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) free from the surface of hydroxypatite (Ca₁₀(PO₄)₆(OH)₂; HAP) at the adsorption equilibrium of sodium dodecyl sulfate (SDS) were determined as a function of equilibrium concentration of dodecyl sulfate ion ([DS^-]_f). When [DS^-]_f was lower than its critical micelle concentration (cmc), [Pi]_f increased with increase in [DS^-]_f owing mainly to the ion-exchange between dodecyl sulfate ion (DS^-) and surface phosphate ion. However, [Ca²⁺]_f decreased with increase in [Pi]_f to maintain the solubility product of HAP constant. On the other hand, when [DS^-]_f was higher than the cmc, [Ca²+]_f increased with increase in [DS^-]_f due to the binding of Ca²⁺ to DS^- micelles. Anionic species were concomitantly liberated from the surface of HAP to maintain the electroneutrality of the surface phase. Therefore, [Pi]_f increased and the amount of adsorption of DS^- decreased with increase in [DS^-]_f.