Surface interactions, corrosion processes and lubricating performance of protic and aprotic ionic liquids with OFHC copper

G

A

So

TGC

a

ARRAA

KCISCL

1

tppo

0h

ARTICLE IN PRESS Model

PSUSC-25236; No. of Pages 20

Applied Surface Science xxx (2013) xxx– xxx

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

urface interactions, corrosion processes and lubricating performancef protic and aprotic ionic liquids with OFHC copper

ulia Espinosa, José Sanes, Ana-Eva Jiménez, María-Dolores Bermúdez ∗

rupo de Ciencia de Materiales e Ingeniería Metalúrgica, Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena,ampus de la Muralla del Mar. C/Doctor Fleming, s/n. 30202-Cartagena, Spain

r t i c l e i n f o

rticle history:eceived 14 January 2013eceived in revised form 12 February 2013ccepted 20 February 2013vailable online xxx

eywords:opper

onic liquidsurface interactionsorrosionubrication

a b s t r a c t

In order to select possible candidates for use as lubricants or as precursors of surface coatings,the corrosion and surface interactions of oxygen-free high conductivity (OFHC) copper with twonew protic (PIL) and four aprotic (APIL) room-temperature ionic liquids have been studied. ThePILs, with no heteroatoms in their composition, are the triprotic di[(2-hydroxyethyl)ammonium]succinate (MSu) and the diprotic di[bis-(2-hydroxyethyl)ammonium] adipate (DAd). The four APILscontain imidazolium cations with short or long alkyl chain substituents and reactive anions: 1-ethyl-3-methylimidazolium phosphonate ([EMIM]EtPO3H); 1-ethyl-3-methylimidazolium octylsulfate([EMIM]C8H17SO4); 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6). Contact angles between the ionic liquids andOFHC copper surface were measured. Mass and roughness changes of OFHC copper after 168 h in con-tact with the ionic liquids have been determined. Copper surfaces were studied by XRD, SEM–EDX andXPS surface analysis. FTIR spectra of the liquid phases recovered after being in contact with the coppersurface were compared with that of the neat ionic liquids. The lowest corrosion rate is observed for thediprotic ammonium adipate PIL (DAd), which gives low mass and surface roughness changes and formsadsorbed layers on copper, while the triprotic ammonium succinate salt (MSu) produces a severe corro-sive attack by reaction with copper to form a blue crystalline solid, which has been characterized by FTIRand thermal analysis (TGA). All imidazolium APILs react with copper, with different results as a functionof the anion. As expected, [EMIM]C8H17SO4 reacts with copper to form the corresponding copper sulphatesalt. [EMIM]EtPO3H produces severe corrosion to form a phosphonate–copper soluble phase. [HMIM]BF4

gives rise to the highest roughness increase of the copper surface. [HMIM]PF6 shows the lowest massand roughness changes of the four imidazolium ionic liquids due to the formation of a solid layer con-taining phosphorus and fluorine. The results described in the present study are in agreement with the
outstanding good tribological performance of the diprotic ammonium adipate (DAd) ionic liquid for thecopper–copper contact, in pin-on-disc tests, preventing wear and giving a very low friction coefficientof 0.01. Under the same conditions, [HMIM]PF6 gives a friction value of 0.03, while the reactivity of MSutowards copper produces maximum friction peaks of 0.05. In contrast with the absence of surface dam-age on copper, an abrasive wear mechanism is observed for MSu and [HMIM]PF6. The results confirm abetter lubricating performance for a lower corrosion rate.
© 2013 Elsevier B.V. All rights reserved.

. Introduction

Among many other scientific and technological applica-ions, room-temperature ionic liquids (ILs) are potential high

Please cite this article in press as: T. Espinosa, et al., Surface interactions, cionic liquids with OFHC copper, Appl. Surf. Sci. (2013), http://dx.doi.org/10

erformance lubricants for engineering tribology [1–10] andrecursors of surface protective coatings. In both fields, the abilityf IL molecules to form stable surface layers is of the outmost

∗ Corresponding author. Tel.: +34 968325958; fax: +34 968326445.E-mail address: [email protected] (M.-D. Bermúdez).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.02.083

importance. However, highly reactive ILs, in particular those con-taining halides, sulphur or phosphorus in the anion can producesevere corrosion on the metal surfaces, leading to systems failure.

The field of IL lubrication was effectively started in 2001 by Liuet al. [11]. They studied alkylimidazolium tetrafluoroborates in avariety of contacts, showing excellent friction reduction. In the caseof steel/copper, a friction coefficient of 0.025 was described using

orrosion processes and lubricating performance of protic and aprotic.1016/j.apsusc.2013.02.083

1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4), oneof the ILs used in the present study. Subsequent studies on lubrica-tion of different materials with ILs containing tetrafluoroborate andhexafluorophosphate anions showed that severe tribocorrosion

dx.doi.org/10.1016/j.apsusc.2013.02.083


http://www.sciencedirect.com/science/journal/01694332

http://www.elsevier.com/locate/apsusc

mailto:[email protected]


ARTICLE IN PRESSG Model

APSUSC-25236; No. of Pages 20

2 T. Espinosa et al. / Applied Surface Science xxx (2013) xxx– xxx

Table 1Ionic liquids.

Nomenclature and abbreviations Cation Anion

Di-(2-hydroxyethylammonium) succinate (MSu)

Di-[bis(2-hydroxyethyl)ammonium] adipate (DAd)

1-Ethyl-3-methylimidazolium ethylphosphonate ([EMIM]EtPO3H)

1-Ethyl-3-methylimidazolium octylsulfate ([EMIM]C8H17SO4)

1-Hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) BF4−

1-Hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6) PF6−

pnp

eo

i

tcps

b

aTtc

hn

erinlw

owt

oat

The PILs used in the present study were synthesized by Igle-sias et al. [37,38] by reaction between Brønsted acids and bases, inparticular by the combination of primary or secondary ammonium

rocesses could take place. As it is well known for these fluori-ated anions, the hydrolysis in the presence of water or moistureroduces HF [12–16].

The corrosion and tribocorrosion processes of ILs on differentngineering metals and alloys have been the subject of a numberf studies [17–25].

The results show that the corrosion activity decreases withncreasing chain length of the alkyl substituent on the cation.

The in situ formation of iron fluoride was described whenetrafluoroborate 1-ethyl-3-methylimidazolium was used as lubri-ant of AISI 52100 steel [13]. When ILs with the hexafluorophos-hate anion are used as poly(ethylene glycol) lubricant additives inteel/Cu–Sn, the formation of CuF2 is observed [14].

Subsequent studies have focused on preventing tribocorrosiony using corrosion inhibitors or less reactive ILs.

Benzotriazole [26–28] was studied as corrosion inhibitordditive in hexafluorophosphate ILs in steel/Cu–Sn contacts.he anticorrosion and antiwear behaviour was explained byhe formation of surface films composed of copper oxide andopper–benzotriazole complexes.

Halogen-free [29] and, in particular, fluorine-free ILs lubricantsave been investigated in order to reduce corrosion, but they areot as effective as those containing fluorine [30].

The performance of ILs as lubricants has been the subject of sev-ral reviews and special issues [1–10]. The results described areeferred to the conventional aprotic ionic liquids (APILs), mostlymidazolium and to some quaternary ammonium and phospho-ium salts. The most recent reviews on room-temperature ionic

iquids (RTILs) [31,7,32] also include protic ionic liquids (PILs)hich are showing great potential [33–39].

The most common cations used in PILs include primary, sec-ndary and tertiary ammonium ions. The large variety of anionshich can be present in PILs include totally organic species such as

he carboxylate salts.


Iglesias and co-workers [35,37,38] have synthesized a familyf PILs from primary amines and organic acids by modifying theliphatic chain of the organic acid and/or using secondary and ter-iary hydroxyamines. These new PILs present some very relevant

features such as a simple synthetic route, low cost and low toxicity[40,41].

The presence of proton donor and acceptor sites in the PILsmolecules can build up a hydrogen bonded network which couldimprove their lubricating performance [42].


Fig. 1. (a) SEM micrograph of the copper surface before the tests; (b) X-ray diffrac-tion pattern of the copper sheet before the tests.




T. Espinosa et al. / Applied Surface Science xxx (2013) xxx– xxx 3

a

Dat a evaluated using CasaXPS

Block No.1

x 102

380

390

400

410

420

430

440

450C

PS

970 965 960 955 950 945 940 935 930Binding Energy (eV)

Cu 2p


Block No.4

x 103

45

50

55

60

65

CPS

544 542 540 538 536 534 532 530 528 526Binding Energy (eV)

O 1s

x 104

10

15

20

25

CPS

970 965 960 955 950 945 940 935 930Binding Energy (eV)

Cu 2pb

pper

c

2

(


Fig. 2. XPS analysis: (a) copper surface before the tests; (b) co

ations and dicarboxylate anions according to reaction (1).


[HO(CH2)2]xNH3−x + HO(O)C(CH2)nC(O)OH

→ 2[HO(CH2)2]xNH4−x + −O(O)C(CH2)nC(O)O− (1)

for MSu: x = 1; n = 2; for DAd: x = 2; n = 4).

Fig. 3. (a) SEM micrograph and (b) EDX spectrum of the co

surface after removing the oxide layer by sputtering for 30 s.

The scarce PILs previously used as lubricants [43–46] containpotentially corrosive groups, with fluorine or sulphur in their com-


position.Our research group is currently working on the applications

of fully organic PILs as neat lubricants, additives in synthetic baseoils and in water, corrosion inhibitors or cutting fluids in severe

pper surface after the test with [EMIM](C8H17SO4).


ING Model

A

4 rface

ttat

osha

TE

ARTICLEPSUSC-25236; No. of Pages 20

T. Espinosa et al. / Applied Su

ribological environments and for sliding pairs particularly difficulto lubricate such as ceramic–metal pairs, light alloys against steels,nd copper–copper contacts, with a wide interest for electric orransport applications [47].

The present work reports the results of corrosion tests of


xygen-free high conductivity (OFHC) copper in the presence ofix ILs, two protic ammonium dianionic species, derived from 2-ydroxyethylamine and the carboxylic adipic and succinic acids,nd four aprotic methylimidazolium salts with an ethyl lateral

able 2volution of OFHC copper during the tests (For interpretation of the reference to colour in

PRESS Science xxx (2013) xxx– xxx

chain and the reactive anions octylsulfate, ethylphosphonate, orwith an hexyl lateral chain and tetrafluoroborate or hexafluo-rophosphate anions.

[HMIM]BF4 was already described in the first paper on ILlubricants [11]. We have previously described the lubricating


performance of [HMIM]PF6 among a series of imidazolium apro-tic ILs in different contact configurations and environments[48–54]. The use of phosphonate derivatives such as [EMIM]EtPO3His currently being studied as precursors of abrasion-protective

this table legend, the reader is referred to the web version of the article.).


Please cite this article in press as: T. Espinosa, et al., Surface interactions, corrosion processes and lubricating performance of protic and aproticionic liquids with OFHC copper, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.02.083




Data evaluated using CasaXPS

x 103

65

70

75

80

85

90

95

100C

PS

965 960 955 950 945 940 935 930 925

Binding Energy (eV)

Cu 2p


x 103

12

14

16

18

20

22

CPS

296 294 292 290 288 286 284 282 280 278

Binding Energy (eV)

C 1s


x 103

70

80

90

100

110

120

130

CPS

540 538 536 534 532 530 528 526 524 522

Binding Energy (eV)

O 1s


x 102

90

100

110

120

130

140

150

160

CPS

174 172 170 168 166 164 162 160 158 156

Binding Energy (eV)

S 2p


x 101

240

245

250

255

260

265

270

275

280

285

CPS

412 410 408 406 404 402 400 398 396 394

Binding Energy (eV)

N 1s

Fig. 4. XPS binding energies for copper surface after the test with [EMIM](C8H17SO4).

Table 3Contact angles, mass (�m) and surface roughness (�Sa) changes after 168 h.

Ionic liquid Contact angle (◦) (standard deviation) �m (%) �Sa (%)

MSu 31.75 (1.19) −1.40 225.0Dad 62.67 (2.19) −0.04 −3.0[EMIM]EtPO3H 33.36 (1.21) −2.07 137.5[EMIM]C8H17SO4 4.58 (0.43) −0.44 935.7[HMIM]BF4 20.22 (0.72) −0.25 2632.1[HMIM]PF6 17.65 (1.28) 0.12 −3.6


ING Model

A

6 rface

cmcwp

omipl

2

al(aw

(



oatings for magnesium alloys [47]. Octylsulfate ILs are consideredore environmentally friendly or ‘greener’ than the fluorine-

ontaining ILs [55]. The sulphate derivative [EMIM]C8H17SO4as selected due to its expected reactivity towards cop-er.

We have recently described some of the preliminary resultsf the use of some ammonium PILs as lubricants [47]. Theain purpose of the present work is to study the surface

nteractions and corrosion processes in order to establish theotentially best candidates to be selected as high performance

ubricants.

. Experimental

Table 1 shows the chemical structures, nomenclature andbbreviations for the ILs used in the present work. The PILs ioniciquids MSu and DAd were kindly supplied by Dr. M. Iglesias [7,32]currently at the Federal University of Bahia, Brazil) and were used


s received. Imidazolium APILs (>98% purity) (Solvionic, France)ere used as received.

Corrosion tests [56] were carried out on OFHC copper sheets20 mm × 20 mm × 1 mm) covered by 2 ml of the ILs at room

Fig. 5. (a) SEM micrograph; (b) EDX spectrum and (c) Magnifi


temperature in air (30–50% HR) (Figs. 1 and 2). Contact angles weremeasured with a DSA30B (Krüss, Germany).

X-ray diffraction patterns were recorded in a Bruker D-8 Advance diffractometer using a wavelength of 1.542 A fromCu-K�, with an angular speed of 120 s/◦, at room tempera-ture.

FTIR spectra were recorded with a Nicolet Magna 5700spectrophotometer. Thermal stability curves, weight losses anddegradation temperatures were determined with a ShimadzuTGA-50 equipment in air or in a nitrogen atmosphere (heatingrate 30 ◦C/min). SEM images and EDX analysis were obtainedusing a Hitachi S3500N scanning electron microscope. X-rayphotoelectron (XPS) spectra were obtained using a VG-MicrotechMultilab 3000. The binding energies (B.E.) [57] were referenced tothe C 1s peak (285.0 eV) used as internal standard. The precisionin the binding energy is estimated to be ±0.1 eV.

For tribological tests [58], OFHC copper (>99.95% Cu; 105HV) discs (12.5 mm diameter; 3.2 mm thickness; surface rough-


ness Ra < 0.08 �m) were tested in a pin-on-disc tribometer(Microtest, Spain) against OFHC copper pins (0.8 mm sphereradius), at room temperature (23–25 ◦C; 50%HR) under a nor-mal applied load of 0.49 N (maximum contact pressure 0.9 GPa),

cation and P map after the test with [EMIM](EtPO3H).


ING Model

A

rface

waot

i2m



ith a sliding radius of 9 mm, at a sliding velocity of 0.10 m s−1,nd a sliding distance of 500 m, in the presence of 0.5 mlf the lubricants added immediately before the start of theests.

Friction coefficients were continuously recorded with slid-ng distance for each test. One friction record is taken every


.5 s by the friction sensor which measures lateral displace-ents.


x 103

56

58

60

62

64

66

68

70

72

74

CP

S

970 965 960 955 950 945 940 935 930

Binding Energy (eV)

Cu 2p


x 103

60

65

70

75

80

CP

S

544 542 540 538 536 534 532 530 528 526

Binding Energy (eV)

O 1s


x 101

245

250

255

260

265

270

275

280

285

CP

S

412 410 408 406 404 40

Binding Energy (eV

N 1s

Fig. 6. XPS binding energies for copper surfa

PRESSScience xxx (2013) xxx– xxx 7

3D surface topography images, surface roughness (Sa) accordingto ISO 25178 standard [59] and volume loss measurements wereobtained by means of a Talysurf CLI optical profiler. Wear volumeswere determined as the difference between the loss volume belowthe base line and the volume of the material accumulated abovethe base line by plastic deformation.


Mean friction coefficients and wear rates are obtained after atleast three tests under the same conditions.


x 103

10

12

14

16

18

20

22

24

26

28

30

CP

S

298 296 294 292 290 288 286 284 282 280

Binding Energy (eV)

C 1s


x 102

82

84

86

88

90

92

CP

S

146 144 142 140 138 136 134 132 130 128

Binding Energy (eV)

P 2p

2 400 398 396 394

)

ce after the test with [EMIM](EtPO3H).





15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

[EMIM]EtPO3H

Fig. 7. FTIR spectra of [EMIM](EtPO3H) before (red) and after (blue) the test. (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of the article.)

3

3

(s(mfi

stpCC5

s

3

3

co

re(

mtl

s

. Results and discussion

.1. OFHC copper

Fig. 1 shows the SEM micrograph of the OFHC copper surfaceFig. 1a) and the X-ray diffraction pattern (Fig. 1b) of the copperheet before the tests. The diffraction peaks correspond to planes1 1 1) and (2 0 0) of the face-centred cubic cell characteristic of

etallic Cu. The same diffraction pattern is obtained after the testsor all ILs, thus showing that no new crystalline phases are formedn sufficient proportion.

Fig. 2 shows the XPS binding energies (B.E.) for the copperheet before the corrosion tests. Fig. 2a shows that, as expected,he copper surface is covered by a layer of copper oxides. Theeak at higher energy (934.96 eV) can be assigned Cu (II) ofuO, while the peak at 932.98 eV could be due to Cu (I) fromu2O [60].Only one peak corresponding to O 1s is observed at32.03 eV.

Cu 2p peak of metallic copper only appear at 933.38 eV after theurface was cleaned by sputtering (Fig. 2b).

.2. Imidazolium aprotic ionic liquids

.2.1. 1-Ethyl-3-methylimidazolium derivativesThe ionic liquids containing the 1-ethyl-3-methylimidazolium

ation produce severe corrosion, but the corrosion products dependn the anion nature.

[EMIM](C8H17SO4) reacts completely with copper, as no liquid isecovered after the test, and the copper surface appears totally cov-red by the reaction products, in particular, a blue solid precipitateTable 2).

In contrast with the original copper surface (Fig. 1a), the SEMicrograph after the test with [EMIM](C8H17SO4) (Fig. 3a) shows


he presence of the corrosion reaction product as a non-uniformayer, completely covering the copper surface.

The EDX spectrum (Fig. 3b) confirms the presence of oxygen andulphur from the octylsulfate anion.

The XPS analysis (Fig. 4) shows the Cu 2p3/2 binding energy at935.5 eV, which corresponds to copper sulphate and is in agreementwith the S 2p3/2 peak at 169.3 eV, and with the O 1s peak of thesulphate anion at 532.0 eV.

[EMIM]EtPO3H presents a relatively high initial contact angle(Table 3) but, at the end of the test, it completely covers the coppersurface, turning deep blue and leaving a slightly tarnished coppersurface (Table 2).

As can be observed in the SEM micrograph (Fig. 5),[EMIM](EtPO3H) produces a very severe corrosive attack, removingthe surface layers and leaving a porous surface with numerouspittings. The EDX spectrum (Fig. 5b) shows only the peaks cor-responding to copper, but the phosphorus element map (Fig. 5c)shows that the copper surface is completely covered by phosphorusfrom the phosphonate anion of the ionic liquid.

The XPS spectra (Fig. 6) shows the Cu 2p B.E. can be deconvolutedinto two peaks at 935.2 and 933.2 eV, which could be due to Cu(II)and Cu(I) species, respectively [60,61].

The ionic liquid on the copper surface shows C 1s peaksat 285.0 eV (aliphatic carbon); 286.2 and 287.9 eV (tenta-tively assigned to N C N and C C N carbons in the imi-dazolium ring, respectively) and 289.5 eV (carbon bonded tothe phosphonate group in the anion). The N 1s at 399.8 eVis in agreement with nitrogen in cationic imidazolium rings[62].

The P 2p3/2 and P 2p1/2 B.E. at 133.4 and 134.8 eV could cor-respond to the phosphonate anion. The O 1s core peak can bedeconvoluted to give two peaks at 531.6 and 532.7 eV, in a 2:1 pro-portion, in agreement with the oxygen atoms of the phosphonateanion.

The FTIR spectrum of the liquid phase after the test (Fig. 7) showsclear changes with respect to that of the neat [EMIM](EtPO3H).Intense bands due to water absorption during the test are observed.The 1571 cm−1 band, due to the �(C N) stretching vibration in the


imidazolium ring [63], remains unchanged. The band that appearedat 929.6 cm−1 in the ionic liquid, shifts to higher frequency,appearing at 940.3 cm−1, being assignable to P OCu. The intenseabsorption at 1226.6 cm−1 is characteristic of the P O stretch.






Fig. 8. (a) SEM micrograph; (b) EDX spectrum and (c) element maps of the copper surface after the test with [HMIM]PF6.






x 103

10

15

20

25

30

CPS

298 296 294 292 290 288 286 284 282 280

Binding Energy (eV)Data evaluated using CasaXPS

x 103

55

60

65

70

75

80

85

CPS

970 965 960 955 950 945 940 935 930

Binding Energy (eV)

Cu 2p C 1s


x 103

50

52

54

56

58

60

CPS

698 696 694 692 690 688 686 684 682 680

Binding Energy (eV)

F 1s


x 103

55

60

65

70

75

80

85

90

95

CPS

544 542 540 538 536 534 532 530 528 526

Binding Energy (eV)

O 1s


x 102

74

76

78

80

82

84

86

88

90

CPS

146 144 142 140 138 136 134 132 130 128

Binding Energy (eV)Data evaluated using CasaXPS

x 101

240

250

260

270

280

290

300

310

CPS

412 410 408 406 404 402 400 398 396 394

Binding Energy (eV)

N 1s P 2p

er su

Tiw

3

rc

Fig. 9. XPS binding energies for copp

his band shifts to lower frequency and appears at 1200.6 cm−1,ndicating a change in the phosphonate group due to interaction

ith copper.


.2.2. 1-Hexyl-3-methylimidazolium derivativesAmong the four imidazolium salts tested, the lowest corrosion

ate is observed for [HMIM]PF6, with lower mass and roughnesshanges (Table 3), with very mild colour changes (Table 2).

rface after the test with [HMIM]PF6.

[HMIM]PF6 produces a continuous surface layer on the areawhich was covered by the ionic liquid (Fig. 8a). The EDX spectrumof this region shows intense C, F and P peaks (Fig. 8b), and is con-firmed by the element maps (Fig. 8c) showing the interface between


the original copper surface and the new surface layer generated byinteraction with the ionic liquid.

XPS analysis (Fig. 9) is in agreement with the presence ofboth the imidazolium and the hexafluorophosphate anion on





10

20

30

40

50

60

70

80

90

100

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

[HMIM]PF6

F retatiov

tasact

t�ab

(aaaTbc

b

ig. 10. FTIR spectra of [HMIM]PF6 before (red) and after (blue) the test. (For interpersion of the article.)

he copper surface. The Cu 2p3/2 peak at 935.3 eV, would bessignable to copper phosphate, while the 934.2 eV peak corre-ponds to CuO. The P 2p peak can be deconvoluted in two peakst 136.4 and 137.5 eV, respectively, which could be assigned toopper phosphate and to the hexafluorophosphate anion, respec-ively.

FTIR spectra of the liquid phase are the same before and afterhe test (Fig. 10). Peaks at 1574 and 1168 cm−1 are due to the(C N) and �(C H) of the imidazolium ring [58,63]. Intense peakst 816 cm−1 and 554 cm−1 are assignable to P F stretching andending vibrations, respectively.

When the anion is changed to BF4, with the same cationTable 1), the ionic liquid turns deep blue during the test (Table 2)nd the copper surface shows the highest roughness increase ofll ionic liquids tested (Table 3). [HMIM]BF4 was described [11]s lubricant of steel/Cu, with a low friction coefficient of 0.025.he good lubricating behaviour was attributed to the formation of


oron and fluorine containing protective layers under the slidingonditions of the tribological tests.

In this case, SEM observation shows the copper surface coveredy a discontinuous solid precipitate (Fig. 11a) which, according

Fig. 11. (a) SEM micrograph and (b) EDX spectrum of t

n of the references to colour in this figure legend, the reader is referred to the web

to the EDX spectrum (Fig. 11b) is composed of copper oxide. Nofluorine was detected.

XPS analysis (Fig. 12) is in agreement with the presence of theimidazolium cation and with copper oxide. No boron or fluorinebinding energies were detected.

Although a colour change is observed during the tests, the FTIRspectra of the liquid phase are identical before and after the tests(Fig. 13), with bands due to the imidazolium cation and intensebands at 1047 cm−1 due to the BF4

− anion.These results could be explained by the decomposition of the

tetraflouroborate anion in the presence of moisture, as the corro-sion tests were carried out in air.

3.3. Protic ionic liquids (PILs)

3.3.1. Di-[bis(2-hydroxyethyl)ammonium] adipate (DAd)The diprotic ammonium dianionic adipate (DAd) (Table 1)


shows the lowest corrosion rate of all six ionic liquids, as mea-sured both from mass and roughness changes (Table 3). DAd is alsothe ionic liquid which gives the highest initial contact angle on thecopper surface (Table 3). The liquid remains colourless during the

he copper surface after the test with [HMIM]BF4.







x 103

85

90

95

100

105

110

115

CPS

970 965 960 955 950 945 940 935 930

Binding Energy (eV)

Cu 2p


x 103

15

20

25

30

35

CPS

300 298 296 294 292 290 288 286 284 282

Binding Energy (eV)

C 1s


x 103

100

105

110

115

120

125

130

135

140

CPS

544 542 540 538 536 534 532 530 528 526

Binding Energy (eV)

O 1s


x 101

355

360

365

370

375

380

385

CPS

414 412 410 408 406 404 402 400 398 396

Binding Energy (eV)

N 1s

Fig. 12. XPS binding energies for copper surface after the test with [HMIM]BF4.

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

[HMIM]BF4

Fig. 13. FTIR spectra of [HMIM]BF4 before (red) and after (blue) the test. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of the article.)






Fig. 14. (a) SEM micrograph and (b) EDX spectrum of the copper surface after the corrosion test with DAd.


x 103

48

50

52

54

56

58

60

62

64

66

CPS

970 965 960 955 950 945 940 935 930

Binding Energy (eV)

Cu 2p


x 103

15

20

25

30

35

40

CPS

298 296 294 292 290 288 286 284 282 280

Binding Energy (eV)

C 1s


x 103

60

65

70

75

80

85

90

CPS

544 542 540 538 536 534 532 530 528 526

Binding Energy (eV)

O 1s


x 102

120

125

130

135

140

145

150

CPS

412 410 408 406 404 402 400 398 396 394

Binding Energy (eV)

N 1s

Fig. 15. XPS binding energies for copper surface after the test with DAd.





30

35

40

45

50

55

60

65

70

75

80

85

90

95

%T

ran

sm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

DAd

Fig. 16. FTIR spectra of DAd before (red) and after (blue) the corrosion test. (For interpretation of the references to colour in this figure legend, the reader is referred to thew

cpt

bts

scwa2t

at2

eb version of the article.)

orrosion tests (Table 3) and no precipitates are formed on the cop-er surface, which only shows a slight colour change at the end ofhe test.

SEM observation showed some crack initiation at the interfaceetween the region which was covered by the ionic liquid andhe uncovered one (Fig. 14a). Nevertheless, EDX analysis (Fig. 14b)hows only the presence of copper.

XPS spectra of the copper surface after being covered by DAd arehown in Fig. 15. The Cu 2p level shows a broad Cu 2p3/2 signal thatan be decomposed into two contributions at 934.7 eV and 933.6 eV,ith the corresponding shake-up satellite peaks. The contribution

t a higher binding energy (934.7 eV) and the observed I(Cusat)/I(Cup) intensities ratio indicate that copper is mainly as Cu(II), whilehe contribution at 933.6 eV can be assigned to Cu(I) species [60,61].


The three deconvoluted peaks for C 1s appear at 285.0 eV,ssignable to aliphatic carbon, but also to possible contamina-ion, at 286.3 eV, attributable to aliphatic carbon or C O, and at88.4 eV, assignable to C O and COO−.

Fig. 17. (a) SEM micrograph and (b) EDX spectrum of the

Only one O 1s peak is observed at 531.8 eV, which could beassigned to CO or COO−, but also to the hydroxyl groups in thecation. The N 1s peak of the ammonium cation appears at 399.8 eV.

These results are in agreement with the formation of a sur-face layer containing the DAd molecules. This kind of interaction,together with the low corrosion rate, would be favourable for lubri-cating purposes.

Fig. 16 shows the superposition of the FTIR spectra of neat DAdbefore the test and of the liquid phase recovered from the coppersheet after the test. No significant changes are observed, indicatingthat there is no new soluble phase formed by reaction between DAdand copper.

3.3.2. Di-(2-hydroxyethylammonium) succinate (MSu)


The results obtained for DAd are in contrast with those found forthe triprotic ammonium succinate MSu, which forms a dark bluecorrosion product that completely covers the copper surface at theend of the test (Table 2).

copper surface after the corrosion test with MSu.






x 103

75

80

85

90

95

CPS

970 965 960 955 950 945 940 935 930

Binding Energy (eV)

Cu 2p


Block No.2

C1s C1s

x 103

14

16

18

20

22

24

26

28

30

32

34

CPS

298 296 294 292 290 288 286 284 282 280

Binding Energy (eV)

C 1s


x 101

320

330

340

350

360

370

380

390

400

CPS

412 410 408 406 404 402 400 398 396 394

Binding Energy (eV)

N 1s


x 103

80

85

90

95

100

105

110

115

CPS

544 542 540 538 536 534 532 530 528 526

Binding Energy (eV)

O 1s

Fig. 18. XPS binding energies for copper surface after the corrosion test with MSu.

oieo

ttf

5oi

t

ei

ttpwt

In this case, SEM/EDX analysis (Fig. 17a and b, respectively)f the dry copper surface after the test shows high roughness,n agreement with the �Sa described in Table 2, and the pres-nce of oxygen and carbon peaks, together with the coppernes.

The XPS analysis (Fig. 18) shows peaks at 933.8 eV, assignableo Cu(I) and at 935.2 eV, assignable to Cu(II). The deconvolution ofhe C 1s peak shows similar binding energies to those describedor DAd, with a new peak at higher energy, tentatively assigned toCOOCu.

In this case, the O 1s peak can be deconvoluted in two peaks, at31.5 and 533.1 eV. The lower energy peak is attributable to copperxide [26], while the peak at higher energy could be due to COOCunteractions.

As in the case of DAd, only one N 1s peak, at 399.8 eV, assignableo the ammonium cation, is observed.

The FTIR spectra (Fig. 19a), shows that the liquid phase recov-red after the test does not contain reaction products, as it isdentical to that of neat MSu.

The corrosion product was separated from the liquid after theest as blue-violet crystals, designed as (MSu + Cu). The FTIR spec-


rum of (MSu + Cu) can be observed in Fig. 19b. The two intenseeaks at 3235 and 3150 cm−1 are assignable to �(OH) stretching,hile medium bands at 2954, 2938 and 2864 cm−1 can be due to

he stretching vibrations �(NH) and �(CH). Intense peaks at 1561

and 1419 cm−1 correspond to asymmetric and symmetric �(OCO)stretching, respectively, while the bending frequencies �(OCO) areobserved at 738 and 750 cm−1.

Thermal analysis (TGA) was carried out both for neat MSuand for the new corrosion product (Fig. 20). The successiveweight losses are similar, with a shift to higher tempera-tures for the corrosion product (MSu + Cu) with respect to neatMSu. The ionic liquid MSu shows three mass loss steps withonset temperatures at 120.2 ◦C (−12.8% weight loss), 182.1 ◦C(−38.2%) and 239.2 ◦C (−47.9%). The corrosion product (MSu + Cu)shows also three main mass loss steps, with onset at 144.9 ◦C(−4.4%), 171.1 (−18.4%) and 214.5 (−49.7%). The main dif-ference is the weight percentage of the final residues, 0.98%for neat MSu and 24.75% for the (MSu + Cu) crystalline solid.If a 1:1 MSu/CuO proportion is considered, the calculatedvalue (24.88%) would be in very good agreement with CuOas the final residue, after the total loss of the MSu moleculeby thermal decomposition of the blue crystalline (MSu + Cu)compound.


3.4. Tribological tests

In order to establish a correlation between corrosion behaviourand tribological performance, both PILs were selected to be






a

b

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

%T

ran

sm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

MSu

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

(MSu+Cu)

Fig. 19. FTIR spectra of (a) MSu before (red) and after (blue) the corrosion test and (b) of the (MSu + Cu) blue crystals. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of the article.)

Fig. 20. Comparative TGA in N2 for neat MSu and for (MSu + Cu) crystals.






Fig. 21. DAd: (a) friction-sliding distance record; (b) 3D surface profile of copper disc surface after the tribological test.

Fig. 22. MSu: (a) friction-sliding distance record; (b) 3D surface profile of copper disc surface after the tribological test.





Fig. 23. [HMIM]PF6: (a) friction-sliding distance record; (b) 3D surface profile of copper disc surface after the tribological test.

uooesctttb

vwpm2idl

b

sed as lubricants of copper–copper contacts under a pin-n-disc configuration. We have very recently described theutstanding performance of DAd, the IL which produces the low-st corrosive attack, as copper–copper lubricant [47]. Fig. 21ahows the friction coefficient-sliding distance record for theopper–copper contact lubricated with DAd. A mean fric-ion coefficient of 0.013, with a 0.009 value at the end ofhe test is obtained. The 3D profile (Fig. 21b) shows thathere is no wear path and the surface damage is negligi-le.

In contrast, when MSu was tested under the same conditions, aery irregular friction-sliding distance record is obtained (Fig. 22a)ith a mean friction value of 0.029, but with maximum frictioneaks of 0.050 from 100 to 500 m, and with an abrasive wearechanism, as can be observed in Fig. 22b, with a wear rate of

.1 × 10−7 mm3 m−1. This increasing friction and surface damages attributed to the reactivity between MSu and copper, which


isrupts the formation of a stable surface adsorbed lubricatingayer.

Among the imidazolium APILs, [HMIM]PF6 was selected for tri-ological tests due to the lower corrosion rate and to the formation

of the P- and F-containing surface layer (see Section 3.2.2) whichcould be protective against wear.

A mean friction coefficient of 0.026, similar to that describedfor MSu is obtained (Fig. 23a), but in this case, the friction recordis smooth and it maintains constant with sliding distance, thusindicating the absence of severe tribocorrosion processes. Thewear scar profile shown in Fig. 23b is also indicative of abrasivewear, but the wear damage is clearly milder than that found forMSu (Fig. 22b), with a wear rate value for the imidazolium IL of7.2 × 10−8 mm3 m−1.

In all cases the wear tracks are too narrow to allow XPS analysisinside and outside them after the tribological tests.

In the case of [HMIM]PF6, SEM observation and EDX analysisof the wear path (Fig. 24a) shows a mild abrasive surface damagewith the presence of metallic copper inside the wear track. Theelement maps (Fig. 24b) show no phosphorus and a slight presenceof fluorine over the surface.


The order of lubricating ability, from lower to higher frictioncoefficients and wear rates: DAd > [HMIM]PF6 > MSu, is in agree-ment with the order from lower to higher copper corrosion rateobtained for these ILs.





ectru

4

ci

d

mhlsr

[a

alrc

nXauafsc

aa

Fig. 24. [HMIM]PF6: (a) SEM micrograph and EDX sp

. Conclusions

The mass and roughness changes of OFHC copper after being inontact with six ILs, two protic ammonium (PIL) and four aproticmidazolium species, have been determined.

All four imidazolium ILs show reactivity towards copper, withifferent mechanism related to the nature of the anions.

The highest mass loss is found for [EMIM]EtPO3H, due to the for-ation of a soluble phosphonate-copper species. As expected, the

igh reactivity of [EMIM](C8H17SO4) towards copper produces theowest contact angle, and gives rise to the precipitation of copperulphate on the copper surface, thus producing a very high surfaceoughness increase.

The highest surface roughness increase takes place forHMIM]BF4 due to the decomposition of the tetrafluoroboratenion, which probably forms soluble species with copper.

The lowest mass and roughness changes of the imidazolium ILsre obtained for [HMIM]PF6, due to the formation of a uniform solidayer on the copper surface, due to the reactivity with the hexafluo-ophosphate anion. This surface layer could act as a wear protectiveoating.

The lowest corrosion rate is observed for the diprotic ammo-ium (PIL) di-[bis-(2-hydroxyethyl)ammonium] adipate (DAd).RD, SEM–EDX, XPS and FTIR studies show that DAd formsdsorbed layers on the copper surface, but no corrosion prod-cts. The low corrosion and the formation of adsorbed layers aremong the conditions which should meet a potential candidateor high performance lubrication. In fact, tribological tests havehown ultralow friction coefficient of 0.01 and negligible wear for


opper-copper contacts.The second PIL studied, the triprotic (2-hydroxyethyl)

mmonium succinate (MSu), presents an initial contact anglebout a 50% lower than that of DAd, a mass loss 35 times higher

m inside the wear track; (b) Cu and F element maps.

and a surface roughness change 75 times higher than those foundfor DAd. This is due to the higher reactivity of MSu towards copper,which gives rise to the formation of a precipitate by combinationof the PIL and copper oxide. MSu shows a mean friction coefficientabout three times higher than that found for DAd.

The results of the tribological tests for DAd, MSu and [HMIM]PF6confirm a better lubricating performance for a lower corrosion rate.The use of protic ammonium carboxylate PILs as lubricants has beenthe object of a patent [64].

Acknowledgments

The authors wish to express their gratitude to Dr. M. Igle-sias (Universidad Federal de Bahia, Brazil) for the synthesis ofthe PILs, and Ministerio de Economía y Competitividad (Spain)(MAT2011-23162) for financial support. T. Espinosa is gratefulto the Ministerio de Educación, Cultura y Deporte (Spain) for aresearch grant (AP2010-3485).

References

[1] I. Minami, Ionic liquids in tribology, Molecules 14 (2009) 2262–2269.[2] F. Zhou, Y. Liang, W. Liu, Ionic liquid lubricants: designed chemistry for engi-

neering applications, Chemical Society Reviews 28 (2009) 2590–2599.[3] M.D. Bermúdez, A.E. Jiménez, J. Sanes, F.J. Carrión, Ionic liquids as advanced

lubricants, Molecules 14 (2009) 2888–2908.[4] T. Torimoto, T. Tsuda, K. Okazaki, S. Kuwawata, New frontiers in materials

science opened by ionic liquid, Advanced Materials 22 (2010) 1196–1221.[5] M. Palacio, B. Bushan, A review of ionic liquids for green molecular lubrication

in nanotechnology, Tribology Letters 40 (2010) 247–268.


[6] E. Schluecker, P. Wasserscheid, Ionic liquids in mechanical engineering, ChemieIngenieur Technik 83 (2011) 1476–1484.

[7] C.A. Angell, Y. Ansari, Z.F. Zhao, Ionic liquids: past, present and future, FaradayDiscussions 154 (2012) 9–27.

[8] Ionic liquids in tribology, Tribology Letters 40 (2010) 213–284.


ING Model

A

2 rface

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[63] Q.G. Zhang, N.N. Wang, Z.W. Yu, The hydrogen bonding interactions betweenthe ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate and water, Journal


0 T. Espinosa et al. / Applied Su

[9] Ionic liquids as lubricants, Proceedings of the Institution of Mechanical Engi-neers, Part J: The Journal of Engineering Tribology 226 (2012) 889–1006.

10] T. Predel, B. Pohrer, E. Schlucker, Ionic liquids as alternative lubricants for spe-cial applications, Chemical Engineers and Technology 33 (2010) 132–136.

11] C.F. Ye, M. Liu, Y.X. Chen, L.G. Yu, Room-temperature ionic liquids: a novelversatile lubricant, Chemical Communications 224 (2001) 4–224, 5.

12] R.P. Swatloski, J.D. Holbrey, R.D. Rogers, Ionic liquids are not alwaysso green: hydrolysis of 1-butyl-3-methylimidazolium hexafluorophosphate,Green Chemistry 5 (2003) 361–363.

13] M. Uerdingen, C. Treber, M. Balser, G. Schmitt, C. Werner, Corrosion behaviourof ionic liquids, Green Chemistry 7 (2005) 321–325.

14] J. Sanes, F.J. Carrión, M.D. Bermúdez, G. Martínez-Nicolás, Ionic liquids aslubricants of polystyrene and polyamide 6-steel contacts. Preparation andproperties of new polymer–ionic liquid dispersions, Tribology Letters 21 (2006)121–133.

15] M.R. Cai, Y.M. Liang, F. Zhou, W.M. Liu, Anticorrosion imidazolium ionic liquidsas the additive in poly(ethylene glycol) for steel/Cu–Sn alloy contacts, FaradayDiscussions 156 (2012) 147–157.

16] Z. Zhao, Y.W. Shao, T.M. Wang, T.M.D.P. Feng, W.M. Liu, Study on corrosion prop-erty of a series of hexafluorophosphate ionic liquids on steel surface, CorrosionEngineering, Science and Technology 46 (2011) 330–333.

17] I. Perissi, U. Bardi, S. Caporali, A. Lavacchi, High temperature corrosion proper-ties of ionic liquids, Corrosion Science 48 (2006) 2349–2362.

18] U. Bardi, S.P. Chenakin, S. Caporali, A. Lavacchi, I. Perissi, A. Tolstogouzov, Sur-face modification of industrial alloys induces by long-term interaction with anionic liquid, Surface and Interface Analysis 38 (2006) 1768–1772.

19] I. Perissi, U. Bardi, S. Caporali, A. Fossati, A. Lavacchi, Ionic liquids as diathermicfluids for solar trough collector’s technology: a corrosion study, Solar EnergyMaterials and Solar Cells 92 (2008) 510–517.

20] S. Sowmiah, V. Srinivasadesikan, M.C. Tseng, Y.H. Chu, On the chemical stabil-ities of ionic liquids, Molecules 14 (2009) 3780–3813.

21] P.J. Scammells, J.L. Scott, R.D. Singer, Ionic liquids: the neglected issues, Aus-tralian Journal of Chemistry 58 (2005) 155–169.

22] M.D. Bermúdez, A.E. Jiménez, G. Martínez-Nicolás, Study of surface interac-tions of ionic liquids with aluminium alloys in corrosion and erosion-corrosionprocesses, Applied Surface Science 253 (2007) 7295–7302.

23] M.D. Bermúdez, A.E. Jiménez, Surface interactions and tribochemical processesin ionic liquid lubrication of aluminium–steel contacts, International Journal ofSurface Science and Engineering 1 (2007) 100–110.

24] C. Gabler, C. Tomastik, J. Brenner, L. Pisarova, L.N. Doerr, G. Allmaier, Corrosionproperties of ammonium based ionic liquids evaluated by SEM–EDX, XPS andICP-OES, Green Chemistry 13 (2011) 2869–2877.

25] K. Marczewska-Boczkowska, M. Kosmulski, The effect of chloride and wateron the corrosion of copper in 1-butyl-3-methylimidazolium tetraflouroborate,Materials and Manufacturing Processes 24 (2009) 1173–1179.

26] X. Liu, F. Zhou, Y. Liang, W. Liu, Benzotriazole as the additive for ionic liquidlubricant: one pathway towards actual application of ionic liquids, TribologyLetters 23 (2010) 191–196.

27] B. Yu, F. Zhou, C. Pang, B. Wang, Y. Liang, W. Liu, Tribological evaluation of �,�-diimidazoliumalkylene hexafluorophophate ionic liquid and benzotriazole asadditive, Tribology International 41 (2008) 797–801.

28] H. Kamimura, T. Kubo, I. Minami, S. Mori, Effect and mechanism of additivesfor ionic liquids as new lubricants, Tribology International 40 (2007) 620–625.

29] P. Wasserscheid, R. van Hal, A. Bosmann, New, halogen-free ionic liquids —synthesis, properties, and applications, in molten salts, in: P.C. Trulove, H.C.DeLong, R.A. Mantz, G.R. Stafford, M. Matsunaga (Eds.), XIII Book Series: Elec-trochemical Society Series, 2002, 2002, pp. 146–154.

30] I. Minami, T. Inada, Y. Okada, Tribological properties of halogen-free ionic liq-uids, Proceedings of the Institution of Mechanical Engineers, Part J: The Journalof Engineering Tribology 226 (2012) 891–902.

31] T.L. Greaves, C.J. Drummond, Protic ionic liquids: properties and applications,Chemical Reviews 108 (2008) 206–237.

32] T.L. Greaves, D.F. Kennedy, A. Weerawardena, N.M.K. Tse, N. Kirby, C.J. Drum-mond, Nanostructured protic ionic liquids retain nanoscale features in aqueoussolutions while precursor Bronsted acids ad bases exhibit different behaviour,Journal of Physical Chemistry B 115 (2011) 2055–2066.

33] U.A. Rana, R. Vijayaraghavan, M. Walther, J.Z. Sun, A.A.J. Torriero, M. Forsyth,D.R. MacFarlane, Protic ionic liquids based on phosphonium cations: com-parison with ammonium analogues, Chemical Communications 47 (2011)11612–11614.

34] A. Pinkert, K.L. Ang, K.N. Marsh, S.S. Pang, Density, viscosity and electricalconductivity of protic alkanolammonium ionic liquids, Physical ChemistryChemical Physics 13 (2011) 5136–5143.

35] V.H. Alvarez, S. Mattedi, M. Martin-Pastor, M. Aznar, M. Iglesias, Thermophy-sical properties of binary mixtures of {ionic liquid 2-hydroxy ethylammoniumacetate plus (water, methanol, or ethanol)}, The Journal of Chemical Thermo-dynamics 43 (2011) 997–1010.

36] M.S. Miran, H. Kinoshita, T. Yasuda, M. Abu Bin, H. Susanz, M. Watan-abe, Hydrogen bonds in protic ionic liquids and their correlationwith physicochemical properties, Chemical Communications 47 (2011)12676–12678.


37] M. Iglesias, R. Gonzalez-Olmos, I. Cota, F. Medina, Bronsted ionic liquids: studyof physico-chemical properties and catalytic activity in aldol condensations,Chemical Engineering Journal 162 (2010) 802–808.

[


38] V.H. Alvarez, S. Mattedi, M. Martin-Pastor, M. Aznar, M. Iglesias, Synthesis andthermophysical properties of two new protic long-chain ionic liquids with theoleate anion, Fluid Phase Equilibria 299 (2010) 42–50.

39] L. Tamar, L. Greaves, A. Weerawardena, C. Fong, I. Krodkiewska, C.J. Drum-mond, Protic ionic liquids: solvents with tunable phase behavior andphysicochemical properties, Journal of Physical Chemistry B 110 (2006)22479–22487.

40] B. Peric, E. Marti, J. Sierra, R. Cruanas, M. Iglesias, M.A. Garau, Terrestrial eco-toxicity of short aliphatic protic ionic liquids, Environmental Toxicology andChemistry 30 (2011) 2802–2809.

41] S. Mattedi, P.J. Carvalho, J.A.P. Coutinho, V.H. Alvarez, M. Iglesias, High pressureCO2 solubility in N-methyl-2-hydroxyethylammonium protic ionic liquids,Journal of Supercritical Fluids 56 (2011) 224–230.

42] R.A. Asencio, E.D. Cranston, R. Atkin, M.W. Rutland, Ionic liquid nanotribology:stiction suppression and surface induced shear thinning, Langmuir 28 (2012)9967–9976.

43] J. Qu, J.J. Truhan, S. Dai, H. Luo, P.J. Blau, Ionic liquids with ammonium cationsas lubricants or additives, Tribology Letters 22 (2006) 207–214.

44] H. Kondo, Protic ionic liquids with ammonium salts as lubricants for magneticthin film media, Tribology Letters 31 (2008) 211–218.

45] E.D. Cranston, O. Werzer, R. Alvarez, R. Atkin, M.W. Rutland, Nanotribologyof protic ionic liquids: green lubricants for micro-/nano-electromechanicaldevices, in: Abstr. Pap. Am. Chem. Soc. 241st National Meeting and Expositionof the American-Chemical-Society (ACS), vol. 241, Anaheim, CA (USA), 2011,pp. 1133–1140.

46] Q. Zhao, G. Zhao, M. Zhang, X. Wang, W. Liu, Tribological behaviour of proticionic liquids with dodecylamine salts of dialkyldithiocarbamate as additives inlithium complex grease, Tribology Letters 48 (2012) 133–144.

47] M.D. Bermúdez, F.J. Carrión, A.E. Jiménez, J. Sanes, G. Martinez-Nicolas, C.Espejo, T. Espinosa, J. Arias, G. Ojados, N. Gonzalez, M. Jiménez, Tribologicalperformance and surface interactions of new ionic nanofluids and nanomate-rials, in: Chemistry and Physics in Tribology. ACS Tribology Symposium. 244stACS National Meeting and Exposition of the American Chemical Society (ACS),Philadelphia, PA, USA, 2012, 205-COLL.

48] A.E. Jiménez, M.D. Bermúdez, P. Iglesias, F.J. Carrión, G. Martínez-Nicolás, 1-N-alkyl-3-methylimidazolium ionic liquids as neat lubricantsand lubricant additives in steel-aluminium contacts, Wear 260 (2006)766–782.

49] A.E. Jiménez, M.D. Bermúdez, F.J. Carrion, G. Martínez-Nicolás, Room-temperature ionic liquids as lubricant additives in steel-aluminium contacts:influence of sliding velocity, normal load and temperature, Wear 261 (2006)347–359.

50] J. Sanes, F.J. Carrión, A.E. Jiménez, M.D. Bermúdez, Influence of temperatura onPA6-steel contacts in the presence of an ionic liquid lubricant, Wear 263 (2007)658–662.

51] A.E. Jiménez, M.D. Bermúdez, Imidazolium ionic liquids as additives of the syn-thetic ester propylene glycol dioleate in aluminium-steel lubrication, Wear 265(2008) 787–798.

52] A.E. Jiménez, M.D. Bermúdez, Ionic liquids as lubricants of titanium-steel con-tact, Tribology Letters 33 (2009) 111–126.

53] A.E. Jiménez, M.D. Bermúdez, P. Iglesias, Lubrication of Inconel 600with ionic liquids at high temperature, Tribology International 42 (2008)1744–1751.

54] A.E. Jiménez, M.D. Bermúdez, Ionic liquids as lubricants of titanium-steelcontact. Part 2: Friction, wear and surface interactions at high temperature,Tribology Letters 37 (2010) 431–443.

55] P. Wasserscheid, R. van Hal, A. Bosmann, 1-n-Butyl-3-methylimidazolium([bmim]) octylsulfate-an even ‘greener’ ionic liquid, Green Chemistry 4 (2002)400–404.

56] ASTM D130-12, Standard test method for detection of copper corrosion frompetroleum products by the copper strip tarnish test, 2012.

57] M. Zhang, X.D. Xu, M.L. Zhang, Hydrothermal synthesis of sheaf-like CuO viaionic liquids, Materials Letters 62 (2008) 385–388.

58] ASTM G99-05, Standard Test Method for Wear Testing with a Pin-on-DiskApparatus, 2010.

59] ISO 25178-2, Geometrical product specifications (GPS). Surface texture: Areal.Part 2: Terms, definitions and surface texture parameters, 2012.

60] J.M. Lázaro-Martínez, E. Rodríguez-Castellón, R.M. Torres, L.R. Denaday, G.Y.Buldain, V. Campo Dall’Orto, XPS studies on the Cu(I,II)-polyampholyte het-erogeneous catalyst: an insight into its structure and mechanism, Journal ofMolecular Catalysis A: Chemical 339 (2011) 43–51.

61] F. Parmigiani, G. Pacchioni, F. Illas, P.S. Bagus, Studies of the Cu-O bond in cupricoxide by X-ray photoelectron spectroscopy and ab initio electronic-structuremodels, Journal of Electron Spectroscopy and Related Phenomena 59 (1992)255–269.

62] S. Caporali, U. Bardi, A. Lavacchi, X-ray photoelectron spectroscopyand low energy ion scattering studies on 1-buthyl-3-methyl-imidazoliumbis(trifluoromethane) sulfonimide, Journal of Electron Spectroscopy andRelated Phenomena 151 (2006) 4–8.


of Physical Chemistry B 114 (2010) 4747–4754.64] M.D. Bermúdez, A.E. Jiménez, J. Sanes, Protic ionic liquids. Patent # P201131590,

Spain, 2011.


Documents

Surface interactions, corrosion processes and lubricating performance of protic and aprotic ionic liquids with OFHC copper