Interaction Models and Molecular Simulation Systems of Steel–Organic Friction Modifier Interfaces

Pominov, Arkadii; Müller-Hillebrand, Johannes; Träg, Johannes; Zahn, Dirk

doi:10.1007/s11249-020-01384-9

Cited by 12 publications

(14 citation statements)

References 23 publications

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“…Subsequently, the temperature was reduced back to 300 K for another 2 ns. The final coverage was then estimated by determining the number of adsorbed and desorbed molecules, based on the distances of the hydroxyl oxygen atom of the headgroup to the hematite surface . The threshold distance (0.4 nm) was chosen such that for low concentrations the number of adsorbed molecules at the end was equal to the number of initially placed molecules.…”

Section: Methodsmentioning

confidence: 99%

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

et al. 2021

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The performance of organic friction modifiers (OFMs) depends on their ability to adsorb onto surfaces and form protective monolayers. Understanding the relationship between OFM concentration in the base oil and the resulting surface coverage is important for improving lubricant formulations. Here, we use molecular dynamics (MD) simulations to study the adsorption of three OFMsstearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO)onto a hematite surface from two hydrocarbon solventsn-hexadecane and poly(α-olefin) (PAO). We calculate the potential of mean force of the adsorption process using the adaptive biasing force algorithm, and the adsorption strength increases in the order SA < GMS < GMO. We estimate the minimum area occupied by OFM molecules on the surface using annealing MD simulations and obtained a similar hard-disk area for GMS and GMO but a lower value for SA. Using the MD results, we determine the adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one previous experimental data set for SA on hematite. For two other experimental data sets for SA, lateral interactions between surfactant molecules need to be accounted for within the MTT framework. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations but also has the highest plateau coverage. We validate the adsorption energies from the MD simulations using high-frequency reciprocating rig friction experiments with different concentrations of the OFMs in PAO. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the simulations and the experiments. The results deviate for OFMs containing Z-unsaturated tailgroups (GMO) due to the additional steric hindrance, which is not accounted for in the current simulation framework. This study demonstrates that MD simulations, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid−liquid interfaces.

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Section: Methodsmentioning

confidence: 99%

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

et al. 2021

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“…The final coverage was then estimated by determining the number of adsorbed and desorbed molecules, based on the distances of the hydroxyl oxygen atom of the headgroup to the hematite surface. 81 The threshold distance (4.0 Å) was chosen such that for low concentrations, the number of adsorbed molecules at the end was equal to the number of initially placed molecules. Varying the number of the initially placed molecules allowed us to determine the maximum coverage and thus the cross-sectional area of each molecule.…”

Section: Hard-disk Areamentioning

confidence: 99%

“…The same observation was made in previous MD simulations of SA monolayer stability on iron oxide surfaces. 81…”

Section: Hard-disk Areamentioning

confidence: 99%

“…The final coverage, or alternatively the final area per molecule, can be obtained by counting the number of molecules adsorbed at the end of the simulation. 81 The threshold distance to consider a molecule as desorbed was 4 Å, measured from the oxygen of the hydroxyl group to the COM of the first layer of surface atoms. Increasing the threshold distance above 3.5 Å did not change the results significantly, whereas decreasing it below this value underestimated the number of adsorbed molecules at low coverages, where all the molecules are assumed to be adsorbed, as shown in the Supporting Information.…”

Section: Hard-disk Areamentioning

confidence: 99%

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Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

Acero

Mohr

Bernabei

et al. 2021

Preprint

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The performance of lubricant additives, such as organic friction modifiers (OFMs), depends critically on their ability to adsorb onto the surfaces of moving components and form protective self-assembled layers (SAMs). Therefore, understanding the relationship between the concentration of the additive in the base oil and the resulting surface coverage is extremely important for lubricant formulations, as well as many other surfactant applications. Here, we use molecular dynamics (MD) simulations to study the adsorption isotherms of three different OFMs, stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO), onto a hematite surface from hydrocarbon solvents, n-hexadecane and poly-α-olefin (PAO). First, we calculate the potential of mean force (PMF) of the adsorption process using MD simulations with the adaptive biasing force (ABF) algorithm. Our MD simulations show that SA has the weakest adsorption energy on hematite, followed by GMS, and finally GMO, due to the increasing number of functional groups available to bind to the surface. We also estimate the area occupied by each OFM molecule on the surface in the high-coverage limit using MD simulations of the annealing of OFM films with different initial surface coverages. We obtain a similar hard-disk area for GMS and GMO, but a lower value for SA, which is due to its smaller headgroup size. Based on the adsorption energy and surface area, we determine the corresponding adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one available experimental data-set for SA. Two other experimental data-sets for SA require lateral interactions between surfactant molecules to be accounted for. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations (due to a smaller adsorption energy), but also has the highest plateau coverage (due to a smaller hard-disk area). We validate the adsorption energies from the MD simulations using high frequency reciprocating rig (HFRR) friction experiments with different concentrations of the OFMs in PAO. We use the Jahanmir and Beltzer model to estimate the surface coverage at each concentration and the adsorption energy of each OFM from the HFRR friction data. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the predictions made by the simulations and the experiments. The MD simulation and experimental results deviate for OFMs containing Z-unsaturated tailgroups (GMO), with the former suggesting stronger adsorption for GMO than GMS, while the latter predicts the opposite trend. We suggest that this is can be attributed to the higher steric barrier of adsorption of the OFMs with kinked Z-unsaturated tailgroup through a partially formed monolayer, an aspect which was not captured in the current simulations. This study demonstrates that MD simulations with the ABF algorithm, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid-liquid interfaces.

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“…[41][42][43][44][45][46][47] Alternatively, force fields are system-specifically adapted by refitting various LJ-pair interactions or replacing certain terms by Morse or Buckingham potentials. [48][49][50][51][52] This leads to a tedious system setup, which has to be adapted for each molecule/surface pair studied due to a lack of transferability.…”

Section: Introductionmentioning

confidence: 99%

Transferable Gaussian Attractive Potentials for Organic/Oxide Interfaces

et al. 2021

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Organic/oxide interfaces play an important role in many areas of chemistry, and in particular for lubrication and corrosion. Molecular dynamics simulations are the method of choice for providing complementary insight to experiments. However, the force fields used to simulate the interaction between molecules and oxide surfaces tend to capture only weak physisorption interactions, discarding the stabilizing Lewis acid/base interactions. We here propose a simple complement to the straightforward molecular mechanics description based on "out-of-the-box" Lennard-Jones potentials and electrostatic interactions: the addition of an attractive Gaussian potential between reactive sites of the surface and heteroatoms of adsorbed organic molecules, leading to the GLJ potential. The interactions of four oxygenated and four amine molecules with the typical and widespread hematite and γ-alumina surfaces are investigated. The total RMSD for all probed molecules is only 5.7 kcal/mol, and the corresponding percentage 23% over hematite, while on γ-alumina the RMSD is 11.2 kcal/mol, despite using a single parameter for all five chemically inequivalent surface aluminum atoms. Applying GLJ to the simulation of organic films on oxide surfaces demonstrates that mobility of the surfactants is overestimated by the simplistic LJ potential, while GLJ and other qualitatively correct potentials show a strong structuration and slow dynamics of the surface films, as could be expected from the first-principles adsorption energies for model head-groups.

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Interaction Models and Molecular Simulation Systems of Steel–Organic Friction Modifier Interfaces

Cited by 12 publications

References 23 publications

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

Transferable Gaussian Attractive Potentials for Organic/Oxide Interfaces

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