Effect of added salt on the adsorption of dodecyl sulfate ion and concurrent release of phosphate and calcium ions at the surface of hydroxyapatite.

Concentrations of phosphate ion ([Pi]f) and calcium ion ([Ca2+]f) liberated from the surface of hydroxyapatite (Ca10 (PO4)6(OH)2; HAP) during the adsorption of dodecyl sulfate ion (DS-) onto HAP were determined at various concentrations of added salt (NaCl or Na2HPO4). The effects of added salt on the relationships among [Pi]f, [Ca2+]f, the concentration of DS-([DS-]f), and the adsorbed amount of DS- (XDS-) are discussed. The amount of XDS- increased monotonously with the concentration of NaCl added ([NaCl]i). However, the equilibrium concentrations of phosphate ion (Pi) and calcium ion (Ca2+) changed in a complex manner with [NaCl]i. It was concluded that ions on the surface of HAP were partly ion-exchanged with those of sodium dodecyl sulfate and NaCl, and that Ca2+ and Na+ were bound competitively as counter-ions to DS- micelles. On the other hand, when Na2HPO4 was added to the solution, added Pi (i.e., one of the lattice ions for HAP) had a drastic effect on XDS- and [Ca2+]f. The adsorbed amount of DS- decreased with increasing concentration of added Na2HPO4 owing to competitive adsorption between DS- and added Pi. The equilibrium concentration of Ca2+, [Ca2+]f, decreased for two reasons. First, adsorbed Pi inhibits the release of Ca2+ from the surface of HAP by virtue of the electrostatic attractive force. Secondly, the excess Pi remaining in the solution prevents Ca2+-release from the surface of HAP since the solubility product for HAP must remain constant.