Ion Adsorption and Hydration Superlubricity

The surface-water interactions are pivotal in interfacial functions and properties relevant in many natural processes, particularly the lubrication systems of living organisms, like the articular cartilage, oral mucosa, cardiovascular system, muscle, and tendon tissue. The ion adsorption often determines interfacial lubrication performance, especially hydration superlubricity, which depends on the surface charge characteristics and ion types. The hydration superlubricity mechanism has been well understood over the past 30 years, which further promotes the exploration and development of artificial hydration lubrication materials inspired by nature. This review first introduces the basic properties of the solid–liquid interface and the superlubricity system related to hydrated ions. Then, the influencing factors and mechanism of interfacial ion adsorption on hydration superlubricity are presented. Subsequently, negatively charged materials based on overall modification are summarized, including modified PEEK and modified UHMWPE with ultra-low friction coefficient and ultra-low wear rate. Finally, the potential application of such materials in artificial joints and underwater equipment systems are put forward. It is projected that interest in ion adsorption and hydration superlubricity from the basic science and engineering communities will continue to accelerate in the near future, accompanied by the concept of superlubricitive engineering.