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Cooperative adsorption behavior of fatty acid methyl esters from hexadecane via coefficient of friction measurements
Author(s) -
Kurth Todd L.,
Biresaw Girma,
Adhvaryu Atanu
Publication year - 2005
Publication title -
journal of the american oil chemists' society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.512
H-Index - 117
eISSN - 1558-9331
pISSN - 0003-021X
DOI - 10.1007/s11746-005-1069-2
Subject(s) - adsorption , chemistry , hexadecane , langmuir adsorption model , thermodynamics , activity coefficient , langmuir , alkyl , fatty acid , organic chemistry , aqueous solution , physics
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 adsoprtion energies: MSt>MP>MO≥ML. Application of Langmuir, Temkin, and Frumkin‐Fowler‐Guggenheim adsorption models via nonlinear fitting of a general cooperative model demonstrates the necessary inclusion of cooperative effects in the applied model. In agreement with the qualitative features of steady‐state COF‐concentration plots, MSt modeling requires minimal cooperative interaction terms. However, MO, MP, and ML data require large attractive interaction terms to be adequately fitted. Primary adsorption energies calculated via the cooperative model are necessarily decreased, whereas total adsorption energies correlate well with values obtained via critical concentration analyses. These results and comparisons with previous adsorption studies of MO and MSt suggest that primary (ester‐surface) and secondary (alkyl‐surface) adsorbate‐adsorbent, adsorbate‐adsorbare, and (free‐additive) adsorpt‐adsorpt interactions collectively determine both the calculated primary and the cooperative interaction energies.