Tribology Letters Vol. 11, No. 1, 2001 49 The boundary lubrication properties of model esters P.M. M c Guiggan Ceramics Division ∗ , National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 14 November 2000; accepted 28 January 2001 The friction of three chemically distinct esters was measured in order to determine how molecular architecture influences friction. The friction coefficients μ of mica surfaces separated by a thin film (<2 nm) of ω-chlorodecyl benzoate, ω-chlorodecyl pentafluoro benzoate, and ω-chlorodecyl perfluoro hexanoate were measured to be 0.15 ± 0.015, 0.13 ± 0.012, and 0.12 ± 0.02, respectively. The friction coefficients for the esters are lower than the previously measured friction coefficients of simple hydrocarbon liquids such as n-tetradecane (μ = 0.8), but are comparable to the friction coefficients of surfactant monolayer coated surfaces (μ = 0.001–0.2). The results suggest that the ester molecules adsorb onto the mica surface with the (phenyl or hexyl) carbonyl next to the surface and the hydrocarbon tail pointing away from the surface. Hence, the friction is controlled by the packing density and properties of the hydrocarbon tail. Changes in the chemistry and structure of the carboxylic acid portion of the ester only give rise to small changes in the friction coefficient. KEY WORDS: lubrication; friction; ester; surface forces apparatus; monolayer 1. Introduction When two surfaces are rubbed together, some amount of lubrication is generally required to minimize friction and wear [1,2]. Ideally, surfaces are lubricated by a micron thick film of lubricant and the friction is determined by the hy- drodynamic properties of the fluid film [1,2]. However, in many applications, the fluid lubricant is pressed out by an applied load, and the surfaces are separated by a lubricant boundary layer that may be only a few molecular dimensions thick [1,2]. A variety of liquid and solid materials have been used through the years to lubricate surfaces. Mineral oils, sil- icones, polyphenyl ethers, esters, and fluorocarbons are among the types of fluids commonly used as lubricants [3]. Additives are often mixed into lubricants to provide the re- quired performance [4]. For the lubrication of magnetic hard disks, perfluoroethers, dithiocarbaminates, amides, amines, silanes, and esters have been investigated [5–7]. Some of the first detailed experiments on friction were performed in 1690 by Amontons’ who found that the fric- tion is proportional to the applied load and independent of the area of contact [8]. In those experiments, Amontons’ investigated the sliding of copper, iron, lead, and wood sur- faces in the presence of a lubricating layer of pork fat [2,8]. Since pork fat is primarily composed of saturated and un- saturated hexadecyl and octadecyl glycerol esters, one may conclude that Amontons’ was one of the first to study the friction of esters. In this paper, the friction of three chemically distinct esters was measured in order to determine how molec- ular structure influences friction. The friction force of these esters is interesting since these specific chemistries are patented for lubricants for magnetic hard disks [5,6]. All ∗ Currently in the Polymers Division. the esters contain a fluid ω-chlorodecyl tail, but the acid part of the ester has varying structure (linear or cyclic) and degree of fluorination. The results show that the friction coefficients of mica surfaces separated by a thin ester film (<2 nm) are lower than the previously measured friction co- efficients of simple hydrocarbon liquids, where the hydro- carbon molecule lays parallel to the mica surface. However, the results are comparable to the value of the friction co- efficients measured for adsorbed hydrocarbon monolayers, where the monolayers are oriented perpendicular to the mica surface. 2. Materials and methods 2.1. Synthesis of ω-chlorodecyl esters The esters were synthesized following standard organic synthesis techniques [9]. Acid chlorides readily react with primary and secondary alcohols to produce esters with high yield, quite often without the need of a solvent. The reaction proceeds via nucleophilic acyl substitution according to R 1 (C=O)Cl + R 2 –OH → R 1 (C=O)O–R 2 + HCl In accordance with this reaction, the esters were synthe- sized using equimolar amounts of acid chlorides and alco- hols. No solvents were used in the reaction. An example of the procedure used to make ω-chlorodecyl pentafluoro ben- zoate is described below. A similar procedure was used to synthesize the other two esters. All the esters are liquids at room temperature. To make ω-chlorodecyl pentafluoro benzoate, 20.9 g (0.108 moles) of 10-chlorodecanol (Aldrich) was added dropwise to 25 g (0.108 moles) pentafluorobenzoyl chloride 1023-8883/01/0700-0049$19.50/0  2001 Plenum Publishing Corporation