Although polyethyleneglycol mono-p-isooctylphenyl ether (TX-100) was not adsorbed on the surface of hydroxyapatite(HAP) in an aqueous phase, it was adsorbed in the presence of sodium dodecylsulfate(SDS) or arginine dodecylphosphate(DP). This fact was explained in terms of modification of the surface of HAP by the adsorption of SDS or DP and complex formation on the surface through hydrophobic interaction between hydrophobic tails of TX-100 and SDS or DP. The effects of the mixing ratio between them and of a concentration of added NaCl were discussed.

Zeta potential of hydroxyapatite (HAP, Ca10(PO4)6(OH)2) particle in an aqueous phase was measured in the presence of various concentrations of NaCl, sodium dodecylsulfate (SDS), and polyethylene glycol mono-p-isooctyl-phenyl ether (TX-100). It was found that the intact particle was negatively charged. Zeta potential of the particle became more negative with a concentration of SDS because of its adsorption. The potential decreased furthermore with a concentration of TX-100 at a given concentration of SDS when NaCl was not added to the system. On the other hand, the potential became less negative with a concentration of TX-100 at a constant concentration of SDS when 500 mmol/L NaCl was added. The mechanisms to explain these facts and the effect of NaCl in particular were discussed, taking into consideration the mixed micelle formation, hydration, adsorption of SDS and TX-100, and shifting of the slipping plane into an aqueous phase.

Adsorption of polyethyleneglycol mono-p-isooctylphenylether(TX-100) on hydroxyapatite(HAP) was accelerated in the presence of sodium dodecylsulfate(SDS). This is because TX-100 was captured into hemimicelle/ admicelle of SDS formed on the surface of HAP. The adsorption amount on and affinity to HAP of TX-100 increased with the adsorption amount of SDS, forming the mixed micelle on the surface. Sodium dodecylbenzenesulfonate (SDBS) was also adsorbed on HAP in the same manner as that of SDS. However, polyoxyethylene(23) laurylether(C12n23) was little adsorbed by itself and scarcely accelerated the adsorption of SDBS. Anionic surfactants such as SDS and SDBS were adsorbed directly on the surface, while nonionic surfactants such as TX-100 and C12n23 required hydrophobic area to be adsorbed on the surface. The hydrophobic patch on the surface should be formed after the adsorption of the anionic surfactant on H.A.P. The roles of hydrophobic interaction and cooperative adsorption in the adsorption of nonionic surfactants were discussed.

Abstract:Adsorption mechanisms of sulfonated organic compounds by hydroxylapatite (HAP) were studied. Alkylsulfonates such as sodium dodecyl sulfonate were easily adsorbed after ion exchange (i.e.,isomorphous substitution) between their terminal sulfonate groups and phosphate ions on the surface of HAP, because the size of the sulfonate group is quite similar to that of phosphate ion. Azo dyes such as methyl orange (MO) and Ponceau R(PR) were adsorbed, because these compounds also have sulfonate groups in their chemical structure. The adsorption amount of PR was larger than that of MO. This fact is suggesting that the amount increases with the number of sulfonate groups in an adsorbate molecule. The adsorption amount of each of these compounds was, however, smaller than that of a long chain alkylsulfonate. The role of hydrophobic group in an alkyl sulfonate was emphasized. Hydrophobic interaction on the surface of HAP resulted in formation of hemimicelle or admicelle, which accelerated the adsorption. The adsorption amounts of MO and PR were increased in the presence of bovine serum albumin (BSA), because these dyes formed an intermolecular complex with BSA on the surface of HAP which promotes their adsorption.

Fresh hydroxyapatite(HAP) was prepared in an aqueous phase by mixing CaCl2 with K2HPO4 in the presence of dodecyl phosphate(DP) and arginine (Arg), where DP/Arg = 1/3 in molar ratio. Arg was added in order to enhance the solubility of DP. The degree of crystallinity of the HAP decreased with a concentration of DP, because DP was adsorbed to the growth sites on the nuclei through its terminal phosphate group as a crystal poison. The effect of adsorption of DP on a ripened HAP was also studied. Though polyethylene glycol p-isooctylphenyl ether (TX-100) was scarcely adsorbed on the raw surface of HAP, it was adsorbed after pretreatment of the surface with DP. This fact was explained in terms of surface modification by the adsorption of DP. Alkyl chains of DP protruding from the surface captured those of TX-100 through hydrophobic interaction on the surface. It was concluded that the phosphate group of DP plays an important role in regulation of the crystal growth and surface modification by virtue of isomorphous substitution of phosphate group of DP for phosphate ion on the surface of HAP.