Hydroxyapatite, dehydration

Dicalcium Phosphate Dihydrate (DPD). Dicalcium phosphate cHhydrate is completely nonreactive at room temperature. At 65—71°C and in the presence of water, it dehydrates and decomposes into hydroxyapatite and acidic monocalcium phosphate, or a free phosphoric acid (18). It is used to some extent in cake mixes in combination with faster acting acid. Its primary function is to provide acidity late in the baking cycle and thus produce a neutral and palatable product. DPD has an NV of 33. It provides sufficient acidity only in products requiring long baking times. 

The dehydration of alcohols on stoichiometric and nonstoichiometric (calcium-deficient) hydroxyapatite (series 8 and 9 in Table II) gave results consistent with the above findings. Although there is a difference in the reaction temperature, it is evident that with the nonstoichiometric catalyst, which must be more acidic, the slope found is more negative than that with the stoichiometric calcium phosphate. 

Arthropathies associated with crystals deposition are acute gouty arthritis, chronic gout and chronic tophaceous gout due to monosodium urate crystals. Then there is acute pseudogout and chronic pyrophosphate arthropathy caused by calcium pyrophosphate dehydrate crystals. Acute calcific periarthritis, acute hydroxylapatite arthritis and chronic hydroxyapatite arthritis including Milwaukee-shoulder-knee syndrome are due to basic calcium-phosphate-hydroxyapatite crystals. 

Because of the importance of the surface chemistry of bone mineral in physiological systems, we have undertaken a series of gas adsorption studies on hydroxyapatite in the form of anorganic bone. In a recent publication from this laboratory (4) results of calorimetric studies of the adsorption of water and methanol vapors on bone mineral and on synthetic hydroxyapatite were reported. The adsorption potential for nitrogen on dehydrated hydroxyapatite, whether from bone or from synthetic sources, was rather profoundly altered by the addition to the surface of chemisorbed methanol or water prior to the adsorption of nitrogen at —195° C. This effect was reflected in the specific surface areas, in the BET C values, and in the resultant values of Ex — EL (net heats of adsorption) as shown in Table I of the above paper. 

Differential cross-polarization Dipolar suppression period Calcium hydroxyapatite Calcined HA (dehydrated)... 

Linear Free Energy Relationships. - Kibby and Hall studied the dehydration of fifteen acyclic alcohols on a stoicheiometric (HA) hydroxyapatite [Caio(P04)6(OH)2] and a non-stoicheiometric (NHA) hydroxyapatatite for which Ca/P= 1.58. The former gave both dehydrogenation and dehydration but the latter gave only dehydration. In the case of the NHA catalyst the dehydration rate constants correlated with the Taft constants for a-carbon substitution giving p = — 5 at 230 °C, a-propanol being the reference alcohol so that the Taft equation was of the form (equation 2). 

Kibby et al compared the behaviour of a hydroxyapatite (Ca/P = 1.58) and an alumina catalyst for the dehydration of threo- and erythro- 3- x -butan-2-ol. Both catalysts gave but-2-enes preferentially, but water can be eliminated by a syn- or anft -mechanism, the former occurring, when groups are removed from the same side of the molecule and the latter, when they are removed from opposite side. Furthermore, syn- and fl/ift -eliminations occur respectively with eclipsed and gauche conformations. In the case of homogeneous reactions, anti-elimination is preferred since the gauche form is thermodynamically more stable. 

The nonstoichiometry of CaHAP has received extensive attention in medical and solid-state science, because the CaHAP formed in the calcified tissues or synthesized is mostly non- stoichiometric. Bett et al.[8] have reported that the catalytic activity of Ca-deficient CaHAP for the dehydration of butanol can be related to the surface acidic hydrogens of HPO4" produced to maintain the electrical neutrality. Joris and Amberg [9, 10] have pointed out that the catalytic activity of cation-deficient CaHAP and strontium hydroxyapatite (abbreviated SrHAP) for the similar dehydration reaction is due to the acidic hydrogens of the H2O molecules located in OH -defects caused by the deficiency of Ca + or Sr " " ions rather than those of the surface HPO ions. 

Non-thermal deposition methods are defined as those ones carried out at temperatures much below the incongruent melting point of hydroxyapatite, in particular at or near ambient temperature. However, frequently post-depositional heat treatment must be applied to either crystallise amorphous calcium phosphate (ACP), transform (dehydrated/dehydroxylated) precursor phases such as octacal-cium phosphate (OCP) to hydroxyapatite, and/or to remove organic compounds used in coating preparation, for example during sol-gel, dip coating, electrochemical and EPD. 

The hydroxyapatite-catalyzed dehydration of 2-butanol, examined by Bett and Hall in 1968 (34), was shown to be amenable to a special method of obtaining the site density. They determined the number of product molecules which desorbed from the surface at infinite flow rate using a microcatalytic reactor. 

FIG. 2 Mass spectrometer temperature-programmed dehydration (MSTPD) spectra showing the evolution of water as a function of temperature from (1) hydroxyapatite precipitated from an aqueous solution via an amorphous precursor phase, (2) hydroxyapatite prepared in a hydrothermal bomb, and (3) apatite from a mineral source. (After Ref. 28.)... 

The decrease of consuni d above about (fC for both bone and dehydrated bone appears to be caused by the increase of tonic conductivity, which neutralizes the piezoelectric polarization in collagen hbets. Figure 11 shows the dependence ctf coostaru d at room temperature on the water content. The water content dense structure embedded in a matrix of hydroxyapatite. 

Although the base sites on nonstoichiometric hydroxyapatite are not strong compared with those on the stoichiometric hydroxyapatite, the dehydration of alcohols involves an acid and a base site, where a nearly concerted elimination occurs. It should be noted that the acid and base sites participating in the dehydration are not the same as those in dehydrogenation, as evidenced by poisoning experiment. 

The deposition of crystals of monosodium urate (gout), calcium pyrophosphate dehydrate (CPPD chondrocalcinosis, pseudogout) and calcium hydroxyapatite (HADD calcifying tendinitis) within synovium, articular cartilage, fibrocartilages and para-articular soft tissues may produce a spectrum of clinical conditions ranging from asymptomatic entities to severe rapidly destructive arthropathy (Steinbach 2004 Clement et al. 2005). The accumulation of crystals takes months to years to develop and leads to the establishment of a chronic synovitis with intermittent acute flares. 

Steinbach LC (2004) Calcium pyrophosphate dehydrate and calcium hydroxyapatite crystal deposition diseases imaging perspectives. Radiol Clin North Am 42 185-205 Steiner GM, Sprigg A (1992) The value of ultrasound in the assessment of bone. Br J Radiol 65 589-593 Stieber JR, Dormans JP (2005) Manifestations of hereditary multiple exostoses. J Am Acad Orthop Surg 13 110-120 Stone M, Bergin D, Whelan B et al (2001) Power Doppler ultrasound assessment of rheumatoid hand synovitis. J Rheumatol 28 1979-1982... 

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