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.

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.