Adsorption of Hydroxypropylcellulose on Hydroxyapatite Via Formation of Surface Complex with Sodium Dodecylsulfate

Abstract

Adsorption of hydroxypropylcellulose (HPC) on hydroxyapatite (HAP) in the presence of sodium dodecylsulfate (SDS) and its effect on stability of the HAP suspension were studied. Although the adsorption amount of HPC was low in the absence of SDS, it increased with the amount of SDS after the formation of the intermolecular complex between them on the surface of HAP. The adsorption of SDS by HAP is due to electrostatic attractive force between dodecylsulfate anion (DS-) and Ca²⁺ on the surface and due to isomorphous substitution of the sulfate group of DS- for the surface phosphate ion. Formation of the surface complex is by virtue of hydrophobic interaction between hydrophobic groups of DS- thus adsorbed and those of HPC captured from the bulk solution. Adsorption of SDS by HAP was reversible with respect to dilution with its solvent (a NaCl solution). As for HPC, however, the desorption was rather complicated, depending on the dilution procedure whether with water or with an aqueous solution of a given concentration of SDS. This observation suggests that the adsorbed SDS is offering the adsorption sites for HPC on the surface after implantation of the hydrophobic groups on the surface. Mean diameter (d) of the secondary particles of HAP was determined by means of a Coulter counter as a function of concentrations of SDS and HPC. The d-value decreased after attaining a maximum with an HPC concentration when an SDS concentration was kept constant. This result suggests that an SDS-HPC complex plays a role of a dispersing/flocculating agent against the HAP suspension. In fact, the behavior of the complex is quite similar to that of a polyelectrolyte as a dispersing/flocculating agent.