No abstract

Fatty acids and their derivatives have been used as model organic friction modifiers for almost a century, but there is still debate as to the nature of the boundary films that they form on rubbed surfaces. In this study, in situ liquid cell atomic force microscopy (AFM) is used to monitor the self-assembly of boundary films from solutions of fatty acids in alkanes on to mica surfaces. Because the mica surfaces are wholly immersed in solution it is possible to study directly changes in the morphology and friction of these films over time and during heating and cooling.It has been found that stearic and elaidic acid, that are able to adopt a linear molecular configurations, form irregular islands on mica that are tens to hundreds of microns in diameter and typically 1.6 nm thick, corresponding to domains of tilted single monolayers. At a relatively high concentration of 0.01M, stearic acid in hexadecane forms an almost complete monolayer, but at lower concentrations, in dodecane solution and for elaidic acid solutions these films remain incomplete after prolonged immersion of more than a day. The films formed by fatty acids on mica are displaced by repeated scanning in contact mode AFM but can be imaged without damage using tapping mode AFM.Rubbed quartz surfaces from a sliding ball-on-disc test were also scanned ex-situ using AFM and these showed that stearic acid forms similar monolayer island films on quartz in macroscale friction experiments as are found on mica.Oleic acid solutions behave quite differently from stearic acid and elaidic acid, forming irregular globular films on both mica and rubbed quartz surfaces. This is believed to be because its cis-double bond geometry means that, unlike its trans-isomer elaidic acid or saturated stearic acid, it is unable to adopt a linear molecular configuration and so is less able to form close-packed monolayers.

The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundary friction values that vary with alkyl group structure in a similar fashion to rolling-sliding MTM tests. These variations in friction occur immediately on rubbing, before any measurable tribofilm can develop. This study suggest that ZDDPs control boundary friction by adsorbing on rubbing steel or tribofilm surfaces in a fashion similar to organic friction modifiers. However it is believed that, for primary ZDDPs, residual alkoxy groups still chemically bonded to the phosphorus atoms of newly-formed polyphosphate/phosphate tribofilm may also contribute to boundary friction. This understanding will contribute to the design of low friction, fuel efficient crankcase engine oils. Graphical Abstract

Deposition of carbonaceous materials, such as asphaltene, is a major problem in petroleum production. During production, changing environmental conditions destabilize asphaltene, resulting in dispersions that are out of equilibrium, where asphaltene is aggregating or flocculating. Key to developing the most effective strategies for tackling this problem, is a fundamental understanding of asphaltene deposition behaviour. A quartz crystal microbalance with dissipation monitoring (QCM-D) is used to study asphaltene deposition from non-equilibrium dispersions generated by in-line mixing of asphaltene in toluene (a solvent) with n-heptane (a precipitant). The effects of heptane:toluene ratio and destabilization time are investigated. At high heptane:toluene ratio the rate of asphaltene aggregation is faster and large flocs form by the time the flowing liquid reaches the QCM cell. In this case, the rate of deposition decreases with deposition time. At low heptane:toluene ratio the rate of asphaltene aggregation is slower, hence large flocs do not form before the flowing liquid reaches the QCM cell, and deposition of smaller aggregates occurs. Here the deposition rate is constant with time. The deposited mass is greatest before the formation of large flocs and at short destabilization times, where the particle distribution is furthest from equilibrium. Destabilized small particles existing immediately after a destabilization event pose a greater deposition problem than the flocs which subsequently form. This may be a contributing factor in the existence of deposition "hotspots" at certain locations in the production pipeline. Pushing destabilized dispersions to their new equilibrium distributions as quickly as possible may be a preventative strategy to combat deposition. The dissipation-frequency relationship monitored by QCM-D is sensitive to the nature of deposited asphaltene films and may be used as a diagnostic tool.

Liquid cell AFM has been applied to study in situ the formation and properties of selfassembled films formed on mica surfaces by octadecylamine from alkane solution.Mica surfaces immersed in hexadecane or dodecane at room temperature show no identifiable surface films. However when octadecylamine solution is injected into the cell a boundary film forms almost immediately. This film takes the form of irregular islands of mean diameter approximately 1 to 3 µm and thickness typically 1.5 nm when measured in contact mode. These islands are believed to correspond to patches of vertically-oriented but tilted octadecylamine or ammonium salt held together primarily by van der Waals forces between adjacent alkyl chains. These films are quite labile in that during scanning of the tip in both tapping and in contact mode changes to the shape of the islands takes place, including consolidation of the island density in the scanned region and depletion from around this area.In situ experiments in which the temperature of the cell is varied over time show that the initially-formed islands disappear at a temperature of ca. 35ºC but are reformed when the cell is re-cooled. Similar tests on samples that remain immersed in solution for extended periods show more stable films, with islands being lost only above about 50-60ºC.The work shows that liquid cell AFM has great promise for studying the formation and properties of the boundary films formed by organic friction modifiers.