Miscibility, corrosion and environmental properties of six hexanoate- and sulfonate-based protic ionic liquids

Article abstract of DOI:10.1016/j.molliq.2020.114561

In this work, six different types of protic ionic liquids (PILs) have been studied. For all these PILs, the bacteria toxicity and environmental biodegradability have been investigated, as well as their corrosive behavior and miscibility in different types of lubricating base oils. The PILs studied showed poor to moderate biodegradability (BOD₅/COD from 0.072 ± 0.002 to 0.18 ± 0.01) and slight toxicity (TU values between from 1.18 ± 0.04 and 2.7 ± 0.1). However, these environmental properties are much better than those obtained for the reference lubricant additive used in industry (ZDDP). In addition, five of the PILs did not show corrosive activity on steel. Only one of the PILs showed some miscibility in an ester as a base oil and the rest of the mixtures were highly unstable.

Full text of DOI:10.1016/j.molliq.2020.114561

Journal of Molecular Liquids 322 (2021) 114561
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Miscibility, corrosion and environmental properties of six
hexanoate- and sulfonate-based protic ionic liquids
J.L. Viesca ⁎, P. Oulego , R. González , H. Guo , A. Hernández Battez a,b, P. Iglesias e
a,b,
c
b,d
e
a
Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain
Faculty of Science and Technology, Bournemouth University, UK
Department of Chemical and Environmental Engineering, University of Oviedo, Asturias, Spain
Department of Marine Science and Technology, University of Oviedo, Asturias, Spain
b
c
d
e
Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, NY, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2020
Received in revised form 24 September 2020
Accepted 10 October 2020
Available online 17 October 2020
In this work, six different types of protic ionic liquids (PILs) have been studied. For all these PILs, the bacteria tox-
icity and environmental biodegradability have been investigated, as well as their corrosive behavior and miscibil-
ity in different types of lubricating base oils. The PILs studied showed poor to moderate biodegradability (BOD /
5
COD from 0.072 ± 0.002 to 0.18 ± 0.01) and slight toxicity (TU values between from 1.18 ± 0.04 and 2.7 ± 0.1).
However, these environmental properties are much better than those obtained for the reference lubricant addi-
tive used in industry (ZDDP). In addition, five of the PILs did not show corrosive activity on steel. Only one of the
PILs showed some miscibility in an ester as a base oil and the rest of the mixtures were highly unstable.
© 2020 Elsevier B.V. All rights reserved.
Keywords:
Protic ionic liquids
Miscibility
Corrosion
Bacterial toxicity
Biodegradability
1. Introduction
fluoride (HF) [20], which may damage the surfaces. Blanco et al. [21]
2
also described the corrosive behavior of some [NTf ] anion-based ILs.
Ionic liquids are formed by organic cations and organic or inorganic
Among the ILs, there are a subset denominated protic ILs (PILs). They
can be synthesized combining a Bronsted acid and a Bronsted base and
they can be tailored to satisfy specific requirements [22–26]. This makes
the cost of the synthesis of PILs much lower than that of aprotic ILs
(APILs) [26,27]. Walden reported the first PILs in 1914 [28] and now
there is an increasing interest to use them in lubrication [24,26,27,29].
Espinosa et al. [25,31] investigated the tribological behavior of some
PILs as lubricant additives in sapphire-stainless steel and copper-copper
contacts. They found that the PIL forms a boundary film on steel surface,
which leads to an ultra-low friction value [31]. They concluded that the
reason of this low friction value is due to hydrogen bonds that occur be-
tween sliding surfaces and hydroxyl substitutes for ammonium groups.
Vega et al. [32] used three oleate-based PILs as lubricants for alumin-
ium, aiming to apply them in metal forming processes. These PILs
formed uniform tribofilms leading to low friction values and a wear re-
duction of 98% in comparison with dry lubrication conditions.
Recently, Guo et al. [30,33] synthesized and studied three novel
hexanoate-based PILs as neat lubricants and additives in the lubrication
of a steel-steel contact. All the PILs greatly reduced friction and wear
when using them as neat lubricants compared to mineral oil with and
without additives [30]. Furthermore, the PILs also decreased friction
and wear when they were used as additives, in this case compared
with a biodegradable base oil with and without additives [33]. This
anions, which melt under 100 °C [1–3]. Unlike conventional salts, the
ionic liquids (ILs) are made up of asymmetric ions, which make weaker
the interaction forces between them. Because of this, ILs have very
interesting properties such as: high viscosity, low volatility, low melting
point and high boiling point, which makes them liquid in a wide
temperature range, high both thermal and chemical stability, non-
flammability, etc. The number of studies dealing with ILs in lubrication
has hugely increased since 2001 [4–9]. However, the high cost of ILs
has limited their use as neat lubricants in comparison to hydrocarbon
oils. Because of this, several research works have employed ILs as lubri-
cant additives in base oils [5–11]. However, the low miscibility in non-
polar hydrocarbon oils has been described as their major problem to
be used as additives [12–19].
Despite the good tribological behavior that many ILs have shown,
another aspect of interest that limits the utilization of some ILs as lubri-
cants or lubricant additives is related to their corrosive activity.
−
−
Imidazolium-based ILs with [PF
6 4
] and [BF ] anions are hydrophilic
and react with water leading to the production of corrosive hydrogen
⁎
Corresponding author at: Department of Construction and Manufacturing
Engineering, University of Oviedo, Asturias, Spain.
E-mail address: viescajose@uniovi.es (J.L. Viesca).
https://doi.org/10.1016/j.molliq.2020.114561
0167-7322/© 2020 Elsevier B.V. All rights reserved.

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
research work showed that PILs contribute to forming protective
tribofilms against tribocorrosion even used as additives at very low
concentrations.
have been published, in which different aquatic organisms including
bacteria, algae, aquatic plants, terrestrial plants, vertebrates, inverte-
brates or even frogs were used (Table 1). In these studies, a relationship
between IL toxicity and different parameters, such as the length of the
alkyl chain, short second alkyl chain of the cation, functionalization of
side chain, cationic part and type of anion, was established (Table 1).
The inhibition assays, specifically bacterial bioassays using Vibrio
fischeri (formerly known as Photobacterium phosphoreum), are the
most widely used methods to determine the toxicity in aqueous
medium according to various authors [52,53]. Vibrio fischeri test is
characterized for being the most rapid, cost effective, sensitive and
The excellent tribological behavior shown by the PILs, together with
their cheap synthesis, makes them very interesting candidates for in-
dustrial applications. However, some properties such as, their corrosive
action on metals [20,21], and their miscibility in different types of base
oils [11,13] should be studied before. Besides, it is also necessary to eval-
uate their biodegradability and toxicity in order to avoid environmental
damage if they are released into the aquatic environment. In this re-
spect, several works related to the aquatic toxicity of ionic liquids
Table 1
Evaluation of the aquatic toxicity of ionic liquids.
Ionic liquid
Aquatic organism
Conclusions
Ref.
Phosphonium, imidazolium- and
ammonium-based
Marine bacteria: Vibrio fischeri
Enterobacteria: Escherichia coli
Cationic and anionic parts exerted significant effect on toxicity, although it
was mainly determined by the cation.
[34]
Increase in the toxicity with higher side chain length of the cation and the
presence of fluorine in the anion.
E. coli was more sensitive to the stress caused by the ILs.
The increase in elongation of the alkyl chain of the PILs caused a negative
N-methyl-2-hydroxyethylammonium-based
Marine bacteria: Vibrio fischeri
[35]
Terrestrial plant: lettuce (Lactuca sativa) impact on both the marine bacteria and Lactuca sativa seeds.
seeds
Imidazolium-based
Green algae: Scenedesmus obliquus
Growth inhibition rate increased with increasing IL concentration and
increasing alkyl chain lengths.
[36]
[37]
Amine-, imidazolium- and pyridinium-based
Terrestrial plant: monocotyledon plants: Imidazolium- and pyridinium-based ILs are more toxic than amine PILs.
onion (Allium cepa) and grass (Lolium
perenne) seeds
Terrestrial plant: dicotyledon, radish
All imidazolium- and pyridinium-based ILs are toxic for onion.
Molecular weight and the size of the anionic part have more influence on
the PILs toxicity.
(
Raphanus sativus) seeds
Amine-, imidazolium- and pyridinium-based
Marine bacteria: Vibrio fischeri
Green microalgae: Pseudokirchneriella
subcapitata
Amine-based PILs are less toxic than imidazolium- and pyridinium-based ILs [38]
Voluminous heterocyclic cation with alkyl side chains in their structure
increase toxicity.
Aquatic plant: Lemna minor (duckweed) Hydrophilic functional groups in the side chain reduces the toxicity.
Imidazolium-, pyridinium-, piperidinium, and
pyrrolidinium-based
Marine bacteria: Vibrio fischeri
Invertebrate: freshwater crustacean
Daphnia magna
Cations with a six-member ring (pyridinium) are more toxic than those
with a five member ring (imidazolium)
[39]
Green microalgae: Pseudokirchneriella
subcapitata
Imidazolium-, phosphonium- and
pyrrolidinium-based
Marine bacteria: Vibrio fischeri
Toxicity increases with alkyl chain length.
Anionic part influences IL toxicity as follows: bis(trifluoromethylsulfonyl)
imide > hexafluorophosphate > tetrafluoroborate > chloride
[40]
[41]
Amine-based
Terrestrial plant: monocotyledon plants: Toxicity increases with the increase of complexity of the PILs molecule:
onion (Allium cepa) and grass (Lolium
perenne) seeds
branch and length of aliphatic chain
R sativus was the most sensitive to the presence of PILs.
Terrestrial plant: dicotyledon, radish
(
Raphanus sativus) seeds
Imidazolium-, pyridinium- and
pyrrolidinium-based
Imidazolium-, pyridinium- and
pyrrolidinium-based
Marine bacteria: Vibrio fischeri
Toxicity increases with alkyl chain length
[42]
[43]
Green microalgae: Pseudokirchneriella
subcapitata
Invertebrate: freshwater crustacean
Daphnia magna
Vertebrate: fish Danio rerio
Vertebrate: frog Rana nigromaculata
Toxicity followed this order:
pyridinium- > imidazolium- > pyrrolidinium-based ILs
Introduction of a more electronegative atom (chlorine or oxygen) on the
longer alkyl chain of the imidazolium ring significantly reduces the acute
toxicity to D. magna and P. subcapitata.
1-Methyl-3 octylimidazolium bromide has toxic effects on the early
embryonic development of the frog.
Imidazolium-based
[44]
Morphological malformations were also observed in the spinal area and
abdomen of treated embryos.
Imidazolium-based
Marine bacteria: Vibrio fischeri
Marine bacteria: Vibrio fischeri
The shorter the chain length of side chain R
Negligible effect of anionic part.
Functionalization of side chain (e.g. oxygenated chains) leads to lower
2
, the lower the toxic effect.
[45]
[46]
Pyridinium-, imidazolium-, morpholinium-,
piperidinium, pyrrolidinium and quaternary Aquatic plant: Lemna minor (duckweed) toxicity.
ammonium-based
Imidazolium-based
Green algae: Scenedesmus vacuolatus
Marine bacteria: Vibrio fischeri
Invertebrate: freshwater crustacean
Daphnia magna
Toxicity increases with alkyl chain length.
Functionalization of side chain (e.g. oxygenated chains) leads to lower
toxicity.
[47]
[48]
Imidazolium- and pyridinium-based
Imidazolium-based
Marine bacteria: Vibrio fischeri
Toxicity increases with alkyl chain length.
Negligible effect of anionic part.
Toxicity increased with the alkyl chain length.
Toxicity increased with higher hydrophobicity.
[49]
[50]
Marine bacteria: Vibrio fischeri
Invertebrate: freshwater crustacean
Daphnia magna
Imidazolium- and pyridinium-based
Invertebrate: freshwater pulmonated
snail Physa acuta
Positive relationship between alkyl chain length and toxicity.
Toxicity was highest for Ils with eight-carbon alkyl chains attached to both
imidazolium and pyridinium rings and decrease with shorter alkyl chains
[51]
2

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
Table 2
2. Materials and methods
Properties and description of the base oils.
Base
oils
Density
Viscosity
Viscosity Trade
Supplier Designation
2.1. Materials
at 15 °C (cSt)
index
name
3
(
g/cm )
Two
dodecylbenzenesulfonic acid (>99%)) and three bases (ethanolamine
>99%), N-methyl ethanolamine (>98%) and N, N-
acids
(2-ethylhexanoic
acid
(>99%)
and
4-
40 °C 100 °C
Ester
0.916
0.826
29.9 5.8
139
Priolube CRODA A2
(
1
936
PAO 6
Mineral 0.875
31.0 5.9
32.0 5.2
135
–
REPSOL S2
REPSOL M1
dimethylethanolamine (>99.5%)), available from Sigma-Aldrich,
were used as chemical reagents to synthesize the PILs. The miscibility
of the PILs was evaluated in three base oils of similar viscosity: an
ester, a polyalphaolefin (PAO) and a mineral oil, whose properties are
listed in Table 2. On the other hand, REPSOL, S.A. also supplied the
zinc dialkyldithiophosphate (ZDDP) used as a reference lubricant
additive.
>98
SN-150
reproducible assay. It is also the standard (eco) toxicological bioassay in
Europe: DIN EN ISO 11348: “ISO 11348-3. Water quality. Determination
of the inhibitory effect of water samples on the light emission of Vibrio
fischeri (Luminescent bacteria test). Part 3: Method using freeze-dried
bacteria,” 2007. Besides, it has been used to determine the toxicity of
conventional organic compounds for more than two decades [54].
This work studies six different PILs by using different tests in order to
characterize their environmental impact (bacteria toxicity and biode-
gradability), corrosion behavior and miscibility in different types of lu-
bricating base oils. This research is important for both the academia
and industry in order to explore the feasibility of using these PILs in lu-
brication, not only fulfilling tribological criteria.
2.2. Synthesis and spectroscopic characterization of PILs
Three hexanoate-based PILs: 2-hydroxyethylammonium 2-
ethylhexanoate (PIL-1), 2-hydroxymethylammonium 2-ethylhexanoate
(PIL-2), and 2-hydroxydimethylammonium 2-ethylhexanoate (PIL-3)
were synthesized using the procedure published in [33]. Briefly, the
stoichiometric amount of 2-ethylhexanoic acid was added dropwise
to each amine base. A water bath was used for keeping temperature
below 80 °C during the process of reaction.
Fig. 1. Simplified procedure of synthesis of each PIL.
3

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
1
The three sulfonate-based PILs: 2-hydroxyethylammonium 4-
dodecylbenzenesulfonate (PIL-4), 2-hydroxymethylammonium 4-
dodecylbenzenesulfonate (PIL-5), and 2-hydroxydimethylammonium
PIL-4 H NMR (chloroform‑d, 500 MHz): d = 7.80–7.82 (d, 2H, CH,
CH), 7.15–7.20 (d, 2H, CH, CH), 6.41 (s, 1H, NH), 3.72–3.74 (t, 2H,
CH
2H, CH
0.80–0.89 (m, 2H, CH
2
), 2.99–3.01 (t, 2H, N-CH
), 1.08–1.24 (m, 16H, CH
), 0.71–0.74 (t, 3H, CH
3
2
), 2.42–2.68 (t, 2H, CH
, CH , CH , CH , CH
).
2
), 1.47–1.70 (m,
, CH , CH , CH ),
4
-dodecylbenzenesulfonate (PIL-6) were prepared in our lab with the
2
2
2
2
2
2
2
2
2
following synthesis route: each amine base was charged to a 500 mL
round-bottom flask under Argon atmosphere; and then, a stoichiomet-
ric amount of 4-dodecylbenzenesulfonic acid was added dropwise to
the amine base, where the reaction mixture was held with stirring
under Argon at less than 120 °C for 1 h.
2
1
PIL-5 H NMR (chloroform‑d, 500 MHz): d = 7.77–7.79 (d, 2H, CH,
CH), 7.17–7.23 (d, 2H, CH, CH), 6.68 (s, 1H, NH), 3.93–3.95 (t, 2H,
CH
2
), 3.14–3.16 (t, 2H, N-CH
), 1.50–1.70 (m, 2H, CH ), 1.07–1.23 (m, 16H, CH
, CH , CH ), 0.81–0.88 (m, 2H, CH ), 0.72–0.75 (t, 3H, CH
PIL-6 H NMR (chloroform‑d, 500 MHz): d = 7.76–7.78 (d, 2H, CH,
CH), 7.15–7.28 (d, 2H, CH, CH), 6.47 (s, 1H, NH), 3.94–3.96 (t, 2H,
CH ), 3.25–3.27 (t, 2H, N-CH ), 2.91 (s, 6H, CH , CH ), 2.41–2.52 (t,
2H, CH ), 1.49–1.60 (m, 2H, CH ), 1.07–1.23 (m, 16H, CH , CH , CH
CH , CH , CH , CH , CH ), 0.82–0.88 (m, 2H, CH ), 0.72–0.75 (t, 3H, CH
Since PIL1-PIL6 are generally hygroscopic, the water content for each
2
), 2.72 (s, 3H, CH
3
), 2.42–2.57 (t, 2H,
, CH , CH , CH
).
CH
2
2
2
2
2
2
,
The simplified procedure of the synthesis of each PIL, including their
name, molecular structure and designation, is shown in Fig. 1.
The molecular structures of the PILs were confirmed through
CH
2
, CH
2
2
2
2
3
1
1
proton nuclear magnetic resonance ( H NMR) characterization
2
2
3
3
where a Bruker Ultra-shield 500 MHz spectrometer was used.
Since PIL1-PIL3 have been previously studied in Dr. Iglesias' labora-
tory, 1H NMR spectra of the three hexanoate-based PILs can be
2
2
2
2
2
,
2
2
2
2
2
2
3
).
1
found in [30,33], and H NMR spectra of the three sulfonate-based
PIL was estimated through Karl Fisher titration, the results can be found
in Table 3. From the results of purity and water content of the PILs, it can
be concluded that most of the impurities are constituted by water.
PILs are shown in Fig. 2. In addition, NMR spectra of these PILs
with peak integrals can be observed in Figs. S1 and S2 of the
Supporting Information document. The peak integration value of
proton on N confirmed the complete transfer of proton for each
PIL. The purity of PIL was calculated from the corresponding NMR
spectra with cation and anion specific peak integrals, where the pu-
rity of PIL-1 to PIL-6 is 99.83%, 98.66%, 99.93%, 97.82%, 97.59%, and
2.3. Environmental characteristics of PILs
2.3.1. Preparation of the PIL solutions
0.25 g of each type of PIL were mixed with 250 g of distilled water.
Firstly, they were manually stirred for 1 min in order to facilitate mixing
9
7.45%, respectively.
Fig. 2. 1H NMR spectra of sulfonate-based PIL-4, PIL-5, and PIL-6.
4

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
Table 3
Water content.
steady temperature of 15 °C, which was kept using a water bath.
Serial two-fold aqueous dilutions in the range of 20 to 210, the
h
i
PIL
mgwater
gPIL
starting concentration of the PIL samples being 1 g/L, were analyzed.
Prior to the analysis, it was necessary to adjust both pH and salinity
of the diluted samples at approximately 7 and 2%, respectively.
Results were expressed in terms of lethal-end-point (EC50), which
indicates the concentration of PIL that brings the luminescence of
Vibrio fischeri down to half of the original value following a 15 min
exposure period. A 2% solution of NaCl was employed as a control
sample. In order to facilitate the understanding of toxicity results,
another parameter named as toxic units (TU) was also calculated
according to Eq. (1) [38].
1
2
3
4
5
6
1.4 ± 0.6
8.0 ± 1.1
0.6 ± 0.4
14.9 ± 1.8
18.1 ± 3.0
12.2 ± 1.3
and then were centrifuged (5000 rpm for 3 min). In the case of PIL-1,
PIL-2 and PIL-3, as they are highly soluble in water, only one phase
was observed after centrifugation (Fig. 3a).
The PIL-4, PIL-5 and PIL-6 are semisolids at room temperature and
are also soluble in water but formed a significant amount of foam
1
EC50
TU ¼
ꢀ 100
ð1Þ
(
(
Fig. 3b). Therefore, it was necessary to add an antifoam agent
Sigma-Aldrich antifoam 289) to prevent foam formation. Its addition
Taking into account TU values, four levels of toxicity can be identi-
fied: non-toxic compounds for values below 1; toxic compounds for
values between 1 and 10; very toxic compounds for values from 10 to
was kept at the minimum possible (3 drops) in order to not affect
COD (chemical oxygen demand) and toxicity values. The negligible ef-
fect of the antifoam agent was evaluated in a solution of distilled
water with 3 drops of the antifoam.
1
00; and extremely toxic compounds for values above 100 [38,57]. Fur-
ther details can be found in Oulego et al. [34]. All measurements were
carried out thrice.
2
.3.2. Biodegradability analysis
Chemical oxygen demand (COD) was determined spectrophotomet-
2.4. Corrosion tests
rically at 600 nm by means of a dichromate method employing a Hach
spectrophotometer (model DR/2500) [55]. Biochemical oxygen de-
The corrosion behavior of the PILs was tested on the surface of AISI
2100 steel disks (see properties in Table 4). Before corrosion tests,
the disks were cleaned in hexane for 5 min using an ultrasonic bath,
and then rinsed in ethanol and dried in hot air. Three drops (around
5
5
mand (BOD ) was determined using a Lovibond respirometric system
(
model BD 600). The PIL solutions (0.1 g/L) were adjusted to pH values
around 7 with diluted HCl (0.5 M) using Crison Basic 20 pH-meter. The
PIL samples and sodium acetate (0.1 g/L), as control sample, were inoc-
ulated with the oxic bioreactor effluent (1 mL) from an urban wastewa-
ter treatment plant. After this, the inoculated samples (244 mL) were
mixed and transferred to BOD bottles. Then, PIL samples were incubated
in the dark at 20 °C for 5 days. All analyses were performed thrice.
0
.15 mL) of the PILs were applied on the surface of the disks. The corro-
sion tests were performed for 15 and 60 days at room temperature.
After corrosion tests, the disks were cleaned following the same proce-
dure described above.
The corrosive activity of the PILs on the steel surfaces was evaluated
by three complementary methods. The first method consisted of a sim-
ple visual inspection of the clean surfaces of the disks. The second
method used consisted of evaluating the loss of mass of the disks
using a precision balance. Finally, a Leica 3D confocal microscope was
used to analyze the surface topography of the disks and measure their
roughness according to a predefined pattern.
2.3.3. Bacterial toxicity: Vibrio fischeri analysis
The Biofix®Lumi-10 commercial test was employed to assess the
toxicity of different PIL samples on the marine bacteria Vibrio fischeri
according to DIN EN ISO 11348-3 [56]. This test was performed at a
a)
b)
Fig. 3. Solutions of PIL-1, PIL-2 and PIL-3 (a) and PIL-4, PIL-5 and PIL-6 (b).
5

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
Table 4
Properties of the steel disks used in corrosion tests.
Chemical composition (%)
C
Si
Mn
Cr
P
S
AISI 52100 steel
Disks
0.98–1.1
0.15–0.30
0.25–0.45
1.30–1.60
0.025
0.025
Dimensions
10-mm diameter
Hardness
190–210 HV30
Roughness (Ra)
<0.02 μm
3
-mm thickness
2.5. Miscibility of the PILs
complex/substituted cationic cores [41]. This can explain why PIL-6
shows the lowest biodegradability of the six PILs studied.
Despite the good lubricant behavior shown by both protic and apro-
If the biodegradability of the PILs is now analyzed in relative terms, it
should be noted that the scientific studies that have evaluated the bio-
degradability of some APILs have concluded that it is generally poor
[34,63,64] and lower than that of the PILs here studied. In this sense,
the percentage of biodegradability was found to be around 1% for
imidazolium- and pyridinium-based APILs [41]. Stolte et al. [65]
tic ionic liquids, their low miscibility in base oils has limited their use as
lubricant additives. For this reason, this work includes the study of the
miscibility of the PILs in three base oils (a mineral oil, an ester and a
polyalphaolefin) of similar viscosity (Table 2).
As a first approximation, 5 wt% mixtures of each PIL in each base oil
were prepared. These mixtures were prepared at 80 °C and mechani-
cally stirred (500 rpm, 2 min) in order to achieve some miscibility.
The resulting mixtures were visually monitored for 1 h and those mix-
tures which shown certain phase separation were discarded.
6
alsoobserved that imidazolium-based ILs with short alkyl (⩽C ) and
short functionalised side chains were highly resistant to be mineralized.
Wells and Coombe [66] evaluated the biodegradability of imidazolium-,
phosphonium-, quaternary ammonium- and pyridinium- based ILs and
The stable mixtures in the above-mentioned first approximation
were studied with an additional test. Two mixtures were prepared
4
found that cation cores with short side chains (C ) can be classified as
non-biodegradable compounds. Atefi et al. [67] observed that phospho-
nium ILs exhibited low levels of biodegradation (<12%), this being neg-
ligible when halide or triflimide anions were present. Likewise,
analyzing the biodegradability of these six PILs with respect to the one
of the reference lubricant additive (ZDDP), it is worthy to note that
this environmental property is greatly improved in all of them. The
low biodegradability of ZDDP can be explained based on its high molec-
ular weight (772.5) and the presence of Zn in its composition [68]. In
short, the industrial use of PILs as potential substitute of ZDDP as lubri-
cant additive would greatly minimize the environmental impact due to
their greater biodegradability.
(
mechanically stirred at 500 rpm for 2 min) at 1.0 and 2.0 wt% of
each PIL in each base oil and then the mixtures stability was measured
by using “Multiple Light Scattering” measurement technology in a
Turbiscan station. 12 h-measurements were made, and the possible
separation of phases was checked for 14 days.
3. Discussion of results
3.1. Biodegradability: chemical and biochemical oxygen demand
Biodegradation is the process by which organic substances are
decomposed by micro-organisms (mainly aerobic bacteria) [38,58].
The biodegradability index (BOD /COD) is a quantitative indicator of
3.2. Bacterial toxicity: Vibrio fischeri tests
5
the tendency of a substance to be biodegraded. Thus, a value of 0.2 has
been considered [59,60] as the minimum threshold value that deter-
mines whether or not a substance is biodegradable.
BOD /COD values of the six PILs studied, as well as the reference lubri-
5
cant additive (ZDDP), are depicted in Table 5. As can be seen, the three
hexanoate-based PILs (PIL-1, PIL-2 and PIL-3) show higher values of the
biodegradability index. Specifically, PIL-1 shows a BOD /COD value that
5
is within the order of magnitude of the biodegradability limit, so it
could be considered moderately biodegradable. On the other hand, PIL-
Some researchers have analyzed how the chemical structure of the
cationic [34,46,60,69] and anionic [70] part of ILs control toxicity.
Table 6 shows the environmental toxicity values of the six PILs stud-
ied, as well as the reference lubricant additive (ZDDP), expressed as
both EC50 and TU units.
Analogously to what was observed when analyzing the biodegrad-
ability behavior, the three hexanoate-based PILs (PIL-1, PIL-2 and PIL-
3) also had a better environmental behavior to bacterial toxicity, being
PIL-1 the one that obtained the value closest to non-toxic threshold.
This is maybe due to the fact that the chemical structure of PIL-1 does
not present substitution in the ammonium group. In any case, if we
rely on the Globally Harmonized System of Classification and Labeling
2
and PIL-3, would show a slightly poorer behavior to biodegradability.
In contrast, sulfonate-based PILs (PIL-4, PIL-5 and PIL-6) have shown
5
lower BOD /COD ratio values. This lower biodegradability could be
related to the presence of the benzenesulfonate aromatic group in
its anionic part [34,61,62]. Besides, the complexity of the cationic
part can also affect the IL biodegradability, thus being lower for more
Table 6
EC50 and TU values from the Vibrio fischeri analyses of PILs (exposure time: 15 min).
Samples
EC50 (mg/L)a
TUb
Table 5
ZDDP
PIL-1
PIL-2
PIL-3
PIL-4
PIL-5
PIL-6
0.9 ± 0.1
111.11 ± 0.1
1.18 ± 0.04
2.04 ± 0.08
2.1 ± 0.1
2.15 ± 0.07
2.5 ± 0.1
5
BOD /COD values of the studied PILs and ZDDP.
85 ±
49 ±
47 ±
47 ±
40 ±
37 ±
2
2
3
2
2
1
Samples
COD
mg O
BOD
(mg O
5
5
BOD /COD
(
2
/L)
2
/L)
ZDDP
PIL-1
PIL-2
PIL-3
PIL-4
PIL-5
PIL-6
596 ±
328 ±
339 ±
343 ±
316 ±
323 ±
328 ±
2
6
5
5
2
4
4
16 ± 0.1
0.027 ± 0.009
0.18 ± 0.01
58 ±
47 ±
4
3
2.7 ± 0.1
0.139 ± 0.008
0.115 ± 0.003
0.101 ± 0.004
0.080 ± 0.004
0.072 ± 0.002
a
EC50 classification for aquatic life: non-toxic: EC50 > 100 mg/L; harmful (acute 3):
EC50: 10–100 mg/L; toxic (acute 2): EC50: 1–10 mg/L; and very toxic (acute 1):
EC50 < 1 mg/L.
39.5 ± 0.7
32 ±
26 ±
2
2
b
TU classification: non-toxic: TU < 1; toxic: TU: 1–10; very toxic: TU: 10–100 and ex-
23.5 ± 0.7
tremely toxic: TU > 100.
6

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
Table 7
values observed in the PILs contrast with the extreme toxicity of the ref-
erence lubricant additive (ZDDP).
EC50 values from Vibrio fischeri for imidazolium and phosphonium cation-based APILs
found in the literature.
Ionic liquid
EC50
average
Ref.
3.3. Corrosion tests
(
mg/L)
2.29
.25
Fig. 4 shows the steel disks used in the corrosion tests at the time of
starting the test, after 15 days of exposure to each of the PILs (once
cleaned according to the procedure described above) and after
1-Methyl-3-octylimidazolium tetrafluoroborate
[71]
[72]
[72]
[71]
7
1.55
4.65
1
1
-Methyl-3-nonylimidazolium tetrafluoroborate
-Methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)
imide
6
0 days of exposition. As can be seen, only the PIL-6 showed evident
corrosive activity, both at 15 and 60 days. The rest of the PILs apparently
did not show corrosive behavior. Beyond the simple visual inspection,
the disks showed little or no variation in mass (including the case of
the PIL-6).
Finally, the surface topography of the disks exposed to the PILs was
analyzed using a confocal microscope (Leica 3D) and the surface rough-
ness was measured. Fig. 5 shows the corrosive effect of the PIL-6 on the
disk surface topography.
1
1
1
-Dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl) 7.3
imide
[34]
[34]
,3-Didecyl-2-methylimidazolium bis
trifluoromethylsulfonyl) imide
6.8
(
-Methyl-3-octylimidazolium hexafluorophosphate
1.71
[45]
[50]
[71]
[71]
[45]
[46]
[50]
[46]
[46]
[34]
3
6
.03
.05
1-Methyl-3-octylimidazolium chloride
1.13
2
2
3
.01
.36
.57
a q
Fig. 6 shows surface roughness (R and R ) values obtained from the
reference surface, as well as from all steel surfaces exposed to the PILs.
The surface roughness measured on the disk's surface after 15 and
1
1
-Methyl-3-decylimidazolium chloride
-Methyl-3-hexadecylimidazolium chloride
0.15
0.58
1.3
6
0 days of corrosion test was very similar for all PILs, with the exception
Trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)
imide
Trihexyltetradecylphosphonium dicyanamide
Trihexyltetradecylphosphonium chloride
of the PIL-6, and also for the reference surface (surface not exposed to
PILs). In the case of the PIL-6, the corrosive activity was already evident
after 15 days, with increased surface roughness values. As expected, the
corrosive effect of the PIL-6 on the steel disk increases with the exposure
time. However, a more complete characterization of the corrosion be-
havior of the studied PILs could be carried out using electrochemical
tests [73].
4.4
7.8
[34]
[34]
of Chemicals (GHS), the six PILs studied would be categorized as
harmful” (EC50: 10–100 mg/L; acute 3).
“
The aquatic life toxicity (EC50) of the six PILs was similar to that
observed for some fatty acid-based aprotic ILs [70]. Nevertheless, it
was significantly lower than that shown by some imidazolium and
phosphonium cation-based APILs (see Table 7), in which the EC50 values
were in the range of 1–10 mg/L or even below 1 mg/L, thus being
classified as toxic (acute 2) or very toxic (acute 1), respectively
3.4. Miscibility of the PILs
The preliminary miscibility test showed that most of the mixtures
were unstable after 1 h (see Fig. 7). In particular, all the PILs-
containing mixtures were unstable when M1 was used as base oil.
This could be due to the non-polar character of the mineral oil. How-
ever, in the case of the S2 base oil (PAO), despite its non-polar character,
an apparent stability of the mixtures formed with the PIL-2 and PIL-3
was appreciated. However, this apparent visual stability could be due
to the translucent character and the same color of both phases. When
a polar base oil (A2) was used, it was also not possible to stabilize the
mixtures formed with the PILs 1, 4, 5 and 6.
[
34,45,46,50,71,72]. The high toxicity of these ILs can be mainly ex-
plained based on the presence of halides (chloride) and fluorine
atoms in the anionic core and the alkyl chain length of the cationic core.
The reference lubricant additive (ZDDP) showed an EC50 value of 0.9,
that is to say, it is very toxic (acute 1) according to the GHS classification.
From the analysis of the TU data, we obtain similar conclusions. The
six PILs studied showed mild toxicity, the PIL-1 being close to the non-
toxicity threshold. The results obtained are in agreement with those
found for some previously studied APILs [70]. The very low toxicity
Only the PIL-2 and PIL-3 reached an apparent stability in the base
oils A2 and S2 after 1 h. Therefore, the mineral base oil was discarded,
Fig. 4. Disks images during and after corrosion tests.
7

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
a)
b)
Fig. 5. Surface topography of fresh disk (a) and disk exposed to PIL-6 after 60 days (b).
as well as the PILs 1, 4, 5 and 6. Then, mixtures at 1.0 and 2.0 wt% of the
PIL-2 and PIL-3 in the base oils A2 and S2 were prepared to analyze their
stability by means of a Turbiscan station over 14 days.
transmittance and backscattering measurements obtained were
very similar to that shown in Fig. 8. Fig. 9 shows the measurements
obtained when the mixture was formed by the A2 base oil and the
PIL-3. As it can be observed, although in this case the mixtures
were also unstable, the phase separation here was not so immediate,
it lasted several days.
Fig. 8 shows the light transmission and backscattering measure-
ments along the test tube with the S2 + 2% PIL-2 mixture. As can be
seen in the area marked with a green circle, two peaks appear. The
first is characteristic of the bottom of the glass, while the second peak,
which appears from the first measurement, is an indicator of the discon-
tinuous phase separation (PIL). That is to say, despite the fact that the
mixture visually seems stable, the truth is that the mixture is highly
unstable.
4. Conclusions
Three hexanoate- and three sulfonate-based protic ionic liquids
were synthesized, and their bacteria toxicity and biodegradability
have been investigated, as well as their corrosive behavior and miscibil-
ity in different base oils. The main conclusions extracted from this
work are:
The rest of the mixtures studied (at both concentrations), with
the only exception of the mixture formed by A2 and PIL-3, were
equally unstable from the first measurement. In these cases, the
•
The PIL-1 could be considered moderately biodegradable, while PIL-2
and PIL-3, would have a slightly poorer behavior to biodegradability.
0.30
0.25
0.20
0.15
0.10
0.05
0.00
5
• Sulfonate-based PILs (PIL-4, PIL-5 and PIL-6) have shown lower BOD /
COD ratio values than the hexanoate-based PILs (PIL-1, PIL-2 and
PIL-3). This lower biodegradability could be related to the presence
of the benzenesulfonate aromatic group in its anionic part. PIL-6
showed the lowest biodegradability of the six PILs studied.
The toxicity of PILs was studied considering a single trophic level
•
(
marine bacteria Vibrio fischeri); and all of them show mild toxicity,
although the values for PIL-1 are close to the non-toxicity threshold.
This is maybe due to the fact that the absence of substitution in the
ammonium group of PIL-1. The PIL-6 showed the highest relative
toxicity.
Ra (µm)
Rq (µm)
• All the PILs studied greatly outperform the biodegradability and
toxicity of the reference lubricant additive (ZDDP), constituting
them potential future alternatives.
Fig. 6. Surface roughness of steel disks exposed to PILs (ISO 4287).
Fig. 7. Visual miscibility test (mixtures at 5.0 wt% of PIL).
8

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
Transmission
5
0 %
0
%
0
mm
20 mm
40 mm
Backscattering
5
0 %
0
%
0
mm
20 mm
40 mm
Fig. 8. Miscibility study of the S2 + 2.0 wt% PIL-2 mixture using a Turbiscan station.
•
•
With the exception of the PIL 6, no corrosive activity was detected in
the test made with the rest of PILs. However, a more complete charac-
terization of the corrosion behavior of the studied PILs could be car-
ried out using electrochemical tests.
Only the PIL-3 was relatively miscible in the polar base oil (ester, A2),
the other PILs resulted in highly unstable emulsions. This could limit
the use of these PILs as lubricant additives.
Supervision, Project administration, Funding acquisition, Writing - review
& editing. P. Iglesias: Conceptualization, Supervision, Project administra-
tion, Funding acquisition, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
CRediT authorship contribution statement
J.L. Viesca: Conceptualization, Methodology, Investigation, Writing -
original draft, Writing - review & editing. P. Oulego: Methodology, Inves-
tigation, Validation, Writing - original draft, Writing - review & editing. R.
González: Formal analysis, Resources, Methodology, Investigation. H.
Guo: Data curation, Visualization, Resources, Writing - original draft,
Writing - review & editing. A. Hernández Battez: Conceptualization,
Acknowledgments
The authors would like to acknowledge the Ministry of Science, In-
novation and Universities of the Spanish government for supporting
the research works of Jose Luis Viesca at Rochester Institute of Technol-
ogy (NY, USA). They also thank the Government of the Principality of
Transmission
5
0 %
0
%
0
mm
20 mm
40 mm
Backscattering
5
0 %
0
%
0
mm
20 mm
40 mm
Fig. 9. Miscibility study of the A2 + 2% PIL-3 mixture using a Turbiscan station.
9

J.L. Viesca, P. Oulego, R. González et al.
Journal of Molecular Liquids 322 (2021) 114561
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