Hydration layer structure modulates superlubrication by trivalent La ³⁺ electrolytes

Abstract

Water-based lubricants provide lubrication of rubbing surfaces in many technical, biological, and physiological applications. The structure of hydrated ion layers adsorbed on solid surfaces that determine the lubricating properties of aqueous lubricants is thought to be invariable in hydration lubrication. However, we prove that the ion surface coverage dictates the roughness of the hydration layer and its lubricating properties, especially under subnanometer confinement. We characterize different hydration layer structures on surfaces lubricated by aqueous trivalent electrolytes. Two superlubrication regimes are observed with friction coefficients of 10 −4 and 10 −3 , depending on the structure and thickness of the hydration layer. Each regime exhibits a distinct energy dissipation pathway and a different dependence to the hydration layer structure. Our analysis supports the idea of an intimate relationship between the dynamic structure of a boundary lubricant film and its tribological properties and offers a framework to study such relationship at the molecular level.

Macroscale Superlubricity of Hydrated Anions in the Boundary Lubrication Regime

“…The logarithmic variation of COF with sliding velocity was characteristic of hydration lubrication with shear-promoted thermally activated sliding. , It is noteworthy that the COF of hydrated Na + ions was almost identical to that of SO 4 2– ions with the strongest hydration strength in the tested anions, indicating that strongly hydrated anions have excellent hydration lubrication characteristics comparable to cations. ,− It should be noted that the comparison of lubrication performance between hydrated cations and anions was in the boundary lubrication regime (see Table S2), without the influence of hydrodynamic contribution on lubrication. Besides, a typical logarithmic variation of the COF with increasing velocity was found for both strongly hydrated anions (especially SO 4 2– , Figure B,C) and cations (Figure B), representing a rate-activated sliding for both hydrated anions and cations. , Lower thermal activation energy barriers for PTFE than UHMWPE represent lower COF for PTFE (see Supplementary Text 4).…”

“…Besides, a typical logarithmic variation of the COF with increasing velocity was found for both strongly hydrated anions (especially SO 4 2− , Figure 3B,C) and cations (Figure 6B), representing a rate-activated sliding for both hydrated anions and cations. 4,13 Lower thermal activation Different to "hard-on-hard" friction pairs such as Si 3 N 4 / sapphire and steel/steel, 11,55−57 polymer/sapphire forms a "hard-on-soft" or "soft-on-hard" friction pair, which will cause significant elastic deformation or even plastic deformation of "soft" polymers. 58,59 To evaluate the effect of this deformation on hydration lubrication, the friction materials of the ball and the disk were swapped with each other to compare the lubrication performance between the "soft" ball on the "hard" disk and the "hard" ball on the "soft" disk under the lubrication of acidic Na 2 SO 4 solution.…”

“…Electrolyte solutions and ion hydration are ubiquitous and essential in nature, which play vital roles in natural and industrial processes, − especially for interfacial lubrication , and surface adhesion . Hydration lubrication between negatively charged surfaces in aqueous electrolyte solutions accounts for superlubricity with ultralow sliding friction coefficient (COF) of order 0.001 under high pressures exceeding 10–100 MPa, ,, which is attributed to the high load bearing capacity and fluid response to shear by hydration layers. − Superlubricity, as a novel breakthrough, can reduce wear of materials, save energy and resources, prolong the service life of equipment, and decrease the safety accidents of machinery . Whether in the microscale or macroscale conditions, the adsorbed layers of smaller and more hydrated monovalent cations (Li + , Na + , and K + ) exhibited better lubrication performance than the layers of larger and less hydrated monovalent cations (Cs + and NH 4 + ) between hydrophilic mica/mica, , silica/mica, silica/silica, and Si 3 N 4 /sapphire interfaces. , Hydration layers of multivalent cations were also tightly adsorbed on the negatively charged surfaces in aqueous solutions, showing a strong short-range hydration repulsion − and leading to the charge inversion of friction surfaces, which can realize better superlubricity performance than monovalent cations, especially in the boundary lubrication regime .…”

The surface force balance: direct measurement of interactions in fluids and soft matter

“…As the design and fabrication of porous materials for energy and environmental applications such as filtration and osmotic energy generation reaches nanometric and sub-nanometic dimensions, we meet the need to understand more deeply the dynamic properties of water and electrolytes in these extreme degrees of confinement. The subtle details of wall-electrolyte interactions and ion hydration in nanoconfinement can lead to dramatic effects [143][144][145][146][147][148][149]. Inspired by biological mechanisms of signalling, energy storage and transfer, the new field of iontronics was born, as a versatile and energy-efficient soft alternative to electronics.…”

Insight into the hydration friction of lipid bilayers

“…The concept of hydration lubrication then has been proposed and invoked to explain the striking reduction of sliding friction between charged surfaces in aqueous media. 9–13 According to the hydration lubrication mechanism, hydration layer structures formed by water molecules are strongly bound to the charges they surround, and so can withstand large normal pressures without being squeezed out while retaining high rapid relaxation and thus respond to shear in a fluid manner. 10 However, there is little molecular-scale understanding of this mechanism, particularly the frictional energy dissipation within the subnanometer hydration layer structures.…”