the thorium oxide. The qst values for CO at low coverage (say θ = 0.002) are if anything slightly less on the thoria surface. This indicates that in spite of its considerably stronger attraction for chemisorbed water the thoria surface has no greater polarizing power for CO than has the bone mineral surface. The qst values obtained by the pulse and continuous flow methods support the conclusion of an earlier publication from this laboratory that the pulse method gives results which are somewhat in error in cases where the isotherms at low coverage fail to conform to Henrys law.