where f is the measured friction coefficient, θ ≡ Γ/Γ max is the fractional coverage, f a is the friction coefficient at maximum additive coverage and f b is the friction coefficient with no additive present. Although we considered θ = 1 to determine f a from the HFRR measurements, complete coverage is not achieved in practice according to our computational adsorption isotherm results. This observation is in agreement with previous MTT-MD studies. Once f a and f b are determined, the Jahanmir and Beltzer model can be applied to the friction data to obtain a relation between the OFM bulk concentration and the fractional surface coverage, to which an adsorption isotherm model can be fitted. Jaishankar et al. used the Langmuir model for this purpose. However, according to Jahanmir and Beltzer, the Temkin isotherm model is better suited for these OFMs than the Langmuir isotherm model since it is able to account for inhomogeneous surfaces and lateral interactions. In the Temkin isotherm model the adsorption energy increases linearly with coverage (making the adsorption process less favorable):