# Molecular Science of Lubricant Additives +
## 1 Introduction
### Components in Liquid Lubricants
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Category** & **Classification** & \multicolumn{4}{c}{**Contents**} & \multicolumn{2}{c}{**Viscosity Index**} & **Remarks** \\ \cline{3-8}  & **Saturates, Mass\%** & **Aromatic, Mass\%** & **Solvate, ppm** & **Examples** & & & \\ \hline Group I & Solvent-refined mineral oil & 60-85 & 15-35 & 300-3000 & & & 80-119 & Extraction of impurities by solvents \\ \hline Group II & Hydro-processed mineral oil & \(\geq\)93 & \(<\)7 & 5-300 & & & 80-119 & Decomposition of organic sulfides by catalytic hydrogenolysis \\ \hline Group III & Hydro-cracken mineral oil & \(\geq\)95 & \(<\)5 & 0-30 & & & \\ \hline Group IV & Cligomers of 1-alkene & - & - & - & & & Synthetic hydrocarbons,so-called \\ \hline Group V & All fluids not included in groups I-IV & - & - & - & & & Non-PAO synthesis, plant oils, and some low quality mineral oils \\ \hline \hline \end{tabular}
\end{table} 
Table 1: API base oil category and examples. In addition to MOs and TGs, there is growing demand for base fluids prepared artificially, so-called synthetic fluids (SF). Huge types of SFs have been introduced and are being developed for lubricants. Among them, synthetic hydrocarbons prepared by the oligomerization of 1-alkens form an individual category, Group IV. They are called \"poly-alpha-olefin (PAO)\" as a market code. This pseudo-chemical naming is an example that makes chemists confused. Other synthetic hydrocarbons, synthetic esters, and poly-ethers are also being used as base fluids. The importance of SF is, in contrast to MOs and TGs, the possibility of molecular design for various purposes [30]. Similar to the issue with TGs, conventional additive for MOs cannot always be applied to SFs, as discussed in Section 4.4. Although the concept of molecular design can create molecules with required properties as lubricants, it is difficult to support all requirements for a lubrication system with base fluid alone in a cost effective manner. Here we shall recognize the importance of task-specific lubricant additives. They are defined as those substances that improve the total performances if added to base fluids properly. The performances include both tribological and material related ones. The former contribute to reducing friction and preventing wear and seizure during machine operation. The latter mainly contribute to the lifetime of the lubricant and/or tribo-materials. The method of improvements is ideal to enhance the advantages of base fluids with a synergistic effect. In the real world, additives make up for the shortages in specific properties of base fluids in most cases. Additives as liquid forms have advantages of handling in liquid lubricants. Solid additives, either as solution or as suspension, are also commonly used. Greases could be used in some cases. The application of lubricant additive is relatively recent in the long history of lubrication engineering. To the best of the author's knowledge, the first publication of the phenomena in a scientific journal was nearly 100 years ago [31]. According to the publication, lubricant additives have been practiced in the market since the 1950s [26]. Since then, lubricant additives have been developed by engineering knowhow. This might be the main reason why the descriptions in this field are less-scientific; confusing terminology, unclear chemical structure, complexity of contents, etc.