Overbased micelles

Several types of nanoparticles can be envisaged as lubricant additives. The chapters of this book describe different possible nanolubricants nested nanoparticles (fuUerenes, onions and nanotubes) made on metal dichalcogenides or carbon, reverse overbased micelles, metallic nanoparticles and boron-based nanolubricants. In some cases, the authors chose to study these particles because their structure is close to well-known lamellar structures in tribology (for example inorganic fuUerenes of M0S2). However, dispersion of nanoparticles in oil is not always simple. Thus, in a first part, some rules will be shown about the dispersion of solid nanoparticles in a liquid. 

Detergents in oils form micelles, which are aggregates of surfactant molecules. Polar molecules are made of a polar part (hydrophilic head) and a nonpolar hydrophobic part. According to the nature of the solvent (polarity), they can form micelles or reverse micelles. Hydrocarbon chains create a steric barrier, preventing the particles from aggregation. Overbased micelles have a mineral core made, for example, of calcium carbonate CaCOa (Figure 1.2). 

Owing to their lipophilic chains and their very small size, micelles are very easily dispersed in base oils and dispersion is stable for a long time. Overbased micelles, used initially as detergents, also present antiwear properties (see Chapter 4). Overbased micelles can be used in several ways (a) as thin coatings deposited on a solid surface, (b) as a colloidal dispersion in apolar medium (oil, grease, etc.) and (c) in gas phase lubrication. 

The ability of overbased micelles to lubricate to some extent compared with molecules can be explained on the basis of the following features ... 

Figure 1.2 Schematic representation of an inverse micelle and overbased micelle structure...

Figure 1.3 TEM image of Langmuir film made of overbased micelles...

This experiment demonstrates the static state of the OMeABS, commonly called overbased micelles . Effectively in the case of true dynamic micelles in equilibrium with single molecules, the continuous extraction with pure solvent would have completely extracted the molecules and the rebuild micelles would then be present in the boiler. 

Encapsulated Nano-Sized Particles, also Called Overbased Reverse Micelles ... 

Figure 4.7 Overbased lead octanoate structure. The dynamic micelles (B) have a well-ordered core (C) (deduced from EXAFS) with a diameter of 1 nm (deduced from light scattering and cryo microscopy). Deposited on a carbon support (A), micelles will rearrange during the solvent evaporation and lead to lamellar aggregates (D)...

Martin, J.M., Mansot, J.L., and Hallouis, M. Energy filtered electron microscopy (EFEM) of overbased calcium alkylaryl sulfonate micelles, Ultramicroscopy, 30, 1990, 321-328,... 

Mansot, J.L., Martin, J.M., Dexpert, H., Faure, D., Hoomaert, P and Gallo, R. Local stmcture analysis in overbased reverse micelles, Physica B,158,1989,237. 

Mansot, J.L., Hallouis, M. and Martin, J.M., Colloidal antiwear additives. Part One stmctural study of overbased calcium alkylbenzene sulfonate micelles. Colloids and Surfaces A, 71,1993,123-134. 

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