EFFECT OF SULFATE ION AND DODECYL-SULFATE TON ON NONSTOICHIOMETRIC DISSOLUTION OF HYDROXYAPATITE

The non-stoichiometric dissolution of hydroxyapatite (HAP) in aqueous solutions of NaCl, Na2SO4, and sodium dodecyl sulfate (SDS) was studie d. Molar ratios of Ca2+ to phosphate ion (Pi) released from HAP varied depending on species and concentration of an added salt. The order of salts increasing [Pi] was SDS>Na2SO4>NaCl, while that increasing [Ca2 +] was NaCl>Na2SO4>SDS, where the concentration of SDS was lower than its cmc (critical micellization concentration). These results were exp lained as follows: Pi on the surface of HAP is more easily replaced by SO42- than Cl- through a mechanism of ion exchange with isomorphic su bstitution, because the ion size of SO42- is quite similar to that of PO43-, On the other hand, the chemical structure of dodecyl sulfate io n (DS-) is made up by a covalent bond between -SO4- and a hydrocarbon chain (-C12H25). The adsorption of organic DS-, therefore, was enhance d more than that of inorganic SO42- by both the hydrophobic/cooperativ e effect of the hydrocarbon chains and the isomorphous replacement of the terminal -SO4- with PO43- on the surface of HAP. Excess amounts of negative charge due to the adsorbed DS- accelerate the release of Pi from HAP in order to relax the high electrochemical potential of the s urface. This mechanism results in more release of Pi in the presence o f SDS than in the presence of Na2SO4. The sequence of the salts for th e Ca2+ releasing was in the reverse of that for the Pi releasing by vi rtue of restriction of the solubility product of HAP.