ADSORPTION OF LOW-MOLECULAR-WEIGHT POLY(ACRYLIC ACID) ON HYDROXYAPATITE - ROLE OF MOLECULAR ASSOCIATION AND APATITE DISSOLUTION

Adsorption of low molecular weight poly(acrylic acid) from aqueous solution on hydroxyapatite shows that the isotherm first rises, reaches a maximum, then decreases, and is irreversible. This is qualitatively explained on the basis of the increasing self-association of polymeric molecules with concentration and the inability of associated molecules to adsorb as their hydrogen-bonding capability is used up. As surface effects decrease with the increase in initial concentration of the acid, the Ca to P ratio in solution rises and eventually reaches the experimental ratio in the apatite. The initial part of the isotherm satisfies the Langmuir plot. The ratio of the geometrical area of the molecule to the area derived from the Langmuir plot is about the same as that obtained by DiMarzio on the basis of theoretical considerations.