Understanding Adsorption Behaviors of Organic Friction Modifiers on Hydroxylated SiO2 (001) Surfaces: Effects of Molecular Polarity and Temperature

Cu(II) inhibited the transport of tetracycline in porous media: role of complexation

Tetracycline (TC) and Cu(II) coexist commonly in various waters, which may infiltrate into the subterranean environment through runoff and leaching, resulting in substantial ecological risks. However, the underlying mechanisms why Cu(II) affects the transport of TC in porous media remain to be further explored and supported by more evidence, especially the role of complexation. In this study, the transport of TC with coexisting Cu(II) was comprehensively explored with column experiments and density functional theory (DFT) calculation. At natural environmental concentrations, Cu(II) significantly inhibited the transport of TC in the quartz sand column. Cu(II) augmented the retention of TC in the column mainly via electrostatic force and complexation. The interaction between TC and TC-Cu complexes on the surface of SiO2 was investigated with first-principles calculations for the first time. There were strong van der Waals forces and coordination bonds on the surface of complexes and SiO2, leading to higher adsorption energy than that of TC and inhibiting its penetration. This study offers novel insights and theoretical framework for the transport of antibiotics in the presence of metal ions to better understand the fate of antibiotics in nature.

Position of Carbonyl Group Affects Tribological Performance of Ester Friction Modifiers

The tribological properties of lubricants can be effectively improved by the introduction of amphiphilic molecules, whose performance is largely affected by their polar head groups. In this work, the tribological performance in steel-steel contacts of two isomers, glycerol monostearate (GMS) and stearyl glycerate (SG), a glyceride and a glycerate, were investigated as organic friction modifiers (OFM) in hexadecane. SG exhibits a much lower friction coefficient and wear than GMS despite their similar structures. The same applies when comparing the performance of oleyl glycerate (OG) and its isomer, glycerol monooleate (GMO). Surface chemical analysis shows that SG forms a polar, carbon-based, tribofilm of around tens of nanometers thick, while GMS does not. This tribofilm shows low friction and robustness under nanotribology test, which may contribute to its superior performance at the macro-scale. The reason for this tribofilm formation can be due to the stronger adsorption of SG on the steel surface than that of GMS. The tribofilm formation can be stress-activated since lower friction and higher tribofilm coverage can be obtained under high load. This work offers insights into the lubrication mechanism of a novel OFM and provides strategies for OFM design.

Effect of Molecular Weight on Tribological Properties of Polyether Amine Derivatives under Different Contact Modes

The requirements for the fuel economy of modern industry continue to drive the progress of low-viscosity lubricants. The present work reports the application of polyether amine derivatives as friction modifiers to improve the tribological properties of low viscosity poly-alpha-olefin. Three polyether amine derivatives with different molecular weights were synthesized, the tribological properties of which were systematically investigated under three different contact modes. These functionalized polymers exhibited significant friction reduction and wear resistance properties in the point-on-flat and line-on-flat friction tests, but just showed anti-wear performance in the severe point-to-point contact mode. The results exhibited that molecular weights of the polymers had a direct effect on their tribological properties. The increase of molecular weight in a certain range was beneficial to the improvement of tribological properties, but further undue increase will rather reduce the friction reduction and wear resistance performances. It can be indicated that the number of oxygen atoms increased with the molecular weight of the polymer, which will be conductive to the adsorption of the polymer on the metal surface. However, when the molecular weight of the polymer exceeds a certain value, the steric hindrance of the molecules adsorbing to the metal surface increases, which in turn has a negative impact on the tribological properties.

Insights into the Mechanical Properties of Ultrathin Perfluoropolyether-Silane Coatings

Ultrathin perfluoropolyether-silane (PFPE-silane) films offer excellent functionality as antifingerprint coatings for display touchscreens due to their oleophobic, hydrophobic, and good adhesion properties. During smartphone use, PFPE-silane coatings undergo many abrasion cycles which limit the coating lifetime, so a better understanding of how to optimize the film structure for improved mechanical durability is desired. However, the hydrophobic and ultrathin (1-10 nm) nature of PFPE-silane films renders them very difficult to experimentally characterize. In this study, the cohesive fracture energy and elastic modulus, which are directly correlated with hardness and better wear resistance of 3.5 nm-thick PFPE-silane films were, respectively, measured by double cantilever beam testing and atomic force microscopy indentation. Both the cohesive fracture energy and modulus are shown to be highly dependent on the underlying film structure. Both values increase with optimal substrate conditions and a higher number of silane groups in the PFPE-silane precursor. The higher cohesive fracture energy and modulus values are suggested to be the result of the changes in the film chemistry and structure, leading to higher cross-linking density. Therefore, future work on optimizing PFPE-silane film wear resistance should focus on pathways to improve the cross-linking density. Subcritical fracture testing in humid environments reveals that humidity negatively affects the fracture properties of PFPE-silane films.