Stearic acid monomolecular films

A recent report describes the radioactive labeled chromium (iii) ion adsorption on stearic acid LB films. The adsorption of chromium (iii) on a stearic acid monolayer on the surface of CrCl3 was described. Stearic acid monomolecular films on I0 3 M CaCl2 subsolutions were deposited in paraffin-coated microscope glass slides by the LB technique (pH range 2-9). 

Fatty acids spread spontaneously on water to form a monomolecular film. A benzene solution containing 0.10 mm3 of stearic acid is dropped into a tray full of water. The acid is insoluble in water, but spreads on the surface to form a continuous film covering an area of 400 cm2 after all of the benzene has evaporated. What is the average film thickness in (a) nanometers and (b) angstroms ... 

When 1 cm of a solution containing 8.5 mg per 100 cm of stearic acid (mol. wt. = 284.3) dissolved in a volatile organic solvent is placed on the surface of water in a Langmuir trough, the solvent evaporates off, leaving the stearic acid spread over the surface as an insoluble monomolecular film. If the surface area occupied by the film is 400 cm, calculate the area occupied by each molecule of stearic acid in the film. 

Isotherms for monomolecular films of a series of nitroxide stearic acid and nitroxide palmitate and stearate (methyl ester) films are reported for the temperature range 9° 50°C. 

No single description or definition adequately covers all lubricants or lubrication processes. For example, we have seen that a fluid can function as a lubricant by virtue of its viscosity as a liquid. We have also seen that a monomolecular film of stearic acid deposited on a metal surface acts as a lubricant and dramatically reduces the coefficient of friction. A polymeric substance such as polymethylmethacrylate when dissolved in an oil will increase its viscosity and improve its loadcarrying performance in the hydrodynamic lubrication process. Stearic acid dissolved in a carrier oil can effect a strong reduction in the coefficient of friction for metal rubbed against metal. 

The nature of an oriented film of monomolecular thickness can be typically exemplified by the properties and behavior of stearic acid, one of the most thoroughly studied film-forming substances. Stearic acid (n-octadecanoic acid) in extended, rigid form has the molecular configuration seen in Fig. 10-1. The total chain length is 2.442 nm (24.42 8) and the cross-sectional area is 18.4 x 10 cm (16.4 82). A regularly packed array of these molecules, with their hydrocarbon chains adlineated and their carboxylic heads located as shown in Fig. 10-1, constitutes an oriented film of monomolecular thickness. The physical evidence for the existence and the nature of such films is well known and is described in the standard texts on the chemistry and physics of surfaces [6]. 

Another concept that has been used freely and frequently in analyzing the coefficient of friction in lubricated sliding involves the dependence of the shear strength of a monomolecular film of fatty acid on the contact pressure, but no detailed model has ever been advanced to show the relation between shear strength and the molecular structure of the film. Bowers and Zisman [45] wrote the following relation for the "bulk" coefficient of friction of stearic acid ... 

Bowers and Zisman obtained an experimental value of 0.04 for the coefficient of friction of steel lubricated by a monomolecular film of stearic acid, whereas the value calculated from Eqn 10-14 was 0.00006. The relation... 

Under some circumstances, a material can spread out into a monatomic or mono-molecular layer or film. For example, solutions of stearic or oleic acid (both long-chain fatty acids) in a hydrocarbon solvent can be carefully dripped onto water when the solvent evaporates, the remaining fatty acids can arrange themselves into a monomolecular film on the water s surface. Such films have a definite surface coverage (that is, a definite area) depending on the number of fatty acid molecules present. 

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