Experimental and Ab Initio Characterization of Mononuclear Molybdenum Dithiocarbamates in Lubricant Mixtures.

Gabriele Losi,Benoit Thiebaut,Stefan Peeters,Sophie Loehlé,Franck Delayens,Hervé Vezin,Maria Clelia Righi

doi:10.1021/acs.langmuir.1c00029

Abstract

Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures.

Highlights

Friction is an unavoidable phenomenon which results in massive economic and environmental costs.[1]
For a systematic description of the chemical properties of the mononuclear compounds, we carried out static density functional theory (DFT) calculations and we evaluated for the first time geometric and electronic properties of mononuclear MoDTC (mMoDTC), estimated the strength of its bonds, and studied its interaction with metallic substrates and its possible decomposition mechanism on iron
We described the properties of one of the components of lubricant mixtures containing Molybdenum dithiocarbamates (MoDTCs): the mononuclear structure referred to as mMoDTC

Summary

■ INTRODUCTION

Friction is an unavoidable phenomenon which results in massive economic and environmental costs.[1]. The calculated values for EI, obtained by subtracting the total energies of the neutral complexes from the structures with one missing electron, turned out to be 5.3 and 6.2 eV for mMoDTC and sMoDTC, respectively, making the mononuclear compounds easier to ionize than the binuclear compounds. Such an electronic configuration makes the mMoDTC cation stickier than the neutral compound, and the stronger binding between the complex and the substrate can hinder the subsequent dissociative steps which can be useful to form the desired MoS2 clusters These features can be an explanation for the behavior observed in the tribological tests. On the other hand, starting to build a MoS2 tribolayer from the dimeric MoDTC structures can be more beneficial as the central units of the dimers require one recombination step less than the monomers

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES