Structured free-water clusters near lubricating surfaces are essential in water-based lubrication

Abstract

Water-based lubrication provides cheap and environmentally friendly lubrication and, although hydrophilic surfaces are preferred in water-based lubrication, often lubricating surfaces do not retain water molecules during shear. We show here that hydrophilic (428 water contact angle) quartz surfaces facilitate water-based lubrication to the same extent as more hydrophobic Si crystal surfaces (618), while lubrication by hydrophilic Ge crystal surfaces (448) is best. Thus surface hydrophilicity is not sufficient for water-based lubrication. Surface-thermodynamic analyses demonstrated that all surfaces, regardless of their water-based lubrication, were predominantly electron donating, implying water binding with their hydrogen groups. X-ray photoelectron spectroscopy showed that Ge crystal surfaces providing optimal lubrication consisted of a mixture of -O and ¼O functionalities, while Si crystal and quartz surfaces solely possessed -O functionalities. Comparison of infrared absorption bands of the crystals in water indicated fewer bound-water layers on hydrophilic Ge than on hydrophobic Si crystal surfaces, while absorption bands for free water on the Ge crystal surface indicated a much more pronounced presence of structured, free-water clusters near the Ge crystal than near Si crystal surfaces. Accordingly, we conclude that the presence of structured, free-water clusters is essential for water-based lubrication. The prevalence of structured water clusters can be regulated by adjusting the ratio between surface electron-donating and electron-accepting groups and between -O and ¼O functionalities.

Extracellular Polymeric Matrix Production and Relaxation under Fluid Shear and Mechanical Pressure in Staphylococcus aureus Biofilms

“…The water stretching band ( Fig. 4d) was decomposed into two components for bound (around 3,308 cm Ϫ1 ) and free (around 3,469 cm Ϫ1 ) water (14,15). Absorption band areas and IR absorption band wavenumbers represent the amount and bond stiffness of the corresponding molecules, respectively, and have been plotted in Fig.…”

Effect of Infrared Light on Protein Behavior in Contact with Solid Surface

“…The correlation between the protective effect of IR, interfacial water structuring and presence of the strongly water-holding residues was demonstrated by results of our previous study [23]. That study indicated that IR-induced increase in cooperativeness and strength of H-bonds of protein interfacial water correlated with the protonation state of the basic amino-acid residues and that enhanced structuring of the hydration shell (acting as lubricant [69,70]) protected proteins from non-specific aggregation in solution. In this context it is interesting to consider different structural response of BSA and OVA to IR light (Fig.…”

Achieving Greener, Super-Robust Performance Water-Based Lubrication: Obtained by a Multifunctional Ionic Liquid Aqueous System and Formation of Unique Interfacial Interactions

“…Moreover, the nonpolar groups in the molecule show certain lateral cohesion; 26 thus, the adsorption film with suitable strength and thickness is necessary, which signifies the limitation of the ILs−water solubility and the balance between solubility and adsorption. In addition, the interaction between water and ILs is mainly manifested in the hydrolysis stability and salting-out effect, 27 as well as various ILs behave differently in aqueous lubrication systems. Consequently, an intensive analysis of the interfacial competitive adsorption behavior and tribochemical mechanisms of IL water-based lubricating systems is urgently desired and extremely beneficial.…”