Journal of Dental Research, Vol 65, 706-711, Copyright © 1986 by International & American Associations for Dental Research Online Journals
Adsorption of benzoic acid on pure and cupric ion-modified hydroxyapatite: implications for design of a coupling agent to dental polymer composites
D. N. Misra
The adsorption isotherms of benzoic acid on synthetic hydroxyapatite
(containing about 1.5 monolayers of physisorbed water) were studied from
ethanol, dimethylsulfoxide, p-dioxane, methylene chloride, and benzene to
discern the role of solvent in the process. The adsorption is reversible
from the first three solvents and follows the Langmuir plots. It is
irreversible from the last two, and a constant amount of absorbent is
removed from solutions above a certain concentration. The isotherms of
potassium benzoate on the apatite from ethanol and dimethyl sulfoxide were
reversible. The isotherms of the acid on cupric ion-modified apatite
surfaces from ethanol and benzene were identical with those obtained on the
pure hydroxyapatite. This may demonstrate that any "surface chelation" with
the cation may not be a significant factor for adsorption to occur. The
adsorptive behavior seems to depend upon the interplay of hydrogen-bonding
among the solute, the solvent, and the hydrated apatite surface. The
capability of a solvent to hydrogen-bond may determine whether adsorption
from it will be reversible or irreversible. Based upon its compatibility
with a solvent, the benzene ring is upright or lies flat on the surface.
The adsorbed molecules rotate about the center of the carboxylate groups
which are hydrogen-bonded to the surface. These factors should be
considered in designing or selecting a suitable surface-active moiety for a
coupling agent between tooth mineral and a restorative resin.
