Adsorption of dodecyl sulfate ion on hydroxyapatite and concurrent release of phosphate and calcium ions from the surface of hydroxyapatite.

Abstract

Concentrations of phosphate ion ([Pi] f) and calcium ion ([Ca2+] f), liberated from the surface of synthetic hydroxyapatite (Ca10 (PO4) 6 (OH) 2; HAP) during the adsorption of dodecylammonium chloride (DAC), were determined as a function of equilibrium concentration of dodecylammonium ion ([DA+] f). When [DA+] f was lower than its critical micelle concentration (cmc), [Ca2+]f increased with increase in [DA +] f owing mainly to ion-exchange between dodecylammonium ion (DA +) and calcium ion (Ca2+) on the surface of HAP. However, [Pi] f decreased with increase in [Ca2+] f to keep the solubility product for HAP (Ksp) constant. On the other hand, when [DA +] f was higher than the cmc, [Pi] f increased with increase in [DA+] f due to the binding of phosphate ion (Pi) to DA+ micelles. Cationic species (i.e., Ca2+ and DA+) were concomitantly liberated from the surface of HAP to maintain the electroneutrality of the surface phase. Therefore, [Ca2+] f increased and the adsorbed amount of DA+ (XDA+) decreased with increase in [DA+]f. When CaCl2+ was added to the solution, XDA+ decreased with increasing concentration of added CaCl2 owing to the competitive adsorption between DA + and added Ca2+. The release of Pi was strongly depressed because of the electrostatic interaction between surface Pi and adsorbed Ca2+, and because of the requirement of constant Ksp in the bulk solution.