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

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.

Figure 1

Molecular structure of mMoDTC.

Figure 2

Chromatogram of the flash separation of S525. The collected fractions are marked on the bottom of the plot, with fractions 7, 8, and 9 highlighted with their corresponding colors.

Figure 3

Top panel: mass spectra of fractions 7, 8, and 9. Different families of the compound are grouped by color. Bottom panel: comparison between the theoretical spectrum of mMoDTC and the corresponding region in the spectrum of fraction 8.

Figure 4

EPR spectra of the MoDTC samples. (A) Echo field sweep EPR spectrum of the commercial S525 mixture measured at the X-band and at 20 K, with the Q-band echo spectrum at 20 K as an inset. (B) 2D-HYSCORE spectrum at 20 K of the commercial S525 mixture. (C) Echo field sweep EPR spectrum of the separated fractions measured at the X-band and at 5 K. (D) Intensity of the Mo(V) signal vs the temperature, indicating the Curie behavior and, therefore, the paramagnetic nature of the species in fractions 8* and 10*.

Figure 5

Tribological test of the considered fractions and the reference mixture.

Figure 6

Electronic properties of mMoDTC. Top left: frontier molecular orbitals of the mMoDTC cation. The isovalue for the LUMO and SOMO-1 is 0.002, while for the SOMO, the isovalue is 0.0005 to better visualize the details of the plot. The red and blue colors of the isosurfaces represent the positive and negative signs, respectively, of the electronic wave function. Bottom left: spin polarization plot, where the blue coloring of the isosurface around the Mo atom indicates an excess of negative spin. The isovalue for this plot is 0.01. Top right: DOSes for cationic (solid line) and neutral (dashed line) mMoDTC. Bottom right: densities of spin-up (red) and spin-down (blue) states in cationic mMoDTC. The DOSes were shifted in order to set the Fermi energy to 0 in all the plots.

Figure 7

Schematic representation of the dissociation patterns of mMoDTC.

Figure 8

Optimized configuration for mMoDTC adsorbed on the clean (left) and oxygen-passivated (right) iron (110) surfaces.

Figure 9

Fragments of mMoDTC originating by each individual step of the dissociation of the ligand units: central metallic units with one ligand unit attached and central metallic units alone and ligand units alone obtained from cut 1 and 3, respectively.

Figure 10

Scheme of two hypothetical reactions where the mononuclear and binuclear MoDTC molecules in the mixture (A) or their central units on iron (B) are in equilibrium.