Adsorption Modeling of Fatty and Oleic Estolide Esters via Boundary Lubrication Coefficient of Friction Measurements

The frictional behaviors of a variety of fatty esters (methyl palmitate (MP), methyl laurate (ML), and 2-ethylhexyl oleate (EHO)) and oleic estolide esters (methyl oleic estolide ester (ME) and 2-ethylhexyl oleic estolide ester (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: EHE ≥ ME > EHO > MP > ML. Application of a general adsorption model indicates slight cooperative adsorption of EHE, ME, and EHO. MP and ML data require larger attractive interaction terms (α ≤ −2.3) to be adequately fit. Irrespective of ester functionality increasingly negative adsorption energies appear to correlate with molecular weight. This suggests that multiple site coverage and multiple adsorptive interactions are likely for each of the esters studied.