1

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)

3

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

4

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

5

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

6

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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