The role of hydrated anions in hydration lubrication

Hydration lubrication has long been invoked to account for the ultralow sliding friction between charged surfaces in aqueous environments, but still not well understood at molecular-level. Herein, we explored the lubrication effect of hydrated halogen anions on positively charged surface at the atomic scale by using three-dimensional atomic force microscopy and friction force microscopy. Atomically resolved three-dimensional imaging revealed that the anion layer was topped by a few hydration layers. The mechanical properties of the hydration layers were found mainly dependent on the concentration of electrolyte solutions and independent of the species of hydrated anions. Atomic-scale friction experiments showed that the hydration friction coefficient and friction dissipation at low concentrations were orders of magnitude lower than that at high concentrations and in pure water. Superlubricity can be achieved in low concentration electrolyte solution. These results indicated that the changes of electrolyte solution concentrations led to different adsorption state of anions on the positively charged surface which gave rise to the difference of the friction behaviors. The findings in this study reveal the role of hydrated anions in hydration lubrication and provide deep insights into the origins of hydration lubrication.