The objective of this study is to prepare and characterize hydroxyapatite (HAP) particles having hydrophobic surface. Alkylphosphate (AP) was incorporated to the surface of HAP by virtue of electrostatic interaction and/or isomorphous substitution , resulting in formation of hydrophobic surface on HAP . The polar head group of AP was oriented to the HAP while the nonpolar tail group oriented to an aqueous phase . H ydrophobic properties of the surface were discussed mainly by means of contact angle measurement of a water drop placed on the HAP pellet . It was confirmed that various organic and hydrophobic compounds were adsorbed to the surface of the HAP prepared here . Th e driving force was hydrophobic interaction . The adsorption amount of calcium cation and phosphate anion, which are the component ions of HAP , decreased with the degree of h ydrophobicity of the surface, i.e. , surface concentration of AP, because nonpolar groups of AP on the surface prevented these lattice ions from approaching to the surface .

Hydroxyapatite (HAP) was prepared from CaCl 2 , Na 2 HPO 4 and NaOH in an aqueous phase mixed with a chloroform solution of cetylphosphate (CP). The CP was incorporated mainly on the surface of HAP but not to its bulk phase, resulting in formation of a hydrophobic surface. This surface-modified HAP has various characteristic properties different from those of the intact HAP. Some of the examples are as follows; an increase in a contact angle of water droplet, a decrease in a mean diameter of the HAP particle, and increases in the amounts of adsorption of various organic compounds. The aim of this study was of characterization of the surface-modified HAP through determining the adsorption amounts of various organic compounds on the surface. As for the adsorption of a surfactant to the modified HAP, it was concluded that hydrophobic interaction between alkyl chains of a surfactant and those of CP implanted on the surface became more important than hydrophilic interaction between polar/ionic groups of a surfactant and ions exposed on the intact area of the HAP. In the adsorption of bovine serum albumin (BSA), it was denatured by the CP implanted on the surface of the modified HAP (i.e., surface denaturation). Adsorption properties of the surface-modified HAP were quite different from those of the intact HAP.