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United States Department of Agriculture

Agricultural Research Service

Title: Adsorption Behavior of Fatty Esters and Oleic Estolides from Hexadecane Via Coefficient of Friction Measurements

Authors
item Kurth, Todd
item Cermak, Steven
item Biresaw, Girma

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 16, 2005
Publication Date: September 16, 2005
Citation: Kurth, T.L., Cermak, S.C., Biresaw, G. 2005. Adsorption behavior of fatty esters and oleic estolides from hexadecane via coefficient of friction measurements [abstract]. World Tribology Congress III. Available: http://www.conferencetoolbox.org/WTC05/index.cfm.

Technical Abstract: The frictional behaviors of a variety of fatty esters {methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), methyl stearate (MSt), and ethylhexyl oleate (EHO)} and oleic estolides {methyloleic estolide (ME), ethylhexyl oleic estolide (EHE)} as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: MSt > EHE greater than or equal to ME > EHO > MP > MO greater than or equal to ML. Application 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 Coefficient of Friction (COF)-concentration plots, MSt, EHE, ME, and EHO display minimal cooperative adsorption. MO, MP, and ML data require large attractive interaction terms (beta greater than or equal to 4) to be adequately fit. Primary adsorption energies calculated via a general cooperative model are necessarily decreased while total adsorption energies correlate well with values obtained via critical concentration analyses. A comparison of the ester data with previous adsorption studies of MO and MSt suggest that secondary alkyl-surface-, adsorbent-adsorbent-, and adsorpt-adsorpt-interactions cumulatively determine the calculated adsorption and cooperative interaction energies. Evaluation of the relative calculated parameters for the methyl fatty esters and the larger oleic estolides is tenuous. The larger systems are not only more massive but contain multiple sites at which adsorptive interactions may occur. To account for multiple surface-site adsorption due to mass or chemical functionality a multiple-site adsorption model was defined and applied. The results of this application indicate that an assumption of multiple-site adsorption of more massive components would result in deceptively increased calculated adsorption energies for typically applied models (e.g. FFG, Langmuir). Unconstrained fitting of the simple multi-site adsorption model further demonstrates the necessary inclusion of lateral interaction parameters.

Last Modified: 9/2/2014
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