Bio-inspired Ionic Liquids and Deep Eutectic Solvents ......containing biocompatible cations based...
Transcript of Bio-inspired Ionic Liquids and Deep Eutectic Solvents ......containing biocompatible cations based...
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Bio-inspired Ionic Liquids and Deep Eutectic Solvents based on sulfur
for tribological applications: synthesis and characterization
Mariana Freire1,2,3, Benilde Saramago1, Rogério Colaço2, Luís C. Branco3
1Centro de Química estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 2Departamento de Engenharia Mecânica e IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001
Lisboa, Portugal 3Requimte, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa,
Campus da Caparica, 2829-516 Caparica, Portugal
Abstract: The research for Ionic Liquids as lubricants is rapidly increasing due to their physical
and chemical properties, such as low-pressure vapor and low melting point (liquids below 100ºC).
However, this kind of liquids is expensive which makes their application as pure lubricants economically
unviable. Therefore, in this work we intend to develop ILs that are simultaneously economically more
viable and possessing sulfur groups in the structure. Thus, it has been chosen to synthesize, characterize -
in terms of their physical properties (water content, viscosity and wettability) - and to test tribologically ILs
containing biocompatible cations based on vitamin B1 called thiamine and an amino acid derivative, S-
carboxymethyl -L-cysteine, or alternatively an imidazolium-type cation with a sulfonic substituent group
([(C4SO3H)MIM]). In this context the following ILs were synthesized: thiamine docusate,
[Thiamine][AOT]2; thiamine triflate, [Thiamine][TfO]2; S-carboxymethyl-L-cysteinium mesylate, [S-
carboxyMeCis][MsO]; 1- ethyl-3-methylimidazolium S-carboxymethyl-L-cysteíne [EMIM][S-
carboxyMeCis]; 1-butylsulfonic-3-methylimidazolium hidrogenosulfate [(C4SO3H)MIM][HSO4]; and 1-
butylsulfonic-3-methylimidazolium triflate [(C4SO3)HMIM][TfO]. In [S-carboxyMeCis][MsO],
[EMIM][S-carboxyMeCis], [Thiamine][AOT]2 and [(C4SO3H)MIM][HSO4] cases, solids products or
liquids with very high viscosity that rendered their tribological studies unfeasible either as pure or as
additives were obtained at room temperature. Thus, the characterization and tribological assays were
performed only for the LIs [Thiamine][TfO]2 and [(C4SO3H)MIM][TfO], allowing the evaluation of
cation’s effect in ILs. Both LIs were tested as additives to PEG200. It has been shown that IL
[(C4SO3H)MIM][TfO] have a better tribological performance because their larger side chain.
Another aspect evaluated in this work was the application of eutectic solvents (DES) as
lubricants. This class of solvents presents physical characteristics very similar to ILs with advantages in
their easy way of preparation and reduce cost associated. As for the case of the ILs synthesized, DES were
prepared containing sulfur units in their structure. The DES prepared were: [N4,4,4,4][Br]:Sulfolane;
ChCl:Urea; ChCl:Thiourea; [N4,4,4,4][Br]:PEG200; ChCl:EG; ChCl:PEG200; ChCl:Sulfolane;
[N4,4,4,4][Br]:EG; [S4,4,2][EtSO4]:PEG200; [C2-THT][EtSO4]:PEG200. DES ChCl:Sulfolane was not
characterized by its viscosity because of the scarcity of the product, which made its tribological study
impossible.The remaining DES were all characterized as pure liquids relative to their water content,
viscosity and wettability, and tribological tests were performed. [S4,4,4][EtSO4]:PEG200 and [C2-
THT][EtSO4]:PEG200 were the DES that shown a best tribological performance due to the presence of the
ethylsulfate group.
Keywords: Ionic Liquids, Deep Eutectic Solvents, Lubrication, Tribology, Additives
1. Introduction
Nowadays, the environmental issue associated with
the use of traditional base oil lubricants has deserved
wide attention and the search for a new environmental
and more efficient solution has been undertaken. Ionic
liquids (ILs) have been tested as a new class of
lubricants since 2001 [1] due to their physical and
chemical properties such as low volatility, higher
thermal and chemical stability, non-flammability,
broad electrochemical window and large miscibility
with several materials [2]. This kind of liquids are
organic salts with low melting points (liquids below
100 ºC) composed by cation-anion pairs [3] [4] [5].
The most studied cations in the literature are
ammonium, phosphonium, imidazolium, pyridinium
and picolinium. ILs have a wide range of applications,
such as alternative solvents for synthesis and catalysis,
electrolytes in batteries, in liquid-liquid extractions,
and as lubricating oils, among others.
Recently, it has been discovered that the presence of
sulfur units improved the tribological performance of
the ILs. [6] [7] [8] [9] [10]
The main problem associated with ILs is the high price
and to minimize it, their use as additives in a
traditional base oil has been investigated [1].
On this work, we intend to develop a new kind of ILs
that simultaneously incorporate units with sulfur and
biocompatible cations such a thiamine (vitamin B1)
and S-carboxymethyl-L-cysteine (aminoacid
2
derivative). The ILs were studied as additives to
PEG200 in order to minimize their associated costs.
Another kind of liquids that has been studied as net
lubricants in this work are the deep eutectic solvents
(DES). DESs are systems consisting of eutectic
mixtures of Lewis or Brønsted acids and bases which
can contain a variety of anionic and/or cationic
species. [11] The most widely spread application of
DES is as an alternative media for processing of
metals that are traditionally difficult to plate or process
and as an environmental alternative for synthesis. [12]
They also present similar physical properties to ILs
and, so, they could be a good alternative to ILs as
lubricants. Until now, only one article about this
subject was published. [13].
The characteristics of ILs and DES (price and electric
conductivity) make them especially adequate for the
lubrication of micro- and nano-mechanical and
electrical systems (M/NEMS). The tribological tests
with the synthetized ILs and DES were performed
using steel–silicon contacts.
2. Experimental
2.1. Materials
The following reagents were used in the synthesis of
ILs: thiamine (Sigma Aldrich, >99%), S-
carboxymethyl-L-Cysteíne (Alfa Aesa, 97%); Sodium
Docusate (Sigma Aldrich, 99%), Sodium
triflate,Sulfuric Acid (MERCK, 95-97%), Methane
sulphonic acid (Sigma Aldrich, 98%), 1-
Methylimidazole (Sigma Aldrich, 99%) and 1-ethyl-
3-methylimidazolium bromide (Solchemar, >98%).
The following reagents were used in the synthesis of
DES: ethylene glycol (Sigma Aldrich, >99%), tetra-n-
butylammonium bromide (Sigma Aldrich, 99%), 1,4-
Butane sultone (Sigma Aldrich, 99%, Choline
Chloride (Sigma Aldrich, 98%), Sulfolane (Fluka AG,
98%), Polyethylene glycol 200 (Fluka AG), urea
(Sigma Aldrich, 95%), thiourea (Sigma Aldrich,
99%), [S4,4,2][EtSO4] (Solchemar, >98%) and [C2-
THT][EtSO4] (Solchemar, >98%). It was also used an
ion exchange resin, Amberlyst IRA-400 (Supelco).
2.2. Ionic liquids synthesis
Synthesis of thiamine docusate,
[Thiamine][AOT]2:
0.2053g of thiamine (0.59 mmol) and 0.5545g of
sodium docusate (1.25 mmol) were dissolved in 50
mL of methanol. The mixture was kept in vigorous
stirring for 72 h at room temperature. At the end of the
reaction the solvent was evaporated, and the mixture
was re-dissolved in 20 mL of ethanol. The solution
was filtrated, and the solvent evaporated. The final
product was dried in vacuum with stirring, at 70 ºC for
3 days. The desired product was obtained as white
solid (0.609 g; 93% yield).
1H RMN (DMSO, 400MHz): δ= 9.91 (s; 1H); 9.06 (br
s; NH3); 8.35 (s; 1H); 5.55 (s; 2H); 3.89 (m; 8H); 3.66
(m; 2H); 3.61 (m; 2H); 3.08 (t; J= 5,2 Hz; 2H); 2.96-
2.88 (m; 2H); 2.56 (s; 3H); 2.53 (s, 3H); 1.50 (m; 4H);
1.24 (m; 32H); 0.85 ppm (m; 24H).
Synthesis of thiamine triflate, [Thiamine][TfO]2:
0.2033g of thiamine (0.59 mmol) and 0.236g of
sodium triflate (1.25 mmol) were dissolved in 30 mL
of ethanol. The mixture was kept in vigorous stirring
for 72 h at room temperature. At the end of the
reaction the solvent was evaporated, and the mixture
was re-dissolved in 20 mL of ethanol. The solution
was filtrated, and the solvent evaporated. The final
product was dried in vacuum with stirring, at 70 ºC for
3 days. The desired product was obtained as a white
solid (0.375g; 94% yield).
1H RMN (DMSO, 400MHz): δ= 9.92 (s; 1H); 9.15 (br
s; NH3); 8.36 (s; 1H); 5.58 (s; 2H); 3.66 (t; J=5,6 Hz;
3H); 3.08 (t; J= 5,6 Hz; 2H); 2.57 (s; 3H); 2,53 ppm
(s, 3H)
Synthesis of S-carboxymethyl-L-cysteine mesylate,
[S-carboxiMeCis][MsO]:
1.250g of S-carboxymethyl-L-cysteine (0.59 mmol)
were dissolved in 150 mL of water on a 200mL
balloon. 5.58 mL of a solution previously prepared of
methanesulfonic acid (1M) were dissolved in 10mL of
water. This solution was added to the balloon and the
mixture was kept in vigorous stirring for 72h at
50ºC.The final product was dried in vacuum. The
desired product was obtained as a viscous liquid
(1.350g; 88% yield).
1H RMN (D2O, 400MHz): δ= 4.23 (m; 3H); 3.42 (s;
2H); 3.22 (m; 1H); 3.06 (d; H=8 Hz; 1H’); 2.71 ppm
(s; 3H)
Synthesis of 1- ethyl-3-methylimidazolium S-
carboxymethyl-L-cysteíne, [EMIM] [S-
carboxiMeCis]:
Previously the 1- ethyl-3-methylimidazólium
hydroxide, [EMIM]OH, was prepared, dissolving
400mg of 1- ethyl-3-methylimidazolim bromide,
[EMIM]Br, in 20mL of water and adding 4.6mL of
Amberlyst IRA 400 resin. The mixture was kept in
vigorous stirring for 15min and then we verified, with
a pH indicator, that the solution was basic. The
exchange of the anion was made by adding 190mg of
S-carboxymethyl-L-cysteine previously dissolve in
20mL of water. The intermediate product was filtrated
a kept in vigorous stirring for 24h. At last, the solution
was filtrated again, and the solvent were evaporated.
The product final was obtained as a viscous brown
liquid. (0.587g; 70% yield)
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1H RMN (D2O, 400MHz): δ= 8.62(s; 2H); 7.40(d;
J=26.8Hz; 4H); 4.14 (m; 4H); 3.85(m; 1H); 3.80 (s;
6H); 3.27 (s; 2H); 3.10-3,07(m; 1H); 3-2.94 (m; 1H)
1.41 ppm (t; 6H)
Synthesis of 1-butylsulfonic-3-methylimidazolium
hydrogenosulfate [(C4SO3H)MIM][HSO4]:
The first part of this synthesis is the preparation of the
cation [(C4SO3H)MIM]: 0.548g of 1-methylimidazole
was weighed to a balloon and then 1g of 1,4- butane
sultone was added. The mixture was kept in vigorous
stirring for 24h at 60ºC. At the end the product was
washed 3 times 20mL with diethyl ether. The product
was dried in vacuum for 1h.
Then, to 600mg of this product was added 0.147mL
of sulfuric acid and the mixture were kept with
vigorous stirring for 24h at 80ºC. (0.788g; 91% yield)
1H RMN (DMSO, 400 MHz): δ: 9.11 (s; 1H); 7.78 (d;
J=26Hz; 2H); 4.18 (m; 2H); 3.27 (s; 3H); 2.55 (m;
2H); 1.88 (m; 2H); 1.55 ppm (m; 2H)
Synthesis of 1-butylsulfonic-3-methylimidazolium
triflate, [(C4SO3H)MIM][TfO]:
600 mg of the 1-methylimidazole previously
functionalized, was dissolved in 50mL of acetone and
475.8mg of sodium triflate was added to the mixture.
The mixture was kept in vigorous stirring for 48h. The
final product was filtrated, the solvent was evaporated
and finally the product was dried in vacuum for 1h.
(0.638g; 50% yield)
1H RMN (D2O, 400MHz): δ= 9.15 (s; 1H); 7.78 (d;
J=26.4Hz; 2H); 4.19 (t; J= 6.8Hz; 2H); 3.86 (s; 3H);
1.88 (m; 2H) 1.57 ppm (m; 2H)
19F RMN (DMSO, 282MHz): δ=-77.84 ppm
Figure 1-Synthetized ILs.
2.2. Deep Eutectic Solvents preparation
Preparation of [N4,4,4,4][Br]:Sulfolane:
1.0038g of tetra-n-butylammonium bromide was
weight and put on a flask and the 2.063mL of
sulfolane was added. The mixture was kept in
vigorous stirring for 24h. The product was dried in
vacuum for 2 days. The final product was obtained as
a liquid.
FTIR (NaCl): ν̃= 3587.29; 2962.03; 2877.1; 1465.26;
1415.50; 1300.61; 1147.72; 1109.39; 1032.91;
906.62; 734.82; 568.38
Preparation of ChCl:uera (1:2):
1.0131g of choline chloride and 0.8753g of urea were
weight and put on a flask. The mixture was kept in
stirring for 24h. The product was dried in vacuum for
2 days. The final product was obtained as a liquid.
FTIR (NaCl): ν̃= 3334.39; 3200.04; 2963.76;
1676.49; 1615.44; 1445.80; 1275.71; 1168.24;
1081.45; 1005.97; 956.07; 870.13; 789.34
Preparation of ChCl:Thiourea (1:2):
0.500g of choline chloride and 0.545g of thiourea
were weight and put on a flask. The mixture was kept
in stirring for 24h. The product was dried in vacuum
for 2 days. The final product was obtained as a liquid.
FTIR (NaCl): ν̃=3372.91; 3272.85; 3169.33; 2924.92;
2857.37; 2688.37;1594.46; 1468.80; 1396.99;
1086.84; 954.16; 868.81; 733.06; 624.80; 508.68
Preparation of ChCl:EG (1:2):
1.019g of choline chloride and 0.7986 mL of ethylene
glycol were added to a flask. The mixture was kept in
stirring for 24h. The product was dried in vacuum for
2 days. The final product was obtained as a liquid.
FTIR (NaCl): ν̃=3327.00; 2940.01; 2875.76; 2124.45;
1649.78; 1481.48; 1415.66; 1203.11; 1084.36;
1043.59; 956.50; 876.64
Preparation of [N4,4,4,4][Br]:EG (1:4):
1.0021g of tetra-n-butylammonium bromide was
weight and put on a flask and 0.692mL of ethylene
glycol was added. The mixture was kept in vigorous
stirring for 24h. The product was dried in vacuum for
2 days. The final product was obtained as a liquid.
FTIR (NaCl): ν̃=3357.55; 2990.99; 2875.43; 1464.91;
1384.05; 1088.57; 1040.50; 883. 67; 740.16; 585.60;
436.78
Preparation of ChCl:PEG (1:4):
0.5073g of choline chloride was put in a flask and 2.58
mL of polyethyleneglycol 200 were added. The
mixture was kept in vigorous stirring for 24h. The
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product was dried in vacuum for 2 days. The final
product was obtained as a liquid.
FTIR (NaCl): ν̃=3370.78; 2872.40; 2139.81; 1954.33;
1649.52; 1466.22; 1355.16; 1294.27; 1247.84;
1131.14; 944.75; 884.67; 834.50
Preparation of [N4,4,4,4][Br]:PEG (1:4):
0.5067g of tetra-n-butylammonium bromide was
weight and put on a flask and 1.104mL of
polyethylene glycol 200 was added. The mixture was
kept in vigorous stirring for 24h. The product was
dried in vacuum for 2 days. The final product was
obtained as a liquid.
FTIR (NaCl): ν̃=3356.02; 2874.59; 1956.13; 1648.50;
1460.75; 1381.29; 1350.78; 1295.50; 1247.80;
1122.96; 938.28; 866.75; 833.38; 738.63
Preparation of ChCl:Sulfolane (1:12):
0.5249g of choline chloride was weighted to a flask
and 3.414mL of sulfolane was added. The mixture
was kept in vigorous stirring for 24h. The product was
dried in vacuum for 2 days. The final product was
obtained as a liquid.
FTIR (NaCl): ν̃=3306.25; 2956.42; 2881.96; 1635.98;
1454.25; 1416.39; 1304.14; 1145.08; 1109.98;
1032.97; 986.94; 905.27; 734.72; 672.17; 570.18;
435.85
Preparation of [S4,4,2][EtSO4]:PEG (1:4):
0.500g of [S4,4,2][EtSO4] was added to a flask and
1.186mL de PEG 200, polyethyleneglycol 200 was
pipped to that flask. The mixture was kept in vigorous
stirring for 24h. The product was dried in vacuum for
2 days. The final product was obtained as a liquid.
FTIR (NaCl): ν̃=3419.06; 2874.17; 2361.35; 2337.52;
1952.54; 1651.23; 1460.06; 1351.96; 1249.03;
1118.73; 1026.29; 939.21; 887.88; 834.59; 767.26
Preparation of [C2-THT][EtSO4]:PEG (1:4):
1.0130g of [C2-THT][EtSO4] was added to a flask and
2.94mL of polyethyleneglycol 200 was pipped to that
flask. The mixture was kept in vigorous stirring for
24h. The product was dried in vacuum for 2 days. The
final product was obtained as a liquid.
FTIR (NaCl): ν̃=3386.70; 2873.48; 1953.76; 1649.75;
1456.40; 1420.53; 1352.22; 1249.04; 1121.91;
920.07; 887.56; 835.70; 771.64
Figure 2- Prepared DES
2.3. Ionic liquids and Deep Eutectic Solvents
characterization
The water content of the mixture ILs-PEG, DES and
PEG200 was measured by a Karl-Fischer coulometric
(Metrohm) titration. The values obtained were less
than 300ppm for all mixtures PEG-ILs and less than
1500ppm for all the DES. The viscosity of the
mixtures PEG-ILs and DES was measured using a
viscometer DVII+Pro (Brookfield) that applies a
certain rotation speed through a spindle on the liquids.
This equipment was also used for rheological tests.
The viscosity tests were made for the temperature 15,
20, 25, 30, 40 e 50°C. All measurements were done
in triplicate. The contact angles measurement on
silicon substrates was done using the sessile drop
method. The drops were generated inside an ambient
chamber model Ramé-Hart 100-07-00 (Ramé-Hart
Succasunna) at room temperature under an inert
atmosphere of dry nitrogen to minimize the water
absorption of the liquids during the measurements.
This equipment has a video camera jAiCV-A50
mounted on a microscope Wild M3Z (Leica
Microsystems) that allows to capture images of the
drops during the stabilization time. The images were
analyzed by running the ADSA-P (Axisymetric Drop
Shape Analysis, Applied Surface Thermodynamics
Research Associates) software. The measurements
were done at room temperature and a minimum of 7
drops were analyzed for each liquid. The stabilization
time for each liquid varied between 15 and 30 min,
after which it was possible to obtain the static contact
angle. Both substrates and spheres used as counter
bodies in tribological tests were submitted to the
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following cleaning procedure: 2 x 15 min sonication
in a detergent solution intercalated with 10 min
sonication in water, followed by 3 x 10 min sonication
with water, rinsing with distilled and deionized water
and drying inside a vacuum oven at room temperature
overnight.
2.4. Tribological tests
The friction coefficients of the mixtures PEG-ILs and
DES were measured in a nanotribometer (CSM
Instruments) with a tribological pair steel – silicon (S-
Si). The spheres used as counter bodies were glued to
a cantilever of medium load. The silicon substrates
were placed on a metal support and 5 drops of liquid
were uniformly distributed on the substrate. The
following parameters were chosen: forces of 15 and
30 mN, five sliding velocities between 0.2 and 1.6
cm/s. Three replicas were performed for each
measurement, at room temperature, under an inert
atmosphere of dry nitrogen.
3. Results
3.1. Ionic Liquids and Deep Eutectic Solvents
characterization
The ionic liquids [Thiamine][TfO]2 and
[(C4SO3H)MIM][TfO] were characterized and
tribologically tested as additives to PEG 200. The
other ILs synthetized were not tested because they
were obtained as solids or viscous liquids with very
low solubility. The DES were characterized and tested
as net lubricants. The liquids characterization was
based in terms of viscosity.
The results here presented refer to the mixture PEG-
[Thiamine][TfO]2 (2% IL mass concentration) and
PEG-[(C4SO3H)MIM][TfO] (2 and 5% IL mass
concentrations) as well as DES [N4,4,4,4][Br]:Sulf,
ChCl:PEG; [N4,4,4,4][Br]:PEG, ChCl:Sulf,
[S4,4,2][EtSO4]:PEG and [C2-THT][EtSO4]:PEG. It
was not possible to determine the viscosity of
ChCl:Sulf because there was not enough liquid.
Rheological tests were performed to test the
Newtonian or non-Newtonian behavior of the liquids.
The tests were made at constant temperature and
variable speed of rotation. The results are presented in
Figure 3 and Figure 4.
Figure 3- Variation of the viscosity with velocity for the
mixtures PEG-ILs, at 25 ° C.
Figure 4-Variation of the viscosity with velocity for the DES
prepared, at 25ºC.
As shown on Figure 3 and Figure 4 the mixture PEG-
[Thiamine][TfO]2 (2%) and PEG-
[(C4SO3H)MIM][TfO] (2 and 5%) as well as the DES
[N4,4,4,4][Br]:PEG and [S4,4,2][EtSO4]:PEG behave as a
Newtonian liquids; in contrast the results associated
with the DES [N4,4,4,4][Br]:Sulf, ChCl:PEG and [C2-
THT][EtSO4]:PEG indicate that they behave as non-
Newtonian because there is some changes in the
viscosity with velocity. However, these results
should be confirmed with a rheometer that allows us
to achieve higher shear stress values.
The viscosity was measured in the temperature range
15-50 ºC and the data were fitted to the Arrhenius
equation (1),
𝜂 = 𝜂0𝑒𝐸𝑎𝑅𝑇 (1)
where η0 is a pre-exponencial factor and Ea the
activation energy for viscous flow. The viscosity
values at 20 ºC and the parameters of this equation are
presented in Table 1.
50
55
60
65
70
75
80
85
1 1,5 2 2,5 3
η(m
Pa.
s)
v (rpm)
[(C4SO3H)MIM][TfO]_2% [(C4SO3H)MIM][TfO]_5%
[Thiamine][TfO]2_2%
80
85
90
95
100
105
110
115
120
1 1,5 2 2,5 3
η(m
Pa.
s)
v (rpm)[N4,4,4,4][Br]:Sulf ChCl:PEG
[N4,4,4,4][Br]:PEG [S4,4,2][EtSO4]:PEG
[C2-THT][EtSO4]:PEG
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Table 1- Viscosity at 20 ºC, η, Arrhenius equation parameters for studied mixtures PEG-ILs and DES and
correlation coefficients, Ea R2.
Líquido
Ea
(kJ/
mol)
𝜼𝟎
(mPa.s
)
𝑹𝟐
𝜼
(mPa.s
; 20ºC)
Ionic Liquids
[Tiamina]
[TfO]2_2% 88.7 4411.6 0.999 79.7
[(C4SO3H)MIM] [TfO]_2%
92.0 4550.6 0.998 85.7
[(C4SO3H)MIM]
[TfO]_5% 95.2 4719.1 0.998 108.5
Deep Eutectic Solvents
[N4,4,4,4][Br]:
Sulf 91.5 4679.9 0.993 139.3
ChCl: PEG
87.8 4498.4 0.998 122.0
[N4,4,4,4][Br]:
PEG 87.4 4558.2 0.997 153.0
[S4,4,2][EtSO4]: PEG
91.7 4696.3 0.996 146.5
[C2-THT][EtSO4]:
PEG 88.0 4587.3 0.996 163.5
PEG 60
The mixtures PEG-ILs have viscosities higher than
pure PEG200, which is a clue for ILs being more
viscous than PEG. Unfortunately, it was not possible
to measure the viscosity of the pure LIs to compare
with the DES values. The viscosities of DES are
higher than PEG. Moreover, the mixtures PEG-[(C4SO3H)MIM][TfO]
(2 and 5%) are more viscous than the mixture PEG-
[Thiamine][TfO]2 (2%) which allow us to conclude
that the cation [(C4SO3H)MIM] confers more
viscosity to the mixture than the cation [Thiamine].
Besides, the viscosity increases with the increase of IL
concentration. The contact angles were measured on silicon
substrates for the same liquids whose viscosity was
measured plus ChCl:Sulf and PEG. The results are
presented in Figure 5 and Figure 6.
Figure 5- Contact angles of mixtures PEG-ILs on silicon
substrates. The error bars represent the standard deviation
Figure 6- Contact angles of DES on silicon substrates. The
error bars represent the standard deviation
All liquids present lower contact angles than PEG.
The mixture PEG-[Thiamine][TfO] presents higher
contact angles than PEG-[(C4SO3H)MIM][TfO].
Besides the increase of the IL [(C4SO3H)MIM][TfO]
concentration in the mixture does not effect on the
contact angle.
The DES with the lowest contact angle is [C2-
THT][EtSO4]:PEG. In general, all liquids present
contact angles below 10º and so we can conclude that
all of them have a good wettability on silicon surfaces.
3.2. Tribological tests
The friction coefficients were plotted as a function the
Sommerfield parameter, z, defined as:
𝑍 = 𝜂𝑣𝑟/𝑁 (2)
where η is the lubricant viscosity, v the sliding speed,
r the radius of the counter body and F the
applied load, leading to the so-called Stribeck curves. Shown in Figure 7Figure 12, it may be concluded that
the tests were done in a elastrohydrodynamic or mixed
lubrication regime, once CoFs do not vary
significantly with z.
Furthermore, the mixture that presents the best
tribological performance is PEG-
[(C4SO3H)MIM][TfO] (2% concentration), because it
has the lowest CoF with the lowest dispersion.
The DES that present the best tribological
performance are [S4,4,2][EtSO4]:PEG and [C2-
THT][EtSO4]:PEG because their Stribeck curves are
below the PEG’s curve. In contrast, the DES
[N4,4,4,4][Br]:Sulf presents a curve with a high
dispersion and the DES [N4,4,4,4][Br]:PEG, presents
CoF higher then PEG.
0
2
4
6
8
10
12
14
16
18
20
θ(°
C)
0
2
4
6
8
10
12
14
16
18
20
θ(°
C)
7
Figure 7-Stribeck curves for PEG-ILS mixtures and PEG.
Figure 8-Stribeck curves for DES [N4,4,4,4][Br]:Sulf and PEG.
Figure 9- Stribeck curves for DES ChCl:PEG and PEG.
Figure 10- Stribeck curves for DES [N4,4,4,4][Br]:PEG and
PEG.
Figure 11- Stribeck curves for DES [S4,4,2][EtSO4]:PEG and
PEG.
Figure 12- Stribeck curves for DES [C2.-THT][EtSO4]:PEG
and PEG.
4. Discussion
The tribological performance of the ILs depends on
the chemical structure of the cations and anions. which
in turn determines the physical properties, such as
viscosity and interfacial behavior. In this work
mixtures of PEG-ILs were characterized for these
three properties. In the literature there are different
studies in which the effect of lateral chain length on
both the cation and the anion is evaluated and it was
concluded that the longer the chain the better the
tribological performance of liquids [14] [15] [16]. On
the other hand, there are also studies that associate a
good tribological behavior to the presence of sulfur
and to the presence of double bonds on IL structure
[6] [17].
Both ILs studied in this work contain sulfur in the
cation and the anion. However only [(C4SO3H)MIM]
[TfO] presents an S-O bond that allows a stronger
interaction between the silica surface and the ILs
mixture by the formation of Si-O-S bonds. This IL
presents also a larger side chain of the cation which
promotes the formation of a thicker and more cohesive
protective film which protects better the surface of the
silicon substrate from wear and friction. Both factors
contributed to the better tribological performance
presented by [(C4SO3H)MIM][TfO] in comparison
with [Thiamine][TfO]2. It is important note that the
tribological performance of PEG improved when
[(C4SO3H)MIM][TfO] was added because, even in a
0
0,05
0,1
0,15
0,2
0,25
0,3
0,00 0,50 1,00
CoF
zx104
[(C4SO3H)MIM][TfO]_2%
[Thiamine][TfO]2_2%
PEG
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,00 0,50 1,00 1,50 2,00
CoF
zx104
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,00 0,50 1,00 1,50
CoF
zx104
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,00 0,50 1,00 1,50 2,00
CoF
zx104
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,00 0,50 1,00 1,50 2,00
CoF
zx104
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,00 0,50 1,00 1,50 2,00
CoF
zx104
8
small fraction, the ILs tends to be adsorbed on the
surface and form a protective layer that reduces
contact between the surfaces and, in consequence,
lower friction is achieved.
The effect of IL’s concentration when added to PEG
is another parameter that has been investigated in
several articles [15] [18]. It is believed that the effect
of increasing IL’s concentration is not synonymous of improving tribological performances. In fact, CoF
tends to decrease till a certain concentration value and
then it begins to increase. In the case of
[(C4SO3H)MIM][TfO] added to PEG, the increasing
concentration of 2 to 5% led to an increase of about
14% in CoF. This behavior may be explained by the
fact that, at lower concentrations, physical adsorption
of IL molecules occurs, and a protective film is
formed; on the other hand, for higher concentrations,
the film formed is thicker but less strongly bound to
the substrate which leads to an easier film removal. In
this case friction of three bodies cases may be present.
Another factor that may contribute to these results is
the increase of viscosity that occurs when the
concentration of [(C4SO3H)MIM][TfO] is higher
because, under elastrohydrodynamic conditions, the
lubrication is not only influenced by the adsorption
but also by the lubricant’s viscosity. In fact, for high
viscosities internal friction of IL ions can occur and
may lead to a CoF increase. The good tribological performance of the PEG +
[(C4SO3H)MIM][TfO] mixture is explained by its
good ability to wet the surface of the silicon, whose
contact angle value is 5° which contributes to the
formation of a stable lubricating layer.
From the tribological tests carried out with DES, it
was observed that only [S4,4,2][EtSO4]: PEG and [C2-
THT][EtSO4]: PEG presented lower CoF than PEG. It
is caused by the presence of the anion [EtSO4] which
was previously observed with on the IL
[EMIM][EtSO4] [17]. On that study it was showed
that [EtSO4] adsorbs strongly to the surface of the
silicon. Though the reaction of the oxygen of the S-O
bond present in the anion, with the Si surface forming
Si-O-S bonds. These results agree with the results of
contact angles obtained, since both DES present a
good ability to wet the silicon surface. On the other
hand, the viscosity of both mixtures is relatively high,
being approximately 115mPa.s in the case of
[S4,4,2][EtSO4]: PEG and 110mPa.s for [C2-
THT][EtSO4], which may contribute to a better ability
of the lubricant to separate the two surfaces in relative
motion.
The DES [S4,4,2][EtSO4]:PEG and [C2-THT] [EtSO4]:
PEG were prepared for the first time in this work and,
in order to be sure of the formation of hydrogen
bonding between the HBO:HA, a calorimetry analysis
should be made. Therefore, the good results obtained
with these two liquids could be attributed only to the
good performance of the mixtures of PEG-
[S4,4,2][EtSO4] and PEG-[C2-THT][EtSO4],
previously studied, and not to the hypothetical eutectic
solvents.
5.Conclussion
In this work six ILs were synthetized in moderate to
high yields except for the cases of [EMIM]2[S-
carboxyMeCis] and [(C4SO3H)MIM][TfO]. The
viscosities of mixtures of the ILs with PEG are higher
than that of pure PEG, while the opposite relation
occurs for the contact angle on Si. The
PEG+[(C4SO3H)MIM][TfO] mixture presented better
tribological performance than PEG+[Thiamine][TfO]
mixture whose may be due to the larger side chain
present on [(C4SO3H)MIM] cation. It was concluded
that in the case of the PEG+[(C4SO3H)MIM][TfO]
mixture the increase in IL concentration from 2% to
5% causes a 14% increase in CoF which may result
from the internal friction caused by the increase of
viscosity.
DES present also a good wettability, less thant 10º,
and DES’s viscosity is higher than the viscosity
obtained with IL-PEG mixtures.
The DESs with the better tribological performance
were [S4,4,2][EtSO4]:PEG and [C2-THT][EtSO4]:PEG
which may be associated with the presence of the
ethylsulfate group that interacts with the silicon
surface establishing a stable protective film.
However, as the performance of DES was not
confirmed, the good tribological performances
accomplished with the mixtures cannot lead to the
conclusion that DES are a viable alternative to ILs.
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