Ionic liquids composed of phosphonium cations and organophosphate, carboxylate, and sulfonate anions as lubricant antiwear additives

Article abstract of DOI:10.1021/la5032366

Oil-soluble phosphonium-based ionic liquids (ILs) have recently been reported as potential ashless lubricant additives. This study is to expand the IL chemistry envelope and to achieve fundamental correlations between the ion structures and ILs physiochemical and tribological properties. Here we present eight ILs containing two different phosphonium cations and seven different anions from three groups: organophosphate, carboxylate, and sulfonate. The oil solubility of ILs seems largely governed by the IL molecule size and structure complexity. When used as oil additives, the ranking of effectiveness in wear protection for the anions are organophosphate > carboxylate > sulfonate. All selected ILs outperformed a commercial ashless antiwear additive. Surface characterization from the top and the cross-section revealed the nanostructures and compositions of the tribo-films formed by the ILs. Some fundamental insights were achieved: branched and long alkyls improve the ILs oil solubility, anions of a phosphonium-phosphate IL contribute most phosphorus in the tribo-film, and carboxylate anions, though free of P, S, N, or halogen, can promote the formation of an antiwear tribo-film.

Full text of DOI:10.1021/la5032366

Article
pubs.acs.org/Langmuir
Ionic Liquids Composed of Phosphonium Cations and
Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant
Antiwear Additives
Yan Zhou,†,‡ Jeffrey Dyck,§ Todd W. Graham,§ Huimin Luo,∥ Donovan N. Leonard,† and Jun Qu*,†
†Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States
‡Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States
§Application Technology Group, Cytec Canada, 9061 Garner Road, Niagara Falls, Ontario L2H 6S5, Canada
∥Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: Oil-soluble phosphonium-based ionic liquids
(ILs) have recently been reported as potential ashless lubricant
additives. This study is to expand the IL chemistry envelope and
to achieve fundamental correlations between the ion structures
and ILs’ physiochemical and tribological properties. Here we
present eight ILs containing two different phosphonium cations
and seven different anions from three groups: organophosphate,
carboxylate, and sulfonate. The oil solubility of ILs seems largely
governed by the IL molecule size and structure complexity.
When used as oil additives, the ranking of effectiveness in wear
protection for the anions are organophosphate > carboxylate > sulfonate. All selected ILs outperformed a commercial ashless
antiwear additive. Surface characterization from the top and the cross-section revealed the nanostructures and compositions of
the tribo-films formed by the ILs. Some fundamental insights were achieved: branched and long alkyls improve the IL’s oil
solubility, anions of a phosphonium-phosphate IL contribute most phosphorus in the tribo-film, and carboxylate anions, though
free of P, S, N, or halogen, can promote the formation of an antiwear tribo-film.
Ionic liquids (ILs) have recently been studied for their
potential applications in lubrication.5−7 Imidazolium-based ILs
with anionic fluorophosphate8−10 and fluoroborate5,11,12 were
the focus of the early work. The hydrolyzation products of those
anions are highly corrosive and toxic that makes those ILs only
applicable to water-free conditions.13 Many ILs possess high
thermal stability, allowing them to potentially be applied to high-
temperature lubrication in which traditional hydrocarbon oils are
unstable.
Ionic liquids are ashless and have stronger absorbance to
metallic bearing surfaces (due to ionic nature) compared to the
common ZDDP and therefore may potentially provide superior
antiwear functionality with less engine deposits. Using ILs as
lubricant additives requires less change in the production process
for lubricants. In earlier studies, ILs mainly were explored in neat
forms or as base stocks because most ILs have little or no
solubility (≪1%) in nonpolar hydrocarbon oils.14−18 In 2012,
our group first reported phosphonium-based ILs that are fully
miscible with nonpolar hydrocarbon oils and provide effective
wear protection as oil additives.19−21 The improved oil-solubility
is attributed to the 3D large ionic molecules that sufficiently
1. INTRODUCTION
The effects of friction and wear account for ∼6% of US GNP. In
2009, it was estimated that 208,000 million liters of fuel was
consumed to overcome friction in passenger cars worldwide.1
Lubrication of vehicle engines and components is necessary to
protect them from damage caused by friction and wear.
Lubrication is classified into three regimes: boundary, mixed,
and elastohydrodynamic/hydrodynamic. In an internal combus-
tion engine, the wear of the piston rings and cylinder liners occurs
primarily in the boundary and mixed regimes, but parasitic
friction loss is contributed mainly by elastohydrodynamic drag,
which consumes 10−15% of the total energy generated by an
engine2 and is proportional to the viscosity of the lubricating oil.
The use of lubricating oils of lower viscosity boosts the efficiency
of engines, but the engine surfaces are challenged by the lower
degree of wear protection. Antiwear (AW) additives are usuaully
used in the oil and the reactive agents, e.g., P, S, N, or halogen
elements, in the AW molecules interact with the metallic surfaces
to form protective tribo-films.3 Zinc dialkyldithiophosphate
(ZDDP)4 is the most commonly used AW additive with effective
wear protection but is known to generate deposits (“ash”) from
thermal decomposition and hence poison emission-system
catalysts. Therefore, developing ashless and effective AW
additives is of great interest from the practical perspective of
energy savings.
Received: August 13, 2014
Revised: October 15, 2014
Published: October 20, 2014
© 2014 American Chemical Society
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Figure 1. Structures of selected ILs and a commercial amine-phosphate.
reduce polarity to allow better miscibility with neutral oil
molecules. Since then, phosphonium cations were paired with
phosphate anions, such as diphenylphosphate,22 dimethylphos-
phate,23 diethylphosphate, and tris(pentafluoroethyl)-
trifluorophosphate,24 and studied as lubricant additives.
Ammonium-carboxylates have been studied as neat lubri-
cants,25,26 grease additives,27 and water additives.28 Espinosa et
al. have reported reductions in friction and wear for copper
samples using synthetic base oils supplemented with 1%
ammonium-adipate.29
The goal of this study is to further expand the chemistry
envelope for using ILs as lubricant additives and to achieve
fundamental correlations between the ion structures and ILs’
physiochemical and tribological properties. This study synthe-
sized eight ILs containing two different phosphonium cations
and seven different anions from three groups: organophosphate,
carboxylate with linear and branched alkyls, and sulfonate. Via
characterization of the selected ILs, we expect to address three
fundamental questions:
• Phosphorus compounds have consistently been detected
in the tribo-films formed by phosphonium-phosphate
ILs.19,21,30 However, the origin of the P is unclear; it could
come from either the cationic phosphonium, the anionic
phosphate, or both. The new groups of phosphonium-
carboxylate and phosphonium-sulfonate ILs, only cations
containing P, allow us to identify the source of P and
furthermore the role of anions.
• For AW additives, it is widely believed that P, S, N, or
halogen elements are critical in the tribo-film formation.3
Can a phosphonium-carboxylate IL, whose anion is free of
the known active agents, produce an effective AW tribo-
film?
The density, viscosity, oil solubility, thermal stability, and
corrosivity of each IL were characterized as a screening measure.
Selected ILs were added to a base oil to test their AW
performance. A molecular dynamics tool was used to calculate
the ILs’ solubility parameters. Morphology examination and
chemical analysis were conducted on selected tribo-films.
• We have previously proposed the correlation between
cation and anion alkyl chain length and an IL’s oil
solubility, which was confirmed by a couple of phosphate
anions.18 The carboxylate anions containing linear and
branch alkyls allow investigations of the effects of the alkyl
structure on the IL’s oil solubility.
2. EXPERIMENTAL SECTION
2.1. Synthesis of Ionic Liquids. Eight ILs were selected for this
research: tetrabutylphosphonium bis(2-ethylhexyl)phosphate ([P4444]-
[DEHP]), trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate
([P66614][DEHP]), trihexyltetradecylphosphonium 2-ethylhexanoate
([P66614][i-C7H15COO]), trihexyltetradecylphosphonium octanoate
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Table 1. Physiochemical Properties of Selected ILs and Their Blends of a Base Oil
η (mPa s) of PAO−IL
blends
chemicals
T
dec (°C)
ρ (g/mL)
η (mPa·s) @ 40 °C
IL (wt %) added into PAO
40 °C
100 °C
PAO 4
250
310
347
348
308
330
327
0.80 (23 °C)
0.92 (20 °C)
0.94 (20 °C)
0.91 (23 °C)
0.88 (20 °C)
0.93 (23 °C)
0.91 (23 °C)
0.89 (25 °C)
0.88 (23 °C)
15.38
1107
320
15.38
15.80
15.44
15.56
15.71
3.27
3.30
3.28
3.29
3.30
amine-phosphate
1.67
0.75
1.03
[P4444][DEHP]
[P66614][DEHP]
556
a
[P66614][i-C7H15COO]
[P66614][n-C7H15COO]
[P66614][i-C9H19COO]
[P66614][n-C9H19COO]
[P66614][n-C17H35COO]
[P66614][RSO3]
185
1.65
122
127
solid
175
308
431
1.98
2.44
15.44
16.04
3.27
3.38
a
Treat rate is slightly higher than what is supposed to be (1.61%) due to a miscalculation of the molecular weight, but such a small discrepancy
should not make any significant impact on the physical, chemical, or lubricating properties.
([P66614][n-C7H15COO]), trihexyltetradecylphosphonium 4-methylno-
nanoate ([P66614][i-C9H19COO]), trihexyltetradecylphosphonium dec-
anoate ([P66614][n-C9H19COO]), trihexyltetradecylphosphonium octa-
decanoate ([P66614][n-C17H35COO]), and trihexyltetradecylphospho-
nium sulfonate ([P66614][RSO3]). The prefixes “i” and “n” denote
branched and linear carbon chains, respectively. A commercial ashless
AW additive, amine-phosphate, was chosen for comparison. The
molecular structures of the selected ILs and the commercial amine-
phosphate are shown in Figure 1, though the exact alkyls of the amine-
phosphate are unavailable.
For preparing [P4444][DEHP], a 5 L three-neck jacketed flask fitted
with an addition funnel, condenser, thermal well, bottom drain valve,
and nitrogen inlet was charged with aqueous [Bu4P]Cl (76.8 wt %, 906
g, 2.360 mol), bis(2-ethylhexyl)phosphate (HDEHP, 723 g, 2.242 mol)
and 515 g of toluene; and the mixture was then heated to an internal
temperature of 70 °C. A solution of NaOH (90.1 g, 2.253 mol, in 184
mL of H2O) was added dropwise while the internal temperature was
maintained at <75 °C (over 20 min). After the addition was completed,
504 g of H2O and 412 g of toluene were added and the mixture was
stirred at 70 °C overnight. The lower aqueous phase was removed, and
the mixture was washed four times with H2O. Vacuum stripping of the
material (max temp. 144.5 °C at 2.1 mbar) afforded 1.281 kg (yield:
98%, purity: 99%).
[P66614][DEHP] was synthesized via the same procedure as
[P4444][DEHP], using [(hexyl)3P(C14H29)]Cl instead of [Bu4P]Cl,
with a yield rate of ∼99% and a purity of 96.4%.
[P66614][i-C7H15COO] was synthesized via the same procedure as
[P66614][DEHP], using 2-ethylhexanoic acid instead of HDEHP, with a
yield rate of ∼88% and a purity of 99%.
For preparing [P66614][n-C7H15COO], octanoic acid (5.53 g, 38.4
mmol) and [P66614]Cl (19.94 g, 38.4 mmol) were dissolved in 75 mL of
hexane. A solution of NaOH (1.54 g, 38.4 mmol, in 75 mL of H2O) was
added dropwise at room temperature (RT). The mixture was stirred at
RT overnight. The upper organic phase was separated and washed four
times with H2O. Solvents were distilled off by rotary evaporator and the
product was dried at 70 °C under vacuum for 4 h to yield [P66614][n-
C7H15COO] as a viscous liquid (23.60 g, 37.6 mmol, yield: ∼98%,
purity: ∼97%).
∼106% and a purity of 93%. The >100% yield rate was probably caused
by nonvolatile impurities in the sulfonic acid starting material.
Proton nuclear magnetic resonance (NMR) analysis was carried out
on the ILs using a Bruker MSL-400 at 400 MHz. Spectra were obtained
1
in CDCl3 with reference to TMS (0 ppm) for H, and results are
summarized in Table S1 in the Supporting Information.
2.2. Methods. Polyalphaolefin (PAO) 4 (supplied by Mobil Oil
Company) was chosen as a baseline lubricant and the base stock. ILs
were used at different treat rates to ensure 800 ppm P in the final blends,
which is the upper limit of P in automotive engine oil specifications
(ILSAC GF-5). The solubility of ILs in PAO 4 was determined by direct
observation after centrifugation of the blends at 13,000 rpm for 5 min.
Thermogravimetric analysis (TGA) was performed with a TGA-2950
(TA Instruments) at a 10 °C/min heating rate in air. The viscosity of
PAO 4 with and without ILs was measured on a Petrolab Minivis II
viscometer at 23, 40, and 100 °C.
The corrosivity of the ILs was tested by applying a droplet of each IL
to the surfaces of AISI E52100 hardened steel balls and CL35 cast iron
flats in an ambient environment and monitoring the surface morphology
over 14 days.
Ball-on-flat tribological tests were carried out on a reciprocating
tribometer (Plint TE77, Phoenix Tribology Ltd.) using a 10 mm
diameter AISI E52100 hardened steel ball sliding against a CL35 cast
iron flat. The compositions of these two materials are shown in Table S2
in the Supporting Information. The microindentation Vickers hardness
(at 100 g-f) is 976 and 213 HV for the steel ball and iron flat,
respectively. The flats were polished using P1200 SiC abrasive paper in
distilled water. Tests were performed under a 100 N normal load and at a
temperature of 100 °C. The maximum Hertzian contact stress at the
beginning of the test was calculated to be 1.68 GPa, much higher than
yield strength (∼950 MPa) of CL35 cast iron at 100 oC, and therefore
plastic deformation is inevitable. The contact stress dropped rapidly
during the wear process with increasing contact area, eventually to the
level of 100-200 MPa (depending on the wear scar size) at the end of the
test. The oscillation frequency used was 10 Hz with a stroke of 10 mm,
and the sliding distance was 1000 m. Two tests were first carried out for
each lubricant, and a third one was added when repeatability was in
question. The worn samples were ultrasonically cleansed, using acetone
and isopropyl alcohol sequentially, before wear quantification using an
optical interferometer (Wyko NT9100) and surface characterization as
described below.
The worn surfaces were first examined using scanning electron
microscopy (SEM) and energy-dispersive spectroscopy (EDS) on a
Hitachi S4800 field-emission SEM equipped with an EDAX SDD EDS
system. X-ray photoelectron spectroscopy (XPS) analysis was
conducted on the wear scars using a Thermal Scientific K-Alpha XPS
system. A 400 μm diameter spot was focused during the acquisition of
the data. The depth profilings of the tribo-films were obtained by using
an argon-ion sputter gun at 3.5 keV. The high-resolution spectra of iron
Fe 2p, oxygen O 1s, carbon C 1s, and phosphorus P 2p were obtained
[P66614][i-C9H19COO] was synthesized via a similar procedure as
[P66614][n-C7H15COO], but using 4-methylnonanoic acid in stead of
octanoic acid, with a yield rate of ∼96% and a purity of ∼97%.
[P66614][n-C9H19COO] was synthesized via a similar procedure as
[P66614][DEHP], but using decanoic acid in stead of HDEHP, with a
yield rate of ∼93% and a purity of 96.7%.
[P66614][n-C17H35COO] was synthesized via a similar procedure as
[P66614][n-C7H15COO], but using stearic acid in stead of octanoic acid,
with a yield rate of ∼97% and a purity of ∼97%.
[P66614][RSO3] was synthesized via the same procedure as [P66614]-
[DEHP], using dinonylnapthalenesulfonic acid (this material is a
mixture of isomers and the composition is proprietary), with a yield rate
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Figure 2. (a) TGA curves of PAO 4, amine-phosphate, and oil-miscible ILs; (b) isothermal scans of two oil-miscible ILs at 175 °C.
after sputtering the worn surfaces for 50 s. All of the XPS data were
analyzed using the Thermo Avantage software (v4.61, Thermo Fisher
Scientific). An iterated Shirley background was applied to subtract the
backgrounds. A symmetric Gauss-Lorentz product line shape was used
in the curve fitting. A Hitachi NB5000 focused ion beam (FIB) system
with a gallium ion source was used to prepare the cross-section sample
for transmission electron microscopy (TEM) examination. The TEM
system was a Hitachi HF-3300 TEM/STEM equipped with an EDS
detector.
For an IL to stably dissolve in a hydrocarbon oil, ideally all individual
cations and anions are separated by oil molecules. Hildebrand solubility
parameter (δ) is defined as the root square of the energy needed to
completely separate molecules in a unit volume, which may serve as an
indicator of solubility based on the like-dissolve-like rule. The δ values of
ILs may be determined experimentally using enthalpies of vapor-
ization,31 surface tension measurement,32 chemical reactivity,33 and
viscosities.34 Computational chemistry provides a platform to prescreen
new chemicals. Molecular dynamics simulation has demonstrated its
feasibility in calculating the Hildebrand parameters of ILs35−37 and is
used here to help understand the ILs’ oil solubility. In this study,
Materials Studio 6.0 (Accelrys, San Diego) was used to calculate the
solubility parameters. The geometry and charge distribution of the
molecules were determined using the DMol3 module. Amorphous Cell
was used to assemble the molecules in a cubic box of 2.4 nm according to
the experimental density. The solubility parameters were calculated by
using Forcite with a pcff force field, in which the charges were fixed using
the DMol3 calculation.
was 8% compared to 39% of [P66614][i-C7H15COO], confirming
the higher thermal stability of the phosphate than the
carboxylate, as observed in the TGA temperature scans (Figure
2a).
No hint of corrosion was observed on either steel or iron
surfaces after 25 days’ exposure to any IL. The tested cast iron flat
surfaces are shown in Figure S1 in the Supporting Information.
3.2. Oil Miscibility. Table 2 lists the experimental solubilities
and calculated solubility parameters. The solubilities of [P4444]-
Table 2. Experimental Solubility and Calculated Solubility
Parameters of ILs
a
solubility parameter
(MPa1/2
δes
)
chemicals
solubility (wt %)
δd
δ
C10H22
PAO 4
13.28
0.42
13.29
b
16.55
amine-phosphate
>10
<1
[P4444][DEHP]
16.09
14.86
14.45
14.98
14.33
14.90
14.18
20.82
16.84
20.29
21.23
20.25
20.68
18.72
26.32
22.46
24.91
26.00
24.81
25.48
23.48
[P66614][DEHP]
>10
>10
<1
[P66614][i-C7H15COO]
[P66614][n-C7H15COO]
[P66614][i-C9H19COO]
[P66614][n-C9H19COO]
[P66614][n-C17H35COO]
[P66614][RSO3]
>2, <5
<1
>10
<2
3. RESULTS AND DISCUSSION
a
3.1. Density, Viscosity, Thermal Stability, and Corro-
sivity. Table 1 shows the densities (ρ) and viscosities (η) of the
ILs in a neat form, the concentrations of ILs in the PAO base oil,
and the viscosities of the oil−IL blends. Evidently, the addition of
an ILs at such low concentrations cause little change in the oil
viscosity.
δd, dispersive parameter; δes, electrostatic parameter; δ, Hildebrand
b
solubility parameter. ExxonMobil chemical PAO product bulletin
[DEHP] and of [P66614][DEHP] were <1% and >10%,
respectively, values consistent with the hypothesis that IL cations
connected with longer alkyl chains had higher solubility in oil.20
The oil solubilities of [P66614][i-C7H15COO], [P66614][i-
C9H19COO], and [P66614][n-C17H35COO] were >10%, >2%,
and >10%, respectively. The IL anions of either branched or long
alkyl chains had high oil solubility. On the other hand, the IL
anions of either linear or short alkyl chains had low oil solubility:
[P66614][n-C7H15COO] and [P66614][n-C9H19COO]. Although
most carboxyl ILs were liquid at RT, [P66614][n-C9H19COO] was
in the solid phase. Ritchie et al. have demonstrated that ILs of an
anionic carboxylate [RCOO] can be oil soluble but restricted the
R to being C9−C17 alkyls or alkenyls.38 Here we have shown
that [P66614][i-C7H15COO] and [P66614][i-C9H19COO] were oil
soluble because of the charge dilution resulting from the
branched carboxylates (U.S. patent pending; Application Serial
No. 14/444,029).
Table 1 also lists the decomposition temperature (Tdec) of the
PAO oil and ILs. TGA curves are shown in Figure 2a. The onset
of the decomposition temperatures of PAO 4, amine-phosphate,
[P4444][DEHP], [P66614][DEHP], [P66614][i-C7H15COO],
[P66614][n-C7H15COO], [P66614][i-C9H19COO], [P66614][n-
C17H35COO], and [P66614][RSO3] was at 250, 310, 347, 348,
308, 330, 327, 308, and 431 °C, respectively. In general, the
resistance to thermal decomposition and oxidation ranked
sulfonate > phosphate > carboxylate, though all tested ILs
showed higher thermal stability than the PAO base oil. The
length of the carbon chain seemed to have little influence on Tdec.
Five-hour isothermal tests were carried out on [P66614][DEHP]
and [P66614][i-C7H15COO] at 175 °C in an ambient environ-
ment and results are presented in Figure 2b. Both showed a linear
weight reduction with the testing time though the carboxylate
had a much higher rate. The final weight loss of [P66614][DEHP]
Hildebrand solubility parameter (δ) is expressed as the square
root of the cohesive energy density: δ = c1/2 (MPa1/2). The
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cohesive energy density (c) is the intermolecular attractive
energy that holds the molecules together in a unit volume. Based
on the like-dissolves-like rule, the ILs with a δ similar to that of oil
will be miscible. As proposed by Hansen and coworkers,39 the δ
can be extended to a sum of the dispersive parameter (δd), the
polar parameter (δp), and the hydrogen parameter (δh), which is,
[P66614][DEHP], possibly because its low oil solubility caused
uneven ion distribution in the oil solution. Figure 4e,f shows the
friction behaviors of PAO-[P66614][i-C7H15COO] and PAO-
[P66614][n-C17H35COO]. The COF of PAO-[P66614][i-
C7H15COO] started at 0.06 and then fluctuated around 0.11.
Results for PAO-[P66614][n-C17H35COO] were less repeatable:
the first and third tests performed similarly and produced
significantly lower friction and wear than the second one. A sign
of scuffing (a rapid friction rise around 100−200 m of sliding)
was seen in the second test, and the corresponding wear rate was
much higher than those of the other two. As shown in Figure 4h,
the wear rates of two blends of carboxylate ILs were 10 and 9% of
those without the additives. The average wear rate of the three
repeat tests of PAO-[P66614][RSO3] was 18% of that without the
additive; those results, however, were scattered. The first test
showed little fluctuation in COF and relatively low wear rate; the
second and third tests showed similar performance with higher
friction and wear.
2
2
2
δ2 = δd + δp + δh . Materials Studio does not calculate δp and δh
individually, instead, the electrostatic parameter (δes) is provided
2
(δes2 = δp2 + δh ). δd, δes, and δ of decane were calculated to be
13.28, 0.42, and 13.29, respectively. The weak electrostatic
contribution (δes = 0.42) of decane can be used to predict a low
δes of PAO 4. Figure 3 plots the solubility parameters of the ILs.
As shown by the comparisons in Figure 4h, all selected ILs
provided excellent AW performance and were more effective
than the commercial amine-phosphate. Since the ILs shared the
same [P66614], the differences in their anions were responsible for
their AW performance. The AW ranking is phosphonium-
phosphate > phosphonium-carboxylate > phosphonium-sulfo-
nate.
3.4. Tribo-Film Characterization. 3.4.1. SEM Morphology
Examination and EDS Surface Chemical Analysis. Figure 5
shows the SEM images and EDS spectra of the worn surfaces.
High oxygen contents were observed on all worn surfaces,
suggesting strong oxidation during the tribo-tests. Adhesive wear
and plastic deformation were observed on both the ball and the
flat lubricated by PAO 4, where silicon peaks were detected by
EDS. Since AISI 52100 steel contained a very low level of silicon
(< 0.3%; that would be undetectable for EDS), the silicon on the
steel ball was from the materials transferred from the iron flat
through adhesive wear. The addition of amine-phosphate
reduced scratches and deformation, forming dark-colored
patches of tribo-films on the worn surfaces.
Surfaces lubricated by both PAO-[P4444][DEHP] and PAO-
[P66614][DEHP] showed strong signals of P, implying the
formation of P-containing tribo-films. A weak silicon signal was
seen on the worn ball, suggesting reduced adhesion. Grooving
and plastic deformation were minimized under PAO-[P66614]-
[DEHP] but not under PAO-[P4444][DEHP]. Note that the
surface porosity is not a result of the tribo-tests but an inherent
characteristic of cast iron.
When the phosphonium cation was the only P source
(carboxylate and sulfonate ILs), the P signals were much weaker.
This is possibly because it is much easier for the phosphate anion
to lose alkyls and react with the metal surface or wear debris
particles to form iron phosphates (a major compound in tribo-
films formed by phosphate ILs19,21,30) compared with the
phosphonium cation that would require a complex decom-
position and/or oxidation before reacting with the metallic
components. On the worn surfaces lubricated by PAO-
[P66614][RSO3], the weak P and sulfur signals suggested a
possible competition between them in the formation of the tribo-
film. This may explain the inferior AW performance of
[P66614][RSO3]. The sodium peak on the surfaces lubricated by
PAO-[P66614][DEHP] was likely introduced by the impurity of
the IL.
Figure 3. Solubility parameters of ILs plotted in ascending order of
Hildebrand solubility parameter (δ). The experimental solubility is
marked in percentile.
As expected, owing to the ionic nature of ILs, the range of values
of δes of ILs (16 < δes < 22) was significantly higher than those of
the neutral alkanes. The Hildebrand solubility parameters (δ) of
ILs were in the same range as those reported in the literature (19
< δ < 32).40−42 For phosphonium-based ILs, the empirical
relationships between the experimental solubility and the
solubility parameter were δ ≤ 25 and δes ≤ 20. Those
relationships provide rules of thumb for designing oil-miscible
ILs.
3.3. Friction and Wear Results. Figure 4 summarizes the
friction and wear results for the PAO 4 and blends. The
coefficient of friction (COF) of the PAO 4 started at 0.08 and
transitioned to above 0.13 but quickly dropped below 0.11 and
then fluctuated at around 0.12. The fast transition at the
beginning implies a failure of the lubrication accompanied by
surface scuffing.43,44 A more significant scuffing process was
observed in the second test, consequently resulting in a higher
wear rate of 3.85 × 10−6 mm3/N·m (see Figure 4h). The PAO-
amine-phosphate caused no such initial friction transition; as a
result, the wear rate was 24% of that without the additive (see
Figure 4h).
Figure 4c,d shows the friction traces of the blends of PAO 4
and the two phosphonium-phosphate ILs. The COF of PAO-
[P4444][DEHP] started at 0.08 and slowly increased to 0.12. On
the other hand, after running-in, the COF of PAO-[P66614]-
[DEHP] stabilized around 0.08. The wear rates of both blends
were 8% and 4% of those without the additives, respectively (see
Figure 4h). [P4444][DEHP] appeared to be less effective than
3.4.2. Cross-Sectional TEM Nanostructural Examination
and EDS Elemental Mapping. We conducted cross-sectional
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Figure 4. Friction behavior of (a) PAO, (b) PAO-amine-phosphate, (c) PAO-[P4444][DEHP], (d) PAO-[P66614][DEHP], (e) PAO-[P66614][i-
C7H15COO], (f) PAO-[P66614][n-C17H35COO], and (g) PAO-[P66614][RSO3] blends; (h) wear rates of PAO and the blends. Numbers indicating the
sequence of repeat tests.
characterization of the tribo-film on the cast iron flat formed in
PAO-[P66614][i-C7H15COO] using FIB-aided TEM nanostruc-
tural examination and EDS elemental mapping. The thickness of
the tribo-film ranges from 230 to 370 nm in the imaging scope, as
shown in Figure 6. Although limited by the sampling size of the
FIB, the carboxyl IL resulted in a thicker tribo-film than
[P66614][DEHP].19,21 Figure 6b shows a magnified TEM image
of the tribo-film containing a large amount of nanoparticles
embedded in an amorphous matrix. These nanoparticles were
mostly rounded or elliptically shaped with sizes in the range of
1−5 nm. The elemental mapping (Figure 6c) indicates high
concentrations of oxygen and iron but just trace amounts of P,
agreeing well with the EDS analysis in Figure 5. We have
reported a higher P content in tribo-films formed under PAO-
[P66614][DEHP] through EDS elemental mapping on the cross-
section.19,21,30 However, results here suggest that a thick, wear-
protective tribo-film may still form even at low levels of P, which
warrants further investigation of the actual roles of the IL-
induced P-compounds.
The XPS spectra of Fe 2p, O 1s, C 1s, and P 2p on the surface
lubricated by PAO-[P66614][i-C7H15COO] are shown in Figure
7a−d. The deconvolution of Fe 2P3/2 presented four major
peaks. The metallic iron peak, Fe0, is at 706.6 eV. Two iron oxide
peaks were identified: Fe (II) at 709.0 eV and Fe (III) at 710.9
eV. A satellite peak from Fe (II) was found at 715.5 eV. The
oxygen O 1s peak was resolved into two separate peaks: iron
oxides and others possibly including PO, C−O, and CO
bonds. The oxygen in iron oxide was the major contribution to
the O 1s peak. Three types of carbon groups were identified: a
carbon group at 284.9 eV, an alcohol group at 286.4 eV, and a
carboxylate group at 289.4 eV. It has been found that the iron
carboxylate formed on the iron surface from tribo-tests under
both neat alkyl lubrication and carboxylate-added lubrication.
The carboxyl groups were the products of the degradation and
chemical oxidation of the alkyl lubricant. Iron carboxylate
complexes were found in tribo-films on the rubbing steel
surfaces45,46 and in wear debris.47 When carboxylic acid was
applied as an oil additive, it was thought that the iron carboxylate
formed as an insoluble salt on the steel rubbing surfaces.48
Chemisorption of carboxylate on the iron surface,49 was further
confirmed by computational simulation.50 The signal of P 2p
exhibited two peaks: 2p3/2 and 2p1/2. The underlying area of 2p1/2
is half that of 2p3/2. Decomposition of a phosphonium cation
3.4.3. XPS Surface Chemical Analysis. XPS was applied to the
surfaces of cast iron lubricated by PAO-[P66614][i-C7H15COO]
and PAO-[P66614][RSO3], respectively, to probe the binding
energies of the elements that compose the tribo-films.
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Figure 5. SEM surface morphology and EDS spectra of worn surfaces. Scale bar represents 200 μm. The numbers on the SEM images correspond to the
specific tests in Figure 3a−g.
usually generates phosphine oxide,51−53 which might be further
combined with iron ions. The incorporation of phosphine in the
tribo-film was not as easy as the incorporation of phosphate
(resulting from organophosphate), because of the ready binding
tendency between phosphate and iron. Figure 7e shows the XPS
depth-composition profile of the tribo-film enabled by ion
sputtering. Phosphorus started at around 3 at. % on the very top
surface and slowly dropped to below 2 at. %. As expected, the
concentration of metallic iron gradually increased whereas the
concentration of iron oxides decreased along with the depth of
sputtering.
that in the tribo-film for [P66614][i-C7H15COO] agreed well with
the EDS analysis in Figure 5.
Results here provide fundamental insights for the impact of
anion structure on ILs’ critical physiochemical and tribological
properties: (1) for the same molecular weight, anions containing
branched alkyls possess higher oil solubility compared to those
containing linear alkyls; (2) phosphorus in the tribo-film formed
by a phosphonium-phosphate IL was primarily contributed by
the phosphate anion, and (3) carboxylate, though free of P, S, N,
or halogen, can promote the formation of an effective antiwear
tribo-film.
Figure 8 shows the binding energies of the major elements and
depth-composition profile of the tribo-film on the worn surface
of the cast iron lubricated by PAO-[P66614][RSO3]. Again, the
major compounds were Fe(II) and Fe(III) oxides. The
underlying area of S 2p1/2 was half that of S 2p3/2. The peak
position of S 2p3/2 at 161.9 eV indicated metal sulfides, likely a
result of the sulfonate anion decomposition and subsequent
reactions with metallic elements. The depth-composition profile
of the tribo-film in Figure 8f shows that the concentrations of P
and sulfur are 1−1.5 at. % and ∼1 at. %, respectively. The reduced
P content in the tribo-film for [P66614][RSO3] compared with
4. CONCLUSIONS
Phosphonium-based ILs containing different anionsorgano-
phosphate, carboxylate, or sulfonatewere synthesized and
characterized for their candidacy as AW lubricant additives in a
PAO base oil. All the synthesized ILs showed high thermal
stability. The oil solubility of an IL depends on both the cation
and anion structures. Results suggested that using branched
alkyls is another way in addition to longer alkyls to improve an
IL’s solubility in a nonpolar hydrocarbon oil. Oil solubility
parameters by molecular dynamics calculation showed reason-
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Figure 6. Cross-sectional TEM images (a, b) and EDS elemental maps (c) of the worn cast iron surface lubricated by PAO-[P66614][i-C7H15COO]; (b)
corresponding to the red box in (a).
Figure 7. XPS core-level spectra of (a) Fe 2p, (b) O 1s, (c) C 1s, and (d) P 2p on the worn cast iron surfaces lubricated by PAO-[P66614][i-C7H15COO];
(e) depth profile.
able agreement with experimental observations. None of the ILs
was corrosive to cast iron or steel in the ambient environment.
Five ILs were blended into a PAO base oil and all demonstrated
more effective wear protection than a commercial amine-
phosphate AW additive. The ranking of effectiveness in wear
protection for the anions are organophosphate > carboxylate >
sulfonate. Tribo-film characterization revealed minimal phos-
phorus in a tribo-film that was produced by a phosphonium-
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Figure 8. XPS core-level spectra of (a) Fe 2p, (b) O 1s, (c) C 1s, (d) S 2p, and (e) P 2p on the worn cast iron surfaces lubricated by PAO-[P66614][RSO3];
(f) depth profiles.
carboxylate or phosphonium-sulfonate IL. This suggests that the
phosphorus compounds in the tribo-film formed by a
phosphonium-organophosphate IL is primarily contributed by
the phosphate anion. The tribo-film formed by a phosphonium-
carboxylate IL, though containing little P, is relatively thick
(∼300 nm) and effective in wear protection. This suggests that
the carboxylate anion, though free of P, S, N, or halogen, can
effectively promote a tribo-film formation. The revealed
correlations among the IL chemistry, physiochemical properties,
and wear protection performance are expected to guide further
research and development.
Barnhill of ORNL for exposure corrosion testing. Research
sponsored by the Vehicle Technologies Program, Office of
Energy Efficiency and Renewable Energy, U.S. Department of
Energy (DOE). Y.Z. was appointed to ORNL through the Oak
Ridge Associated Universities/Oak Ridge Institute for Science
and Engineering’s Advanced Short-Term Research Opportunity
program. The authors also thank the Laboratory for Molecular
Simulation at Texas A&M University for providing the access to
Materials Studio. This manuscript has been authored by UT-
Battelle, LLC, under Contract No. DE-AC05-00OR22725 with
the U.S. Department of Energy. The United States 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.
ASSOCIATED CONTENT
* Supporting Information
■
S
Summary of results in Table S1. Compositions of materials in
Table S2. This material is available free of charge via the Internet
at http://pubs.acs.org.
REFERENCES
■
AUTHOR INFORMATION
Corresponding Author
*Phone: (865) 576-9304. E-mail: qujn@ornl.gov.
Notes
■
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