Halogen-free pyrrolidinium bis(mandelato)borate ionic liquids: Some physicochemical properties and lubrication performance as additives to polyethylene glycol

Article abstract of DOI:10.1039/c4ra02551b

This work reports on the synthesis and physicochemical characterisation of novel halogen-free boron containing ionic liquids (hf-BILs) with dialkylpyrrolidinium cations [C_nC₁Pyrr]⁺, n = 4-14, and bis(mandelato)borate anion

Full text of DOI:10.1039/c4ra02551b

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Halogen-free pyrrolidinium bis(mandelato)borate
ionic liquids: some physicochemical properties and
lubrication performance as additives to
polyethylene glycol†
Cite this: RSC Adv., 2014, 4, 30617
Mamoun Taher,a Faiz Ullah Shah,a Andrei Filippov,ab Patrick de Baets,c
cd
ae
*
*
Sergei Glavatskih
and Oleg N. Antzutkin
This work reports on the synthesis and physicochemical characterisation of novel halogen-free boron
containing ionic liquids (hf-BILs) with dialkylpyrrolidinium cations [CnC1Pyrr]+,
4–14, and
n
¼
bis(mandelato)borate anion [BMB]ꢀ. All the synthesised compounds are liquids at room temperature. It
was found that the thermal properties and density of these hf-BILs are affected by the length of the
longest alkyl chain connected to the nitrogen atom in the pyrrolidinium cations. Differential scanning
calorimetry measurements revealed that glass transition temperatures of these ionic liquids are in the
temperature range from 218 to 241 K. Interestingly, the glass transition temperatures follow the “odd–
even” effect related to the number of carbons (n) in one of the alkyl chains of [CnC1Pyrr]+. It was also
found that hf-BILs' density decreases with an increase in the alkyl chain length of [CnC1Pyrr]+. It is
suggested that the “odd–even” effect is associated with the difference in packing and specific
interactions of cations and anions of this class of hf-BILs. Their lubricating properties, as 3 wt% additives
in polyethylene glycol (PEG), were evaluated in steel–steel contacts. PEG with hf-BILs additive provided
significantly lower wear and friction compared to the neat PEG and 5W40 engine oil. It was found that
shortening the length of the longest alkyl chain in the cations of [CnC1Pyrr][BMB] significantly reduces
frictional losses. Antiwear properties of [CnC1Pyrr][BMB] in PEG follow the same trend.
Received 24th March 2014
Accepted 20th June 2014
DOI: 10.1039/c4ra02551b
www.rsc.org/advances
lubricants capable to deliver required performance.3 Ionic
liquids (ILs) are attractive candidates to be used as the next
1 Introduction
generation of lubricants or additives to lubricant due to their
unique properties. ILs are salts composed of cations and
anions, which are liquids below 100 ꢁC. A great body of research
work done on ILs have demonstrated their promising functions
as new materials for an extensive number of applications.4
Many ILs have negligible vapour pressure,5 are non-ammable,6
possess attractive electrochemical properties,7 high conductiv-
ities,8 high affinities to metal ions,9 high thermal stabilities,10
and in particular provide outstanding tribological performance
as neat lubricants and as additives to lubricants.11,12
The physiochemical properties of ILs are affected by the
selective combination of cations and anions. Designability of
ILs makes them attractive candidates for a wide range of
applications.13,14 Task specic ionic liquids (TSILs) with
designed physicochemical characteristics are a hot topic in the
eld of ILs research. TSILs are mainly designed by incorpora-
tion of various functional groups into cations, although a few
examples involving functionalised anions are also known.14
Aer the original work of Rogers and coworkers,6 TSILs' appli-
cations in electrochemistry,7 catalysis,7 synthesis,15 separation
analysis,16 lubrication,1,17 and functional materials18 have been
In all industrial applications, there is a rapidly growing trend
towards increased machine service life, durability, reduced
emissions and minimized energy consumption.1 A considerable
amount of energy is wasted to overcome friction in different
machines and mechanisms. In addition, wear of their compo-
nents negatively affects functionality and ultimately forces their
replacement leading to substantial economic losses.2 Avail-
ability of the efficient lubrication technology is the key condi-
tion to meet the challenges.
Consequently, considerable research efforts have being
made to develop novel lubrication technologies and formulate
a
˚
˚
Chemistry of Interfaces, Lulea University of Technology, SE-97187, Lulea, Sweden
bKazan Federal University, 420008 Kazan, Russian Federation
cDepartment of Mechanical Construction and Production, Ghent University, B-9000
Ghent, Belgium
dSystem and Component Design, KTH Royal Institute of Technology, SE-10044
Stockholm, Sweden
eDepartment of Physics, University of Warwick, CV4 7AL, Coventry, UK. E-mail: segla@
kth.se; olan@ltu.se
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02551b.
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1
demonstrated. However, the fundamental understanding of the yields, liquid state H, 13C and 11B NMR data for all other hf-
relationship between TSILs' molecular structure and their BILs are included in the ESI.†
physicochemical properties has not yet been achieved.19
2.2.1 Synthesis of N-butyl-N-methylpyrrolidinium bis-
Halogen-based ILs are the most commonly studied (mandelato)borate, [C4C1Pyrr][BMB]
compounds that are also considered for different applications.
Step 1. N-Butyl-N-methylpyrrolidinium chloride was prepared
However, most of halogen based ILs are sensitive to moisture by admixing 1-chlorobutane (9.26 g, 100 mmol) dropwise to a
and may hydrolyse in the presence of water giving rise to ask with N-methylpyrrolidine (8.51 g, 100 mmol) dissolved in
hydrogen uoride (HF) among other corrosive products. The 15 mL toluene. The reaction mixture was reuxed for 24 hours
hydrolytic products may ultimately damage the system in which under nitrogen atmosphere at 388 K. Toluene was rotary evap-
these ILs are intended to be used. Therefore, functional and orated and a white powder of N-butyl-N-methylpyrrolidinium
environmental regulations place restrictions on halogen con- chloride was obtained in 86% yield.
taining ILs.20 Consequently, there is a strong drive towards the
development of halogen-free ILs.3,5
Step 2. Sodium bis(mandelato)borate was prepared by adding
mandelic acid (15.21 g, 100 mmol) to an aqueous solution of
Chelated orthoborate anions may form ILs when combined sodium carbonate (2.65 g, 25 mmol) and boric acid (3.09 g,
with different cations.3,21–23 However, chelated orthoborate 50 mmol) dissolved in 100 mL distilled water. The solution was
based ILs have signicantly higher viscosities compared to ILs stirred for two hours at 333 K. The reaction mixture was cooled
with the same cations but halogen-containing anions.22,24 Such to room temperature and (8.88 g, 50 mmol) of N-butyl-N-
ILs can be used as efficient additives to lubricating oils.
methylpyrrolidinium chloride was added. An organic layer
We have recently reported on the novel classes of halogen- formed during stirring for 3 hours at room temperature was
free (hf) chelated orthoborate ILs (hf-BILs) with outstanding extracted with 100 mL of CH2Cl2. The organic layer was
tribochemical properties. hf-BILs with tetraalkylphosphonium washed three times with distilled water to remove traces of
cations exhibited signicantly better antiwear and friction sodium chloride. The CH2Cl2 was rotary evaporated under
reducing performance in boundary lubrication of the steel– reduced pressure. A viscous colourless IL, N-butyl-N-methyl-
aluminium contacts compared to 15W40 engine oil.1,3 Other pyrrolidinium bis(mandelato)borate, [C4C1Pyrr][BMB], was
potential applications, for example, cholinium bis(salicylato) obtained in 71% yield (32.2 g). Before characterisation and tri-
borate, can be used for extraction of lanthanum ions from botests, [C4C1Pyrr][BMB] and other synthesised hf-BILs were
aqueous solutions.21
dried under vacuum (<10ꢀ3 mbar) during 5 hours at 343 K.
Anal. calcd for C25H32O6BN (MW 453.33 g molꢀ1) C, 66.23; H,
The objective of this work is to further explore properties of
novel hf-BILs. This time we focus on orthoborate based hf-BILs 7.11; N, 3.09. Found: C, 63.45; H, 7.2; N, 3.4. Only insignicant
with pyrrolidinium cations. hf-BILs composed of the bis(man- impurities were detected in [C4C1Pyrr][BMB] by 13C NMR, see
delato)borate anion [BMB]ꢀ and pyrrolidinium [CnC1Pyrr]+ Fig. SI-2.†
cations with alkyl groups of different chain length, n ¼ 4–14,
Water content: 0.082 ꢂ 0.002 wt%. Chlorine content: 0.0291
were synthesised. Their physicochemical and tribological prop- wt%. Sodium content: 0.0198 wt%.
erties were studied using a range of experimental techniques.
1H NMR (359.92 MHz, CDCl3): 7.62–7.53 (m, 4H, C6H5), 7.33–
7.27 (m, 4H, C6H5), 7.26–7.22 (m, 2H, C6H5), 5.27 (d, 1H, 4JHH
¼
4
1.2 Hz, C6H5–CH), 5.23 (d, 1H, JHH ¼ 3.3 Hz, C6H5–CH), 3.03–
2.96 (m, 4H, N–(CH2–CH2–)2 pyrrolidinium ring), 2.98–2.83 (m,
2H, N–CH2–CH2–, alkyl chain), 2.52 (s, 3H, N–CH3), 1.83–1.76
(m, 4H, N–(CH2–CH2)2, pyrrolidinium ring), 1.39–1.31 (m, 2H,
N–CH2–CH2–CH2–, alkyl chain), 1.30–1.07 (m, 2H, N–CH2–CH2–
CH2–, alkyl chain), 0.82 (t, 3H, 3JHH ¼ 7.5 Hz, –CH2–CH3) ppm.
13C NMR (90.506 MHz, CDCl3): 177.90 (s, 2C, pC]O), 139.90
(s, 2C, –CH(O)–), 129.03–125.22 (12C, aromatic groups), 63.96 (t,
2 Experimental
2.1 Chemicals
Boric acid (Merck, 99.8% purity), sodium hydroxide (Merck,
99% purity), mandelic acid (Aldrich, 99% purity), N-methyl-
pyrrolidine (Fluka, >99% purity), 1-chlorobutane (Sigma
Aldrich, >99% purity), 1-bromopentane (Aldrich, >99% purity),
1-bromohexane (Aldrich, >98% purity), 1-bromoheptane
(Aldrich, >99% purity), 1-chlorooctane (Aldrich, >99% purity), 1-
bromodecane (Aldrich, >98% purity), 1-chlorododecane
(Aldrich, >97% purity), 1-bromotetradecane (Aldrich, >97%
purity), dichloromethane (Fluka, >99.7% purity), toluene
(Fluka, >99.7% purity), diethyl ether (Sigma Aldrich, >99%
purity) were used as received.
1
2C, JCH ¼ 76.6 Hz, N–CH2–, pyrrolidinium ring), 53.52 (t, 1C,
1
1JCH ¼ 88.7 Hz, N–CH2– alkyl chain), 47.73 (q, 1C, JCH ¼ 68.2
Hz, N–CH3), 26.67–18.03 (4C, methylene carbons of the pyrro-
lidinium ring and methylene carbons of the alkyl chain), 13.41
1
(q, 1C, JCH ¼ 54.4 Hz, –CH3) ppm.
11B NMR (115.5 MHz, CDCl3): 11.07 ppm.
2.2 Synthesis
2.3 Physicochemical characterisation
hf-BILs were synthesised using a slightly modied method
2.3.1 Spectroscopic characterisation. Liquid state NMR
reported previously.22,23 As an example, the synthesis and the spectra of hf-BILs were recorded at 5–10 wt% in CDCl3 on an
chemical characterisation of N-butyl-N-methylpyrrolidinium Agilent/Varian/Chemagnetics InnityPlus CMX-360 (B ¼ 8.46 T)
bis(mandelato)borate, [C4C1Pyrr][BMB], is given below, while spectrometer using a 10 mm double-resonance probe for
the synthetic procedures and characterisation details including liquids tuned to resonance frequencies of 1H (359.92 MHz), 13
C
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(90.51 MHz), or 11B (115.48 MHz). 1H and 13C spectra were
The weight loss of hf-BILs as a function of temperature was
internally referenced to 0 ppm resonance peak of TMS and 77.2 determined by thermogravimetric analysis (TGA) using a
resonance peak of CDCl3, respectively. External reference Thermal Advantage TGA Q5000IR (TA Instrument) module. The
(0 ppm) was used in the 11B (Et2O$BF3).
samples (5–10 mg) were measured in platinum pans (100 ml)
2.3.2 Thermal analysis. Thermal behaviour of hf-BILs was heated from 298 to 673 K at a heating rate of 10 K minꢀ1 under
studied using a Thermal Advantage DSC Q1000 (TA Instrument) nitrogen purge. The data was analysed using Universal Analysis
equipped with a cooling system. Indium was used to calibrate 2000 V4.3 soware (TA Instruments).
temperature and the enthalpy of phase transitions. The stan-
2.3.3 Density. An Anton-Paar DMA 4100M density meter
dard/conventional DSC method was used to determine the glass was used to measure densities of BILs in a temperature range
transition temperature (Tg) and the specic heat capacity of the from 293 to 353 K. The density meter was calibrated before the
glass–liquid transition (Cp) of ILs, as exemplied in Fig. SI-27.† measurements using ultra high pure water with a density of
A sample of about 1 mg was accurately weighed and placed into 0.998 g cmꢀ3 at 293 K.
a non-hermetic aluminium pan and crimped. All samples were
scanned in a temperature range from 200 to 360 K using a cool/ 2.4 Lubrication performance
heat cycle at a rate of 10 K minꢀ1 under continuous nitrogen
The lubrication performance of hf-BILs was evaluated using a
ball-on-disc tribometer. The hf-BILs were mixed with technical
grade polyethylene glycol (PEG) at a concentration of 3.0 wt%
using an analytical balance. The mixtures were ultrasonicated
purge (50 mL minꢀ1).
Tg for all these ionic liquids were determined at the onset of
the glass–liquid transition using Universal Analysis 2000 V4.3
soware (TA Instruments). Cp of the glass–liquid transition is
for 30 minutes at 353 K and clear transparent solutions were
calculated at the central peak transition point. All measure-
obtained. [C14C1Pyrr][BMB] is not soluble in PEG. PEG was
ments were performed in duplicate.
characterised by mass-spectrometry (API 3200 LC/MS System, AB
Scheme 1 Synthesis of N-alkyl-N-methylpyrrolidinium bis(mandelato)borate hf-BILs.
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Sciex, MA, USA). The MS spectrum of PEG demonstrates bimodal
distribution of molecular masses centred at 150 and 850 Da
(see Fig. SI-25†). The PEG used in this study has a viscosity value
134 mPas and a density value 1.030 g cmꢀ3 at 313 K.
Friction and wear tests were performed at room temperature
under a load of 15 N and a speed of 0.2 m sꢀ1. The test matrix
was designed in such a way to investigate the variation of wear
as a function of sliding distance. To achieve this, separate runs
were performed for 100, 200, 300 and 2000 m sliding distances.
It means that the 100 m runs were repeated four times, the
200 m runs were repeated three times, etc. The tribo-contact was
achieved by pressing the upper stationary 100Cr6 steel ball
(diameter 6 mm) against the rotating 100Cr6 steel disk (diam-
eter 30 mm). The wear coefficients for the steel discs were
measured with Surfascan SOMICRONIC prolometer using the
following conditions: tip radius 2 mm; tip angle 90ꢁ; cut-off value
(l) 2.5 mm; and lter 50% Gauss.
Fig. 1 DSC thermograms of selected N-alkyl-N-methylpyrrolidinium
bis(mandelato)borate hf-BILs.
Table 1 Thermal properties of N-alkyl-N-methylpyrrolidinium bis(-
mandelato)borate hf-BILs
3 Results and discussion
hf-BIL
Tg (K)
Cp (J kgꢀ1 Kꢀ1
)
3.1 Synthesis and characterisation
The synthetic procedure for [C4C1Pyrr][BMB] is described in
detail in the Experimental section. As illustrated in Scheme 1,
the synthesis of hf-BILs started with a N-methylpyrrolidinium
and the corresponding alkyl halides in a 1 : 1 molar ratio. Aer
reuxing for 24 hours at 388 K, the reaction mixture was cooled
down to room temperature and a solid product of N-butyl-N-
methylpyrrolidinium chloride was obtained.
[C2C1Pyrr][BMB]
[C4C1Pyrr][BMB]
[C5C1Pyrr][BMB]
[C6C1Pyrr][BMB]
[C7C1Pyrr][BMB]
[C8C1Pyrr][BMB]
[C10C1Pyrr][BMB]
[C12C1Pyrr][BMB]
[C14C1Pyrr][BMB]
271a
412.2a
225.6 ꢂ 0.1
223.3 ꢂ 0.5
225.8 ꢂ 0.5
218.2 ꢂ 0.3
225.6 ꢂ 0.5
235.6 ꢂ 0.6
235.5 ꢂ 0.4
240.9 ꢂ 0.8
412.2 ꢂ 0.3
442.6 ꢂ 0.7
519.6 ꢂ 0.5
302.7 ꢂ 0.6
288.8 ꢂ 0.8
419.5 ꢂ 0.7
266.5 ꢂ 0.3
462.4 ꢂ 0.5
In the second step, the sodium salt of chelated orthoborate
was obtained by mixing mandelic acid, boric acid and sodium
carbonate in a 2 : 1 : 1 molar ratio dissolved in distilled water.
The reaction mixture was cooled down to room temperature
aer heating for 2 hours at about 333 K. Chloride salt of N-butyl-
N-methylpyrrolidinium prepared in the rst step was added to
the aqueous solution of chelated orthoborate salt. The imme-
diately formed reaction product, an organic layer, was extracted
with dichloromethane (DCM) and washed three times with
distilled water to remove any traces of the by-product, sodium
halide. DCM and residual water were rotary evaporated from the
nal products, dialkylpyrrolidinium bis(mandelato)borates ILs.
Traces of water and DCM were then removed under vacuum
(<10ꢀ3 mbar) during 5 hours at 343 K. Water content in hf-BILs
was found to be within 0.035–0.109 wt% (see Table SI-1 in ESI†).
Sodium content in hf-BILs was found to be within 0.0068–
0.0254 wt% while the halogen content was found to be within
0.0244–1.0710 wt% (see Table SI-1 in ESI†). In the formulated
PEG based lubricants the concentration of these elements is 33
times lower.
a
Ref. 21.
and specic heat capacities of the glass–liquids transition (Cp)
are tabulated in Table 1. All the reported pyrrolidinium bis-
(mandelato)borate hf-BILs are viscous liquids at room temper-
ature with glass transition temperatures between 218 and 241 K.
The chemical structure and purity of synthesised hf-BILs were
characterised by multinuclear (1H, 13C, and 11B) NMR spectros-
copy and the “CHN” elemental analysis. The data obtained
conrmed the structure and purity of all synthesised ILs.
3.2 Thermal analysis
Fig. 2 “Odd–even” effect of the longest alkyl chain length in N-alkyl-
N-methylpyrrolidinium bis(mandelato)borate on the glass transition
DSC curves of selected pyrrolidinium bis(mandelato)borate hf-
BILs are shown in Fig. 1. The glass transition temperatures (Tg) temperature (Tg).
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the longest alkyl chain. A similar “odd–even” effect has been
reported earlier for melting points, solid–solid phase transition,
entropy and enthalpy of vaporisation, viscosity and density for
other types of ILs.10,25 In general, thermal behaviour of ILs is
affected by the structure and nature of both cations and anions,
i.e. by intermolecular forces, molecular symmetry and confor-
mational degrees of freedom of the ions.4,26,27
The thermal stabilities of [C4C1Pyrr][BMB], [C8C1Pyrr][BMB]
and [C14C1Pyrr][BMB] were assessed by the thermogravimetric
analysis. Fig. 3 reveals that [C14C1Pyrr][BMB] is less thermally
stable compared to [C4C1Pyrr][BMB] and [C8C1Pyrr][BMB].
Decomposition of [C14C1Pyrr][BMB] starts just above 370 K,
while the onset of abrupt decomposition is above ca. 485 K. In
the case of [C4C1Pyrr][BMB], weight loss is observed around
480 K. It decomposes through two steps; the onset temperature
of the rst step of decomposition is ca. 480 K and the onset of
the second step is ca. 630 K. The behaviour of [C8C1Pyrr][BMB]
is similar to the behaviour of [C4C1Pyrr][BMB]. No weight loss is
observed until ca. 480 K. Stepwise decomposition behaviour is
revealed at onset temperatures of ca. 480 and 620 K.
Fig. 3 Thermogravimetric analysis of N-alkyl-N-methylpyrrolidinium
bis(mandelato)borate hf-BILs.
The DSC curves show no evidence of melting points. It was
found that Tg and Cp of the hf-BILs are signicantly affected by
the length of one of the alkyl chains attached to the pyrrolidi-
nium ring. A comparative analysis of the thermal behaviour
reveals that [C7C1Pyrr][BMB] has the lowest Tg (218 K), while
[C14C1Pyrr][BMB] shows the highest Tg (241 K).
An IL containing bis(mandelato)borate ([BMB]ꢀ) with tri-
hexyltetradecylphosphonium cation is more thermally stable
than N-alkyl-N-methylpyrrolidinium bis(mandelato)borate ILs.3
Tg changes irregularly with the number of carbon atoms
n ¼ 4–14 in the longest alkyl chain (see Table 1). Tg increases in
the following order [C7C1Pyrr][BMB] < [C5C1Pyrr][BMB] <
[C8C1Pyrr][BMB]
< [C4C1Pyrr][BMB] < [C6C1Pyrr][BMB] <
[C12C1Pyrr][BMB] < [C10C1Pyrr][BMB] < [C14C1Pyrr][BMB]. Cp
changes irregularly with the number of carbon atoms n ¼ 4–14,
increasing in the following order [C6C1Pyrr][BMB] < [C14C1Pyrr]-
[BMB] < [C5C1Pyrr][BMB] < [C10C1Pyrr][BMB] < [C4C1Pyrr][BMB]
< [C7C1Pyrr][BMB] < [C8C1Pyrr][BMB] < [C12C1Pyrr][BMB].
The glass transition temperatures of these hf-BILs if
compared exhibit a distinct “odd–even” effect (see Fig. 2).
Higher Tg were detected for ILs with even number of carbons in
Fig.
5 Effect of the alkyl chain length of N-alkyl-N-methyl-
pyrrolidinium bis(mandelato)borate on densities of BILs at 293 K.
Table 2 Densities of N-alkyl-N-methylpyrrolidinium bis(mandelato)
borate hf-BILs
Density equation: d ¼ b ꢀ aT, g cmꢀ3
hf-BIL
a ꢃ 10ꢀ4
b
R2
[C4C1Pyrr][BMB]
[C5C1Pyrr][BMB]
[C6C1Pyrr][BMB]
[C7C1Pyrr][BMB]
[C8C1Pyrr][BMB]
[C10C1Pyrr][BMB]
[C12C1Pyrr][BMB]
[C14C1Pyrr][BMB]
7.0
7.0
7.0
7.0
7.0
7.0
7.0
6.0
1.3958
1.4017
1.3847
1.3801
1.3624
1.3520
1.3419
1.3195
0.9994
0.9996
0.9995
0.9997
0.9997
0.9989
0.9994
0.9989
Fig. 4 Density of N-alkyl-N-methylpyrrolidinium bis(mandelato)borate
hf-BILs as a function of temperature. From top to bottom: [C4C1Pyrr]-
[BMB], [C5C1Pyrr][BMB], [C6C1Pyrr][BMB], [C7C1Pyrr][BMB], [C8C1Pyrr]-
[BMB], [C10C1Pyrr][BMB], [C12C1Pyrr][BMB] and [C14C1Pyrr][BMB].
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This demonstrates the important role of cations for thermal
stability of orthoborate based ILs.
3.3 Density
Fig. 4 shows a linear decrease in densities of the hf-BILs with
temperature. Density also reduces gradually if the length of the
alkyl chain connected to the pyrrolidinium ring is increased
from 4 to 14 (see Fig. 5).
Density of the hf-BILs decreased in the following order:
[C4C1Pyrr][BMB]
[C7C1Pyrr][BMB]
>
>
[C5C1Pyrr][BMB]
[C8C1Pyrr][BMB]
>
>
[C6C1Pyrr][BMB]
[C10C1Pyrr][BMB]
>
>
[C12C1Pyrr][BMB] > [C14C1Pyrr][BMB]. Interestingly, the “odd–
even” effect for densities of these hf-BILs is very small (compare
data for n ¼ 4–8 in Fig. 5).
Fig. 8 The effect of the alkyl chain length of N-alkyl-N-methyl-
pyrrolidinium bis(mandelato)borate on the wear depth and friction
coefficient. The tests were performed at room temperature with 3 wt%
of a hf-BIL in PEG at 15 N load and sliding speed of 0.2 m sꢀ1 for
2000 m sliding distance.
The decrease in densities of the hf-BILs with an increase in
temperature from 293 to 353 K follows a linear regression (see
Table 2). This behaviour can be explained by a reduction in van
der Waals interactions leading to a less efficient packing of
ions.3 Parameters characterising density of these hf-BILs as a
function of temperature are tabulated in Table 2. The linear
reduction in density with temperature is typical for many ILs.3,28
3.4 Lubrication performance
Fig. 6 shows friction reducing performance of the [CnC1Pyrr]-
[BMB] hf-BILs as 3 wt% additives in PEG in comparison with the
neat PEG and 5W40 oil in the steel–steel contacts. The data
reveals that [CnC1Pyrr][BMB]/PEG mixtures provide signicantly
lower friction than the neat PEG and 5W40 oil. Friction coeffi-
cient is also more stable compared to 5W40 oil. The “odd–even”
effect of the alkyl chain length in the cation is not visible in the
friction traces. But there is a clear inuence of the alkyl chain
length on the friction level.
The effect of sliding distance on the wear rate was studied by
performing the tests at different sliding distances: 100, 200, 300
and 2000 m. The friction curves and wear data for 100, 200 and
300 sliding distances are shown in Fig. SI-28 to SI-31.† Initially
all compounds showed similar wear rates but as the sliding
distance increased the wear rates for the neat PEG and 5W40 oil
signicantly increased. Adding a hf-BIL to PEG considerably
reduced the wear rate increase indicating formation of stable
tribolayers on the contacting surfaces. The wear coefficients,
presented in Fig. 7, were calculated for the wear scars on the
discs for the sliding distance of 2000 m. The discs lubricated
with 5W40 oil or neat PEG oil had the largest wear. The wear
coefficients for the discs lubricated with PEG containing 3 wt%
of [CnC1Pyrr][BMB] are considerably lower.
Fig. 6 Variation of the friction coefficient with the sliding distance in
the steel ball on steel disk contact lubricated by 5W40 oil, neat PEG or
PEG with 3 wt% [CnC1Pyrr][BMB]. Room temperature (298 K), 15 N
load, 0.2 m sꢀ1 sliding speed. Friction curves from top to bottom:
5W40, neat PEG, [C12C1Pyrr][BMB]/PEG, [C8C1Pyrr][BMB]/PEG,
[C7C1Pyrr][BMB]/PEG, [C6C1Pyrr][BMB]/PEG, [C5C1Pyrr][BMB]/PEG and
[C4C1Pyrr][BMB]/PEG.
A comparative analysis of the friction and wear data reveals
that pyrrolidinium cations with the shorter alkyl chains provide
better antiwear and friction reducing performance compared to
cations with the longer alkyl chains (see Fig. 7 and 8). This result
can be explained by the higher mobility of hf-BILs with the
shorter alkyl chains in the base uid. As a result, availability of
hf-BIL at the rubbed surfaces is higher.
Fig. 7 Wear coefficients for the discs lubricated by 5W40 oil, neat PEG
or PEG containing 3 wt% of a hf-BIL. Tests were performed at room
temperature, steel–steel contacts, 15 N load, 0.2 m sꢀ1 sliding speed.
30622 | RSC Adv., 2014, 4, 30617–30623
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Paper
Mechanism of lubrication by ILs can be thought as a
combination of the following processes: (i) ILs tend to adsorb
on rubbed surfaces due to their ionic nature. This process is
encouraged by the electron deciency of the interacting steel
surfaces.13,17 Adsorption leads to formation of easy to shear
overlayers of anions and cations. (ii) ILs can break down and
chemically react with the rubbed surfaces forming protective
overlayers usually called “tribolms”.
As noted above, tribological performance of an IL in the base
uid is affected by its interactions with the base uid. Two
categories of interactions can be distinguished: (i) interactions
between ions in IL and (ii) interactions between the ions of IL
and the molecules of the base oil. In general, less interaction
between an IL and the base uid means higher availability of
RSC Advances
3 F. U. Shah, S. Glavatskih, D. R. MacFarlane, A. Somers,
M. Forsyth and O. N. Antzutkin, Phys. Chem. Chem. Phys.,
2011, 13, 12865–12873.
4 J. Ranke, S. Stolte, R. Stormann, J. Arning and B. Jastorff,
Chem. Rev., 2007, 107, 2183–2206.
5 R. P. Swatloski, J. D. Holbrey and R. D. Rogers, Green Chem.,
2003, 5, 361–363.
6 A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton,
S. Sheff, A. Wierzbicki, J. H. Davis Jr and R. D. Rogers,
Chem. Commun., 2001, 1, 135–136.
7 T. Welton, Coord. Chem. Rev., 2004, 248, 2459–2477.
8 M. Antonietti, D. Kuang, B. Smarsly and Y. Zhou, Angew.
Chem., Int. Ed., 2004, 43, 4988–4992.
9 X. Sun, H. Luo and S. Dai, Chem. Rev., 2011, 112, 2100–2128.
the IL additive at the contacting surfaces, and thus better fric- 10 W. Zheng, A. Mohammed, L. G. Hines Jr, D. Xiao,
tion and wear performance.13,29
O. J. Martinez, R. A. Bartsch, S. L. Simon, O. Russina,
A. Triolo and E. L. Quitevis, J. Phys. Chem. B, 2011, 115,
6572–6584.
4 Conclusion
11 X. Liu, F. Zhou, Y. Liang and W. Liu, Wear, 2006, 261, 1174–
Halogen-free boron-based ILs, namely pyrrolidinium bis(man-
1179.
delato)borates with different alkyl chain lengths in the pyrroli- 12 H. Arora and P. Cann, Tribol. Int., 2010, 43, 1908–1916.
dinium cation, were successfully synthesised and characterised. 13 A. E. Somers, B. Khemchandani, P. C. Howlett, J. Sun,
The synthesised ILs are liquids at room temperature with glass
transition temperatures ranging from 218 to 241 K. The length
D. R. MacFarlane and M. Forsyth, ACS Appl. Mater.
Interfaces, 2013, 5, 11544–11553.
of the longest alkyl chain bonded to the nitrogen atom of the 14 S. Fang, Z. Zhang, Y. Jin, L. Yang, S. Hirano, K. Tachibana
pyrrolidinium cation has a signicant inuence on the physi-
and S. Katayama, J. Power Sources, 2011, 196, 5637–5644.
cochemical properties of this class of hf-BILs. Glass transition 15 T. Welton, Chem. Rev., 1999, 99, 2071–2084.
temperatures varied as a function of the number of carbon 16 A. Berthod, M. Ruiz-Angel and S. Carda-Broch, J. Chromatogr.
atoms in the longest alkyl chain in pyrrolidinium cations. An
“odd–even” effect on Tg of these hf-BILs was discovered. Lower 17 A. E. Somers, P. C. Howlett, D. R. MacFarlane and
Tg values for n ¼ 5 and 7 and higher Tg values for n ¼ 4, 6, 8, 10, M. Forsyth, Lubricants, 2013, 1, 3–21.
12 and 14 were observed. Density of hf-BILs decreases 18 T. Fukushima and T. Aida, Chem.–Eur. J., 2007, 13, 5048–
linearly with the increasing temperature in the temperature 5058.
range typical for lubrication industry. [CnC1Pyrr][BMB] hf-BILs 19 R. L. Gardas, H. F. Costa, M. G. Freire, P. J. Carvalho,
A, 2008, 1184, 6–18.
(n ¼ 4–8 and 12) if added to PEG at 3 wt% signicantly reduce
I. M. Marrucho, I. M. Fonseca, A. G. Ferreira and
J. A. Coutinho, J. Chem. Eng. Data, 2008, 53, 805–811.
friction and wear in the steel–steel contacts in comparison with
the neat PEG and 5W40 oil. The best performance is provided by 20 P. Wasserscheid, A. Bosmann and R. V. Hal, US Patent,
the [C4C1Pyrr][BMB] hf-BIL.
7,863,458, 2011.
21 F. U. Shah, S. Glavatskih, P. M. Dean, D. R. MacFarlane,
M. Forsyth and O. N. Antzutkin, J. Mater. Chem., 2012, 22,
6928–6938.
Acknowledgements
The Knut and Alice Wallenberg Foundation and the Swedish 22 M. Yang, F. Xu, Q. Liu, P. Yan, X. Liu, C. Wang and U. Welz-
Research Council are gratefully acknowledged for supporting
Biermann, Dalton Trans., 2010, 39, 10571–10573.
this work. The Foundation in memory of J. C. and Seth 23 S. Yu, S. Lindeman and C. D. Tran, J. Org. Chem., 2008, 73,
M. Kempe is gratefully acknowledged for the grants for NMR 2576–2591.
equipment at LUT, stipends for MT and FUS and chemicals. 24 D. R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth and
Swedish Research School in Tribology is acknowledged for G. B. Deacon, Chem. Commun., 2001, 273, 1430–1431.
nancing MT's research visits to Ghent University, Belgium. We 25 M. A. Rocha, J. A. Coutinho and L. M. Santos, J. Phys. Chem.
thank Prof. Velaga and Dr Shimpi for assistance with DSC and
TGA experiments at LUT.
B, 2012, 116, 10922–10927.
26 P. Wasserscheid and W. Keim, Angew. Chem., 2000, 39, 3772–
3789.
27 M. Nieuwenhuyzen, K. R. Seddon, F. Teixidor, A. V. Puga and
C. Vinas, Inorg. Chem., 2009, 48, 889–901.
References
1 F. U. Shah, S. Glavatskih and O. N. Antzutkin, Tribol. Lett., 28 C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. Aki and
2013, 51, 281–301. J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, 954–964.
2 C. Ye, W. Liu, Y. Chen and L. Yu, Chem. Commun., 2001, 21, 29 R. Gusain, R. Singh, K. Sivakumar and O. P. Khatri, RSC Adv.,
2244–2245.
2014, 4, 1293–1301.
This journal is © The Royal Society of Chemistry 2014
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