Adsorption Behavior and Nanotribology of Amine-Based Friction

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Adsorption Behavior and Nanotribology of AmineBased Friction Modifiers on Steel Surfaces Prathima C. Nalam, Alex Pham, R. Veronica Castillo, and Rosa M. Espinosa-Marzal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02097 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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The Journal of Physical Chemistry

Adsorption Behavior and Nanotribology of Amine-Based Friction Modifiers on Steel Surfaces Prathima C Nalam1,2, Alex Pham1, R. Veronica Castillo3 and Rosa M. Espinosa-Marzal1* 1

Department of Civil and Environmental Engineering, University of Illinois at UrbanaChampaign, Urbana 61801, Illinois, USA 2

3

Department of Materials Design and Innovation, University at Buffalo, NY, USA.

TOTAL MARKETING SERVICES - Centre de Recherche de Solaize - Chemin du Canal – BP 22 - 69360 Solaize – France

* Corresponding Author: [email protected] Abstract: This work describes the effect of the adsorption behavior of fatty amine-based organic friction modifiers on their tribological performance. The adsorption of four different fatty amines with a varying number of amine functional groups (two vs. three) and different anchoring structures (straight vs. branched) on stainless steel surfaces in n-hexadecane was monitored using quartz crystal microbalance (QCM). It is shown that the fatty amines form weakly adsorbed and disordered boundary films and the molecules lay horizontally on the surface. A higher number of amine functional groups at the anchoring end of the fatty amines results in higher adsorbed masses, however, their larger steric hindrance results in slower adsorption kinetics. Atomic force microscopy shows that the loosely-packed adlayers decrease the adhesion between the tip and the stainless steel surface and yield a linear increase in friction force with load at low loads. Comparing adsorbed masses and adsorption kinetic constants with the coefficients of friction measured by lateral force microscopy reveals that faster surface-adsorption kinetics of the additive molecules enables a more effective healing of the worn track, which dictates the friction force in the boundary lubrication regime. 1. Introduction: Rapid evolution of engine technologies and stringent emission policies in automotive engineering present constant challenges for the lubricant industry to develop high performance, low-pollutant additives. Efficient lubrication, especially when the engine is operating at low speeds, depends on the ‘oiliness’ of the lubricant film formed at the interface1,2. This oiliness results from the formation of a low interfacial shear stress film consisting of amphiphilic organic friction modifiers (OFMs) 3,4. OFM molecules are typically composed of a polar head group attached to an alkyl chain with more than 10 carbons (tail) and form an ultra-thin, dense, self-assembled film on the rubbing surfaces5. Covalent head group–surface bonds determine the amount of irreversibly bound, i.e. chemically adsorbed, molecules, while parameters such as alkyl chain length, degree of 1 ACS Paragon Plus Environment

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unsaturation of alkane chain, steric effects of the head group and the solubility of OFMs in the base oil determine the film thickness and the packing density of the adsorbed molecules in case of physisorption 6,7. High inter-chain van der Waals interactions between alkyl chains can enhance the order and the rigidity of the surface-adsorbed film 8,9, and thereby, the resistance of the film to compression and interdigitation, which facilitates shear at the tail-tail interface. Fatty chains with acid-, amine- or alcohol- based end groups have been investigated as OFMs on several metallic and other engineering-relevant surfaces2,5,10. Fatty acids, one of the most widely researched friction modifiers, readily adsorb on oxidized metallic surfaces to form a thin, viscous layer of fatty acid soap 11,12. Optical interferometer shows that the surface-adsorption of a monolayer of stearic acid (0.001 M in hexadecane) is sufficient to reduce friction by at least 75% in comparison to neat hexadecane13. At higher temperature, fatty acids are shown to form either mono- or bidentate bonds between the carboxyl group and the iron-based surface with an interaction strength of ~ -360 kJ/mol 14. Fatty amines (FAs), on the other hand, are used as corrosion-resistance additives 15,16 and antiwear pad growth inhibitors 17,18, and hence, they are widely used in lubricant formulation along with amides and mono-oleates 2. Spikes et al. studied long-chain fatty amines as friction modifiers on stainless steel and showed that the lubrication failure of the amine films below a critical concentration was associated with the desorption of the molecules 19. While alkyl amines remain unprotonated in non-polar oils, they are cationic surfactants in aqueous medium. Self-assembly of FAs on hydrated mica surfaces led to the formation of patchy islands via protonated amino end groups 19,20. The activation of amino end groups through acid-base reaction in presence of either surface-adsorbed water21 or atmospheric CO222 led to the formation of ionic hydrogen bonds with an interaction strength of ~-156.1 kJ/mol, which was essential for the formation of low-shear, highcoverage monolayer islands. However, friction measurements by atomic force microscopy led to immediate damage of the FA films at the smallest applied shear stress20, indicating the formation of a weakly interacting film on the mica surface. Wood et al. studied hexadecylamine adsorption on iron oxide surfaces in hexadecane and demonstrated the high affinity between amine molecules and surface iron (III) ions via nitrogen lone pair transfer23. MD simulations confirmed the formation of a surface-anchored disordered amine film on iron surfaces with a thickness of ~ 1620 Å and an average molecular tilt angle of ~ 40o with respect to the surface24. In this study, we investigate the performance of fatty amines as friction modifiers on steel surfaces. Previous studies have shown that the structure of the anchoring groups, i.e. the number and charge of end functional groups, can play an important role in improving the anchoring strength and sticking probability of OFM molecules to metal oxides surfaces. Hence, we designed four different end group chemistries to modulate the fatty amine-steel interaction strength. The number of amino groups, their charge and anchor structure were varied. Quartz crystal microbalance with dissipation (QCM-D) was employed to quantify both the equilibrium adsorbed mass and the adsorption rate constants of the fatty amines on stainless steel surfaces, and these results were correlated to the 2 ACS Paragon Plus Environment

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tribological properties of the films studied using an atomic force microscope (AFM). The results show that, besides the molecule-surface interaction strength, the rate of adsorption of the fatty amine molecules from solution onto the locally worn boundary film defines the coefficient of friction. 2. Experimental Description: 2.1 Materials Fatty amine derivatives, labelled as di-FA (N-Tallowalkyl-1,3-propanediamine), charged di-FA (N-Tallowalkyl-1,3-propanediamine diamine), tri-FA (N- Tallow alkyl- Dipropylene triamine) and branched tri-FA (N-Aminopropyl-N-tallow alkyl trimethylenediamine) were provided by TOTAL MS (Lyon, France). The alkyl chains employed in this study are derived from Tallow triglycerides, which are a combination of saturated and unsaturated C16-C18 fatty chains. The structures of the head groups used in this study are shown in schematic 1. Stainless steel coated quartz crystals (purchased from Biolin Scientific, USA) were used as substrates for both QCM and AFM measurements. The detailed surface composition of steel-coated crystals (Swedish standard steel 2342), measured using X ray photon electron spectroscopy (XPS), was described elsewhere25. These XPS measurements revealed an oxygen atomic concentration of ~ 57 % on the oxide layer at the surface. Organic solvents n-hexadecane (ReagentPlus, 99%), n-heptane (anhydrous, 99%) and ethanol (absolute) were purchased from Sigma-Aldrich (USA). All solvents were stored at room temperature and were used as received. The concentrations of the fatty amine molecules were varied from 0.05 wt. % to 1 wt. % in n-hexadecane. The solutions were magnetically stirred for at least 45 minutes on a hotplate at 55oC to obtain stable and clear solutions.

Schematic 1: Nomenclature and corresponding anchoring group structures for the selected fatty amines. 3 ACS Paragon Plus Environment


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2.2 Methods 2.2.1 Dynamic light scattering (DLS): DLS measurements were conducted using a Zetasizer ZS90 (Malvern, UK) at a wavelength of 633 nm. The scattering measurements were performed in glass cuvettes that were pre-cleaned with sodium dodecyl sulfate solution (20% in DI water), dilute hydrochloric acid and DI water. The laser was aligned at a scattering angle of 90o to pass through the solution in the cuvette (edges were avoided). The measurements were conducted after heating the solutions in the DLS chamber to 55 ºC. 100 scans per test and 3 tests per solution were performed to evaluate the solubility of the friction modifier in the solution. All solutions were filtered through 0.2 m filters and were stirred at 55 oC until a clear solution was observed before transferring the solutions into the cuvettes. The average hydrodynamic radius was obtained from the highest-intensity peak in the DLS spectrum. 2.2.2 Quartz Crystal Microbalance (QCM): QCM (Biolin Scientific, Q-sense Analyzer E4, US) was used to measure the adsorption behavior of the fatty amines on steel crystals (Biolin Scientific, US). The cleaning protocol for the steel crystals involved sonication of crystals for 15 minutes in ethanol, followed by UV-O3 (Bioforce Nanoscience, Chicago, IL) cleaning for 45 minutes and rinsing with ethanol to remove organic contamination from the surface. The crystals were further sonicated in heptane and sodium dodecyl sulfate (20% in DI water) for 15 minutes each, followed by excessive rinsing with ultrapure water to remove traces of base oil from the crystals. The crystals were sonicated again in ethanol for 10 minutes and UV-O3 cleaned for 10 minutes, just before transferring them to the QCM flow modules. The quartz crystals were re-used for adsorption studies at least 4-5 times before discarding. The flow modules and QCM tubing were rinsed thoroughly with n-heptane and ethanol solutions before use. All the measurements were conducted at 55oC and a flow rate of 75 l/min. Estimation of adsorbed mass: The frequency changes measured by QCM were converted to adsorbed mass (ng/cm2) using the Sauerbrey equation26:

f  

2 f 02 A q q

m

Eqn. 1

where 𝑓0 is the resonant frequency (Hz), 𝑓 the frequency change, 𝑚 the mass change, 𝐴 the active crystal area (cm2), 𝑞 the density of the quartz (2.648 g.cm-2) and 𝑞 the shear modulus of the AT cut quartz crystal (2.947 ∙ 1011 g·cm-1·s-2), respectively. For Q-Sense crystals, the above equation can be reduced to 𝑚 = ―17.7𝑓. This equation was employed only when the influence of the viscoelastic dissipation of the adsorbed mass was found to be negligible, i.e. ∆𝐷 < 2 ∙ 10-9).

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Two types of adsorption isotherms were measured, single-injection and series-injection isotherms. In single-injection measurements, the baseline was first obtained in neat n-hexadecane at 55oC, then a solution of the friction modifier in n-hexadecane at the selected concentration was injected into the QCM cell, and the adsorption of the fatty amines was measured, and finally the cell was rinsed in n-hexadecane solution. In series-injection measurements, solutions of increasing concentrations in n-hexadecane were introduced into the QCM cell in consecutive injections. Here, the concentration of the fatty amine in the solution was increased systematically from 0.05 to 0.5 wt. % through a series of injections on the same crystal with intermediate rinsing steps with neat (additive-free) hexadecane. The baseline in series-injection measurements was also obtained in nhexadecane solution at 55oC. Drifts in the baselines due to the high operating temperature was taken into account through appropriate baseline corrections. In all measurements, n-hexadecane was employed as the base oil as the adsorption of n-hexadecane molecules on the steel surface was found to be negligible 25 (Supporting information 1). 2.2.3 Atomic Force Microscopy (AFM): AFM measurements were conducted on a Nanowizard AFM (JPK Instruments, Germany). Force-distance maps were acquired using naturally-oxidized silicon AFM tips (HQ:CSC 37, Mikromash, spring constant kn=0.11 N/m) to determine the change in pull-off force upon adsorption of the fatty amines on the steel surface. An approach/separation speed of 0.5 m/s and a distance of ~1 m were employed to acquire ~ 100 force curves per condition in order to estimate the average pull-off force. Friction forces mediated by the adsorbed fatty amines were measured by lateral force microscopy. A sharp, naturally-oxidized silicon AFM tip (HQ:CSC 37, Mikromash, kn=0.3 N/m) was employed to acquire the friction loops. At least ten lateral force loops per condition were measured by recording the lateral deflection of the cantilever tip in the forward (trace) and reverse (retrace) directions. The friction force in a loop was calculated by averaging over the half width of the trace and retrace scans. First, speed-dependent friction force measurements were conducted at a constant normal load of 10 nN with sliding velocities ranging from 0.05 m/s to 40 m/s. Then, friction measurements were performed at a sliding speed of 0.5 µm/s and a sliding distance of 1 µm with normal loads varying from 0 to 40 nN. All measurements were performed in n-hexadecane solution and the temperature of the sample cup was held at 55 ºC. The normal force calibration for the cantilevers was conducted using the thermal noise method, as described elsewhere27. The lateral spring constant was calibrated according to the wedge method 28,29 (TGT 01 grid, Nano and More, USA) and the ratio of the lateral stiffness to the lateral sensitivity was determined to quantify the friction force. 3. Results and Discussion: 3.1 Solubility studies of the FA molecules in n-hexadecane The absence of light scattering by 1 wt. % fatty amine (FA) solutions in n-hexadecane in DLS measurements indicates that the investigated FA molecules are soluble in n-hexadecane at 55 ºC. In addition to this, unsaturated fatty chains are prone to oxidization, and hence, they can be reduced to low molecular weight fragments at elevated temperatures that then aggregate. Hence, the absence of light scattering also demonstrates the high stability of the selected fatty amines in 5 ACS Paragon Plus Environment


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hexadecane. When the temperature of the solution was lowered to 26 ºC, tri–FA solutions showed the formation of aggregates with a hydrodynamic radius of ~ 4 m. In contrast, the DLS intensity of di-FA solutions showed negligible scattering even at room temperature. A summary of the DLS measurements is given in the supporting information 2. 3.2 Adsorption studies of the FA molecules on steel surfaces Figure 1a represents the adsorption of di-FA on steel surfaces at a concentration of 0.5 wt.% in nhexadecane in a single injection measurement. A before-rinse adsorbed mass was estimated from the frequency change after injecting the FA solution in the fluid cell using Eqn. 1. The before-rinse mass represents the equilibrium adsorbed mass of the FA molecules on the surface in presence of excess fatty amine molecules in the solution, which consists of both the physically- and chemically-adsorbed mass on the steel surface. When the FA solution is exchanged with nhexadecane, the desorption of the loosely-bound molecules from the surface takes place. This leads to a decrease in the adsorbed mass, which is defined as the after-rinse mass. The after-rinse mass represents the mass of (likely) chemisorbed molecules in the absence of FA molecules in the solution. Figure 1b shows the adsorption of di-FA on steel surfaces during a series-injection measurement; 6 different overtones are displayed here. The di-FA concentration in n-hexadecane was systematically varied from 0.05 to 0.5 wt.% and before-rinse and after-rinse frequency changes were estimated at each concentration. The injection events and the rinsing steps are indicated by black and red arrows, respectively. The changes in frequency and dissipation as a function of the overtone at different concentrations of di-FA are summarized in Supporting information 3. The measurements showed only small changes in frequency as a function of overtone and the magnitude of dissipation changes was as low as 0.4E-6, justifying the use of the Sauerbrey’s equation (Eqn. 1) to estimate the adsorbed mass at each concentration.


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Figure 1: (a) Representative single-injection QCM measurements showing the change in frequency during the adsorption of the FA molecules on stainless steel surfaces. Before-rinse and after-rinse frequency changes are indicated. (b) Representative series-injection QCM measurement with the change in frequency across 6 overtones as a function of time at six different FA concentrations. The black arrows represent the injection events, during which the amine solution at the selected concentration is introduced into the QCM cell, and the red arrows indicate the rinsing steps with neat n-hexadecane. The corresponding dissipation changes are shown in the supporting information 3.



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Effect of adsorption history: Figures 2a and 2b display the adsorption isotherms of fatty amines obtained from series-injection measurements, in presence (before-rinse) and absence (after-rinse) of FA molecules in the solution, respectively. Isotherms reached a plateau beyond 0.3 wt. % in the case of di-FA, charged di-FA and branched tri-FA in n-hexadecane, while the adsorption of triFAs showed no saturation within the investigated concentration range. Figure 2c compares the adsorbed masses of the FAs measured in single-injection experiments at a concentration of 0.5 wt.%, i.e. on bare steel surfaces, and in series-injections experiments at the same concentration, i.e. on a steel surface after adsorption of FAs in preceding injections at lower concentrations (0.050.4 wt%). A summary of the adsorbed mass obtained in single- and series-injection measurements at a concentration of 0.5 wt.% is given in the supporting information 4. The before-rinse adsorbed mass of di–FA, charged di-FA and tri-FA was higher (~20%, ~37% and ~66%, respectively) in series-injection measurements than in the respective single-injection measurement. Only the adsorbed mass of branched tri - FA showed the opposite trend, i.e. a smaller adsorbed mass in series-injection than in single-injection measurements. Similar trends were also observed for the after-rinse adsorbed masses. The alkane chain of the fatty amines investigated here has similar lengths and unsaturation, and hence, the observed trend reversal stems likely from the end-group chemistry. While previous works25 employed series-injection methods to obtain equilibrium adsorbed masses, our results demonstrate that the adsorption history of the surface has a significant impact on the adsorbed masses on steel surfaces. In series-injection experiments, the concentration of the FA molecules available near the bare surface is low during the first injection, and it gradually increases with every consecutive injection at higher concentration. Loehle et al. modeled the adsorption of stearic acid molecules on iron oxide surface arriving randomly to the surface one after another simulating a low concentration. Molecular dynamic snapshots showed that the random adsorption process leads to a thicker film of horizontally-orientated molecules after short equilibration times, while a self-assembled monolayer forms after longer equilibrium times14. Hence, the smaller steric hindrance experienced by the FA molecules in the proximity of the surface in dilute solutions compared to higher concentrated solutions justifies the enhanced adsorption of di-, charged di- and tri- FAs in seriesinjection experiments. On the other hand, anchoring a branched (Y-structured) multi-functional end group to the surface, like the branched tri-FAs in this study, requires the availability of multiple attachment sites next to each other for adsorption to happen. The sticking probability is, hence, higher on bare surfaces in single-injection experiments compared to series-injection experiments, where adsorbed molecules are present already on the surface, which decreases the sticking probability. This explains the higher adsorbed masses of tri-FAs in single-injection measurements. According to these results, both the concentration of fatty amines close to the surface and the branching of the head group are shown to influence the adsorbed mass. The before-rinse adsorbed mass of fatty amines was in the range ~ 37 – 57 ng/cm2 (table 1), which is comparable or even higher than that reported for densely-packed oleic acid on steel at a concentration of 0.5 wt.% in n-hexadecane (~ 25-30 ng/cm2) 30,25. Previously, the adsorbed mass 8 ACS Paragon Plus Environment

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of hexadecyl amine on iron/iron oxide surfaces was measured using neutron reflectivity and led to 54.5 ng/cm2 and 131.8 ng/cm2 at the lowest and highest examined concentrations of the amines in oil, respectively23. The lower adsorbed masses observed in our QCM studies might result from the higher steric hindrance provided by the unsaturated tallow alkyl chains in comparison to the linear, saturated alkane chains used in that previous work 23. Further, both single- and series-injection measurements showed significantly smaller after-rinse adsorbed masses in comparison to beforerinse adsorbed masses (Figure 2c). Up to a ~ 80% of the adsorbed fatty amine molecules were removed from the surface during the rinsing step, which indicates the weak interaction of the FA films with the stainless steel surface.

Figure 2: Adsorbed mass of FA molecules as a function of concentration in n-hexadecane (a) before-rinse and (b) after-rinse during series-injection measurements. (c) Comparison of the adsorbed mass of FA molecules in single and series-injection measurements at a concentration of 9 ACS Paragon Plus Environment


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0.5 wt. %. (d) Schematic representation of three possible configurations of FA molecules on stainless steel surfaces, with 𝜃 is the tilt angle. Structure of the FA adlayers on steel surfaces: Table 1 summarizes the before-rinse adsorbed mass, film thickness and possible configurations of FA films on the steel surface. First, a uniformly dense FA adlayer was assumed to estimate the before-rinse film thicknesses. The thickness of the adlayer was estimated by dividing the adsorbed mass at 0.5 wt.% by the density of the molecule. Under this assumption, the largest average adlayer thickness was obtained for branched tri-FA (~ 0.71 nm), whereas the di-FAs exhibit the smallest film thickness (~ 0.47 nm), indicating that the higher number of end-functional groups may promote adsorption. The structure of the end functional group also has a significant effect on the film thickness, since branched tri FA adlayers are ~ 20 % thicker than linear tri-FA adlayers. Our results also show that the charge on the end group (e.g. charged di-FA) has no significant effect on the adsorbed mass on steel surfaces. If the molecules are assumed to adsorb in a perfectly upright configuration (see Figure 2d), with an approximate end-to-end distance of the surfactant molecule of ~ 2.5-2.7 nm (calculated assuming a C-C bond length of 0.154 nm and a chain length of 16-18 carbons), the estimated number of layers of all FAs is less than 1, which indicates a partial surface coverage of the surface. In contrast, the formation of ~1-2 layers is obtained under the assumption that the FA molecules lay horizontally on the steel surface (height of one FA molecule ~ 0.372 nm, which is estimated as ~ 2 x (length (H-C) + length (C-C)). If the FA molecules are assumed to interact with the iron/iron oxide surface via nitrogen lone pair electron to form a monolayer that provides full surface coverage, then the molecules would need to lay at an angle ranging between 10o and 16o with respect to the surface (assuming the stretched length of the FA molecule to be ~ 2.5 nm) (table 1, Figure 2d). These values are remarkably different to the reported values for hexadecylamine in hexadecane, which showed an average film thickness of 2 nm and a tilt angle of ~ 68o24. Nevertheless, it is known that longer equilibration times result in a more upright configuration of the adsorbed single alkyl end chain amines. In comparison, the short experimental duration of our QCM measurements (~ 30 minutes) might result in a non-equilibrated configuration of the adsorbed FA molecules, which either lay almost parallel to the surface or exhibit very small tilt angles. It is also worth mentioning reported measurements of the surface adsorption of sodium dodecyl sulfate (SDS), a surfactant with an anionic head attached to a C14 fatty tail) on stainless steel surfaces from aqueous solution31 Adsorption isotherms measured using QCM revealed three adsorption stages eventually leading to the formation of highly-dense surface micellar structures with SDS tails interacting with the steel surface. At weight percentages similar to those used in this study, i.e. 0.25 wt.%, the measured adsorbed mass was ~ 130 ng/cm2, which is at least 1.5 - 8 times higher than the adsorbed mass of the FAs selected in this work (Figure 2a). The absence of the different adsorption stages, the small dissipation values along with the small adsorbed mass of FAs let us exclude the formation of similar densely-packed micellar FA structures on stainless steel surfaces. 10 ACS Paragon Plus Environment

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Table 1. Before-rinse adsorbed mass in single-injection measurements, corresponding average film thicknesses and number of FA monolayers on the stainless steel (abbreviated as SS in the table) surface. The last column shows the rate constants obtained by fitting the experimental data to Eqn. 2. Adsorption kinetics: The change in adsorbed mass as a function of time can be fit by a twodimensional first-order adsorption kinetics equation32: 𝑘𝑎𝑑𝑠

log (𝑞𝑒 ― 𝑞𝑡) = log 𝑞𝑒 ― 2.303𝑡

Eqn. 2

where 𝑞𝑡 and 𝑞𝑒 are the mass adsorbed at a point of time t and in equilibrium, respectively, and 𝑘𝑎𝑑𝑠 the rate constant of the first-order adsorption kinetics (in s-1). Figure 3 shows log (𝑞𝑒 ― 𝑞𝑡) during the first 200-250 seconds of adsorption for the investigated FA molecules. The dashed lines (black) in the figure indicate the fit to the first-order adsorption kinetics given by Eqn. 2. Supporting information 5 summarizes the estimated rate constants as a function of concentration. The 𝑘𝑎𝑑𝑠 values were found to be independent of additive concentration in the range 0.01 – 0.5 wt.%, thereby confirming the validity of the first-order kinetics equation to describe the adsorption kinetics of the investigated FA molecules. Table 1 shows the rate constants obtained during singleinjection measurements by averaging the rate constants across the measured concentration range. This analysis yields the highest rate constants for di - FA (0.012 s-1) and the smallest values for branched tri - FA (0.001 s-1), that is, the larger end-group size of the tri-amines is associated with slower adsorption kinetics. These values are of the same order of magnitude to those measured for oleyl amine (0.004 s-1) by Lehner et al. 33. It is also interesting to observe that the end-group charge does not enhance the adsorption rate, perhaps due to the repulsive electrostatic force between surface-adsorbed amines and the amines in solution in the proximity of the surface. Although this is still a speculation, we note that such electrostatic repulsion is known to hinder adsorption of polyelectrolytes, and it is the reason why salts are added to screen electrostatic interactions and thereby, to enhance polyelectrolyte adsorption 34.



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Figure 3: Time-dependent adsorption of FA molecules on steel surfaces. The adsorbed mass is shown as log (𝑞𝑒 ― 𝑞𝑡), as in Eqn. 2. The experimental data during the first 250 seconds of adsorption were fit to a first-order adsorption kinetics. Flow rate: 75 µl/min, single injection, concentration: 0.5 wt. %, temperature: 55oC. 3.2 Nanotribological studies: The nanotribological studies were only performed in the presence of excess FA amines in hexadecane (before-rinse conditions in single-injection experiments). Figure 4a represents the average pull-off forces measured on bare steel surfaces and on di-FA films by AFM. The average pull-off force measured on bare steel surface was found to be ~ 85 % higher in comparison to di – FA films. The force maps were acquired across several locations on the sample and the insignificant error bars confirm the formation of a uniformly adsorbed film on the steel surface. Figure 4b represents the speed-dependence of the friction force measured on bare steel and di-FA films with a sharp AFM tip. The speed of the tip was varied by increasing the scan frequency of the piezo from 0.02 Hz to 20 Hz at a constant sliding distance of 1 µm resulting in sliding speeds ranging from 0.05 µm/s to 40 µm/s. Friction continuously decreased (~ 40%) with increasing speed over three decades on bare steel surfaces. In contrast, di-FA films showed velocity-weakening friction only at sliding velocities smaller than ~ 1 m/s, while friction remained constant with a further increase in the sliding speed. The friction forces measured on bare steel and di-FA surfaces showed no onset of hydrodynamic lubrication. This is a reasonable result considering the high Hertzian contact pressure (~ 4.5 GPa) at the single asperity contact. A ~ 50 % reduction in friction was observed for di – FA coated surfaces in comparison to bare steel at all sliding speeds, 12 ACS Paragon Plus Environment

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demonstrating that the FA adlayer is effective in impeding the direct contact between the steel surface and the AFM tip. In contrast to our results, reported macrotribological measurements showed a linear/logarithmic friction - speed dependence at contact pressures of ~ 0.5 GPa in carboxylic acid solutions 35,36. The observed speed-dependence of the frictional response of the FA films is discussed later in more detail.

Figure 4: (a) Average pull-off force. (b) Speed-dependence of friction on bare stainless steel surfaces (SS) and on di - FA films measured using a sharp AFM tip. Solvent: n-hexadecane, temperature: 55 ºC, concentration: 0.5 wt. %, load = 10 nN. A sliding speed of 0.5 µm/s was employed to measure friction as a function of load on bare steel and on FA films in the presence of excess FA molecules in solution (i.e. before-rinse conditions). Figure 5a shows a linear increase in friction with load for all FA films in presence of excess FA molecules in n-hexadecane solution. Amonton’s law was employed to obtain the coefficients of friction (COF) at loads smaller than 15 nN. The normalized COF (i.e. COF for FA films divided by the COF for bare steel) in the inset of Figure 5a reveals that the surface-adsorbed di–FAs are more lubricious than tri – FAs. A change in the slope (indicated by arrows) was observed around ~ 10-15 nN for all the FA films. Assuming a tip radius of ~ 15 nm and an elastic modulus of the adsorbed films of ~ 2 GPa 37,38, a pressure of ~ 0.54 – 0.63 GPa is estimated at the transition of the coefficient of friction. Similar transition pressures have been observed for chemisorbed alkanethiol monolayers on Au(111) (~ 0.8±0.1 Gpa) 38 and for carboxylic fatty acids on steel surfaces (~ 0.4 – 0.75 GPa) 8. These studies proposed that the transition in the COF results from the elastic deformation of the adlayer due to the pressure-induced change in molecular tilt and/or from the compression of the adlayer with the AFM tip. Doig et al. calculated the local density profiles of hexadecylamine adlayers in nhexadecane and showed the interdigitation between oil molecules and adsorbed alkyl amine molecules24. In our friction-force measurements, it is possible that both the squeeze-out of n13 ACS Paragon Plus Environment


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hexadecane molecules and the compression of the FA molecules are responsible for the observed transition of the coefficient of friction. Beyond the transition pressure, a reversible displacement/re-adsorption of FA molecules in the contact zone has been proposed to occur38, which may also happen here. Figure 5b represents the friction force measured upon first increasing (forward loading) and then decreasing load (i.e. reverse loading) at the same location. The similar friction values measured with the silicon AFM tip on bare stainless steel surfaces upon load reversal indicates the negligible wear of the AFM tip under these conditions. Furthermore, the frictional response was also recovered upon load reversal when excess FA molecules were present in the solution. The small error bars for each data point in Figure 5 (for an average of at least 10 friction loops) and the negligible change in friction upon load reversal support that the removed FA molecules are immediately replaced by FA molecules from the bulk solution, which allows maintaining a constant friction coefficient. This let us conclude that wear of the boundary film might happen above the transition pressure, but readsorption of the FA molecules “re-heals” the boundary film as long as FA molecules are present in the solution. Such transition point was not observed when the friction measurements were carried out in pure n-hexadecane (not shown) after FA adsorption. The linear increase in friction with load indicated that wear of the FA molecules already occurred at lower contact pressures.


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Figure 5: (a) Friction force as a function of normal load on bare steel and on FA films. Inset: Normalized COF of FA adlayers obtained before the transition load. (b) Friction force vs. normal load curves measured on bare steel, di – FA and branched tri – FA surfaces using a sharp AFM tip. Closed symbols represent friction force upon an increase in normal load (forward loading) and open symbols represent friction upon a decrease in normal load (reverse loading). Concentration: 0.5 wt. %, temperature: 55 ºC, sliding speed: 0.5 µm/s. Figure 6 compares the equilibrium adsorbed mass (right Y-axis) and the adsorption rate constants (left Y-axis) with the measured normalized coefficients of friction (Figure 5a, inset). It is intuitive to expect that a thicker boundary film should protect the steel surface better, which would result in lower friction. In contrast, our results show that surfaces with higher adsorbed masses lead to 15 ACS Paragon Plus Environment


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higher COFs (yellow circles). One possible explanation for this correlation relies on the packing density and structural arrangement of the FA molecules in the boundary films. Film packing density and the molecular order in the film depend on alkyl chain length, which dictates the strength of interchain van der Waals interactions and with hexadecane, on the anchoring structure, which determines the sticking probability, as well as on the conformation and interactions with the surface, and the degree of unsaturated bonds along the chain length 14. The adsorption studies conducted by Spikes et al. showed a sublinear increase in the adsorbed mass of fatty amines with concentration, indicating the weaker adsorption of amines on metal substrates in comparison to fatty acids 19. In spite of similar or even higher (before-rinse) adsorbed masses of fatty amines in our work compared to the fatty acids, a weaker interaction forces between fatty amines and iron surfaces can still be expected. It has been previously shown that densely-packed, strongly surface-adsorbed films lead to positive friction vs. speed response 36. Interestingly, Campen et al. showed that elaidic acid lost its positive friction-speed response when the adsorption temperature was reduced from 100 °C to 35 °C, which was attributed to the increase in the disorder of the adsorbed molecules at 35 °C 39. In the same study, the friction coefficient vs. sliding speed for amine-based surfactants exhibited a negative slope, consistent with our results at the nanoscale (Figure 4b). Considering the competition between the rates of adsorption and desorption of the surfactant molecules to/from the sliding surfaces, Drummond et al. demonstrated that a velocity-weakening frictional response indicates that interactions between the boundary film and the counter-surface (in aqueous solution in this previous work) is limited by the rate of re-adsorption to the counter surface 40. It is reasonable to expect that such rate of re-adsorption is hindered in films composed of disordered molecules that need to rearrange to interact with the counter-surface. Thus, the decrease in friction with velocity observed in our experiments supports that the boundary films are not well-ordered. In summary, the random adsorption of molecules from the solution along with the weak interaction forces between amines and the steel surface and the insufficient equilibration time may explain the formation of boundary films composed of disordered, mostly horizontally-oriented physisorbed FA molecules forming a multilayer or a highly-tilted monolayer with a tilt angle of 10-16 degrees. The almost horizontally-lying surfactant molecules expose their alkyl chains and anchoring groups (here, amines) to the solution, which enhances the number of intermolecular interactions with the AFM tip, thereby justifying that higher frictional forces on boundary films may originate from a higher number of adsorbed molecules. Therefore, the higher adsorbed mass of tri – amines, in comparison with di- amines may explain the higher COFs (Figure 6, right Y-axis). On the other hand, Figure 6 reveals that the FA molecules with greater adsorption rate constants provide lower COFs. This result is consistent with our hypothesis that wear of the adsorbed molecules can occur during friction-force measurements, and that, in presence of excess FA molecules in the solution, the worn surface can be replenished with new molecules from the solution. A faster re-adsorption rate, hence, can more efficiently maintain a low COF.


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70

2

0.01

60

kads

6 5 4

Branched tri - FA

50

3

tri - FA 2

2

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di - FA

Charged di - FA

Mass Adsorbed (ng/cm )

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30 6 5

0.0

0.2

0.4 0.6 Normalised COF

0.8

1.0

Figure 6: Normalized COF as a function of adsorption rate constant (kads) (left, black squares) and adsorbed mass (right, yellow circles) of FA adlayers. It is important to emphasize that, in an engine, the organic friction modifiers in the lubricant oil are always available in excess and at a fixed concentration. Therefore, before-rinse, singleinjection measurements are an appropriate model system to correlate adsorption with the tribological performance of the boundary films. As the FA molecules are worn out during tribological loading, one can expect that the molecules re-adsorbing on the sliding counter-surfaces will not have sufficient time to organize themselves into an upright configuration since the equilibrium time is limited by the frequency of the piston movement. According to our results, it is reasonable to expect that, in engines, the boundary films will be constituted of FA molecules either lying parallel to the surface or in a highly tilted configuration. Our study indicates that the adsorption kinetics of the weakly interacting FA amines with the stainless steel surface dominates the tribological performance of the investigated boundary films. Conclusions: Stable solutions of FAs in n-hexadecane were obtained at 55 oC. The adsorbed mass and kinetic constants of charged vs. uncharged, two vs. three and linear vs. branched end functional groups of FAs on stainless steel surfaces were determined via QCM experiments. The adsorption history of the surface, i.e. single injection vs. series-injections of FAs from low to high concentration influenced the total adsorbed mass. Single injection, before-rinse measurements, which includes physi- and chemisorbed masses, showed no significant impact of the charge of the end functional group on the adsorbed mass. However, linear FAs with two end functional groups (di-FAs) 17 ACS Paragon Plus Environment


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exhibited smaller adsorbed masses with higher adsorption rate constants than branched FA with three end functional groups (branched tri-FAs). The structural analysis of the FA films and the negative friction vs. speed dependence measured by AFM demonstrated that the films are disordered and weakly interacting with the stainless-steel surface. The horizontally lying molecules enhanced the interactions with the AFM tip and the increase in adsorbed mass of the tri-FAs films resulted in higher coefficients of friction. A continuous wear of the adsorbed FAs from the surface due to the high applied contact pressures was also concluded. Here, di –FAs with higher adsorption rate constants, in comparison to tri-FAs, were shown to more rapidly “re-heal” the boundary film, i.e., FA molecules in solution rapidly adsorbed to occupy the space of the worn-out molecules, thereby maintaining a low coefficient of friction. In this study, we have presented a combinatorial approach of QCM-D with AFM, which allows the development of a rapid but efficient screening method for formulating high-quality lubricants using an efficient selection of additives and by further avoiding the expenses incurred from timeconsuming dedicated engine tests. Supporting Information: Supporting information figures include QCM measurements to prove the absence of n-hexadecane adsorption on the steel surface, the formation of rigid (non-dissipative) films of FA on SS surface and the adsorption rate constants as a function of FA concentration in n-hexadecane. Supporting information also presents DLS measurements to demonstrate the solubility of FA in n-hexadecane. The supporting information is available free of charge on the ACS publication website. Acknowledgements: This project was supported by the TOTAL MS under TOTAL-UIUC collaboration (Research agreement U15-012 PC15-039). The authors gratefully acknowledge Benoît Thiébaut and Sophie Loehle at Total M&S, Solaize Research Center (CRES), France for providing FAs and for the useful discussions. References: (1) (2) (3) (4) (5) (6)

Ratoi, M.; Anghel, V.; Bovington, C.; Spikes, H. A. Mechanisms of Oiliness Additives. Tribol. Int. 2000, 33, 241–247. Spikes, H. Friction Modifier Additives. Tribol. Lett. 2015, 60, 5. Gellman, A.; Spencer, N. D. Surface Chemistry in Tribology; Carnegie Institute of Technology: Pittsburg, 2002. Spikes, H. A. Direct Observation of Boundary Layers. Langmuir 1996, 12, 4567–4573. Okabe, H.; Masuko, M.; Sakurai, K. Dynamic Behavior of Surface-Adsorbed Molecules Under Boundary Lubrication. E Trans. 1981, 24, 467–473. Jahanmir, S. Chain Length Effects in Boundary Lubrication. Wear 1985, 102, 331–349. 18 ACS Paragon Plus Environment

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(7) (8) (9)

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Table of Contents Graphic

AFM tip sliding along a stainless steel surface removes amine-based friction modifiers adsorbed on the surface while re-adsorption of the molecules concurrently re-heals the wear track and helps maintain low friction.


Adsorption Behavior and Nanotribology of Amine-Based Friction

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