Macroscale Superlubricity of Hydrated Anions in the Boundary Lubrication Regime

Abstract

Cations can achieve excellent hydration lubrication at smooth interfaces under both microscale and macroscale conditions due to the boundary layer composed of hydration shells surrounding charges, but what about anions? Commonly used friction pairs are negatively charged at the solid/solution interface. Achieving anionic adsorption through constructing positively charged surfaces is a prerequisite for studying the hydration lubrication of anions. Here we report the hydration layer composed of anions adsorbed on the positively charged polymer/ sapphire interface at acidic electrolyte solutions with pH below the isoelectric point, which contributes to the hydration lubrication of anions. Strongly hydrated anions (for the case of SO 4 2−) exhibit stable superlubricity comparable to cations, with strikingly low boundary friction coefficient of 0.003−0.007 under contact pressures above 15 MPa without a running-in period. The hydration lubrication performance of anions is determined by both the ionic hydration strength and ion adsorption density based on the surface potential and tribological experiments. The results shed light on the role of anions in superlubricity and hydration lubrication, which may be relevant for understanding the lubrication mechanism and improving lubrication performance in acidic environments, for example, in acid pumps, sealing rings of compressors for handling acidic media, and processing devices of nuclear waste.

Disparate External Electric Field Effect on the Adsorption and Shear Behavior of Monovalent and Trivalent Ions in Electrolyte Solution

“…In contrast, when HOPG was positively charged, more Cl – ions were adsorbed on HOPG, while excessive La 3+ ions adsorbed on SiO 2 microspheres; this led to the disappearance of ion–ion attractive interactions that originated from high valence ions, and thus a more repulsive interaction (Figure g) and lower COF (Figure f). Besides, although the hydration ability of Cl – is weaker than that of metal cations (low hydration energy of ∼340 kJ/mol and low hydration number of 1.0–2.0), our previous study found that the adsorption of Cl – anions on the positively charged surface could also reduce the COF and obtained superlubricity . Moreover, the primary adsorption of anions, resulting in a flat and low energy barrier surface, may contribute to the reduction of the COF.…”

“…Besides, although the hydration ability of Cl − is weaker than that of metal cations (low hydration energy of ∼340 kJ/mol and low hydration number of 1.0−2.0), our previous study found that the adsorption of Cl − anions on the positively charged surface could also reduce the COF and obtained superlubricity. 41 Moreover, the primary adsorption of anions, resulting in a flat and low energy barrier surface, may contribute to the reduction of the COF. Interestingly, we found that trivalent ionic solutions (CrCl 3 , AlCl 3 ) showed a similar trend of tribology and electrostatic potential as La 3+ (see Figures S13 and S14).…”

Electric Potential Controlled Ionic Lubrication

“…The ions in water-based lubricants can form an adsorption layer through the electrostatic interaction, which can bear a large applied load and reduce friction at the interface. Ionic aqueous lubricants even provide favorable conditions for the realization of superlubricity under all lubrication regimes, including boundary lubrication (BL) [69,82,[110][111][112][113][114], mixed lubrication (ML) [14,45,55,107,115,116], and elastohydrodynamic lubrication (EHL) [46,[117][118][119][120]. In a BL regime, the repulsive force (like hydration repulsion) generated by the adsorption of ions on the friction surfaces is conducive to bearing the applied load and reducing the friction.…”

A Review of Electric Potential-Controlled Boundary Lubrication

“…In aqueous solutions, these molecules tightly associate with charged or amphoteric ions, forming hydrated layers [65,71]. Research has demonstrated the effective load-bearing and lubricating properties of metal cation hydration layers [72][73][74]. By modulating the adsorption and desorption of hydrated ions on electrode surfaces, significant alterations in the tribological characteristics between the electrode and contact substrate can be achieved.…”