Coupling of 10-methacryloyloxydecyldihydrogenphosphate to tetragonal zirconia: Effect of pH reaction conditions on coordinate bonding

Objectives Identification of the mechanism of chemical coupling of a phosphate ester monomer to zirconia via a computational modeling approach enables materials scientists to design new coupling agents that can resist hydrolytic degradation of bonds made by methacrylate resins to zirconia. We investigated the possibility of chemical bonding between 10-methacryloyloxydecyldihydrogenphosphate (MDP) and tetragonal zirconia, and the effect of pH reaction conditions on such prospective chemical bonds. Methods A tetragonal zirconia crystal model was created. An "Our-own N-layered integrated molecular orbital and molecular mechanics" (ONIOM) method was used to simulate two potential configurations of the MDP-ZrO₂ system: double-coordinate and single-coordinate. Thermodynamic calculations were used to ascertain if the reaction could proceed spontaneously and to compare the stability of the two possible configurations. Short-term testing of shear bond strength (SBS) was done to evaluate bonding improvement of MDP to alumina-sandblasted zirconia surfaces in neutral, acidic or alkaline environments. Results Digital models of coordinate bonds between MDP and tetragonal zirconia were constructed. Thermodynamic calculations indicated that the Gibbs free energy for forming double-coordinate and single-coordinate configurations were -461.2 kJ/mol and -450.9 kJ/mol, respectively. Equilibrium constants for the double-coordinate and single-coordinate configurations were 6.4 × 10⁸⁰ and 9.8 × 10⁷⁸, respectively. Application of MDP in alkaline conditions showed the highest SBS, whereas acid conditions resulted in lower SBS. Significance MDP can establish a "true" chemical bond with zirconia spontaneously. The double-coordinate configuration was identified to be more energetically favorable than the single-coordinate configuration for the coupling of MDP to zirconia. Alkaline conditions may positively affect formation of MDP-ZrO₂ coordination bonds.