Monica Ratoi

Acute Toxicity and Prothrombotic Effects of Quantum Dots: Impact of Surface Charge

Background Quantum dots (QDs) have numerous possible applications for in vivo imaging. However, toxicity data are scarce. Objectives To determine the acute in vivo toxicity of QDs with carboxyl surface coating (carboxyl-QDs) and QDs with amine surface coating (amine-QDs), we investigated the inflammatory properties, tissue distribution, and prothrombotic effects after intravenous injection. Methods We performed particle characterization by transmission electron microscopy and dynamic light scattering. Carboxyl-QDs and amine-QDs were intravenously injected in mice (1.44–3,600 pmol/mouse). At different time intervals, analyses included fluorescence microscopy, blood cell analysis, bronchoalveolar lavage, wet and dry organ weights, and cadmium concentration in various organs. We examined the prothrombotic effects in vivo by assessing the effect of pretreatment with the anticoagulant heparin and by measuring platelet activation (P-selectin), and in vitro by platelet aggregation in murine and human platelet-rich plasma exposed to QDs (1.44–1,620 pmol/mL). Results At doses of 3,600 and 720 pmol/mouse, QDs caused marked vascular thrombosis in the pulmonary circulation, especially with carboxyl-QDs. We saw an effect of surface charge for all the parameters tested. QDs were mainly found in lung, liver, and blood. Thrombotic complications were abolished, and P-selectin was not affected by pretreatment of the animals with heparin. In vitro, carboxyl-QDs and amine-QDs enhanced adenosine-5′-diphosphate–induced platelet aggregation. Conclusion At high doses, QDs caused pulmonary vascular thrombosis, most likely by activating the coagulation cascade via contact activation. Our study highlights the need for careful safety evaluation of QDs before their use in human applications. Furthermore, it is clear that surface charge is an important parameter in nanotoxicity.

Boundary lubrication of oxide surfaces by poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) in aqueous media

In this work, we have explored the application of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) as an additive to improve the lubricating properties of water for metal-oxide-based tribo-systems. The adsorption behavior of the polymer onto both silicon oxide and iron oxide has been characterized by optical waveguide lightmode spectroscopy (OWLS). Several tribological approaches, including ultra-thin-film interferometry, the mini traction machine (MTM), and pin-on-disk tribometry, have been employed to characterize the frictional properties of the oxide tribo-systems in various contact regimes. The polymer appears to form a protective layer on the tribological interface in aqueous buffer solution and improves both the load-carrying and boundary-layer-lubrication properties of water.

Lubricating Properties of Aqueous Surfactant Solutions

This paper describes a study of the elastohydrodynamic and boundary film-forming properties of solutions of simple surfactants in water. Such solutions are finding increasing use both as highly fire-resistant and environmentally-friendly hydraulic fluids. They also provide a vehicle to help clarify the origins of the film-forming behavior of oil-in-water emulsions. Film formation was found to be dependent upon the type and concentration of surfactant and also on the pH of the solution. Two different types of behavior were seen. At slow speeds, mono-layer-type boundary films were observed while at higher speeds, speed-dependent elastohydrodynamic-type films were produced. The latter were formed at entrainment velocities much lower than might be predicted from the viscometric properties of the bulk surfactant solutions and were similar to films formed by some oil-in-water emulsions at high speed. They show that water is entrained into high pressure, rolling contacts in accord to the predictions of elastic i...

Mechanisms of Oiliness Additives

Ultrathin film interferometry has been used to measure the boundary film-forming behaviour of long chain, carboxylic acid oiliness additives. It has been shown that in dry conditions, these acids form very thin films of around 2–3 nm thickness. However when water is present, some acids form significantly thicker films, around 10 nm in thickness. The behaviour of these films is very similar to that previously seen with metal carboxylate additive films, including thick film collapse at high rolling speeds followed by film reformation at slow speeds. It is suggested thick films formed by long chain carboxylic acid additives result from reaction of the acids at the rolling solid surfaces in the presence of water to form deposits of insoluble iron carboxylate.

Mechanism of Action of WS2 Lubricant Nanoadditives in High-Pressure Contacts

Because of their excellent tribological properties and potential to replace problematic lubricant additives currently in use, WS2 nanoparticles have spurred considerable interest over the last two decades from academia and industry to decipher their mechanism of action. To elucidate the mechanism, this study carried out tribological tests at low and high temperatures and investigated the wear track and friction properties. It was found that in high-pressure, high-temperature sliding contacts, WS2 nanoadditives react with the metal substrate to generate thick chemical tribofilms which account for their excellent tribological properties. Based on XPS and FIB/SIMS results, a layered structure was proposed for the chemically formed tribofilms. The large amount of W in the composition of the reacted tribofilm could explain the excellent mechanical and antiwear properties, while the exfoliated squashed WS2 NPs which fill the gaps and cover the reacted tribofilm account for the striking reduction in the boundary friction.Graphical Abstract

The impact of organic friction modifiers on engine oil tribofilms

Organic friction modifiers (OFMs) are important additives in the lubrication of machines and especially of car engines where performance improvements are constantly sought-after. Together with zinc dialkyldithiophosphate (ZDDP) antiwear additives, OFMs have a predominant impact on the tribological behaviour of the lubricant. In the current study, the influence of OFMs on the generation, tribological properties and chemistry of ZDDP tribofilms has been investigated by combining tribological experiments (MTM) with in situ film thickness measurements through optical interference imaging (SLIM), Alicona profilometry and X-ray photoelectron spectroscopy. OFMs and antiwear additives have been found to competitively react/adsorb on the rubbing ferrous substrates in a tribological contact. The formation and removal (through wear) of tribofilms are dynamic processes which result from the simultaneous interaction of these two additives with the surface of the wear track. By carefully selecting the chemistry of OFMs, the formulator can achieve lubricants that generate ZDDP antiwear films of optimum thickness, morphology and friction according to the application-specific requirements.

In Situ Study of Metal Oleate Friction Modifier Additives

There has been debate for many years as to whether long-chain surfactant friction modifier additives reduce friction by forming adsorbed films of monolayer thickness or whether they form films equivalent to several or many multilayers thick. In the work described in this paper, a series of metal oleate friction modifier additives has been synthesized and their film-forming properties compared in rolling-contact conditions by means of ultrathin film interferometry. It has been found that some of these additives form thick boundary films while others do not. It is concluded that thick boundary-film formation results from the formation of insoluble iron(II) oleate on the rubbing surfaces and that, for metal oleates, this occurs only for metals lower than iron in the electrochemical series and is due to a redox reaction involving iron from the steel surface and the metal oleate.

Molecular scale liquid lubricating films

A number of recently-developed experimental techniques, such as force balance, atomic force microscopy and ultrathin film interferometry have enabled the direct study of the properties of very thin liquid lubricating films between solid surfaces. These have been used to demonstrate the structure and rheology, and thus the lubricating ability, of monolayer additive films in rolling and sliding contacts. They have also been used to investigate the thin film properties of simple, additive-free fluids such as hydrocarbon base stocks. This paper reviews previous work on the thin film-forming properties of simple lubricant base fluids. Newwork is carried out using ultrathin film interferometry and a rolling-sliding friction test apparatus. It is found that the quasi-spherical molecules, cyclohexane and OMCTS form enhanced film thicknesses in high pressure, slow speed, rolling contacts. There is also an indication of a step-wise dependence of film thickness on rolling speed, in accord with finding using atomic force microscopy and surface forces apparatus. Friction measurements in mixed rolling-sliding show that these fluids also reduce friction in the boundary film regime.

WS2 nanoadditized lubricant for applications affected by hydrogen embrittlement

Hydrogen is one of the cleanest available vehicle fuels but its small atomic size allows it to diffuse readily through the lattice of solid materials, which can cause catastrophic failure in high strength steels. Metal embrittlement has been identified as a major consequence of hydrogen uptake and represents an extra challenge for lubricated tribological parts in fuel cell vehicles, hydrogen compressors, storage tanks, dispensers and wind turbines that are normally subjected to high stresses. This study has found WS2 nanoparticles as an effective additive candidate to impede the permeation of hydrogen into rolling element bearings at high temperatures and pressures. Compared to the pure polyalphaolefin (PAO) base oil, WS2 nanoadditized oil reduced the concentration of permeated hydrogen in the bearing steel and led to controlled wear and smoother tracks. These effects are attributed to the formation of a chemical tribofilm on the wear track which reduces hydrogen embrittlement and extends the life of steel through several mechanisms: (1) its continuous generation impedes formation of nascent catalytic surfaces during rubbing and thus prevents the decomposition of oil/water molecules and generation of atomic hydrogen; (2) acts as a physical barrier to hydrogen permeation through the wear track; (3) the low coefficient of diffusion of hydrogen through the tungsten compounds found in the tribofilm further reduces hydrogen permeation; (4) some of the atomic hydrogen is used up in redox reactions during the formation of the tribofilm and (5) the tribofilm reduces the total amount of water in the steel formed by the reaction of hydrogen atoms with oxides and thus extends the fatigue life. WS2 nanoadditized lubricants can lead to improved profitability and sustainability of the emerging renewable energy industry.

Langmuir-Blodgett Films in High-Pressure Rolling Contacts

Ultrathin film interferometry has been used to measure the film thickness of deposited Langmuir-Blodgett layers of stearic acid in a rolling, high pressure, ball on flat contact. Deposited multilayers up to five monolayers thick have been found to remain in a vertically-oriented configuration in static contact up to Hertzian pressures of at least 0.7 GPa. It has been shown that one deposited monolayer is extremely durable in rolling conditions and continues to show a thickness of at least 2 nm (the vertically-oriented monolayer) even at high speeds and in the presence of supernatant hydrocarbon solvent. Multilayers showed variable behavior. Two monolayers rapidly reduced to one, while three initial monolayers stabilized after rolling at a film thickness corresponding to between one and two monolayers. A four monolayer initial film reduced only slightly in thickness during rolling, to reach a level corresponding to three monolayers. The results have been interpreted in terms of the transfer of one or more deposited molecular layers from the disc to the ball surface. Presented at the 54th Annual Meeting in Las Vegas, Nevada May 23–27, 1999