Fatty acid ionic liquids as environmentally friendly lubricants for low friction and wear

Article abstract of DOI:10.1039/c5ra25001c

Vegetable oils are environmentally-friendly, sustainable and rich source of fatty acids, and have been used as lubricants since ancient times. The carboxylic group of fatty acids interacts with metal surface and forms the tribo-chemical thin film of low shear strength under the boundary lubrication, which reduces the friction and the wear. Herein, four fatty acids having variable chain length and unsaturated sites are selected as anionic precursors to prepare the tetrabutylammonium-fatty acid ionic liquids. The preparation of these ionic liquids is confirmed by FTIR and NMR (¹H and ¹³C) analyses. The chain length and degree of unsaturation in the fatty acid anions control the viscosity, melting temperature, crystallization temperature and latent heat of fatty acid ionic liquids. These ionic liquids as lubricants exhibited significantly lower friction (18-50%) compared to polyol ester lube base oil. Further, the degree of friction reduction is largely influenced by the structure of the constituent fatty acid anion. The oleate anion showed the best tribo-performance among all fatty acid ionic liquids being studied. The elemental mapping of worn surfaces revealed the formation of fatty acid ionic liquids constituted a tribo-chemical thin film. Being halogen-free and abundantly available sources of fatty acid precursors, these ionic liquids promise immense potential for tribological applications, where the friction and environment are of prime importance.

Full text of DOI:10.1039/c5ra25001c

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Fatty acid ionic liquids as environmentally friendly
lubricants for low friction and wear†
Cite this: RSC Adv., 2016, 6, 3462
ab
Rashi Gusainab and Om P. Khatri
*
Vegetable oils are environmentally-friendly, sustainable and rich source of fatty acids, and have been used
as lubricants since ancient times. The carboxylic group of fatty acids interacts with metal surface and forms
the tribo-chemical thin film of low shear strength under the boundary lubrication, which reduces the
friction and the wear. Herein, four fatty acids having variable chain length and unsaturated sites are
selected as anionic precursors to prepare the tetrabutylammonium-fatty acid ionic liquids. The
preparation of these ionic liquids is confirmed by FTIR and NMR (1H and 13C) analyses. The chain length
and degree of unsaturation in the fatty acid anions control the viscosity, melting temperature,
crystallization temperature and latent heat of fatty acid ionic liquids. These ionic liquids as lubricants
exhibited significantly lower friction (18–50%) compared to polyol ester lube base oil. Further, the degree
of friction reduction is largely influenced by the structure of the constituent fatty acid anion. The oleate
anion showed the best tribo-performance among all fatty acid ionic liquids being studied. The elemental
mapping of worn surfaces revealed the formation of fatty acid ionic liquids constituted a tribo-chemical
thin film. Being halogen-free and abundantly available sources of fatty acid precursors, these ionic liquids
promise immense potential for tribological applications, where the friction and environment are of prime
importance.
Received 25th November 2015
Accepted 17th December 2015
DOI: 10.1039/c5ra25001c
www.rsc.org/advances
controls the viscosity and thermal properties of ionic liquids but
also makes them promising lubricious materials for various
Introduction
solid surfaces. To date, most of tribological studies have been
made using the conventional ionic liquids, which exhibit
halogens in simple/complex form; particularly, tetra-
uoroborate (BF4À), hexauorophosphate (PF6À), tri-
uoromethansulfonate (OTfÀ), bis(triuoromethylsulfonyl)
imide (NTf2À), triuorotris(pentauoroethyl)phosphate (FAPÀ)
anions.6–13 These ionic liquids are considered to be hazardous
for the environment and facilitate the corrosive events.14,15 The
halogen-constituted hydrophilic ionic liquids are prone to be
hydrolyzed in the presence of water molecules and generate the
acid halides, which corrode the tribo-interfaces.16,17 Further, the
disposal of halogenated ionic liquids has been a great concern
and hold back their applications for solving the lubrication
problems. Thus, environmentally-friendly and halogen-free
ionic liquids are gaining large interest for the tribological
applications. Recently, Antzutkin et al. studied the tribo-phys-
ical properties of chelated orthoborate ionic liquids. These ionic
liquids as lubricants exhibited considerably lower friction and
wear under boundary lubrication as compared to fully formu-
lated engine oils.18 The chelated orthoborate ionic liquids as
additive to polyethylene glycol further revealed remarkable
friction-reducing and antiwear properties by forming a tribo-
chemical thin lm on the steel surfaces.19,20 Liu et al. have
prepared and evaluated the lubrication properties of amino
acids anion-based halogen-free ionic liquids. These ionic
Ionic liquids, sterically hindered salts of an organic cation and
organic/inorganic anion, exhibit good conductivity, non-vola-
tility, non-ammability, wide liquid-range, high chemical and
thermal stability, broad electrochemical window, inherent
polarity etc. and have been proved as promising candidates for
various applications such as catalysis, synthesis, drug-delivery,
energy devices, chemical extraction, nuclear fuel reprocessing
etc.1–4 The potential of ionic liquids for lubrication application
was introduced by Ye et al., demonstrating friction-reducing,
antiwear performance and load-carrying capacity of 1-alkyl-3-
hexylimidazolium tetrauoroborate ionic liquids.5 Over the last
decade, several studies have been made addressing the friction
and the wear characteristics of ionic liquids as lubricants,
lubricant additives and thin lms for various solid surfaces
including iron, aluminum, ceramic, and silicon.6–9 The inherent
polarity of ionic liquids is believed to facilitate their interaction
with tribo-interfaces and avoids direct contact between the
interacting surfaces. Furthermore, the molecular exibility of
ionic liquids by varying the constituent cation/anion not only
aChemical Science Division, CSIR-Indian Institute of Petroleum, Mohkampur,
Dehradun-248005, India. E-mail: opkhatri@iip.res.in; Fax: +91 135 2660200
bAcademy of Scientic and Innovative Research (AcSIR), New Delhi, India
† Electronic supplementary information (ESI) available: FTIR spectra, molecular
structure and 13C NMR spectra of ionic liquids. See DOI: 10.1039/c5ra25001c
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liquids as lubricants provided superior tribo-performance salt of each fatty acid is prepared by mixing the respective fatty
compared to synthetic lube base oil polyalphaolen and acid with an aqueous solution of equimolar amount of NaOH at
conventional ionic liquid (1-hexyl-3-methylimidazolium bis(tri- 60 ꢀC for 3 hours. In the subsequent step, TBA-Br (0.2 M) is
uoromethylsulfonyl)imide). The enhanced tribo-characteris- added to an aqueous solution of sodium salt of fatty acid (0.2 M)
tics were attributed to the formation of physically adsorbed and uninterruptedly stirred for overnight. The developed
layers of amino acids ionic liquids on the contact surfaces product as ionic liquid is extracted in dichloromethane (DCM)
under the tribo-stress.21 Although, these ionic liquids are and washed several times with distilled water to remove the
halogen-free, however, high cost of precursors and multi-step non-reactant content and impurities. In the nal step, DCM is
synthesis procedure limit their potential to the lubricant distilled under the reduced pressure and ionic liquid is dried in
applications.
the vacuum at 80 ꢀC for 48 hours. Total four ionic liquids
Vegetable oils, the triglycerides esters of various saturated (Fig. 1): tetrabutylammonium caproate (TBA-C6), tetrabuty-
and unsaturated fatty acids, are abundantly available in the lammonium caprylate (TBA-C8), tetrabutylammonium oleate
nature and are considered as renewable and alternative lubri- (TBA-C18-1) and tetrabutylammonium linoleate (TBA-C18-2) are
cants.22 These fatty acids and their esters exhibit inherent synthesized. Further, the preparation of each ionic liquid is
lubricious properties to form the tribo-chemical thin lm on the conrmed by their FTIR and NMR analyses.
metal surfaces under the tribo-stress. The tribo-chemical thin
lm of fatty acids provides low shearing and prevents direct
contact between the interacting surfaces.23,24 Taking the
Characterization of ionic liquids
advantage of (a) wonderful tribo-chemistry of fatty acids and (b)
their abundant availability from renewable resources (vegetable
Synthesis and chemical structure of each ionic liquid are
conrmed by FTIR and NMR (1H and, 13C) analyses. The FTIR
oils), fatty acid anions-based ionic liquids could be sustainable
spectra of ionic liquids are collected using Thermo-Nicolet 8700
Research spectrometer at a resolution of 4 cmÀ1. A uniform thin
and economic alternate to the conventional ionic liquids for the
tribological applications. The protic ammonium carboxylate
lm of each ionic liquid is prepared on the KBr pallet to collect
the FTIR spectrum. The NMR (1H and 13C) analyses of ionic
ionic liquids as lubricants for the copper surface exhibited
improved friction and wear properties compared to poly-
liquids are carried out using a Bruker Av III 500 MHz spec-
alphaolens and were attributed to the formation of stable
trometer. The NMR was operated at 500.13 and 125.78 MHz
boundary thin lm.25 Qu et al. have revealed that carboxylate
resonance frequencies for 1H and 13C NMR, respectively, using
ionic liquids as lubricant additives to polyalphaolens synthetic
a 5 mm broad band probe. The 30% (w/V) sample of each ionic
lube base oil provides superior tribological properties
compared to that of sulfonate anion-based ionic liquids. It was
attributed to formation of carboxylate anion-based antiwear
thin lm, which reduces both friction and wear.26 Herein, tet-
rabutylammonium ionic liquids having fatty acid anions (cap-
roate, caprylate, oleate and linoleate) with variable chain length
and unsaturation are synthesized and then probed their
thermal properties and viscosity. The tribo-characteristics of
these ionic liquids as lubricants are explored using steel tribo-
pairs and demonstrated the friction and wear characteristics as
function of chain length and degree of unsaturation in the
constituted fatty acid anions.
Experimental section
Materials
Tetrabutylammonium bromide (TBA-Br; 99%, Loba Chemie),
caproic acid (99.5%, Sigma Aldrich), caprylic acid (99%, Loba
Chemie), oleic acid (Extra pure, Loba Chemie) and linoleic acid
(97%, Alfa Aesar) are used without further purication for
synthesis of fatty acid ionic liquids. Pentaerythritol tetraoleate
(polyol), purchased from M/s. Mohini Organic Pvt. Ltd. India is
used as synthetic lube base oil.
Synthesis of fatty acid ionic liquids
All ionic liquids are synthesized by a facile and scalable
metathesis reaction between the TBA-Br (cationic precursor)
Fig. 1 Chemical structure of tetrabutylammonium (TBA) cation and
fatty acid anions: caproate (C6), caprylate (C8), oleate (C18-1) and
and sodium salts of fatty acids (anionic precursors). The sodium linoleate (C18-2).
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liquid is prepared in the CDCl3 for NMR analysis. The thermal
properties of fatty acid ionic liquids including melting and
crystallization points and latent heats during melting and
crystallization events are measured by using a Differential
Scanning Calorimeter (DSC Q200, TA Instruments). The heating
and cooling thermal cycles are performed in the range of À60 to
120 ꢀC at thermal rate of 5 ꢀC minÀ1 under the ow of nitrogen
(50 mL minÀ1). The indium is used to calibrate the temperature
reading. The latent heats of melting and crystallization are
determined by calculating the integrated area of concerned
peaks. Further, the thermal decomposition prole of each ionic
liquid is collected using Diamond (Perkin-Elmer) thermal
analyzer under the ow of nitrogen (50 mL minÀ1) at thermal
rate of 10 ꢀC minÀ1. The viscosity of all ionic liquids as a func-
tion of temperature are measured on Stabinger viscometer
(Anton Paar, model SVM3000).
Tribo-evaluation of ionic liquids
The tribological properties of these ionic liquids for steel tribo-
pair are measured using four-ball tribo-tester (Ducom, India). In
a typical tribo-test, one steel ball is rotated over three stationary
balls establishing the rotating point contact. The each test is
run for one hour duration with rotational speed of 1200 rpm
under the load of 392 N. The temperature of sample pot having
Fig. 2 FTIR spectra of (a) TBA-C8 and (b) TBA-C18-1 ionic liquids.
ꢀ
ionic liquid as lube sample is maintained as 75 C throughout
liquids is conrmed by characteristic vibrations owing to the
carboxylate group. The peaks in the range of 1762–1730, 1625–
1635, and 1475–1460 cmÀ1 are attributed to the C]O, COO
asymmetric and symmetric stretches, respectively and revealed
the presence of carboxylate group in the TBA-fatty acid ionic
liquids.
the test. The worn areas of three stationary balls are used to
examine the antiwear properties of fatty acid ionic liquids. The
polyol lube base oil is used as reference lubricant for
a comparative study. The morphological features of worn
surfaces of steel balls are probed by eld emission scanning
electron microscopy (FESEM, FEI Quanta 200F). In order to
understand the nature of tribo-chemical thin lm deposited on
the worn area, the elemental distribution on the worn surfaces
is examined by Energy Dispersive X-ray spectroscopy (EDX)
coupled to the FESEM.
The molecular structures of fatty acid ionic liquids are
1
further conrmed by H and 13C NMR analyses. The chemical
1
shis of H NMR are extracted in the Table S1 (ESI†) and the
corresponding protons in each ionic liquid are highlighted in
the Fig. S2 (ESI).† The protons associated to terminal methyl
groups in the TBA cation and the fatty acid anions exhibit
chemical shis in the range of 0.75–1.01 ppm and the corre-
sponding carbons (13C NMR, Fig. S3–S6, ESI†) appeared at
ꢁ13–14 ppm. The cationic centre in tetrabutylammonium
attracts electron cloud of adjacent methylene units, as a result
1H NMR of corresponding protons show downeld shis at
ꢁ3.24–3.37 ppm with 13C NMR shis at ꢁ58–59 ppm. Whereas
the methylene units of fatty acid anions adjacent to carboxylate
anionic centre experienced upeld shis of proton and carbon
NMR signals at ꢁ2.23–3.37 and ꢁ34 ppm, respectively. The
carbon in carboxylic group of fatty acid anions shows extreme
upeld shi because of two adjacent electronegative oxygen
atoms and experiences deshielded 13C NMR shi at ꢁ172–
177 ppm. The chemical shis of methylene units in the
carboxylate anion inuenced by presence of electronegative
atoms in the carboxylate group. Consequently, deshielding
effect gradually reduces with increasing distance of methylene
units from the carboxylate centre and exhibits chemical shis of
1H and 13C peaks at upeld position of ꢁ1.17–2.08 and ꢁ15–
30 ppm, respectively. The unsaturation site/s and in oleate
(TBA-C18-1) and linoleate (TBA-C18-1) anions inuences the shi
Results and discussion
Tetrabutylammonium ionic liquids having four variable fatty
acid anions viz. caproate (C6), caprylate (C8), oleate (C18-1) and
linoleate (C18-2) are synthesized by following a facile and scal-
able metathesis reaction. The preparation of each ionic liquid is
conrmed by FTIR and NMR analyses. Fig. 2 depicts FTIR
spectra of TBA-C8 and TBA-C18-1 fatty acid ionic liquids. The
FTIR spectra of other ionic liquids are shown as Fig. S1 (ESI).†
The broad and strong vibrational peaks in the range of 3000–
2800 cmÀ1 are attributed to asymmetric and symmetric C–H
stretches of methylene and methyl units in the alkyl chains of
ionic liquids. The butyl chains of TBA cation and variable alkyl/
alkenyl chains of anions are contributing for the C–H stretches.
Further, the vibrational modes in the range of 1480–1350 cmÀ1
are assigned to bending modes of methylene and methyl units
in the TBA-fatty acid ionic liquids. The presence of unsaturation
in TBA-C18-1 and TBA-C18-2 ionic liquids is deduced from
vibrational mode in the range of at 3008–3004 cmÀ1 owing to ]
C–H stretching. The presence of fatty acid anions in these ionic
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of proton and carbon NMR due to free movement of pi-electrons signicantly lower viscosity than the TBA-C18-1 ionic liquid
and provides deshielded shis. As a result, protons of double (Fig. 3) and was attributed to their packing orientation. Further,
bonded and neighbouring carbon exhibit chemical shis at the van der Waals interactions in fatty acid ionic liquids are
ꢁ5.22–5.42 and ꢁ2.61–2.81 ppm, respectively. While, corre- gradually reduces with increasing temperature (thermal
spondingly carbon NMR signal appeared at ꢁ129 and ꢁ29 ppm, energy), as a result viscosity of fatty acid ionic liquids decreased
respectively. These featured chemical shis associated to signicantly.
constituted cation and anions conrmed the preparation of
Thermal properties such as phase transition and latent heat
fatty acid ionic liquids.
ow during melting and crystallization of fatty acid ionic liquids
The electrostatic interaction between the constituted ions are deduced from their DSC patterns. Fig. 4 shows DSC patterns
and other interactive forces such as van der Waals interaction, of TBA-C8 and TBA-C6 as representative fatty acid ionic liquids.
inductive forces, hydrogen-linkage etc. control the physico- The TBA-C8 ionic liquid melts at 1.3 ꢀC during the heating cycle
chemical properties of ionic liquids.27 The presence of alkyl and crystallizes at À9.4 ꢀC during the cooling cycle. Table 1 lists
groups, unsaturation sites, aromatic and cyclic moieties along the melting point (Tm), crystallization point (Tc) and latent heats
with polarity and size of constituted ions determines both the of fatty acid ionic liquids. It is revealed that both Tm and Tc
electrostatic and the interactive forces in the ionic liquids. The increases with increasing the chain length of fatty acid ionic
variable alkyl chain length and degree of unsaturation in the liquid.28,29 This is attributed to higher van der Waals interaction
fatty acid anions primarily control the viscosity of TBA-fatty acid between the increasing numbers of methylene units in the fatty
ionic liquids. The van der Waals interaction between the acid anion, as a result TBA-C8 ionic liquid exhibit higher latent
methylene units of fatty acid anions increases the interactive heats of melting and crystallization compared to that of TBA-C6
forces, as a result viscosity of these ionic liquids increases with ionic liquid (Table 1). The oleate anion (C18-1) have signicantly
increasing the chain length. The TBA-C8 ionic liquid exhibited higher number of methylene units than caprylate (C8) anion,
higher viscosity than TBA-C6 and was attributed to increasing however, both Tm and Tc for TBA-C18-1 ionic liquid are
van der Waals interaction between the higher number of comparatively low. This was attributed to the presence of
methylene units in the caprylate anion of TBA-C8 ionic liquid unsaturation site in the oleate anion, which distort the orien-
(Fig. 3). Further gradual increase of methylene units leads to tation of methylene units and generates sterically hindered
gel–solid phase of resultant ionic liquids, which are not structure, as a result reduced van der Waals interaction
favourable as lubricant for the tribological application. The diminishes the packing structure of TBA-C18-1 ionic liquid.
introduction of unsaturation in the fatty acid anion leads to Further increasing number of unsaturation sites in the fatty
bending of alkyl chain at sp2 site and disturbed the packing acid anion reduces phase transition temperatures and latent
orientation by diminishing the van der Waals interaction heats as shown in the Table 1. The thermal decomposition
between the associated methylene units. As a result, TBA-C18-1 patterns of fatty acid ionic liquids are measured by thermo-
ionic liquid exhibits liquid phase structure. An increasing of
unsaturation sites further increases the steric constraint and
signicantly reduces the van der Waals interaction. Therefore,
TBA-C18-2 ionic liquid having two unsaturation sites exhibited
Fig. 3 Viscosity of fatty acid ionic liquids as a function of temperature. Fig. 4 Differential scanning calorimetric patterns of (a) TBA-C8 and (b)
Inset graph shows kinematic viscosity of these ionic liquids at 40 ꢀC.
TBA-C6 as representative fatty acid ionic liquids.
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Table 1 The structures and thermal properties of fatty acid ionic liquidsa
Melting feature
Crystalline feature
Ionic liquids
R, anion group
Tm, ꢀ
C
Heat ow, J gÀ1
Tc, ꢀC
Heat ow, J gÀ1
Td50,
ꢀC
TBA-C6
TBA-C8
TBA-C18-1
TBA-C18-2
CH3–(CH2)4–, caproate
CH3–(CH2)6–, caprylate
CH3–(CH2)7–CH]CH–(CH2)7–, oleate
CH3–(CH2)4–CH]CH–CH2–CH]CH–(CH2)7–, linoleate
À17.2
1.3
1.4
150.8
51.0
À27.0
À9.4
2.6
146.3
45.4
222.3
248.9
268.4
262.7
À15.3
À26.2
À50.0
À31.3
14.9
13.6
a
Td50: the temperature, where 50% of ionic liquid thermally decomposed.
gravimetric analysis. These ionic liquids are found to be ther- lube oil for a comparison purpose. Fig. 6 illustrated the coeffi-
mally stable up to 190 ꢀC and then gradually decomposed with cient of friction and wear scar diameter (WSD) of fatty acid ionic
increasing of temperature as shown in Fig. 5. The longer chain liquids and polyol ester under the load of 392 N at a rotating
length of constituted fatty acid anions provides higher thermal speed of 1200 rpm for one hour duration. The polyol lube base
stability (Table 1) to the resultant ionic liquids and it could be oil, a biodegradable pentaerythritol ester consisting of four
attributed to their organized structure driven by increasing van oleate chains, exhibited ꢁ0.086 coefficient of friction and
der Waals interaction. The observed trend is found to be
opposite of imidazolium ionic liquids, where the thermal
decomposition temperature reduces with increasing of
substituted alkyl chain length.30,31 The increasing number of
unsaturation site in fatty acid anions further reduce the
decomposition temperature and was attributed to poorer
packing orientation in TBA-C18-2 compared to TBA-C18-1 ionic
liquid.
Tribological properties of fatty acid ionic liquids in terms of
friction and wear are examined under the rolling contact
between the steel balls. The polyol ester was used as reference
Fig. 6 (a) Coefficient of friction and (b) wear scar diameter of fatty acid
Fig. 5 Thermal decomposition patterns of TBA-C6, TBA-C8, TBA-C18-1 ionic liquids and polyol ester (as a reference lube base oil) under the
and TBA-C18-2 ionic liquids at thermal rate of 10 ꢀC minÀ1 under the rolling contact between steel balls. Load: 392 N, speed: 1200 rpm,
nitrogen flow.
temperature: 75 ꢀC, test duration: 1 hour.
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ꢁ590 mm WSD. The fatty acid ionic liquids as lubricants showed unsaturation site in the ionic liquids, as a result TBA-C18-2 ionic
signicantly lower coefficient of friction (18–50%) compared to liquid showed comparatively higher friction and wear.
the polyol ester (Fig. 6a) under identical experimental condi-
Fig. 8 show the FESEM images of worn area on steel balls
tions. The TBA-C6, TBA-C8, TBA-C18-1 and TBA-C18-2 ionic liquids lubricated with polyol ester and fatty acid ionic liquids. The
exhibited 18, 20, 50 and 31% lower coefficient of friction polyol ester-lubricated steel ball showed larger wear scar with
compared to that of polyol ester. Recently, Espinosa et al. deep scratches. Ionic liquids with inherent polarity interact
demonstrated the lubrication properties of bis(2-hydroxy) with the tribo-interfaces and form a thin lm of low shear
ammonium oleate ionic liquid for copper tribo-pair.25 The bis(2- strength, which not only protect the contact surface against the
hydroxy)ammonium oleate ionic liquid as lubricant showed low wear at some extent but also reduces the friction signicantly.
friction for initial sliding distance and then stabilized to 0.14, As a result, steel balls lubricated with fatty acid ionic liquids
which is even higher than the coefficient of friction for non- showed comparatively lower wear scar (Fig. 8b–e). Furthermore,
polar synthetic lubricant PAO 6. The high coefficient of friction the elemental distribution images on the worn surfaces of steel
could be attributed to presence of hydroxyl groups and short balls lubricated with fatty acid ionic liquids are collected to
alkyl groups in the ammonium cation.
understand the nature of tribo-chemical thin lm. The carbon,
The polyol ester exhibited intrinsic lubricious nature owing oxygen and nitrogen are key constituent elements of fatty acid
to presence of oleate moiety in the ester form. In spite of that ionic liquids. Herein, nitrogen is selected as a characteristic
fatty acid ionic liquids showed lower friction than polyol ester. element and found to be uniformly distributed on the worn
The TBA-C6 and TBA-C8 exhibited coefficient of friction in the surfaces of steel balls (Fig. 9) lubricated with fatty acid ionic
same range, however, a larger deviation of friction was noted
with TBA-C8 ionic liquid. This effect is also reected in the wear
characteristic. Further increasing the chain length of fatty acid
anion provides gel-to-solid like structure, hence couldn't
execute the tribo-tests with longer alkyl chain fatty acid ionic
liquids. However, introduction of unsaturation site/s in the fatty
acid anions (oleate and linoleate in TBA-C18-1 and TBA-C18-2
ionic liquids, respectively) leads to steric constraint in the alkyl
chain and changes the physical state of ionic liquids from solid
to liquids by diminishing the van der Waals interaction between
the associated methylene units. Hence, TBA-C18-1 and TBA-C18-2
ionic liquids could be used as lubricants. The TBA-C18-1 ionic
liquid having oleate anion with one unsaturation site showed
lowest coefficient of friction among all ionic liquids (Fig. 7) and
was attributed to the formation of stable tribo-chemical thin
lm on the steel interfaces induced by tribo-stress. Both coef-
cient of friction and WSD further increases with increasing of
Fig. 7 The evolution of coefficient of friction with contact time for Fig. 8 FESEM images of the worn surfaces of steel balls lubricated
fatty acid ionic liquids and polyol ester (as a reference lube base oil) with (ai–ii) polyol ester, (bi–ii) TBA-C6, (ci–ii) TBA-C8 and (di–ii) TBA-C18-1
under the rolling contact between steel balls. Load: 392 N, speed: and (ei–ii) TBA-C18-2 ionic liquids. Load: 392 N, speed: 1200 rpm,
1200 rpm, temperature: 75 ꢀC, test duration: 1 hour.
temperature: 75 ꢀC, test duration: 1 hour.
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showing immense interest for fundamental understanding to
applications perspective. The fatty acids-enriched vegetable oils
and animal fats are well-established lubricants since ancient
time. The fatty acids interact with steel surface under the tribo-
stress and prepare the thin lm of low shear-strength, which
reduces the friction and protect the surface against the unde-
sirable wear. Herein, fatty acids as renewable precursors having
variable chain length and degree of unsaturation are selected to
prepare fatty acid ionic liquids. The syntheses of TBA-fatty acids
ionic liquids are conrmed by FTIR and NMR analyses. The
viscosity of these ionic liquids increases with increasing the
chain length of fatty acid anions and was attributed to
increasing van der Waals interaction between the associated
methylene units. The introduction of unsaturation sites in fatty
acid anions signicantly diminished the van der Waals inter-
action owing to steric constraint and reduced the viscosity. The
structural parameters of fatty acid anions further inuence the
thermal properties (Tm, Tc, Td50 and latent heats) of these ionic
liquids. The fatty acid ionic liquids as lubricants provide 18–
50% reduction in coefficient of friction compared to that of
polyol ester lube base oil. The inherent polar nature of fatty acid
anions facilitates their interaction promptly with steel surface
under the boundary lubrication and forms the stable tribo-
chemical thin lm of ionic liquids. The FESEM and nitrogen
mapping (Fig. 9) of worn surfaces further conrmed the depo-
sition of tribo-chemical thin lm of fatty acid ionic liquids,
which reduces both the friction and the wear. The TBA-C18-1 and
TBA-C18-2 ionic liquids exhibited comparatively better tribo-
performance than TBA-C6 and TBA-C8 ionic liquids and was
attributed to difference of their chain length. Further, degree of
unsaturation in fatty acid anion found to control the lubrication
characteristics. This study demonstrates that fatty acid ionic
liquids could be excellent alternative to the conventional halo-
genated ionic liquids. Further, signicantly low cost, environ-
mentally-friendly nature, renewable and sustainable resources
of fatty acid precursors promise immense potential of these
ionic liquids for various applications.
Fig. 9 FESEM images and correspondingly distribution of nitrogen on
the worn surfaces of steel balls lubricated with (ai–aii) TBA-C6, (bi–bii)
TBA-C8, (ci–cii) TBA-C18-1 and (di–dii) TBA-C18-2 ionic liquids. Nitrogen
(ammonium cation) is an integral part of TBA-fatty acid ionic liquids
and suggesting the participation of these ionic liquids in tribo-chem-
ical thin film formation.
Acknowledgements
liquids. These images suggested the formation of fatty acid ionic
liquids constituted tribo-chemical thin lm on the worn surfaces.
The polyol molecules is usually decomposed under the tribo-
stress and generates the oleate units, which interact with steel
surface under the tribo-stress and provides good tribological
properties. Herein, fatty acid ionic liquids consisting of fatty acid
anions are believed to promptly interact with steel tribo-inter-
faces and forms the good quality tribo-chemical thin lm
compare to that of with polyol ester. Therefore fatty acids ionic
liquids exhibited signicantly improved friction-reducing and
antiwear properties compared to that of polyol ester lube base oil.
We kindly acknowledge the Director CSIR-IIP for his kind
permission to publish these results. The authors are thankful to
CSIR, India for nancial support. The analytical supports from
Analytical Science Division, CSIR-IIP and IIT Chennai are kindly
acknowledged. R. Gusain thanks the CSIR, India for the
fellowship support.
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