Adsorption of organic friction modifiers at static and sheared interfaces
Organic friction modifiers (OFMs) are additives that can be added to engine oils to reduce friction between rubbing surfaces. OFMs are thought to reduce friction by adsorbing at engine surfaces, producing films that are conventionally thought to be one molecule thick. Although established, the adsorption structure and the mechanism by which OFMs operate is far from proven. In this thesis, the adsorption and film structure of glycerol monooleate (GMO), an industrially relevant OFM, at the iron oxide-dodecane interface is investigated under static conditions and under applied shear. The study of the interface under applied shear is facilitated by the use of a novel sample environment, referred to as the tribometer, for neutron reflectometry (NR) and X-ray reflectometry (XRR). The tribometer can apply shear to an interface at rates of up to 3.8 x 103 s-1 and the thicknesses of thin films adsorbed at the interface can be determined via NR or XRR. The details of the tribometer are described in Chapter 2. Chapter 3 investigates the aggregation of GMO in dodecane with pendant drop tensiometry and small angle neutron scattering. Additionally, the iron oxide surfaces used in the reflectometry studies are characterised through X-ray photoelectron spectroscopy, XRR and NR. Interestingly, a region of solvent depletion was found at the iron oxide-dodecane interface, which was approximately 1 nm thick. This is postulated to arise from the presence of adsorbed gases and depleted solvent. Chapter 4 examines the adsorption of GMO at the iron oxide-dodecane interface under static conditions. The adsorption behaviour has been explored using depletion isotherms and NR. The former technique was used to study the adsorption of GMO as a function of bulk concentration and temperature. The adsorption behaviour was found to be Langmuir-like, indicating the formation of a monolayer film. In agreement, the thickness of the GMO adsorbate film was found to be less than the length of a GMO molecule. NR was then used to investigate the adsorption of GMO at the iron oxide-dodecane interface in the presence of water. Water was found to be present alongside GMO at the interface at 25 and 60 °C, although the film was found to alter from a mixed layer of GMO and water at 25 °C to a distinct bilayer of water and GMO at 60 °C. Chapter 5 focusses on modelling NR and XRR data collected with the tribometer. An NR model is presented that combines conventional reflectivity theory with the summation of reflected intensities to describe reflectivity from thicker films. This model was used to describe the reflectivity of GMO adsorbed at the iron oxide-dodecane interface under shear at 7.0 x 102 s-1. The film thickness was found to be equivalent to the film thickness determined when under static conditions, indicating that any changes to the film structure under this particular shear rate were not resolvable using the tribometer and NR. Finally, a model which describes the XRR data of GMO adsorbed at the iron oxide-dodecane interface at 3.0 x 103 s-1 is presented.