Resin bonding is widely accepted for zirconia adhesion; however, its bond strength still requires improvement. Conventional acid etching has not shown a positive effect on the resin bond with zirconia [1]. Since zirconia is a glass-free polycrystalline material that lacks silica, traditional silane coupling agents used with silica-based ceramics are not very effective on zirconia [2]. Therefore, adhesive monomers are recommended for clinical use to achieve chemical bonding with zirconia [1], [3].

Physical bonding forces are generally weak at the adhesive bonding interface, making it crucial to focus on chemical bonds, as these are significantly stronger [4]. Phosphoric ester monomers are utilized as primers for resin application to enhance zirconia bonding, as phosphate esters are expected to chemically bond with the oxide groups on the Y-TZP surface [5]. By definition, phosphate ester monomers contain a carbon-carbon double bond and a phosphate group [6]. One of the most commonly used phosphate primers in dentistry is 10-methacryloyloxydecyl dihydrogen phosphate (MDP) [6]. This phosphoric ester monomer is believed to interact chemically with zirconia, enhancing bond durability [7], [8].

Previous studies have provided insights into the chemical interactions between phosphoric ester monomers and zirconia. A study by R. Pilo et al. demonstrated that phosphate-based zirconia primers induced phosphate salt formation in a dissociative form on Y-TZP, as shown by reflection Fourier transform infrared microscopy tests [9]. Another study using time-of-flight secondary ion mass spectrometry (ToF-SIMS) indicated the presence of chemical bonds, suggesting P-O-Zr bonding in zirconia treated with primers containing MDP [7], [10]. Furthermore, factors affecting the chemical affinity of 10-MDP to zirconia were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermodynamic calculations. It was confirmed that a solvent is necessary for the formation of Zr–O–P bonds between 10-MDP and Y-TZP, with acetone facilitating the best bonding, followed by ethanol [11].

A solid-state nuclear magnetic resonance (SS NMR) method has been applied to analyze the interaction between phosphoric ester monomers and zirconia. A previous study investigated the structures formed by the adsorption of carboxyalkyl phosphonic acids on TiO2 and ZrO2 powder using 1H fast magic angle spinning (MAS) [12]. In that study, dipole-bound P-OH protons were detected on TiO2, whereas only isolated residual P-OH groups were present on ZrO2 [12]. Regarding the MDP-zirconia interaction, 31P magic angle spinning (MAS) NMR detected that P-OH from MDP could form a strong chemical bond with zirconia, suggesting ionic and hydrogen bonding [13]. Another SS NMR analysis revealed that the chemical bonds in the interaction between MDP and zirconia were primarily P-O-Zr ionic bonds; however, when silane was added to the system, P-OH-Zr hydrogen bonds formed [10].

One advancement in high-resolution SS NMR techniques is their element-specific sensitivity to the local structure surrounding a nucleus (e.g., P) [14]. Particularly, when conducting interfacial sorption experiments, SS NMR studies can effectively discuss the chemical nature of surface-adsorbed inorganic phosphate, which depends on conditions such as pH [15], [16]. The pH can significantly affect the ionization state of phosphoric ester monomers. For instance, the pKa1 of MDP is reported to be 2.2 and pKa2 is 7.0 when water is used as the solvent [17]. This approach is crucial for providing a plausible explanation of how phosphoric ester monomers should react on the zirconia surface. Different pH conditions may influence the chemical reaction between phosphoric ester monomers and zirconia. A previous study verified that the pH of acidic functional monomers containing phosphonate or phosphate groups can influence the bond strength formed between dentin and restorative materials [18].

Despite various studies investigating the bonding efficiency of phosphoric ester monomers with zirconia, the pH factor in the chemical interaction of phosphate ester monomers with zirconia at the atomic level has not been thoroughly discussed. The objective of this study is to analyze the atomic-level interaction between zirconia and phosphoric ester monomers under different pH conditions (acidic or neutral). In this study, we prepared two types of phosphoric ester monomers—MDP and glycerophosphate-dimethacrylate (GPDM)—under different pH conditions. Adjusting the reaction environment under varying pH may influence the chemical bonds between phosphoric ester monomers and zirconia due to different concentrations of H+, which affect monomer ionization. Additionally, contact angle measurements and tensile bond strength tests were conducted using dual-cured resin cement and zirconia disks. To the best of our knowledge, this is the first report investigating whether these phosphoric ester monomers can have chemical interactions with zirconia and comparing contact angle and tensile bond strength under different pH values. This research can enhance our understanding of the clinical performance of phosphoric ester monomers.