Hydration lubrication modulated by water structure at TiO₂–aqueous interfaces

The nature of solid–liquid interfaces is of great significance in lubrication. Remarkable advances have been made in lubrication based on hydration effects. However, a detailed molecular-level understanding is still lacking. Here, we investigated water molecule behaviors at the TiO₂–aqueous interfaces by the sum-frequency generation vibrational spectroscopy (SFG-VS) and atomic force microscope (AFM) to elucidate the fundamental role of solid–liquid interfaces in lubrication. Combined contributions of water structures and hydration effects were revealed, where water structures played the dominant role in lubrication for TiO₂ surfaces of varying hydrophilicity, while hydration effects dominated with the increasing of ion concentrations. Superior lubrication is observed on the initial TiO₂ surfaces with strongly H-bonded water molecules compared to the hydrophilic TiO₂ surfaces with more disordered water. The stable ordered water arrangement with strong hydrogen bonds and the shear plane occurring between the ordered water layer and subsequent water layer may play a significant role in achieving lower friction. More adsorbed hydrated molecules with the increasing ionic concentration perturb ordered water but lead to the enhancement of hydration effects, which is the main reason for the improved lubrication for both TiO₂. This work provides more insights into the detailed molecular-level understanding of the mechanism of hydration lubrication.