Proceedings of WTC2005 World Tribology Congress III September 12-16, 2005, Washington, D.C., USA WTC2005-64073 ADSORPTION MODELING OF FATTY AND OLEIC ESTOLIDE ESTERS VIA BOUNDARY LUBRICATION COEFFICIENT OF FRICTION MEASUREMENTS Kurth, T.L., Cermak, S.C., Byars, J.A., Biresaw, G. National Center for Agricultural Utilization Research United States Department of Agriculture 1815 N. University, Peoria, IL 61604 ABSTRACT 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. INTRODUCTION As part of our effort to develop new bio-based lubricants, members of our lab have sought to characterize a wide range of bio-based materials. Biresaw et al. previously examined a series of natural oils and methyl ester control systems as lubricant additives (in hexadecane) in a boundary lubrication test regime (Ball-on-Disk) [1]. Critical to their performance is their ability to adsorb to a steel surface. To characterize such systems, Jahanmir and Beltzer previously noted the ability to obtain Coefficient of Friction (COF)-derived adsorption isotherms [2]. Subsequently, adsorption energies may be obtained via fits to a variety of adsorption models (e.g. Langmuir, Temkin, Frumkin-Fowler-Guggenheim (FFG)). In our previous work we noted the non-Langmuirian behavior of methyl fatty ester control systems (methyl stearate, methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML)) and applied non-linear iterative fitting and a general adsorption model [3]. In this paper we extend that work to include more complex estolide esters (methyl oleic estolide ester (ME), 2- ethylhexyl oleic estolide ester (EHE)) and an additional ester control system (2-ethylhexyl oleate (EHO)). Estolide esters display good pour points, viscosities, and oxidative stability and are thus suitable choices for bio-based lubricants [4]. Additionally, these systems provide more complex surface interactions for which the theoretical models may be tested and developed. NOMENCLATURE C additive concentration (M). θ additive surface coverage, 0 to 1. E primary adsorption energy (kcal/mole). R molar gas constant (1.987x10 -3 kcal/mole-K). T temperature (298 K). α lateral interaction parameter (kcal/mole). E ads total adsorption energy (kcal/mole). C c critical concentration (M). E c critical concentration adsorption energy (kcal/mole). MW additive molecular weight (g/mole). n number of sites covered by one adsorpt. γ arbitrary dependence parameter. E 0 ester adsorption energy (kcal/mole). RESULTS AND DISCUSSION θ versus additive concentration data were generated for each of the methyl esters studied using Ball-on-Disk and the method of Jahanmir and Beltzer [1-3]. An unconstrained General adsorption model [5] allowing for no lateral interaction (Langmuir), repulsive interaction (Temkin), or attractive interactions (FFG) was applied via iterative fitting: RT RT E e e C αθ θ θ − − − = ) 1 ( (1) Where, E + αθ = E ads . Isotherm fits and relevant parameters are depicted in Fig. 1 and Table 1. It was found that simply determined critical concentrations, indicative of a critical points of effective lubrication (θ = 0.5, α = 0, Eq. 1, Fig. 1), provide adsorption energies, E c , that correlate well with values obtained via iterative methods, Table 1. However, all Langmuir fits are clearly insufficient, Fig. 1. For data exhibiting relatively Langmuirian behavior the adsorption parameters are similar regardless of the method of fitting, e.g. EHE, ME, and EHO. However, when significant 1 Copyright © 2005 by ASME