# Molecular Science of Lubricant Additives +
## 2 Individual Lubricant Additives and Their Working Mechanism
### Tribo-Improvers
#### 2.1.1 Friction Modifiers (FM)  
Historically, carboxylic acids with a straight hydrocarbon chin (higher fatty acids) were the first recognized \"lubricity improving substances\" when they were dissolved in mineral oils [35]. They have been well-studied from both a scientific and engineering perspective in the past 70 years. The benefit of carboxylic acids in scientific research is the availability of pure compounds as commercially available reagents, although their practical applications are trivial [36]. However, an analogous molecular structure could be found in various practical additives, indicating the importance of good understanding in the working mechanism from a scientific viewpoint. Figure 5 explains the effective working mechanism of carboxylic acids (octadecanoic acid, \(n\)-C\({}_{17}\)H\({}_{35}\)COOH is the most frequently employed in fundamental research) for steel as a tribo-material. Engineering surfaces are usually covered with contaminants or oxide layers, which can be removed by rubbing. This exposes active surfaces that interact with the FM molecule. Further rubbing provides the energy to the molecule for surface reactions. It produces organic salt (salt of carboxylic acid), and results in chemisorption (adsorption with chemical bond between adsorbent and adsorbant). Chemisorption possesses a strong interaction with the surface compared to that of physisorption [37], and hence it is beneficial for protecting the film against rubbing [38]. The alkyl group in the molecule has significant role in lubrication. Direct asperities, contact upon rubbing could be mitigated by the adsorbed FM molecules. The Van der Waals attraction force could be generated between oriented molecules. It strengthens the robustness of the film to support the load applied to the contact [35]. In fact, alkyl chains having >12 methylene (-CH\({}_{2}\)- unit) are beneficial for boundary lubrication [39]. This model has been proved under dynamic conditions (so-called \"in lubro\") using optical interferometry [40] or liquid cell scanning probe microscope [41]. Although the interaction of the acid moiety with the surface is a chemical process, the hydrocarbon group (commonly straight hydrocarbons having 15-17 carbons), a major part of the molecule, is supposed to be unreacted. However, common spectroscopy cannot provide clear evidence of additive molecules that are mainly composed of the hydrocarbon moiety. It should be noted that the concentration of FM is usually around 10 mmol\(\cdot\)kg\({}^{-1}\) in hydrocarbons as base fluids and hence it is difficult to identify additive molecules as highly diluted solutions. Nonetheless, clear evidence of chemisorption was obtained by surface mass spectroscopy using deuterium-labeled molecules as model FM [42]. This is frequently misunderstood by tribologists as \"those FMs physisorp on surfaces\", probably because the alkyl group is unreacted. A term \"quasi-physical adsorption\" might be convenient to distinguish those FMs from \"tribo-chemical types\" explained below. Some lubricant additives take advantage of tribo-chemical reactions. Figure 6 illustrates the working mechanism of organic salts of molybdate. The first step is the interaction of additive molecules  
Figure 4: Overlap effects of tribo-improvers. with the rubbing surface, which is an analogous mechanism in Figure 5. The difference appears in the next step. When the tribo-process provides enough energy to the molecule, the molecule decomposes to inorganic salts. Since the reaction is associated with the dissociation of many chemical bonds, it needs relatively higher energy than the chemisorption process. The inorganic salts (molybdenum disulfide as the example in Figure 6) are usually more robust than organic salts to heat and mechanical shearing. Contrary to the chemisorption mechanism, the organic moieties in an additive molecule were decomposed during the tribo-film formation. In this regard, the main role of the organic moiety is to dissolve the precursor of inorganic salt in base fluids (mostly hydrocarbons).