Theoretical insights into adhesion mechanisms of dental adhesives on zirconia surfaces: effects of functional groups in adhesive monomers

This study investigates the adhesion mechanism of dental adhesives to zirconia (ZrO₂) using periodic density functional theory (DFT). ZrO₂ is extensively used as a prosthetic material. Dental adhesives for ZrO₂ typically contain adhesive monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) or 2-(methacryloxy)ethyl phenyl hydrogen phosphate (2-MEP), 2-(methacryloxy)ethyl phenyl hydrogen phosphate (Phenyl-P), and 4-methacryloxyethyl trimellitic acid (4-MET). This study investigates the effects of acidic functional groups, including phosphoric acid, phenyl phosphate, and phthalate, on adhesion. For all adhesive monomers on clean zirconia surfaces, no barrier proton transfer from the acidic functional group to the surface occurred, and a minimal difference in adhesive strength was detected. In contrast, on the hydroxylated surface, only the phosphate group contained in 2-MEP caused proton transfer, resulting in higher adhesive strength. After releasing a proton, the oxygen atom served as a hydrogen-bond acceptor, forming interfacial interactions involving charge transfer. This proton transfer was attributed to the high acidity of the phosphoric acid group. These results provide insights into the molecular mechanisms governing adhesion of zirconia dental materials.