OFMs are one of the earliest and most widely studied types of lubricant additive. Hardy and Doubleday were the first to suggest that OFM friction reduction was due to the formation of vertically-oriented surfactant monolayers on the sliding surfaces. They found that fatty acids with longer chain length (C 4 -C 12 ) showed lower boundary friction and that fatty acids gave lower friction than fatty alcohols . Bowden and Leben showed that the friction of SA (C 18 ) monolayers deposited on steel surfaces with the Langmuir-Blodgett technique was initially the same as that for SAMs formed from a hydrocarbon base oil. Fowkes showed that OFM adsorption strength from hydrocarbon solution onto steel varied in the order carboxylic acid > amine > alcohol, and Groszek showed that this correlated with antiwear performance. Rowe 17 also found that adsorption strength and antiwear performance were correlated for OFMs, but with a slightly different order (amine > carboxylic acid > alcohol). Using this data, Rowe developed a mathematical model to calculate thermodynamic quantities describing adsorption from experimentally-obtained wear rates. Spikes and Cameron showed that adsorption and lubricant failure both had a similar dependence on fatty amine concentration and temperature, suggesting that lubricant failure could be due to desorption of the OFM molecules. Jahanmir and Beltzer studied the adsorption and friction behaviour of a wide range of carboxylic acid, ester, alcohol, amine, and amide OFMs with different alkyl tailgroup lengths, branching, and unsaturation. They found that additives that adsorbed more strongly generally gave lower friction. Jahanmir and Beltzer also built upon the model due to to describe the relationship between the friction coefficient and adsorption energy for a two-component lubricant (i.e. base oil and one additive), assuming that the minimum friction coefficient observed at high OFM concentration was due to the formation of a close-packed SAM. Recent depletion isotherm and polarised neutron reflectometry (PNR) experiments by Wood et al. for the adsorption from n-dodecane onto iron oxide surfaces have shown that OFMs with linear, saturated tailgroups (SA) do form monolayers with a high surface coverage, similar to that expected for a close-packed monolayer, at high concentration. Conversely, OFMs with kinked, Z-unsaturated tailgroups, such as oleic acid (OA), form monolayers with much lower surface coverage. This observation explained the higher friction coefficient for OA than SA reported in nanoscale atomic force microscopy (AFM) by Lundgren et al. and microscale surface forces apparatus (SFA) experiments by Ruths et al. The same observations were made at the macroscale by Campen et al. for OA and glycerol mono-oleate (GMO) compared to SA and glycerol mono-stearate (GMS) using steel substrates and an n-hexadecane solvent. Using a range of OFMs with different headgroups and tailgroups, Fry et al. showed through a combination of quartz crystal microbalance (QCM), ellipsometry, and tribology experiments that OFMs which formed higher coverage monolayers gave lower initial friction coefficients.