# Molecular Science of Lubricant Additives +
## 4 Discussion as Multi-Component Systems
### Additive Technology for Synthetic Fluids  
Historically, additive technology in lubrication engineering arose in the early period of the petroleum age. Mass production of mineral oils, together with naphtha and fuels from crude oils, provided considerable benefits in the market. Liquids with proper viscosity for lubrication had become available at reasonable costs. However, neat mineral oils (without additives) usually display poor lubrication performances, especially in reducing friction and wear, in comparison with natural triglycerides (plant oils and animal tallow that had been the resources of lubricants since the ancient days until the petroleum age). After the advantage of additives had been recognized, the demands of additive technology sprang up. It was proved in many applications that additive technology enables machine operation with enhanced reliability, improved energy efficiency, and prolonged lifetime, etc. It should be emphasized that the improvements in lubricant performance motivated an advanced design of machine elements. This further promoted the improvement of lubricant performances. Today and in the future, additive technology is and will be indispensable in lubrication systems. The majority of base oils in the lubricant industry had been and is still Group I oils. Therefore, additive technology has been focused on improvement of Group I oils. Demands of quality lubricants from users are promoting the shift of base fluids to Group II or III from Group I. Then the problem with the solubility of additives in these base fluids arose. The same problem happens with Group IV. Although Groups I\(-\)IV are composed of hydrocarbons, Group I oils contain a considerable amount of cycloalkanes (naphthenic) and aromatics that are responsible for good solvency. Solvency-improving fluids are usually blended in these poor solvents (Group II\(-\)IV oils). Solvency could be understood by the affinity of a solute to the solvent. If a compound does not dissolve in the base oil, formulation of a liquid lubricant is impossible. On the other hand, high solubility of a solute in the solvent is not desirable. A proper solubility, especially for tribo-improvers, is required in formulating a lubricant. Figure 23 depicts a model of solubility\(-\)adsorptivity relation on lubrication performance, in consideration with an adsorption isotherm. From the working mechanism of tribo-improvers (Figures 5-7), it is possible to ascertain that they start by adsorption on rubbing surfaces. Assume that there is a threshold of surface coverage at C by which adsorption enables the tribo-improver to work. When additive-A and additive-B were compared at the same concentration of XA, the former displays expected performances while the later displays none, due to insufficient amount of molecules on the surface. A higher concentration of XB is needed for additive-B to perform. However, using an additive at high concentration rises the risk of disadvantages by the interaction with other additives. Therefore, additives should have proper solvency to the base fluids. The solubility in the base fluid is the second key factor in choosing additives after the primary lubrication function. Esters and polyethers belong to Group V oils and are receiving growing demands as fast biodegradable fluids [96]. Contrary to Groups I\(-\)IV oils, they contain oxygen atoms in their molecule. Here we discuss the difference between hydrocarbon and oxygen-containing molecules (esters and polyethers) in terms of polarity. Polarity is a result of unequaled sharing of the bonding electron pair, due to the differences in electron negativity of the atoms that form the covalent bond. The electron negativity of hydrogen, carbon, and oxygen are 2.2, 2.5, and 3.5, respectively. Hydrocarbons are being constructed with the combination of a carbon-carbon bond and carbon-hydrogen bonds. Since the differences in the electron negativity of the atoms of these bonds are low (0 for C\(-\)C bond, 0.3 for C\(-\)H bond), hydrocarbons display \"non-polar\" properties.