Time-dependent molecular interaction at the monomer-hydroxyapatite interface

Objectives: ‘Mild' self-etch adhesives bond to dentin through micro-mechanical hybridization and chemical interaction with residual hydroxyapatite (HAp). Since the mechanisms of molecular interaction at the interface are far from understood, we analyzed the chemical interaction of two functional monomers (phenyl-P/MDP) with HAp using solid-state NMR and XRD.

Methods: Two gm of HAp particles were dispersed in 10 gm of phenyl-P or MDP water-ethanol solutions under stirring for 5 min, 1 hr and 24 hrs. The treated HAp particles were separated from the solution by centrifugation, followed by washing three times with absolute ethanol and drying at room temperature. They were then analyzed using an X-ray powder diffractometer (RINT 2500, Rigaku, Japan) and Varian UNITY INOVA300 FT-NMR spectrometer (Palo Alto, USA).

Results: CaHPO₄•2H₂O was formed when phenyl-P interacted with HAp for only 5 min, while phenyl-P-Ca salt was hardly detected, even after 24-hr interaction. On the contrary, MDP-Ca salt was readily formed at 5 min, forming a layered structure on the HAp surface (disclosed by XRD). CaHPO₄•2H₂O remained below the detection limit at 5 min, while a clear peak was only detected at 24 hrs.

Conclusion: Correlated XRD and NMR disclosed a time-dependent molecular interaction at the interface with stable ionic bond formation of the monomer competing in time with the deposition of CaHPO₄•2H₂O that is less stable in water than HAp. The ionic bond of phenyl-P to HAp was not stable, by which abundant phosphoric acid and hydroxyl ions were released and with Ca rapidly deposited as CaHPO₄•2H₂O. On the contrary, MDP formed a stable ionic bond to HAp, leading to a layered structure, while only a small amount of released phosphoric acid and hydroxyl ions resulted in slow CaHPO₄•2H₂O deposition. This interfacial molecular interaction mechanism explains the more intense and stable chemical interaction of MDP versus phenyl-P.