Although there is interest in forming synthetic analogs of hard tissues at physiologic temperature, significant gaps in knowledge exist with respect to the mechanisms by which precursor solids convert to apatites and also with respect to the apatite compositions that may be formed. In this study calcium-deficient HAp [Ca9(HPO4)(PO4)5OH] was prepared by hydrolysis of tricalcium phosphate (TCP), alpha-Ca3(PO4)2. The kinetics of HAp formation were studied as a function of temperature by isothermal calorimetry. TCP hydrolyzed completely within about 12 h, and the hydrolysis reaction evolved 133 kJ/mol of HAp formed. Although the kinetics of hydrolysis exhibited a strong temperature dependence, the mechanistic path taken appeared independent of temperature. The fluoridation of hydroxyapatite compositions having Ca/P ratios higher than 1.59 previously has been investigated. However, little work has been done on the fluoridation of more calcium-deficient hydroxyapatite. Ca9(HPO4)(PO4)5OH was formed at temperatures between 37.4 degrees and 55 degrees C to vary its morphology. These preparations then were reacted in NaF solution and the kinetics of fluoride incorporation studied. Solution chemical analyses were used to determine the amounts of fluoride incorporated. The extent of hydroxyl replacement by fluoride ranged from 17 to 72% and correlated with the surface area of the parent HAp.
The enthalpies of formation were determined for Ca 4 (PO 4 ) 2 O, R-Ca 3 (PO 4 ) 2 , and the hydroxyapatite compositions Ca 10 (PO 4 ) 6 (OH) 2 and Ca 9 (HPO 4 )(PO 4 ) 5 OH by the analysis of isothermal calorimetric results obtained at 37.4°C. Both hydroxyapatite compositions were formed by reaction of Ca 4 (PO 4 ) 2 O with CaHPO 4 or with CaHPO 4 ‚ 2H 2 O in water depending on the molar proportions of the reactant used. Ca 9 (HPO 4 )(PO 4 ) 5 OH was also formed by direct hydrolysis of R-Ca 3 (PO 4 ) 2 in water. Enthalpy of formation values were calculated by a Hess's Law analysis using several sets of equations. In summary, the enthalpy of formation values are: for Ca 4 (PO 4 ) 2 O, -476 2.4 kJ/mol, for R-Ca 3 (PO 4 ) 2 , -409 6.5 kJ/mol, for Ca 9 (HPO 4 )(PO 4 ) 5 OH, -127 19.7 kJ/mol, and for Ca 10 -(PO 4 ) 6 (OH) 2 , -137 44.0 kJ/mol at 37.4°C.
1998 thermodynamic functions, thermochemistry thermodynamic functions, thermochemistry E 3000 07 -007 Enthalpies of Formation of Compounds in the P 2 O 5 -CaO-H 2 O System. -The enthalpies of formation ∆H f (kJ/mol) of biological relevant calcium phosphates: Ca 4 (PO 4 ) 2 O (-476 2.4 ), α-Ca 3 (PO 4 ) 2 (-409 6.5 ) and the hydroxylapatite compositions Ca 10 (PO 4 ) 6 (OH) 2 (-137 44.0 ) and Ca 9 (HPO 4 )(PO 4 ) 5 OH (-127 19.7 ) are determined by isothermal calorimetry at 37.4 • C. -(MARTIN, R. I.; TENHUISEN, K. S.; LEAMY, P.; BROWN, P. W.; J.
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