Adsorption of Cetylpyridinium Chloride to Hydroxyapatite

Abstract

Adsorption of cetylpyridinium chloride (CPC) to hydroxyapatite (HAP) was studied. The amount of adsorbed CPC increased and decreased with the amount of added NaOH, and added NaCl or CaCl2, respectively. It was assumed that cetylpyridinium cation (CP+) is adsorbed to negatively charged sites on HAP surface by electrostatic force. Adsorption of anions such as OH- makes the number of adsorption sites for CP+ increase on the surface of HAP (cooperative adsorption). But the effect of Cl- anion, which may increase the amount of CP+ adsorption, was assumed to be smaller than that of Na+ cation, which decreases the amount of CP+ adsorption, because NaCl, as a whole, inhibits CP+ adsorption. Cations such as Na+ and Ca2+ compete with CP+ for the adsorption site charged negatively on HAP (competitive adsorption). The effect of Ca2+ was more remarkable than that of Na+, since Ca2+ is one of the lattice ions of HAP crystal. As the affinity of Ca2+ to HAP is stronger than that of CP+, the adsorption isotherm of Ca2+ became just one curve irrespective of concentration of CP+ added. The fact mentioned above may be one of the reasons why hard tissues and renal calculi adsorb Ca2+ selectively from body fluid containing many kinds of organic and inorganic ions, and grow up in our human body.