Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2005
Publication Date: 4/1/2005
Citation: Kurth, T.L., Biresaw, G., Adhvaryu, A. 2005. Cooperative adsorption behavior of fatty acid methyl esters from hexadecane via coefficient of friction measurements. Journal of the American Oil Chemists' Society. 82(4):293-299. Interpretive Summary: In order to develop new bio-based lubricants, we need to understand how such materials interact with metal surfaces. The materials studied are components of, and are obtained from, natural oils such as soybean oil. These compounds are methyl oleate, methyl palmitate, methyl laurate, and methyl stearate. We have purposely tested simple compounds as additives in solutions of varying concentration to obtain the least complex data for subsequent analysis. We have found that common friction measurements using these solutions provide insight into the additive interactions with steel surfaces. The data has been analyzed using well established mathematical models in novel ways. The results indicate that the additive molecules adsorb to the surface and rearrange as surface coverage increases. The new methods of analysis and the resulting information have lead to greater understanding of current lubricants and will facilitate the development of new lubricants.
Technical Abstract: The frictional behaviors of methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), and methyl stearate (MSt) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. It was found that the transient attributes of Coefficient of Friction (COF)-time spectra are a sensitive measure of adsorption equilibria. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: MSt > MP > MO greater than or equal to ML. Comparison of Langmuir, Temkin, and Frumkin-Fowler-Guggenheim (FFG) adsorption models via non-linear fitting demonstrates the necessary inclusion of cooperative effects in the applied model. In agreement with the qualitative features of steady-state COF-concentration plots, MSt displays minimal cooperative adsorption. MO, MP, and ML data require large attractive interaction terms (beta greater than or equal to 4) to be adequately fit. These results, and comparison with previous adsorption studies of MO and MSt suggest that reordering of adsorbed molecules, at increased additive concentrations, rather than specific intermolecular interactions determines the observed cooperative behavior. Primary adsorption energies calculated via the cooperative model are necessarily decreased while total adsorption energies correlate well with values obtained via critical concentration analyses.