How surface electronegativity and calcium release in enamel mediate the adsorption and lubrication of salivary proteins: The role of interfacial water

This study investigated the effect of surface electronegativity and calcium release from human enamel on the adsorption and lubrication of salivary proteins from the perspective of interfacial water using three model substrates: calcium-releasing electronegative hydroxyapatite (which represents enamel), calcium-free electronegative silica, and calcium-free electropositive zirconia. The interfacial water layer was probed using attenuated total reflection-infrared (ATR-IR) spectroscopy, and the adsorption and lubrication of salivary proteins were examined using atomic force microscopy (AFM), quartz crystal microbalance with dissipation (QCM-D), and nanoindentation/scratch techniques. The strong affinity of electropositive substrates for water contributed to a thick interfacial water layer, which served as a physical barrier to weaken electrostatic attraction to salivary proteins. Thus, the proteins randomly adsorbed, forming a pellicle without a multilayered structure and good lubricity. The interfacial water layer on electronegative substrates tends to be thin. Driven by strong electrostatic interactions, salivary proteins are adsorbed through self-assembly to form a pellicle with a two-layered structure. While the hydrated calcium ions caused by substrate calcium release thickened the interfacial water layer, they served as a bridge to connect proteins. Consequently, a two-layered pellicle, both stiff and viscoelastic, formed to provide excellent lubricating action. In summary, the surface electronegativity and calcium release of enamel benefit the adsorption and lubrication of salivary proteins by regulating interfacial water.