lubricants

Article Competitive Adsorption of Ionic Liquids Versus Friction Modifier and Anti-Wear Additive at Solid/Lubricant Interface—Speciation with Vibrational Sum Frequency Generation Spectroscopy

1, 2 3 2 1, Dien Ngo †, Xin He , Huimin Luo , Jun Qu and Seong H. Kim * 1 Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA; [email protected] 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; [email protected] (X.H.); [email protected] (J.Q.) 3 Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; [email protected] * Correspondence: [email protected] or [email protected] Current address: Avery Dennison, 8080 Norton Parkway, Mentor, OH 44060, USA. †


Received: 12 October 2020; Accepted: 10 November 2020; Published: 12 November 2020


Abstract: A modern lubricant contains various additives with different functionalities and the interactions or reactions between these additives could induce synergistic or antagonistic effects in tribological performance. In this study, sum frequency generation (SFG) spectroscopy was used to investigate competitive adsorption of lubricant additives at a solid/base oil interface. A silica substrate was used as a model solid surface. The lubricant additives studied here included two oil-soluble ionic liquids (ILs, [N888H][DEHP] and [P8888][DEHP]), an antiwear additive (secondary ZDDP), an organic friction modifier (OFM), and a dispersant (PIBSI). Our results showed that for mixtures of ZDDP and IL in a base oil (PAO4), the silica surface is dominated by the IL molecules. In the cases of base oils containing OFM and IL, the silica/lubricant interface is dominated by OFM over [N888H][DEHP], while it is preferentially occupied by [P8888][DEHP] over OFM. The presence of PIBSI in the mixture of PAO4 and IL leads to the formation of a mixed surface layer at the silica surface with PIBSI as a major component. The SFG results in this investigation provide fundamental insights that are helpful to design the formulation of new lubricant additives of desired properties.

Keywords: ionic liquids; lubricants; sum frequency generation spectroscopy; base oil; additives

1. Introduction Lubrication plays an important role in reliable performance and durability of many engine systems and machineries [1–3]. In practice, commercial lubricants consist of various components with different functionalities such as a base oil, friction modifier, antiwear additive, antioxidant, antifoam, and detergent [4]. The presence of multiple additives in the lubricant can improve the lubrication efficiency (lower friction and wear prevention), but it can also induce unwanted antagonistic effects as reported in some tribological studies [5–10]. When a secondary zinc dialkyldithiophosphate (sec-ZDDP) was added to a lubricant mixture consisting of polyalphaolefin (PAO4) base oil, molybdenum dithiocarbamate (MoDTC), and polyisobutylene succinimide (PIBSI), for example, the friction coefficient and wear of a cast iron were reported to be higher than those in the case without adding sec-ZDDP [10]. Such reduced performances could be due to the reaction or interaction between sec-ZDDP and PIBSI in the mixed liquid. It is, therefore, important to understand chemical interactions between potential

Lubricants 2020, 8, 98; doi:10.3390/lubricants8110098 www.mdpi.com/journal/lubricants Lubricants 2020, 8, 98 2 of 18 additives to predict or design lubricant formulations for specific applications. The adsorption of a compound (base oil or additive) to a solid surface can be affected by other components in lubricants [11] and compositional or structural information of molecular layers at the interface between a solid and a mixed liquid (lubricant) is needed to understand the tribological performance of a lubricant. Oil-soluble ionic liquids (ILs) have recently been developed as a new class of friction-reducing and wear-protecting lubricant additives [12] and compatibilities of some ILs with other lubricant additives have been reported lately [6,9,10]. Specifically, phosphonium-phosphate ILs demonstrated synergistic effects with ZDDP [6] but did not work well when mixed with some other additives such as organic friction modifier (OFM) [9]. In contrast, an ammonium-phosphate IL worked well with OFM [9] to deliver much reduced friction and wear. In our previous study, the competitive adsorption of a base lubricant oil (PAO4) and two oil-soluble ILs, tetraoctylphosphonium bis(2-ethylhexyl)phosphate ([P8888][DEHP]) and trioctylammonium bis(2-ethylhexyl)phosphate ([N888H][DEHP]), at a solid surface (silica) was investigated using vibrational sum frequency generation (SFG) spectroscopy [11]. Being a surface spectroscopic technique [13–15], SFG spectroscopy has provided valuable insights into the adsorption behavior of those compounds at the solid surface. Results in the previous study showed that ILs form well-ordered layers at the silica/IL interface and they replace PAO4 at the silica surface when added to the base oil. In practice, ILs would be combined with other additives in lubricants and it is necessary to understand the structure of an adsorbed layer at the solid/lubricant interface when several additives are used together. SFG spectroscopy can differentiate characteristic vibrational bands of specific molecules at the solid/liquid interface [11,14,16–18], thus it can determine the chemical identity or structure of interfacial molecules present at the solid/liquid interface. In the current study, SFG spectroscopy was further used to investigate surface behavior of real lubricants at the solid surface. This is the first time that adsorption behavior of real lubricants was studied by a surface specific technique. The lubricants consisted of PAO base oil and various additives including ILs, OFM, PIBSI, and ZDDP. The study used silica windows as a model solid substrate because they are optically transparent in the frequency region measured in SFG experiments, enabling access to the buried interface from the solid phase. Our results showed that most additives preferentially adsorb to the silica surface from the liquid mixtures containing one additive to the base oil. For mixtures containing IL and sec-ZDDP, IL was found to dominantly adsorb at the solid/liquid interface. The lubricants containing IL and OFM showed the preferential adsorption of [P8888][DEHP] to the silica surface from (PAO4 + OFM + [P8888][DEHP]) mixture, but the dominance of OFM at the surface in the case of (PAO4 + OFM + [N888H][DEHP]) mixture. When PIBSI was used along with IL in PAO4, the interfacial layer appeared to contain more PIBSI than IL. The SFG results in this study, when combined with results from tribological experiments, would provide valuable information on the tribological performance of various lubricants.

2. Materials and Methods

2.1. Materials

The two oil-soluble ionic liquids (IL), [P8888][DEHP] and [N888H][DEHP], were synthesized following the protocols published previously [19,20]. The PAO4 base oil, OFM, and PIBSI were provided by ExxonMobil (Paulsboro, NJ, USA). Sec-ZDDP was obtained from Lubrizol (Wickliffe, OH, USA) and it contained ~5–10 wt.% mineral oil for flowability [10]. The chemical structures of these compounds are displayed in Figure1. The compositions of mixed liquid samples used in the study are given in Table1. Note that the total amount of antiwear molecules (ILs and/or ZDDP) was the same in any given fluid. The OFM was used at 0.8 wt.%, which is a typical concentration used in engine oils. The concentration of PIBSI was 2 wt.%, similar to that used to provide sufficient suspension of MoDTC in a base oil. The solid substrates used in the study were IR-grade fused silica windows (diameter = 25.4 mm; Lubricants 2020, 8, 98 3 of 18 thickness = 3.0 mm) and they were obtained from Esco Optics, Inc. (Jefferson, NJ, USA). The windows, SFG liquid cell, and glassware were cleaned following the procedure described in Ref. [11]. Lubricants 2020, 8, x FOR PEER REVIEW 3 of 18

FigureFigure 1. Chemical 1. Chemical structures structures of PAO4,of PAO4, OFM, OFM, ZDDP, ZDDP, PIBSI, PIBSI, [N888H][DEHP], [N888H][DEHP], and and [P8888][DEHP] [P8888][DEHP] used in theused study. in the The study. chemical The chemical structure structure of OFM of OFM was was not exactlynot exactly known known and and the the one one shownshown here was suggestedwas suggested by NMR by and NMR Infrared and Infrared spectroscopy spectroscopy [9]. [9] The. The structure structure of of PAO4 PAO4 is is a a genericgeneric one one [11,21] [11,21.].

TableTable 1. Chemical 1. Chemical compositions compositions of of mixed mixed liquidliquid samples used used in in the the study. study. The The weight weight percent percent ratios ratios were chosen so that ZDDP and ionic liquids (ILs) have the same molecular concentration in the base were chosen so that ZDDP and ionic liquids (ILs) have the same molecular concentration in the base oil [10]. The 0.8 wt.% concentration of the organic friction modifier (OFM) is the common value used oil [10]. The 0.8 wt.% concentration of the organic friction modifier (OFM) is the common value used in in engine oils. engine oils. Sample Weight Percent Ratio PAO4 Sample+ [N888H][DEHP] Weight99.13:0.87 Percent Ratio PAO4PAO4+ [N888H][DEHP] + [P8888][DEHP] 98.96:1.04 99.13:0.87 PAO4 +PAO4[P8888][DEHP] + OFM 99.20:0.80 98.96:1.04 PAO4PAO4+ OFM+ PIBSI 98.00:2.00 99.20:0.80 PAO4 + secPIBSI-ZDDP 99.20:0.80 98.00:2.00 PAO4PAO4 + OFM+ sec-ZDDP + [N888H][DEHP] 98.33:0.80:0.87 99.20:0.80 PAO4PAO4+ OFM + OFM+ [N888H][DEHP] + [P8888][DEHP] 98.16:0.80:1.04 98.33:0.80:0.87 PAO4PAO4+ +OFM PIBSI+ +[P8888][DEHP] [N888H][DEHP] 97.13:2.00:0.87 98.16:0.80:1.04 PAO4 + PIBSI + [N888H][DEHP] 97.13:2.00:0.87 PAO4 + PIBSI + [P8888][DEHP] 96.96:2.00:1.04 PAO4 + PIBSI + [P8888][DEHP] 96.96:2.00:1.04 PAO4 + sec-ZDDP + [N888H][DEHP] 99.16:0.40:0.44 PAO4 + sec-ZDDP + [N888H][DEHP] 99.16:0.40:0.44 PAO4PAO4+ sec-ZDDP + sec-ZDDP+ [P8888][DEHP]+ [P8888][DEHP] 99.08:0.40:0.52 99.08:0.40:0.52

Lubricants 2020, 8, 98 4 of 18

2.2. Attenuated Total Reflectance Infrared (ATR-IR) Spectroscopy

LubricantsA Bruker 2020, Vertex8, x FOR PEER 80 FT-IRREVIEW spectrometer equipped with a single reflection ATR4 accessory of 18 (Harrick MVP-Pro, Harrick Scientific Products Inc., Pleasantville, NY, USA) and a diamond crystal was used2.2. Attenuated to acquire Total the Reflectance infrared Infrared spectra (ATR of base-IR) S oilpectroscopy and additives investigated in the current study. The spectrometerA Bruker Vertex was used 80 FT with-IR spectrometer an MCT detector, equipped and with each a single spectrum reflection was ATR an accessory average (Harrick of 100scans. 1 1 The spectralMVP-Pro range, Harrick was Scientific 400–4000 Products cm− andInc., aPleasantville, spectral resolution NY, USA of) and 4 cm a diamond− was applied. crystal was The used incident angleto of acquire unpolarized the infrared IR beam spectra at theof base diamond oil and crystal additives/liquid investigated interface in was the 45 current◦, and study. a spectrum The of crystalspectrometer/air interface was was used used with as an the MCT reference. detector, and each spectrum was an average of 100 scans. The spectral range was 400–4000 cm−1 and a spectral resolution of 4 cm−1 was applied. The incident angle of 2.3. Sumunpolarized Frequency IR beam Generation at the (SFG)diamond Spectroscopy crystal/liquid interface was 45°, and a spectrum of crystal/air interface was used as the reference. A scanning pico-second sum frequency generation spectrometer (EKSPLA, Vilnius, Lithuania) was used2.3. Sum for F SFGrequency measurements Generation (SFG) of the Spectroscopy silica/liquid interface. The experimental geometry and liquid cell wereA described scanning pico in details-second in sum Ref. frequency [11]. The generation liquid sample spectrometer was added (EKSPLA, to the Vilnius, sample Lithuania cell and) the silicawas window used for was SFG then measurements placed on topof the of silica/liquid the liquid. interface. The SFG The spectrometer experimental has geometry a tunable and infrared liquid (IR) lasercell from were an described optical parametricin details in Ref. generator [11]. The and liquid amplifier sample w andas added a visible to the (532 sample nm, cell Vis) and laser the silica from the pico-secondwindow laserwas then (~27 placed ps, 10 on Hz). top of The the SFG liquid. beam The (SFGωSFG spectrometer= ωVis + ω IRhas) wasa tunable generated infrared by (IR) overlapping laser the visiblefrom an and optical IR laser parametric beams generator spatially and amplifier temporally and at a thevisible silica (532/liquid nm, interface.Vis) laser from Both theSSP picoand- PPP polarizationsecond laser combinations (~27 ps, 10 Hz). were The used SFG inbeam SFG (ω experiments.SFG = ωVis + ωIR) was An generatedSSP polarization by overlapping combination the visible means, for example,and IR laserS-polarized beams spatially SFG, andS-polarized temporally 532at the nm, silica/liquidP-polarized interface. IR with Both SSP respect and PPP to the polarization laser incident combinations were used in SFG experiments. An SSP polarization combination means, for example, S- plane (Figure2). More details of the SFG setup, spectral collection, and data processing can be found polarized SFG, S-polarized 532 nm, P-polarized IR with respect to the laser incident plane (Figure 2). in Ref. [11]. More details of the SFG setup, spectral collection, and data processing can be found in Ref. [11].

FigureFigure 2. (a 2.) Illustration(a) Illustration of of liquid liquid sample sample cellcell and experimental experimental geometry geometry used used in SFG in SFGexperiment experimentss of of the silica/liquid interface. The lab coordinate is the XYZ Cartesian coordinates and the incident plane the silica/liquid interface. The lab coordinate is the XYZ Cartesian coordinates and the incident plane of of the laser beams is (XZ) plane. The symbol S indicates that the polarized light has its electric field the laser beams is (XZ) plane. The symbol S indicates that the polarized light has its electric field vector vector aligned along the Y direction (perpendicular to the (XZ) plane). When the electric field vector alignedof the along polarized the Y light direction is in the (perpendicular incident plane, it to is the denoted (XZ) as plane). P, and When it has vector the electric components field vectoraligned of the polarizedalong lightX and is Z in directions. the incident Figure plane, (a) itis isadapted denoted with as Ppermission, and it has from vector Ref. components [11], Copyright aligned © 2020 along X and ZAmerican directions. Chemical Figure Society. (a) is ( adaptedb) Illustration with of permission symmetric and from asymmetric Ref. [11], Copyrightstretch modes© of2020 a methyl American Chemicalgroup Society. at the silica (b) surface. Illustration of symmetric and asymmetric stretch modes of a methyl group at the silica surface. The use of different polarization combinations helps understand the structure of the adsorbed layerThe useat the of silica different surface. polarization At the liquid/amorphous combinations solid helps interface, understand the X and the Y structure directionsof (parallel the adsorbed to layerthe at the surfa silicace) are surface. identical; At the in this liquid case,/amorphous the SSP polarization solid interface, combination the X and preferentially Y directions detects (parallel a to the surface)transition are dipole identical; moment in along this the case, Z direction the SSP (normalpolarization to the combinationsurface, Figure preferentially 2a), while the detectsPPP a transitioncombination dipole measures moment both along surface the- Zparallel direction and perpendicular (normal to the compone surface,nts Figureof the transition2a), while dipole the PPP moment (Figure 2a) [13]. Using different polarization combinations can, therefore, give information combination measures both surface-parallel and perpendicular components of the transition dipole on the orientation of functional groups as well as the packing of the surface layer [13]. The methyl moment (Figure2a) [ 13]. Using different polarization combinations can, therefore, give information group in Figure 2b, for example, has its symmetric axis very close to the surface normal. In this case, on the orientation of functional groups as well as the packing of the surface layer [13]. The methyl

Lubricants 2020, 8, 98 5 of 18 group in Figure2b, for example, has its symmetric axis very close to the surface normal. In this case, the relative SFG intensity of the symmetric methyl stretch mode is strong in SSP but weak in PPP measurements because its transition dipole moment is very close to the surface normal (Z direction). On the contrary, the transition dipole moment of the asymmetric methyl stretch mode (Figure2b) is nearly parallel to the surface and the relative SFG intensity of this vibrational mode is strong in PPP but very weak in SSP measurements.

3. Results and Discussion

3.1. ATR-IR Spectra The chemical structures of PAO4, sec-ZDDP, OFM, and PIBSI are not precisely known in this study, and infrared spectroscopy was used to obtain the spectra of main IR active vibrational modes in these compounds. The results from IR spectroscopy are useful for interpreting SFG spectra of base oil, additives as well as their mixtures at silica/liquid interface. The ATR-IR spectra of individual compoundsLubricants 20 used20, 8, inx FOR this PEER study REVIEW are given in Figure3. 5 of 18 Figure3 shows that PAO4, OFM, and ILs have very strong peaks of symmetric methylene stretch the relative SFG1 intensity+ of the symmetric methyl stretch mode is strong in SSP but1 weak in PPP mode (~2852 cm− , CH2,s, d ) and asymmetric methylene stretch mode (~2920 cm− , CH2,as, d−)[22–28]. measurements because its transition dipole moment is very close to the surface normal (Z direction). The symmetric methyl stretch mode (CH , r+) and the asymmetric methyl stretch mode (CH , r ) are On the contrary, the transition dipole moment3,s of the asymmetric methyl stretch mode (Figure 3,as2b) is− at ~2872 cm 1 (weak) and ~2956 cm 1 (medium), respectively [25–28]. The band centered at ~2920 cm 1 nearly parallel− to the surface and −the relative SFG intensity of this vibrational mode is strong in PPP − 1 + consistsbut very of symmetric weak in SSP methylene measurements. Fermi resonance (~2907 cm− , CH2,s,FR, d FR) and symmetric methyl 1 + Fermi resonance (~2934 cm− , CH3,s,FR, r FR)[24]. Based on the works of Wang et al., the peak at 1 ~29203. cmResults− could and alsoDiscussion be attributed to the Fermi resonance between the symmetric methylene stretch and the CH2 bending overtone [29,30]. Although the methyl group has a larger IR cross-section than the methylene3.1. ATR-IR group,Spectra its intensity is weaker due to a lower density in the molecule. For example, [N888H][DEHP]The chemical has 31structures CH2 groups of PAO4, and sec 7 CH-ZDDP,3 groups OFM, in and each PIBSI molecule. are not Theprecisely IR spectra known of in PIBSI this and sec-ZDDPstudy, areand quite infrared similar spectroscopy to each otherwas used and to are obtain very the diff spectraerent from of main the IR spectra active ofvibrational other compounds modes in in these compounds. The results from IR spectroscopy1 are useful for interpreting SFG spectra of base this study. They both have a shoulder at ~2975 cm− that could be attributed to out-of-plane asymmetric oil, additives as well as their mixtures at silica/liquid interface. The ATR-IR spectra of individual methyl stretch [31], and the intensities of CH2 and CH3 stretches are comparable. Table2 lists the compounds used in this study are given in Figure 3. typical assignments of vibrational modes of CH2 and CH3 in organic compounds.

Figure 3. ATR-IR spectra of compounds used in the study. The spectra of PAO4, [N888H][DEHP], and [P8888][DEHP]Figure 3. ATR-IR are spectra adapted of compounds from Ref. used [11]. in the study. The spectra of PAO4, [N888H][DEHP], and [P8888][DEHP] are adapted from Ref. [11].

Figure 3 shows that PAO4, OFM, and ILs have very strong peaks of symmetric methylene stretch mode (~2852 cm−1, CH2,s, d+) and asymmetric methylene stretch mode (~2920 cm−1, CH2,as, d−) [22–28]. The symmetric methyl stretch mode (CH3,s, r+) and the asymmetric methyl stretch mode (CH3,as, r−) are at ~2872 cm−1 (weak) and ~2956 cm−1 (medium), respectively [25–28]. The band centered at ~2920 cm−1 consists of symmetric methylene Fermi resonance (~2907 cm−1, CH2,s,FR, d+FR) and symmetric methyl Fermi resonance (~2934 cm−1, CH3,s,FR, r+FR)[24]. Based on the works of Wang et al., the peak at ~2920 cm−1 could also be attributed to the Fermi resonance between the symmetric methylene stretch and the CH2 bending overtone [29,30]. Although the methyl group has a larger IR cross-section than the methylene group, its intensity is weaker due to a lower density in the molecule. For example, [N888H][DEHP] has 31 CH2 groups and 7 CH3 groups in each molecule. The IR spectra of PIBSI and sec-ZDDP are quite similar to each other and are very different from the spectra of other compounds in this study. They both have a shoulder at ~2975 cm−1 that could be attributed to out-of-plane

Lubricants 2020, 8, 98 6 of 18

Table 2. Typical assignments of CH2 and CH3 groups in organic compounds. Due to the positive + charge on the phosphorus atom of [ P8888], P–CH2–C is listed in the same group with N–CH2–C and O–CH2–C groups.

Frequency Functional Group 1 Vibrational Mode (cm− ) + 2845–2858 CH2,s (d ), symmetric methylene stretch 2896–2930 CH (d ), asymmetric methylene stretch C–CH –C [22,25,27,28,31–33] 2,as − 2 CH (d+ ), symmetric methylene 2890–2927 2,s,FR FR Fermi resonance + 2869–2925 CH3,s (r ), symmetric methyl stretch 2948–2973 CH (r ), asymmetric methyl stretch C–CH [22,25–28,31–34] 3,as − 3 CH (r+ ), symmetric methyl 2884–2942 3,s,FR FR Fermi resonance + 2870–2875 CH2,s (d ), symmetric methylene stretch 2900 CH (d ), asymmetric methylene stretch O–CH –C, N–CH –C, P–CH –C [29,35–37] 2,as − 2 2 2 CH (d+ ), symmetric methylene 2938–2954 2,s,FR FR Fermi resonance

3.2. SFG Spectra of Individual Compounds at Silica/Liquid Interface The SFG spectra of individual compounds at the silica/liquid interface are given in Figure4 for both SSP and PPP polarization combinations. The low signal to noise (S/N) ratios of PPP spectra compared to those of SSP spectra are due to the low reflectivity of P-polarized light under the experimental conditions used in the current study. The SFG spectra of PAO4, [N888H][DEHP], and [P8888][DEHP] are adapted from Ref. [11] and are given in Figure4 for comparison as well as to support the spectral interpretation of the adsorption data of mixed compounds at the silica/liquid interface presented in other sections. Note that due to the unknown structures of some proprietary additives (esp. sec-ZDDP), comprehensive assignments of spectral features were quite limited for those compounds and their mixtures. The SSP and PPP spectra of silica/PIBSI could not be obtained with a reasonably good S/N ratio because their signals were so weak; this could be due to the extremely high viscosity of PIBSI, so the adsorption-driven interfacial packing was difficult. Without non-centrosymmetric order of molecules, SFG signal is extremely weak. The silica surface is different from oxide surfaces of engineering metal substrates and thus structures of adsorbed layers at these surfaces could be somewhat different. However, the SFG results in this study can still be used as reference for understanding surface behavior of lubricant base oil and additives at the metal oxide/liquid interface [11,38–41]. 1 The SSP spectrum of PAO4 at the silica surface shows a medium intensity peak at ~2875 cm− 1 (CH3,s) and a strong peak at ~2940 cm− (CH3,s,FR), while the PPP spectrum reveals a dominance of 1 SFG signal from the asymmetric methyl stretch mode at ~2960 cm− which is not observed in the SSP spectrum. This means that the symmetry axis of methyl groups of the PAO4 side chains is relatively 1 perpendicular to the surface (Figure2b). The signal of symmetric methylene stretch mode (~2850 cm − ) is strong in SSP measurement but weak in PPP measurement, indicating that the alkyl chains of PAO4 align along the surface normal with gauche defects (Figure5a) [ 11,13]. The segregation of the chain end to the solid surface with no specific chemical interactions provides the minimization loss of conformational entropy at the interface [42,43]. Both SSP and PPP spectra of [N888H][DEHP] and 1 1 1 [P8888][DEHP] show three peaks at ~2850 cm− , 2870 cm− , and 2940 cm− , and the previous study has shown that the charged head groups of these IL molecules are preferentially interacting with the silica surface while their alkyl chains facing the bulk liquid (Figure5b) [11,43]. Lubricants 2020, 8, x FOR PEER REVIEW 7 of 18 Lubricants 2020, 8, 98 7 of 18

Figure 4. Sum frequency generation (SFG) spectra of single compounds at the silica/liquid interface. Figure The4. Sum solid frequency lines are fitted generation curves and (SFG the) fittingspectra results of single are given compounds in the SI. Theat the line silica/liquid spectra of PAO4, interface. [N888H][DEHP], and [P8888][DEHP] are adapted from Ref. [11] and are shown here for comparison. The Lubricantssolid lines 2020 ,are 8, x FORfitted PEER curves REVIEW and the fitting results are given in the SI. The line spectra of8 PAO4,of 18 [N888H][DEHP], and [P8888][DEHP] are adapted from Ref. [11] and are shown here for comparison.

The SSP spectrum of PAO4 at the silica surface shows a medium intensity peak at ~2875 cm−1 (CH3,s) and a strong peak at ~2940 cm−1 (CH3,s,FR), while the PPP spectrum reveals a dominance of SFG signal from the asymmetric methyl stretch mode at ~2960 cm−1 which is not observed in the SSP spectrum. This means that the symmetry axis of methyl groups of the PAO4 side chains is relatively perpendicular to the surface (Figure 2b). The signal of symmetric methylene stretch mode (~2850 cm−1) is strong in SSP measurement but weak in PPP measurement, indicating that the alkyl chains of PAO4 align along the surface normal with gauche defects (Figure 5a) [11,13]. The segregation of the chain end to the solid surface with no specific chemical interactions provides the minimization loss of conformational entropy at the interface [42,43]. Both SSP and PPP spectra of [N888H][DEHP] and [P8888][DEHP] show three peaks at ~2850 cm−1, 2870 cm−1, and 2940 cm−1, and the previous study has shown that the charged head groups of these IL molecules are preferentially interacting with the silica surfaceFigureFigure while 5. 5.Molecular Moleculartheir alkyl adsorptionadsorption chains structurestructure facing ofthe ( a) silica/PAO4,bulk silica/ PAO4,liquid ( (bb ))(Figure silica/IL, silica/IL, and5b) and ( c ([11,43])c )silica/OFM silica/.OFM interfaces. interfaces. TheThe schematics schematics shown shown in in (a (a,b,b)) are are reproduced reproduced with with permission permission from Ref. Ref. [11 [11]],, Copyright Copyright ©© 20202020 AmericanAmerican Chemical Chemical Society. Society.

1 TheTheSSP SSPspectrum spectrum of of OFM OFM has has a a very very weak weak signalsignal atat ~2850~2850 cm−1 (this(this peak peak was was not not captured captured well well 1 inin the the spectral spectral fitting) fitting) whilewhile itsits relative intensity intensity at at ~2950 ~2950 cm cm−1 −(CH(CH2,s,FR2,s,FR or CHor2,as CH of2,as C–CHof C–CH2–O) is2 –O)very is 1 verystro strong.ng. The The peak peak at ~2950 at ~2950 cm cm−1 is− dominantis dominant in the in thePPPPPP spectrumspectrum (see (see Table Table S2 S2in inthe the SI). SI). The The very very 1 weakweak intensity intensity of of the the methylene methylene group group atat 28502850 cmcm−−1 inin both both SSPSSP andand PPPPPP spectraspectra reveals reveals that that OFM OFM molecules form an adsorbed layer at the silica surface with only few gauche defects. The absence of the symmetric methyl stretch peak at ~2875 cm−1 suggests that SFG signals of two opposite CH3 groups cancel out each other, indicating the formation of a multilayer structure of OFM at the interface [44]. The peak at ~2950 cm−1 in the PPP spectrum could then be attributed to a vibrational mode of CH2 in O–CH2–C(OH) of the head groups. The presence of a peak at ~2965 cm−1 (CH3,as) with a relatively small amplitude (see Table S2) indicates some imperfection in dipole cancelation of CH3 groups. The polar head groups of OFM must be attached to the silica surface containing polar functional groups such as Si–OH and Si–O–Si [4,45]. The hydrogen bonds between adsorbed head groups and van der Waals forces between long alkyl chains help the formation of a well ordered layer at silica/liquid interface [4]. This also explains the strong signals of methylene groups (O–CH2– C(OH)) in both SSP and PPP spectra of OFM at the silica surface. The adsorption of OFM molecules at the silica/liquid interface is schematically illustrated in Figure 5c. The thickness and order of an adsorbed layer of OFM at solid surfaces have been shown to be dependent on the molecular structure and its concentration in the lubricant [41,46]. The SSP and PPP spectra of sec-ZDDP in Figure 4 shows dominant peaks at ~2940 cm−1 and weak peaks at ~2870 cm−1 (see Tables S1 and S2 in the SI), while the SFG signal at ~2850 cm−1 is not observed. The absence of SFG signal at ~2850 cm−1 means that the –C–CH2–C– groups in the alkyl chains of sec- ZDDP at the silica surface lie in a centrosymmetric medium, so that they are SFG inactive [13]. The weak peak at ~2870 cm−1 can be attributed to symmetric methylene stretch mode in O–CH2–C and/or symmetric methyl stretch mode (CH3,s), and the dominant peak at ~2940 cm−1 can be assigned to asymmetric methylene stretch mode in O–CH2–C and/or symmetric methyl Fermi resonance (CH3,s,FR, see Table 2). Note that sec-ZDDP contains 5–10 wt.% of a mineral oil with an unknown structure and its competitive adsorption at the silica surface cannot be ruled out. Nonetheless, it can be said, based on the good SFG signal intensity, that the mixture of sec-ZDDP and mineral oil forms a well-ordered layer at the silica surface. The dominance of the SFG peaks attributable to O–CH2–C groups suggests that polar phosphate groups are likely to face toward the silica surface.

Lubricants 2020, 8, 98 8 of 18 molecules form an adsorbed layer at the silica surface with only few gauche defects. The absence of the 1 symmetric methyl stretch peak at ~2875 cm− suggests that SFG signals of two opposite CH3 groups cancel out each other, indicating the formation of a multilayer structure of OFM at the interface [44]. 1 The peak at ~2950 cm− in the PPP spectrum could then be attributed to a vibrational mode of CH2 in 1 O–CH2–C(OH) of the head groups. The presence of a peak at ~2965 cm− (CH3,as) with a relatively small amplitude (see Table S2) indicates some imperfection in dipole cancelation of CH3 groups. The polar head groups of OFM must be attached to the silica surface containing polar functional groups such as Si–OH and Si–O–Si [4,45]. The hydrogen bonds between adsorbed head groups and van der Waals forces between long alkyl chains help the formation of a well ordered layer at silica/liquid interface [4]. This also explains the strong signals of methylene groups (O–CH2–C(OH)) in both SSP and PPP spectra of OFM at the silica surface. The adsorption of OFM molecules at the silica/liquid interface is schematically illustrated in Figure5c. The thickness and order of an adsorbed layer of OFM at solid surfaces have been shown to be dependent on the molecular structure and its concentration in the lubricant [41,46]. 1 The SSP and PPP spectra of sec-ZDDP in Figure4 shows dominant peaks at ~2940 cm − and 1 1 weak peaks at ~2870 cm− (see Tables S1 and S2 in the SI), while the SFG signal at ~2850 cm− is 1 not observed. The absence of SFG signal at ~2850 cm− means that the –C–CH2–C– groups in the alkyl chains of sec-ZDDP at the silica surface lie in a centrosymmetric medium, so that they are SFG 1 inactive [13]. The weak peak at ~2870 cm− can be attributed to symmetric methylene stretch mode 1 in O–CH2–C and/or symmetric methyl stretch mode (CH3,s), and the dominant peak at ~2940 cm− can be assigned to asymmetric methylene stretch mode in O–CH2–C and/or symmetric methyl Fermi resonance (CH3,s,FR, see Table2). Note that sec-ZDDP contains 5–10 wt.% of a mineral oil with an unknown structure and its competitive adsorption at the silica surface cannot be ruled out. Nonetheless, it can be said, based on the good SFG signal intensity, that the mixture of sec-ZDDP and mineral oil forms a well-ordered layer at the silica surface. The dominance of the SFG peaks attributable to O–CH2–C groups suggests that polar phosphate groups are likely to face toward the silica surface.

3.3. Speciation of Silica/Lubricant Interface for PAO4 Containing Single Additive The SFG spectra of the silica/(PAO4 + one additive) interface were measured and displayed in Figure6. The spectra of PAO4 and its mixtures with two ionic liquids at the silica surface are adapted from Ref. [11] and are given in Figure6 for comparison. Results from the previous study showed that [N888H][DEHP] and [P8888][DEHP] were dominant at the interface between silica and (PAO4 + IL) mixtures. The preferential adsorption of ILs at the solid/lubricant interface explains the beneficial effect of the addition of a small amount of ILs in lubrication performance [19,47–49]. As in the case of silica/IL interface, the polar head groups of IL are in contact with the polar solid surface containing Si–OH and Si–O–Si groups while their alkyl chains face to the bulk liquid. The structure of the adsorbed surface layer at silica/(PAO4 + IL) interface, however, is slightly different from that of silica/IL interface [11]. Lubricants 2020, 8, x FOR PEER REVIEW 9 of 18

3.3. Speciation of Silica/Lubricant Interface for PAO4 Containing Single Additive The SFG spectra of the silica/(PAO4 + one additive) interface were measured and displayed in Figure 6. The spectra of PAO4 and its mixtures with two ionic liquids at the silica surface are adapted from Ref. [11] and are given in Figure 6 for comparison. Results from the previous study showed that [N888H][DEHP] and [P8888][DEHP] were dominant at the interface between silica and (PAO4 + IL) mixtures. The preferential adsorption of ILs at the solid/lubricant interface explains the beneficial effect of the addition of a small amount of ILs in lubrication performance [19,47–49]. As in the case of silica/IL interface, the polar head groups of IL are in contact with the polar solid surface containing Si–OH and Si–O–Si groups while their alkyl chains face to the bulk liquid. The structure of the adsorbed surface layer at silica/(PAO4 + IL) interface, however, is slightly different from that of silica/ILLubricants interface2020, 8, 98 [11]. 9 of 18

Figure 6. SFG spectra of two compound mixtures at silica/liquid interface. The solid lines are fitted Figurecurves 6. andSFG thespectra fitting of resultstwo compound are given inmixtures the SI. Theat silica/liquid line spectra interface. of PAO4, The (PAO4 solid+ [N888H][DEHP]),lines are fitted curvesand and (PAO4 the+ fitting[P8888][DEHP]) results are given are adapted in the SI. from The Ref.line spectra [11] and of shown PAO4, here (PAO4 for + comparison. [N888H][DEHP]), The line andspectra (PAO4 of + OFM [P8888][DEHP]) and Sec-ZDDP are areadapted replotted from in Ref. Figure [11]6 toand compare shown with here theirfor comparison. spectra in PAO4. The line spectra of OFM and Sec-ZDDP are replotted in Figure 6 to compare with their spectra in PAO4. 3.3.1. PAO4 + OFM Mixture 3.3.1. PAO4The SSP + OFMand PPPMixturespectra of the silica/(PAO4 + OFM) interface are very similar to those of silica/OFM interfaceThe SSP (see and Figure PPP4 andspectra Tables of S1–S4the silica/(PAO4 in the SI ). It + means OFM) that interface OFM molecules are very similar preferentially to those adsorb of silica/OFMfrom its mixture interface with (see PAO4 Figure to the 4 silica and surface Tables and S1 form–S4 in a well-packed the SI). It surface means layer. that OFM Although molecules the OFM preferentiallyconcentration adsorb is much from smaller its mixture than that with of PAO4 PAO4 to in the the silica bulk mixturesurface and (Table form1), itsa well surface-packed concentration surface layer.at the Although silica interface the OFM is concentration dominant due is to much the strong smaller interaction than that betweenof PAO4 OFMin the polar bulk mixture head groups (Table and 1),the its polarsurface solid concentration surface, as at well the as silica the vaninterface der Waals is dominant interactions due to between the strong long interaction alkyl chains between of OFM OFMmolecules polar head [4]. groups The similarity and the betweenpolar solid SFG surface, spectra as of w silicaell as/ OFMthe van and der silica Waals/(PAO4 interactions+ OFM) between interfaces longalso alkyl indicates chains that offor OFM the OFM molecules concentration [4]. The used similarity in the betweencurrent study, SFG a spectra multilayer of silica/OFM structure of and OFM silica/(PAO4is formed at+ theOFM) interface. interfaces The also conformation indicates that of the for OFM the OFM layer atconcentration the silica/(PAO4 used+ inOFM) the current interface study,would a multilayer be similar tostructure the one illustratedof OFM is informed Figure at5c the except interfac thate. the The bulk conformation liquid is now of a the mixture OFM of layer PAO4 at andthe silica/(PAO4 OFM. The formation + OFM) interface of a surface would layer be by similar the preferential to the one illustrated adsorption in of Figure OFM from5c except its mixture that thewith bulk the liquid lubricant is now base a oil mixture is consistent of PAO4 with and results OFM. reported The formation in the literature of a surface [41,46 ]. layer The adsorbed by the preferentialsurface layer adsorption greatly reducedof OFM thefrom interfacial its mixture friction with inthe tribological lubricant base operation oil is consistent conditions with [41]. results

3.3.2. PAO4 + PIBSI Mixture The SSP spectrum of this mixture at the silica surface is significantly different from that of PAO4. 1 It has a peak at 2956 cm− that is not observed in the spectrum of PAO4. The SSP spectrum of this 1 mixture is very similar to that of silica/hexadecane interface [11]. The strong peak at ~2925 cm− in the SSP spectrum is assigned to symmetric methylene Fermi resonance of C–CH2–C (see Table2) and 1 the only peak at ~2930 cm− in the PPP spectrum could be assigned to asymmetric methylene stretch mode (see Table2). These results suggest that PIBSI molecules lie flat at the silica surface with the symmetry axis of CH2 in C–CH2–C groups aligns relatively along the surface normal (see Figure2b, 1 the CH3 group can be replaced by the CH2 group with a C2 axis). If the peak at 2956 cm− in the SSP spectrum is of asymmetric methyl stretch mode (CH3,as), there should be a strong peak of this Lubricants 2020, 8, 98 10 of 18

1 vibrational mode in the PPP spectrum [30]. The absence of ~2956 cm− peak in the PPP spectrum then suggests that the peak is not due to asymmetric methyl stretch mode. It could, however, be attributed to symmetric methylene Fermi resonance of N–CH2–CH2–N (see Figure1) as in the case of 1,3-propanediol (HOCH2CH2CH2OH) [29]. Although we cannot rule out a small fraction of PAO4 is co-adsorbed at the solid surface, the dominance of PIBSI at the solid/(PAO4 + PIBSI) interface explains the improved tribological performance of this mixture compared to the base oil only case [10,50].

3.3.3. PAO4 + sec-ZDDP Mixture The SSP spectrum of the silica/(PAO4 + sec-ZDDP) interface is quite similar to that of silica/sec-ZDDP interface; however, their PPP spectra are different with an additional peak in the 1 spectrum of silica/mixture interface at ~2972 cm− which can be assigned to out-of-plane asymmetric methyl stretch mode (see Table2). These results suggest that a mixed film of PAO4, sec-ZDDP, and mineral oil could form at the silica surface with a possible dominance of components from sec-ZDDP. The abundance of sec-ZDDP in the interfacial layer is the main reason that it works effectively as the antiwear additive even at such a low concentration in the bulk liquid [51,52].

3.4. Speciation of Silica/Lubricant Interface for PAO4 Containing Two Additives Commercial lubricants contain multiple additives with different functionalities such as friction modifier, antiwear, dispersant, antioxidant, detergent [4,9]. In the current study, the competitive adsorption at the silica surface from PAO4-based lubricants containing IL and another additive (friction modifier, anitwear additive, or dispersant) was investigated using SFG spectroscopy. The compositions of mixed liquids are displayed in Table1. The fitting results of the SFG spectra are available in the SI.

3.4.1. Mixtures of PAO4 + OFM + IL The SFG spectra of silica / (PAO4 + OFM + IL) mixtures are given in Figure7. The spectra of PAO4 and other mixtures are also displayed in Figure7 for comparison. The SSP spectrum of (PAO4 + OFM + [N888H][DEHP]) mixture in Figure7 is very similar to that of silica /(PAO4 + OFM) interface; however, their PPP spectra are slightly different. These results reveal that at the interface between silica and (PAO4 + OFM + [N888H][DEHP]) mixture, OFM is dominant over other two compounds and there could be a small contribution of [N888H][DEHP] to the surface layer. Results from a previous study showed a strong synergistic effect between OFM and [N888H][DEHP]; the tribological test with bronze and the lubricant containing both OFM and [N888H][DEHP] resulted in a significantly lower steady-state friction coefficient and wear rate compared to the test with the lubricant containing OFM or [N888H][DEHP] alone [9]. The outstanding lubrication performance of (OFM + [N888H][DEHP]) mixture was attributed to a physically adsorbed mixed surface film providing the benefits of both additives and the formation of such a film is enhanced by possible hydrogen bonds between OFM and [N888H][DEHP] at the bronze surface [9]. The slight difference in PPP spectra of silica/(PAO4 + OFM) and silica/(PAO4 + OFM + [N888H][DEHP]) could confirm the formation of such mixed film at the silica surface (and the oxide layer on bronze as well [9]). The interface between silica and (PAO4 + OFM + [N888H][DEHP]) mixture is schematically illustrated in Figure8a. Lubricants 2020, 8, x FOR PEER REVIEW 11 of 18 in PPP spectra of silica/(PAO4 + OFM) and silica/(PAO4 + OFM + [N888H][DEHP]) could confirm the formation of such mixed film at the silica surface (and the oxide layer on bronze as well [9]). The interface between silica and (PAO4 + OFM + [N888H][DEHP]) mixture is schematically illustrated in FigureLubricants 8a. 2020, 8, 98 11 of 18

Figure 7. SFG spectra of (PAO4 + OFM + IL) mixtures at silica/liquid interface. The solid lines are fitted Figurecurves 7. SFG and spectra the fitting of results(PAO4 are + OFM given in+ IL) the mixtures SI. The line at spectra silica/liquid of PAO4, interface. PAO4 + [N888H][DEHP],The solid lines are fitted PAO4 curves+ [P8888][DEHP] and the fitting are adapted results from are Ref. given [11 ] in and the shown SI. here The for line comparison. spectra of PAO4, PAO4 + [N888H][DEHP],The SSP and PAO4PPP +spectra [P8888][DEHP] of silica/ (PAO4are adapted+ OFM from+ Ref.[P8888][DEHP]) [11] and shown interface here for in comparison. Figure7 are very similar to those of silica/(PAO4 + [P8888][DEHP]) interface (see Tables S3–S6). Both SSP spectra The SSP and PPP spectra of silica/(PAO41 + OFM1 + [P8888][DEHP])1 interface1 in Figure 7 are very have the same peaks at ~2850 cm− , 2870 cm− , 2915 cm− and 2940 cm− (see Table2 for possible 1 1 1 similarspectral to those assignments) of silica/(PAO4 and their + [P8888][DEHP])PPP spectra contain interface peaks at(see ~2850 Tables cm− S3, 2870–S6). cm Both− , andSSP 2940 spectra cm− have. the sameThese peaks results atshow ~2850 that cm [P8888][DEHP]−1, 2870 cm−1, preferentially2915 cm−1 and adsorbs 2940 tocm the−1 silica(see Table surface 2 from for possible the mixture spectral of assignments)these three and compounds their PPP [11 spectra]. The polar contain head peaks groups at of ~2850 [P8888][DEHP] cm−1, 2870 molecules cm−1, and must 2940 be in cm contact−1. These resultswith show the that silica, [P8888][DEHP] while their alkyl preferentially chains face to the adsorbs bulk liquid to the (see silica Figure surface8b). In from the tribologicalthe mixture study of these three atcompounds the bronze surface,[11]. The the polar adsorption head groups of OFM of to [P8888][DEHP] the solid surface molecules from its mixture must withbe in PAO4 contact and with the silica,[P8888][DEHP] while their was alkyl considered chains toface be atoffected the bulk by strong liquid intermolecular (see Figure hydrogen8b). In the bonds tribological between OFMstudy at and [P8888][DEHP] in the liquid phase, in addition to inevitable competitive adsorption between these the bronze surface, the adsorption of OFM to the solid surface from its mixture with PAO4 and two compounds [9]. However, these hydrogen bonds could also affect the adsorption of [P8888][DEHP] [P8888][DEHP] was considered to be affected by strong intermolecular hydrogen bonds between at the bronze surface. The SFG results in this study suggest that the presence of one more alkyl chain OFM in and [P8888] [P8888][DEHP]+ compared to in [N888H] the liquid+ can cause phase, drastic in addition differences to in inevitable the adsorption competitive of these ILs adsorption to the betweensilica these surface two from compounds their mixtures [9]. However, with PAO4 these and OFM. hydrogen bonds could also affect the adsorption of [P8888][DEHP] at the bronze surface. The SFG results in this study suggest that the presence of one more alkyl chain in [P8888]+ compared to [N888H]+ can cause drastic differences in the adsorption of these ILs to the silica surface from their mixtures with PAO4 and OFM.

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FigureFigure 8. ( 8.a )( Illustrationa) Illustration of silica of / silica/(PAO4(PAO4 + OFM + + OFM[N8888H][DEHP]) + [N8888H][DEHP]) interface interface and (b) that and of silica(b) that/(PAO4 of + OFMsilica/(PAO4+ [P8888][DEHP]) + OFM + [P8888][DEHP]) interface. interface.

3.4.2.3.4.2. Mixtures Mixtures of of PAO4 PAO4+ +sec-ZDDP sec-ZDDP ++ IL TheTheSSP SSPand andPPP PPPspectra spectra of of the the silica silica/(PAO4/(PAO4 + + sec-ZDDPsec-ZDDP + IL)IL) interface interface are are shown shown in in Figure Figure 9.9 . TheseThese spectra spectra are are veryvery similarsimilar to those those of of the the silica/(PAO4 silica/(PAO4 + IL)+ IL) interface interface (see (see Tables Tables S3–S S3–S66 in the in SI). the SI).Note Note that that the thePPPPPP spectraspectra of the of silica/(PAO4 the silica/(PAO4 + sec-+ZDDPsec-ZDDP + IL) interface+ IL) interface do not show do not a peak show at a~2845 peak 1 atcm ~2845−1 and cm the− absenceand the of absence this peak of thiscould peak be due could to a be low due S/N to ratio. a low The S/N results ratio. Thein Figure results 9 reveal in Figure that9 revealthe silica that thesurface silica is surfacedominated is dominated by IL molecules by IL molecules even though even its though concentration its concentration in the bulk in liquid the bulk is liquidvery islow. very Both low. ionic Both liquids ionic liquidscontaining containing polar head polar groups head groupshave stronger have stronger interactions interactions with the with silica the silicasurface surface compared compared to other to other components components in the in mixtures the mixtures leading leading to their to theirpreferential preferential adsorption adsorption at the at theinterface. interface. Synergistic effects between sec-ZDDP and [P8888][DEHP] in tribological experiments were reported in the literature [6], in which the presence of these compounds together in the lubricant lead to ~30% reduction in friction and ~70% reduction in wear compared to lubricants containing base oil + sec-ZDDP (or [P8888][DEHP]) only. The fact that SFG spectra of silica/(PAO4 + [P8888][DEHP]) and silica/(PAO4 + sec-ZDDP + [P8888][DEHP]) interfaces are very similar indicates that the reductions in friction and wear in the case of PAO4 + sec-ZDDP + [P8888][DEHP] mixture could be related to the properties of the tribofilm formed during the tribological test, rather than the initial surface film before tribo-test. It was shown that this tribofilm, compared to the tribofilms formed with the lubricants containing sec-ZDDP or [P8888][DEHP] only, has significantly higher amounts of zinc and iron phosphates while having much lower contents of metal oxides and sulfur compounds [6]. The previous study also suggested that both cations and anions of IL have critical roles in reductions of friction and wear. This also explains why [N888H][DEHP] did not show synergistic effects with sec-ZDDP [6] although it dominates the solid surface before the onset of the tribo-test (as shown in Figure9, assuming that adsorptions of base oil and additives at the silica and metal oxide surfaces are similar).

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Figure 9. SFG spectra of (PAO4 + ZDDP + IL) mixtures at silica/liquid interface. The solid lines are fitted Figurecurves 9. SFG and spectra the fitting of (PAO4 results are+ ZDDP given in+ theIL) SI.mixtures The line at spectra silica/liquid of PAO4, interface. PAO4 + [N888H][DEHP],The solid lines are PAO4 + [P8888][DEHP] are adapted from Ref. [11] and shown here for comparison. fitted curves and the fitting results are given in the SI. The line spectra of PAO4, PAO4 + [N888H][DEHP],3.4.3. Mixtures of PAO4 PAO4 ++ [P8888][DEHP]PIBSI + IL are adapted from Ref. [11] and shown here for comparison. The SFG spectra of the silica/(PAO4 + PIBSI + IL) interface are shown in Figure 10 along with the Synergistic effects between sec-ZDDP and [P8888][DEHP] in tribological experiments were spectra of other mixtures for comparison. The SSP spectrum of the (PAO4 + PIBSI + [N888H][DEHP]) reportedmixture in the is quiteliterature similar [6] to, in that which of the the (PAO4 presence+ PIBSI) of mixture,these compounds while its PPP togetherspectrum in isthe similar lubricant to the lead to ~30%spectrum reduction of the in (PAO4 friction+ [N888H][DEHP]) and ~70% reduction mixture in (see wear Tables compared S3–S6). to This lubricants means that containing at the interface base oil + sec-betweenZDDP (or silica [P8888][DEHP]) and the (PAO4 only.+ PIBSI The+ fact[N888H][DEHP]) that SFG spectra mixture, of silica/(PAO4 PIBSI and [N888H] + [P8888][DEHP]) co-adsorb to and silica/(PAO4form a mixed + sec- layerZDDP with + [P8888][DEHP]) an insignificant contribution interfaces are from very PAO4. similar Tribological indicates tests that on the a cast reductions iron in frictionsurface and showed wear in that the the case friction of PAO4 coeffi +cient sec- andZDDP wear + [P8888][DEHP] volume of cast iron mixture were could higher be when related tested to the propertieswith the of (PAO4 the tribofilm+ PIBSI + formed[N888H][DEHP]) during the mixture tribological compared test, to those rather in than the case the of initial testing surface with the film before(PAO4 tribo+-test.[N888H][DEHP]) It was shown mixture that [ this10]. These tribofilm, higher compared values could to bethe due tribofilms to a reduced formed amount with of the lubric[N888H][DEHP]ants containing at sec the-ZDDP solid surface or [P8888][DEHP] when used together only, has with significantly PIBSI in the lubricanthigher amounts as evidenced of zinc by and iron phosphatesSFG results in while the current having study. much lower contents of metal oxides and sulfur compounds [6]. The The SSP spectrum of (PAO4 + PIBSI + [P8888][DEHP]) mixture at the silica surface is quite previous study also suggested that both cations and anions of IL have critical roles in reductions of similar to that of (PAO4 + PIBSI) and their PPP spectra are also quite similar (except a small peak friction and wear.1 This also explains why [N888H][DEHP] did not show synergistic effects with sec- at 2942 cm− ) in terms of spectral features (see Tables S3–S6). These results suggest that at the ZDDPinterface [6] although between it dominates silica and three the compoundsolid surface mixture, before PIBSI the onset molecules of the possibly tribo-test dominate (as shown the surface in Figure 9, assuminglayer. The that absence adsorptions of [P8888][DEHP] of base oilat the and silica additives surface could at the be silica due to and its reaction metal / oxideinteraction surfaces with are similar).PIBSI in the bulk liquid [10]. For this reason, [P8888][DEHP] may stay in the liquid phase while PIBSI preferentially adsorbs to the silica surface. The depletion of [P8888][DEHP] from the interface 3.4.3.when Mixtures used of together PAO4 with+ PIBSI PIBSI + IL can explain the results obtained in the previous study in which the (PAO4 + PIBSI + [P8888][DEHP]) mixture showed a higher friction and a more wear of cast iron The SFG spectra of the silica/(PAO4 + PIBSI + IL) interface are shown in Figure 10 along with the compared to those of (PAO4 + [P8888][DEHP]) mixture [10]. spectra of other mixtures for comparison. The SSP spectrum of the (PAO4 + PIBSI + [N888H][DEHP]) mixture is quite similar to that of the (PAO4 + PIBSI) mixture, while its PPP spectrum is similar to the spectrum of the (PAO4 + [N888H][DEHP]) mixture (see Tables S3–S6). This means that at the interface between silica and the (PAO4 + PIBSI + [N888H][DEHP]) mixture, PIBSI and [N888H] co-adsorb to form a mixed layer with an insignificant contribution from PAO4. Tribological tests on a cast iron surface showed that the friction coefficient and wear volume of cast iron were higher when tested with the (PAO4 + PIBSI + [N888H][DEHP]) mixture compared to those in the case of testing with the (PAO4 + [N888H][DEHP]) mixture[10]. These higher values could be due to a reduced amount of

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[N888H][DEHP] at the solid surface when used together with PIBSI in the lubricant as evidenced by SFG resultsLubricants in2020 the, 8, current 98 study. 14 of 18

Figure 10. SFG spectra of (PAO4 + PIBSI + IL) mixtures at silica/liquid interface. The solid lines are fitted Figurecurves 10. SFG and thespectra fitting of results (PAO4 are + givenPIBSI in + theIL) SI.mixtures The line at spectra silica/liquid of PAO4, interface. PAO4 + [N888H][DEHP], The solid lines are fittedPAO4 curves+ [P8888][DEHP] and the fitting are adapted results from are Ref. given [11 ] in and the shown SI. here The for line comparison. spectra of PAO4, PAO4 + [N888H][DEHP], PAO4 + [P8888][DEHP] are adapted from Ref. [11] and shown here for comparison. 4. Conclusions

The CompetitiveSSP spectrum adsorption of (PAO4 of additives + PIBSI at + the [P8888][DEHP]) silica/lubricant interface mixture was at studied the silica using surface vibrational is quite similarsum to frequencythat of (PAO4 generation + PIBSI) spectroscopy. and their The PPP results spectra showed are also that quite adsorption similar of an(except additive a small at a solid peak at 2942 surfacecm−1) in can terms be a ffofected spectral by the features presence (see ofother Tables additives S3–S6). in These the base results oil. The suggest adsorption that at behavior the interface of (base oil + additive(s)) mixtures at the silica/lubricant interface is summarized and compared with the between silica and three compound mixture, PIBSI molecules possibly dominate the surface layer. tribological performance data found in the literature in Table3. The correlation between adsorption The absence of [P8888][DEHP] at the silica surface could be due to its reaction/interaction with PIBSI behavior of additives on the silica surface and experimental results from tribological tests can provide in themechanistic bulk liquid insights [10]. For for future this reason, lubricant [P8888][DEHP] development, particularly may stay in important the liquid for phaseguiding while the use PIBSI preferentiallyof ILs. adsorbs to the silica surface. The depletion of [P8888][DEHP] from the interface when used together with PIBSI can explain the results obtained in the previous study in which the (PAO4 + PIBSI + [P8888][DEHP]) mixture showed a higher friction and a more wear of cast iron compared to those of (PAO4 + [P8888][DEHP]) mixture [10].

4. Conclusions Competitive adsorption of additives at the silica/lubricant interface was studied using vibrational sum frequency generation spectroscopy. The results showed that adsorption of an additive at a solid surface can be affected by the presence of other additives in the base oil. The adsorption behavior of (base oil + additive(s)) mixtures at the silica/lubricant interface is summarized and compared with the tribological performance data found in the literature in Table 3. The correlation between adsorption behavior of additives on the silica surface and experimental results from tribological tests can provide mechanistic insights for future lubricant development, particularly important for guiding the use of ILs.

Lubricants 2020, 8, 98 15 of 18

Table 3. Summary of adsorption behavior of (lubricant base oil + additive) mixtures at the silica/liquid interface and comparison with lubrication performance of the same mixtures at tribological interfaces reported in literatures.

Lubricant Interfacial Layer Tribological Property PAO4 + IL IL molecules dominate the interface [11] Beneficial effects [9,10] The interface is dominated by OFM molecules and the surface layer structure PAO4 + OFM Beneficial effects [9,41] is very similar to that of silica/OFM interface. A mixed layer of PAO4 and sec-ZDDP PAO4 + sec-ZDDP forms at the silica surface and sec-ZDDP Beneficial effects [50] dominates this surface layer. PIBSI preferential adsorbs to the PAO4 + PIBSI silica/liquid interface and seems to lie Beneficial effects [10,50] relatively flat at the surface. A mixed layer of OFM and [N888H][DEHP] is formed at the silica PAO4 + OFM + [N888H][DEHP] Synergistic effects [9] surface and OFM is the major component of this mixed layer. The surface layer is dominated by Lubrication efficiency was reduced in [P8888][DEHP] and its structure is similar PAO4 + OFM + [P8888][DEHP] comparison to that of (PAO4 + to that formed at silica/(PAO4 + OFM) [9] [P8888][DEHP]) interface. [N888H][DEHP] molecules adsorb to the No synergistic effect due to the PAO4 + sec-ZDDP + silica surface to form a surface layer that presence of [N888H]+[6] (In Ref. [6] [N888H][DEHP] is similar to that of silica/(PAO4 + the base oil was gas-to-liquid (GTL)) [N888H][DEHP] interface. Reduction of friction by ~30% and wear by >70% [6]. The SFG results The surface layer is dominated by show that this improved performance PAO4 + sec-ZDDP + [P8888][DEHP] and its structure is very must be related to the properties of the [P8888][DEHP] similar to that of silica/(PAO4 + tribofilm formed during the [P8888][DEHP]) interface. tribological test, rather than the initial surface film before tribo-test. PIBSI and [N888H][DEHP] form a mixed Inferior performance, compared to IL PAO4 + PIBSI + [N888H][DEHP] layer at the silica surface. The presence of only [10] PAO4 in the surface layer is insignificant. PIBSI possibly dominates the silica/mixed Inferior performance, compared to IL PAO4 + PIBSI + [P8888][DEHP] liquid interface. only [10]

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-4442/8/11/98/s1, Table S1: Fitting results of SSP spectra of single compounds at solid/liquid interface, Table S2: Fitting results of PPP spectra of single compounds at solid/liquid interface, Table S3: Fitting results of SSP spectra of two compound mixtures at silica/liquid interface, Table S4: Fitting results of PPP spectra of two compound mixtures at silica/liquid interface, Table S5: Fitting results of SSP spectra of three compound mixtures at silica/liquid interface, Table S6: Fitting results of PPP spectra of three compound mixtures at the silica/liquid interface. Author Contributions: Conceptualization, J.Q. and S.H.K.; Formal analysis, D.N.; Funding acquisition, J.Q. and S.H.K.; Investigation, D.N., X.H., H.L., J.Q. and S.H.K.; Resources, J.Q.; Supervision, S.H.K.; Writing—original draft, D.N. and S.H.K.; Writing—review and editing, D.N., X.H., H.L., J.Q. and S.H.K. All authors have read and agreed to the published version of the manuscript. Funding: This manuscript has been co-authored by UT-Battelle, LLC under contract no. DEAC05-00OR22725 with the U.S. Department of Energy. The U.S. Government retains, and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes. The Department of Energy will provide public access to these results of federally sponsored research by the DOE public access plan (http://energy.gov/downloads/ doe-public-access-plan). Lubricants 2020, 8, 98 16 of 18

Acknowledgments: This work was supported by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) as a subcontract to Pennsylvania State University through the Oak Ridge National Laboratory. The authors thank J. Dyck and E. Conrad from Solvay for providing phosphonium cation feedstocks, A.G. Bro and C. Dubin from ExxonMobil for providing the PAO base oil, O. Farng from ExxonMobil for providing the OFM, and PIBSI, and E. Bardasz from Lubrizol for providing the ZDDP. Conflicts of Interest: The authors declare no conflict of interest.

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