Molecules with a TEMPO-based head group as high-performance organic friction modifiers

Abstract

High-performance organic friction modifiers (OFMs) added to lubricating oils are crucial for reducing energy loss and carbon footprint. To establish a new class of OFMs, we measured the friction and wear properties of N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide referred to as C12Amide-TEMPO. The effect of its head group chemistry, which is characterized by a rigid six-membered ring sandwiched by an amide group and a terminal free oxygen radical, was also investigated with both experiments and quantum mechanical (QM) calculations. The measurement results show that C12Amide-TEMPO outperforms the conventional OFMs of glyceryl monooleate (GMO) and stearic acid, particularly for load-carrying capacity, wear reduction, and stability of friction over time. The friction and wear reduction effect of C12Amide-TEMPO is also greatly superior to those of C12Ester-TEMPO and C12Amino-TEMPO, in which ester and amino groups replace the amide group, highlighting the critical role of the amide group. The QM calculation results suggest that, in contrast to C12Ester-TEMPO, C12Amino-TEMPO, and the conventional OFMs of GMO and stearic acid, C12Amide-TEMPO can form effective boundary films on iron oxide surfaces with a unique double-layer structure: a strong surface adsorption layer owing to the chemical interactions of the amide oxygen and free radical with iron oxide surfaces, and an upper layer owing to the interlayer hydrogen-bonding between the amide hydrogen and free radical or between the amide hydrogen and oxygen. Moreover, the intralayer hydrogen-bonding in each of the two layers is also possible. We suggest that in addition to strong surface adsorption, the interlayer and intralayer hydrogen-bonding also increases the strength of the boundary films by enhancing the cohesion strength, thereby resulting in the high tribological performance of C12Amide-TEMPO. The findings in this study are expected to provide new hints for the optimal molecular design of OFMs.

Adsorption Behavior of TEMPO-Based Organic Friction Modifiers during Sliding between Iron Oxide Surfaces: A Molecular Dynamics Study

“…15,47 Throughout the simulations, a constant normal pressure of 0.5 GPa was applied by exerting a constant downward force to the atoms in the outermost layer of the upper substrate and fixing the atoms in the outermost layer of the lower substrate. Although 0.5 GPa might induce elastohydrodynamic-like behavior for PAO, 53,54 it is within the range of 0.4−1.2 GPa used in our experiments 45 and identical to that used in the previous MD study for conventional OFMs, 15 thereby facilitating comparison with the previous study. First, the Nose− Hoover thermostat 55,56 was coupled to the entire system except the fixed layer, and an equilibration run at a temperature of 393 K was executed for 0.5 ns.…”

“…The results are shown in Figure . Because the DFT calculations suggested that O amide and O • are located at an equal distance from the surfaces for the most stable adsorbed structure of C 12 TEMPO molecules, 0.40 nm was selected as one characteristic distance for O amide , which is close to 0.38 nm for O • . The other characteristic distance was selected to be 0.50 nm because beyond this distance, both the number density and its variation are small.…”

“…In our previous studies, we have developed N -(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)­dodecaneamide, which will be simply referred to as C 12 TEMPO hereafter, as a new type of OFM. , As shown in Figure a, it consists of a linear saturated alkyl chain, an amide (−CONH−) linkage, a six-membered ring, and a terminal oxygen radical (O • ). The two functional groups of different types, i.e., the amide group and the oxygen radical, are particularly unique as compared with conventional OFMs, which usually have a single functional group or multiple functional groups of the same type.…”

“…The two functional groups of different types, i.e., the amide group and the oxygen radical, are particularly unique as compared with conventional OFMs, which usually have a single functional group or multiple functional groups of the same type. Pin-on-disk type tribological tests have demonstrated that C 12 TEMPO exhibits a higher load-carrying capacity, a higher wear-reducing effect, and a more stable friction coefficient over time than the conventional OFMs of stearic acid and glycerol monooleate (GMO). , The optimized static structure from quantum mechanical (QM) calculations with the density functional theory (DFT) showed that both the radical and amide group chemically adsorb on iron oxide surfaces, thereby leading to stronger surface adsorption than stearic acid and GMO . Although the QM results correlate with the good performance of C 12 TEMPO, the adsorption behavior of C 12 TEMPO in lubricating oils during sliding and the structure of the adsorbed boundary films are yet to be elucidated.…”

“…44,45 The optimized static structure from quantum mechanical (QM) calculations with the density functional theory (DFT) showed that both the radical and amide group chemically adsorb on iron oxide surfaces, thereby leading to stronger surface adsorption than stearic acid and GMO. 45 Although the QM results correlate with the good performance of C 12 TEMPO, the adsorption behavior of C 12 TEMPO in lubricating oils during sliding and the structure of the adsorbed boundary films are yet to be elucidated.…”