Comparative Study of the High Pressure Thermophysical Properties of

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Comparative study of the high pressure thermophysical properties of 1-ethyl-3-methylimidazolium and 1,3-diethylimidazolium ethyl sulfates for use as sustainable and efficient hydraulic fluids Marzena Dzida, Ma#gorzata Musia#, Edward Zorebski, Sylwia J##ak, Justyna Skowronek, Katarzyna Malarz, Anna Mrozek-Wilczkiewicz, Robert Musiol, Andrzej Cyranka, and Micha# #wi#tek ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02318 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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ACS Sustainable Chemistry & Engineering

Comparative study of the high pressure thermophysical properties of 1-ethyl-3-methylimidazolium and 1,3-diethylimidazolium ethyl sulfates for use as sustainable and efficient hydraulic fluids Marzena Dzida1*, Małgorzata Musiał1, Edward Zorębski1, Sylwia Jężak1, Justyna Skowronek1, Katarzyna Malarz2,3, Anna Mrozek-Wilczkiewicz2,3, Robert Musiol1, Andrzej Cyranka4, Michał Świątek4

1

University of Silesia in Katowice, Institute of Chemistry, Szkolna 9, 40-006 Katowice,

Poland 2

Silesian Center for Education and Interdisciplinary Research, University of Silesia in

Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland 3

University of Silesia in Katowice, A. Chelkowski Institute of Physics, 75 Pułku Piechoty 1,

41-500 Chorzów, Poland 4

Specol Sp. z o.o., Kluczborska 31, 41-508 Chorzów, Poland

Corresponding Author E-mail: [email protected]

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ABSTRACT In this work, we discussed and compared new acoustic (speed of sound), transport (viscosity), surface (surface tension and contact angle on stainless steel and glass), and thermophysical properties with special regard of compressibilities and isobaric thermal expansion of 1,3diethylimidazolium

ethyl

sulfate

([C2C2im][EtSO4])

with

those

of

1-ethyl-3-

methylimidazolium ethyl sulfate ([C2C1im] [EtSO4]), the reference mineral, synthetic, and biodegradable oils as well as hydraulic oils for use as hydraulic fluids. The refractive index, NMR spectra and cytotoxicity of [C2C1im][EtSO4] and [C2C2im][EtSO4] were also investigated. [C2C1im][EtSO4] and [C2C2im][EtSO4] have low isothermal compressibility and isobaric thermal expansion values, which are weakly dependent of the pressure and temperature. The viscosity, surface tension, and contact angle of [C2C2im][EtSO4] are more similar to that of commercial hydraulic oils than [C2C1im][EtSO4]. The investigated ILs have a twenty times lower toxicity on normal fibroblast from human new-born skin than that of bis(trifluoromethylsulfonyl)imide-based

ILs.

The

presented

results

show

that

[C2C2im][EtSO4] can potentially be used as a hydraulic fluid in the same way as [C2C1im][EtSO4]. Keywords: ethyl sulfate-based ionic liquids, green working fluids, compressibility, sustainable hydraulic fluids, cytotoxicity, high pressure


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INTRODUCTION The role of hydraulic fluids is to transmit power to the moving parts of many machines. These fluids should have a low compressibility that is nearly independent of temperature and pressure, as well as a superior lubricating ability. Only a fluid with low compressibility and optimal lubricity can transmit pressure in a system without delay and low energy losses due to compression of the fluid itself.1 Water in hydraulic fluids can cause great damage, i.e., destroying the fluid and even causing further damage to the elements that the fluid flows through in the mechanical system. Examples of the problems related to the presence of water are corrosion and the reduced viscosity and lubricity of the fluid. Moreover, in the presence of water, the ice crystals can form, hydrolysis impeding the operation of the system can occur, higher operating temperatures can force the system to work harder and respond more slowly, and water can cause vapor pockets to form within the fluid, reducing its effectiveness and lifetime. Clean, dry hydraulic fluid is critical to an efficient hydraulic system. Hydraulic fluids should also have a low vapor pressure and low gas solubility, which means that at high temperatures, there is no gas that would significantly increase the compressibility of the system. Hydraulic fluids with low isobaric thermal expansions are also preferred.2 The polyalkylene glycols, polyphenyl ethers, alkylated aromatics, and diesters are mainly used as hydraulic fluids.3 However, in practice fully formulated hydraulic fluids are not composed of one component but consist of a blend of a base fluid and an additive package of various ingredients to obtain/improve required properties. The base fluids are categorized by the American Petroleum Institute (API) into five groups.4 First three groups (I, II and III) are mineral base oils for which resources are finite, while groups IV and V are synthetic base fluids. Synthetic hydraulic fluids are designed to provide excellent stability, lubricity, and other performance-enhancing properties. The disadvantages of most of these fluids include high cost and toxicity. One of the alternatives for such a fluid is the [C2C1im][EtSO4], known



as ECOENGTM 212, which has been successfully introduced into the industry and produced on a ton scale. 5,6 The Linde AG has used IL as a type of liquid piston that can operate at a maximum pressure of 90 MPa (the so-called the ionic compressor 90 MPa – IC90). Linde stated that ‘‘in contrast to a conventional piston compressor, with some 500 moving parts, we now need only eight.’’7 To date, many potential applications of ILs from working fluids through catalysts to solvents in extraction and separation have been discovered but only in some cases translate into later mass industrial use. Accordingly, what distinguishes [C2C1im][EtSO4] from other ILs? Alkyl sulfate-based ILs combine different favorable properties, a low glass transition temperature, a lack of a melting point, halide-free, easy to prepare and relatively cheap i.e., synthesis from cheap and easily available materials, less corrosive to machine equipment i.e., the absence of halogen compounds inhibit corrosion on a metal surface.8,9 Uerdingen et al.10 have reported that [C2C1im][EtSO4] does not corrode stainless steel but does corrodes brass, copper and carbon steel. In addition, more importantly, the ethyl sulfates are relatively non-toxic towards e.g., anaerobic bacteria,11 bacteria Escherichia coli,12 luminescent bacteria 13 compared to other ILs and show no irritating effect on the skin and eyes.13 This non-toxic effect is extremely valuable because many ILs have turned out to be too toxic, thus, research has stopped at a certain stage with no chance of further development and exploitation on a wider scale.14 For example, the well-known 1butyl-3-methylimidazolium hexafluorophosphate [C4C1im][PF6] hydrolyses in contact with moisture forming volatiles hydrogen fluoride (HF) and phosphorus oxytrifluoride (POF3), which can damage materials such as steel (autoclaves and reactors) and glass.15 [C2C1im][EtSO4] has been mostly investigated as a hydraulic fluid that can replace classic hydraulic oils.1,3,16,17 Kambic et al.1 showed that the lower compressibility of [C2C1im][EtSO4] than those of the hydraulic mineral oil ISO VG 46 and the hydraulic fluid Quintolubric 888-68 of the HFD-type as well as water allows its use, e.g., in hydraulic


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systems with increased dynamics. In other study, Regueira et al.3 demonstrated that the lowest compressibilities have the [C2C1im][EtSO4] and 1-ethyl-3-methylimidazolium hexyl sulfate ([C2C1im][HexSO4]) as against the isothermal compressibilities of vegetable (BIO-H01, HOSO-B), mineral (MIN-H01, MIN-H02) and synthetic oils (PAG1, DiPEC7) as well as water. Low compressibility leads to a high efficiency factor of a high-pressure machine and permits a reduction in the size pumps and equipment.16,17 Schlücker et al.17 reported that increasing volumetric efficiency reached 30% after replacing the mineral hydraulic oil by the [C2C1im][EtSO4] in hydraulically driven diaphragm pumps. All above-mentioned studies prove that for high-pressure hydraulic systems, compressibility is one of the crucial properties, and that low compressibility indicates high potential for hydraulic applications. 1,3,16,17

It should be noted that although in developing novel hydraulic fluids, ILs are of great

importance, investigations to develop new and efficient hydraulic fluids, that are free of mineral oil and environmentally friendly, or at least environmentally acceptable, are not limited to ILs only.18 All the advantages of the alkyl sulfate-based ILs have become the motivation for the presented work which will broaden the knowledge regarding the already widely studied [C2C1im][EtSO4] and weakly tested [C2C2im][EtSO4].19 For both ILs, this work extended considerably recent study on heat capacities and related properties executed at atmospheric pressure.20 In this work, the speed of sound in [C2C1im][EtSO4] and [C2C2im][EtSO4] was measured at temperatures ranging from 293 K to 318 K and at pressures up to 101 MPa. Using the experimental speed of sound data, the density, isentropic and isothermal compressibilities, isobaric thermal expansion, isobaric and isochoric heat capacities as a function of pressure and temperature were calculated for the studied ILs. We chose the acoustic method because it is the only experimental one leading directly to isentropic compressibility.

Knowledge of isentropic compressibility allows in a very accurate and



convenient way to determine related thermophysical quantities characterizing the liquid phase under atmospheric conditions or at high pressures. In particular, it makes it possible to calculate the isothermal compressibility of liquids. The compressibility and isobaric thermal expansion of [C2C2im][EtSO4] were compared with those of [C2C1im][EtSO4], other ILs previously proposed as hydraulic fluids

3,5,6

, the reference mineral (MIN-H02), synthetic

(SYN-2T) and biodegradable (BIO-H) oils, as well as commercial hydraulic oils (Specol LHV 22, 32, 46 and Specol L-HL 150). Next, we describe additional studies of [C2C1im][EtSO4] and [C2C2im][EtSO4] including viscosity, surface tension and contact angles on stainless steel and glass. In this part of work, we also examined four commercial hydraulic oils: the first three (Specol L-HV 22, 32, and 46) were formulated to lubricate and protect hydraulic systems operating at high pressures and the last fluid (Specol L-HL 150) operates under moderate conditions. To complete the data, the speed of sound, density, surface tension, contact angle on stainless steel and glass, viscosity of commercial hydraulic fluids were also measured. Lastly, we described the cytotoxicity of [C2C1im][EtSO4] and [C2C2im][EtSO4] on a human normal cell. Thus, based on our results reported in this work and those reported previously,20 we intend to check the suitability of [C2C2im][EtSO4] as a potential hydraulic fluid that could replace the commercial fluids.

EXPERIMENTAL SECTION Materials. Figure 1 shows structures and Table 1 represents the properties of used ILs. We investigated the same batch of both ILs and prior to the measurements samples were prepared in the same way as in previous study.20 The refractive indices, nD (298.15 K), are 1.479110.00002 (conducted by an Anton Paar Abbemat 550 refractometer) and 1.47300.0002 (conducted by an Abbe refractometer RL3) for [C2C1im][EtSO4] and 6 ACS Paragon Plus Environment

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[C2C2im][EtSO4], respectively. The experimental values of the refractive index are listed in Table S1 and presented in details in Figure S1. With the exception of data sets reported by Montalban et al.21, Reddy et al.22 and Larriba et al.

23

the consistency between nD conducted

by us and the literature values 24–30 for [C2C1im][EtSO4] is very good. The relative deviations RD ( RD  100 ( y exp ylit ) / yexp ) vary from -0.01 to 0.04% and the absolute average relative deviation AARD ( AARD  (100 / N )i 1 ( yexp,i  ylit,i ) / yexp,i ) is in the range: 0.02 ÷ 0.03%. n

The literature concerning the hydrolysis stability of [C2C1im][EtSO4] is inconsistent. Some authors described these ILs as stable in presence of water, 9,15,31 while other authors indicated that these ILs are hydrolytically unstable.32-34 Jacquemin et al.32 improved the hydrolysis of [C2C1im][EtSO4] to ethanol and hydrogenate anions and re-formed the desired alkyl sulfate anions by a simple transesterification reaction between the hydrogen sulfate-based ILs and the corresponding alcohol. To carry out the measurements presented in this work and in the previous studies, 20 bottles of [C2C1im][EtSO4] and [C2C2im][EtSO4] were opened on 15.02.2016 and 17.04.2015, respectively. To check the stability of [C2C1im][EtSO4] and [C2C2im][EtSO4], we analyzed the samples after approximately two and three years storage, respectively (16.01.2018). The water content did not deviate significantly from that measured after the bottle was opened for the first time, showing no change in water contamination during storage (see Table 1). The obtained density is in an excellent agreement with our first measurement reported in our previous work

20

(Table 1). We also analyzed the samples of [C2C1im][EtSO4] and

[C2C2im][EtSO4] (after approximately two and three years storage) using NMR spectra (NMR spectra were recorded in CDCl3 with a Bruker Advance 400 MHz instrument) (Figure S2). The structure of [C2C1im][EtSO4] was confirmed by comparison of the 1H-NMR data obtained in this work (Figure S2a) and that obtained by Jacquemin et al. 32 The results show no water and ethanol contamination. 7 ACS Paragon Plus Environment


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Table 1. Specification of Chemical Compounds Liquid

CAS number

supplier

densitya

mass

water b

halidesc

kgm-3

fraction

ppm

ppm

>0.99

22.21/892

< 100

>0.99

131/742

< 100

purity [C2C1im][EtSO4]

342573-75-5

Iolitec

1237.591/ 1237.532

[C2C2im][EtSO4]

516474-04-7

Iolitec

1203.373/ 1203.312

a

298.15 K; b Coulometric Karl Fisher titration, TitroLine 7500; c Declared by a supplier. 1 15.02.2016 20; 216.01.2018; 317.04.2015 20

Speed of sound measurements. The speed of sound, u0, was conducted at 2.75 MHz at ambient pressure and u at 2 MHz under elevated pressures using two measuring sets constructed in our laboratory. The combined expanded uncertainty with an interval of confidence of 0.95 was estimated to be better than U(u0) = 1 m·s-1 (uu0 = 0.5 m·s-1, k = 2) for the speed of sound measurements at ambient pressure (p0 = 0.101325 MPa) and U(u) = 2 m·s1

(uu = 1 m·s-1, k = 2) for measurements at elevated pressures. Detailed description of the high

pressure device and the method can be found in the previous papers.35, 36 Density measurements. Densities were conducted using vibrating-tube densimeter DMA 5000 M (Anton Paar, Austria). Apparatus was calibrated (using the extended calibration procedure) with dry air and re-distilled water (with electrolytic conductivity equals 110-4 Sm1

at T=298.15 K. The densities were conducted in temperatures from 288.15 to 363.15 K. The

viscosities were automatically corrected. The standard uncertainty was estimated to be better than 0.05 kg m−3 with repeatability 0.005 kgm-3. Viscosity measurements. The kinematic viscosity, ν, of the ILs was determined with a calibrated and thermostated rolling–ball viscometer (Lovis 2000 M/ME), which measures the rolling time of a ball through transparent and opaque liquids according to Hoeppler's falling 8 ACS Paragon Plus Environment

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ball principle. The viscosity was measured with accuracy better than ±0.5% (with the same ball). The viscosities were conducted in the temperature range from 293.15 to 343.16 K, and the accuracy of temperature measurements were ± 0.02 K. The kinematic viscosity of the oils was measured using precalibrated capillary Ubbelohde viscometers from Schott (ІІ with K = 0.1004 and ІІa K = 0.5240). The measurements were done in the temperature range from 293.2 K to 323.2 K. The temperature was maintained by a cascade thermostat unit with fluctuations not exceeding ±0.05 K. The uncertainty of the viscosity measurements was ±1%. Surface tension measurements. The surface tension, γ, was measured using the pendant drop method (DSA 100S Krüss Tensiometer with drop shape analysis software). The measurements were conducted in the temperature range from 293.2 to 323.2 K. The uncertainty of the method given by the manufacturer was 0.1 mN∙m–1. The description of the method, all necessary instrumental details and experimental procedures can be found in ref.37. Contact angle. The contact angle, , for ILs and oils on the chosen materials (stainless steel and glass) was measured using the sessile drop method by means of the DSA 100S Krüss Tensiometer with drop shape analysis software. In each case drop was placed on the studied surface located in a thermostated (at a temperature 298 ± 0.5 K) chamber filled with argon as shielding gas. Each simple measurement was repeated approximately 10–15 times and the  reported values are the average of these measurements. Although the resolution of the single measurement of  was 0.1, the standard deviations of the averages vary from 0.5 to 3.4. The materials used for contact angles measurements were optical quality glass BK-7 (smooth surface) and stainless steel (AISI OH18N9). The materials were purified in an ultrasonic cleaner and dried. The contact angle measurements were carried out on the entire material surface and all  values were comparable. The most representative parts of the stainless steel



surface and the weight percentages of C, Ni, Mn, Fe and Cr are presented in Figure S3 and Table S2, respectively. Cytotoxicity test. The details concerning cell culture and determination of the 50% inhibitory concentration (IC50) were given in the previous paper.38 To determine the intracellular levels of the ROS (reactive oxygen species), the NHDF cells were seeded in black 96-well plates (Corning) at a density of 9.000 cells/well and incubated at 310.15 K. After overnight incubation, the solutions of the tested compounds [C2C1im][EtSO4] and [C2C2im][EtSO4] were added and incubated for 24 h. Additional wells were used as the basal control (untreated cells), positive control (H2O2) and negative control (resveratrol). The generation of ROS was measured using a CellROX® green reagent (Molecular Probes™). Additionally, the quantity of cells in each well was determined using Hoechst 33342 (Molecular Probes™). The solutions of the tested compounds were removed, and 100 µL of CellROX Green Reagent and Hoechst 33342 at a final concentration of 5 μM was added to each well. Then, the cells were incubated for 30 min at 310.15 K. The fluorescence was measured using a multi-plate reader (Synergy 4, Bio Tek) at a 485 nm excitation and a 520 nm emission for the CellROX green reagent and a 345 nm excitation laser and a 485 nm emission filter for Hoechst 33342. The experiments were performed three times. The ROS levels were expressed as the percentage of the control cells level. RESULTS AND DISCUSSION Speed of sound and derived properties. The speed of sound in [C2C1im][EtSO4] and [C2C2im][EtSO4] was measured at temperatures from 293 to 318 K and under pressures from 0.1 to 101 MPa, whereas the speed of sound in Specol L-HV 22, 32, and 46 was measured in the same temperature range but at 0.1 MPa only. Since, ILs are often strongly dispersive media, an initial analysis of the dispersion region based on classical ultrasound absorption and u0(T) dependence at 0.1 MPa was performed in our previous work. 20 This analysis showed 10 ACS Paragon Plus Environment

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that for both studied ethyl sulfates low values of classical ultrasound absorption and linear u0(T) dependences at 0.1 MPa did not extort the limitation of the measurement range. A more detailed discussion regarding the ultrasonic dispersion phenomenon in ILs can be found in our previous works.39,40 The speed of sound data for investigated ILs and commercial Specol oils are collected in Table S3 and Table S4, respectively. The quality of the measured speed of sound under atmospheric pressure for [C2Cnim][EtSO4] was checked by comparison given in Table S5 and Figure S4. The result is satisfactory. Note that compared results were obtained by means of two different techniques: the wide-band technique 20 and narrow-band technique used in this work (short comparison of both techniques can be found in ref. 41,42). Apart from the high quality speed data, the selection of the smoothing equation for these data in the (p,T) experimental range is very important for calculation thermodynamic properties by means of acoustics method. Similarly as previously,38,43 a double polynomial equation of the form used by Sun et al.44,45 was chosen for smoothing out the experimental data of the speed of sound, pressure and temperature. The equation and calculated by the least squares method coefficients and the mean deviation from the regression line are given in Table S6. Using the measured in this work speeds of sound (in form the smoothing equation reported in SuppInfo) together with the density, ρ, and isobaric molar heat capacity, Cp, at ambient pressure reported in the previous work,

20

the pρT and pCpT data were calculated for

temperatures from 293.15 to 318.15 K and pressures reached 100 MPa for [C2C1im][EtSO4] and [C2C2im][EtSO4] based on the used routinely in our lab procedure proposed by Sun et al.44 In Tables S7 and S8, the calculated density and isobaric molar heat capacity values are listed. The temperature and pressure dependence of density were correlated by fitting to Tait equation:

 (T , p)  0 (T , p0 ) /[1  C ln(( p  B(T )) /( p0  B(T )))] ,


(1)


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where C is the adjustable constant, and B(T) is the parameter described as B(T )  A1  A2 (T / 100)  A3 (T / 100) 2 . The calculated C, A1, A2 and A3 coefficients along with

standard mean deviations are presented in Table S9. Figure S5 presents the densities of [C2C1im][EtSO4] calculated in this work compared with literature data. Apart from the data reported by Tome et al.46 (AARD = 0.5%), our data are in very good agreement with literature values (the best accordance is observed in the case of Schmidt et al.47, Nieto de Castro et al.48 and Matkowska et al.49 data; AARDs = 0.02%); the details are described in SuppInfo. From the densities and speeds of sound, the isentropic compressibilities were calculated by the Laplace formula:  S   1  u 2 . The results of the calculations for [C2C1im][EtSO4] and [C2C2im][EtSO4] are reported in Table S10. The calculation of the material constants leads to useful information regarding the dependence of the volumetric properties on the temperature and pressure. The isothermal compressibility,

 T , was calculated from the S:

T   S   p2 V  T  C p1 ,

(2)

where p is the isobaric thermal expansion, defined by

 p  1   T  p . The values of

the p and  T of [C2C1im][EtSO4] and [C2C2im][EtSO4] are listed in Tables S11 and S12, respectively. The ρ, p and S of Specol L-HV 22, 32,46 and Specol L-HL 150 at ambient pressure were reported in Table S4. A comparison between the isobaric thermal expansion of [C2C1im][EtSO4] calculated in this work and the data sets reported by Nieto de Castro et al.48, Matkowska et al.49, Matkowska and Hofman 50, Regueira et al.51 and Hofman et al.52 produces AARD in the range (0.4 ÷ 2.1) %. Thus, the consistency is very good. Poor accordance (similarly as in the case of compared density) show the data reported by Tome et al.46 (AARD = 11.4 %) (see Figure S6). A 12 ACS Paragon Plus Environment

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comparison between the isothermal compressibility of [C2C1im][EtSO4] calculated in this work and literature data,48–51 except for data sets reported by Tome et al.46 and Hofman et al.52 produces RDs = (–2.8 ÷ 1.4) % (see also Figure S7). Thus, the consistency is satisfactory. The worst agreement shows data reported by Tome et al.46 with AARD = 6.7 % but it is not surprising in the light of abovementioned poor accordance for densities. Compressibility and thermal expansibility of the ILs and hydraulic fluids. For further comparison, we used four commercial Specol hydraulic oils manufactured by Specol Sp. z.o.o. The first three, Specol L-HV 22, 32 and 46 hydraulic oils are manufactured using highly refined mineral oil. The suitably selected additive package provides a high level of anti-wear properties and a wide range of viscosity and temperature characteristics, resulting in a longer service life and reduced wear on the hydraulic components. Specol L-HV 22, 32 and 46 are used in highly loaded systems operating at pressures of approximately 35 MPa, in vane pumps at pressures up to 20 MPa, in precision hydraulic control systems, and in hydraulic systems that require small changes of viscosity with temperature.53–54 The last oil, Specol LHL 150 is manufactured from high-quality mineral base oil and additives that provide suitable anti-corrosion, anti-foam, and de-emulsifying and antioxidant properties. Specol L-HL 150 is designed for use in low- and medium-loaded power and drive transmission systems, operating at moderate temperature and humidity.54 The working fluids are be characterized by low compressibility and isobaric thermal expansion. As seen in Figure 2, the ILs analyzed in this work have lower values for both coefficients αp and S compared to the commercial Specol oils. The effect of temperature on the isobaric thermal expansion is the weakest for [C2C2im][EtSO4], i.e., αp decreases less than 1.7% in the temperature range from 283.15 to 363.15 K. 20 For [C2C1im][EtSO4], αp decreases less than 2.7% in the same temperature range. 20 For Specol oils αp increases within 5.2% – 5.9% in the temperature range from 288.15 to 363.15 K. The S values range from 2.136 to 13 ACS Paragon Plus Environment


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3.071 Pa–1 and 2.275 to 3.393 Pa–1 at temperatures from 293.15 to 318.15 K and pressures from 0.1 to 100 MPa for [C2C1im][EtSO4] and [C2C2im][EtSO4], respectively (Table S10). The S values of [C2Cnim][EtSO4] are lower by 43% – 53% than the values for Specol oils at 0.1 MPa (see Figure 2). Because hydraulic fluids with low isothermal compressibility and isobaric thermal expansion are preferred 2, the  T vs p under atmospheric pressure was analyzed (Figure 3). Clearly, the ILs, except for trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P66614][(C2F5)3PF3]), show lower κT and p values than those of the reference mineral (MIN-H02), synthetic (SYN-2T) and biodegradable (BIOH) oils. The lowest  T and αp values have 1,3-dimethylimidazolium dimethylphosphate ([C1C1im][(C1O)2PO2]),

[C2Cnim][EtSO4],

1-ethyl-3-methylimidazolium

hexyl

sulfate

([C2C1im][C6SO4]), and 1-ethyl-3-methylimidazolium thiocyanate ([C2C1im][SCN]), while the highest κT and αp values have the reference hydraulic oils (MIN-H02, SYN-2T, BIO-H) and [P66614][(C2F5)3PF3]. High-pressure thermodynamic properties play an important role in assessing the suitability of ILs as hydraulic fluids. As mentioned in the Introduction, low compressibility translates into a fast response time, a high-pressure transmission velocity and a low power loss.1 In Figure 4, the isothermal compressibility and isobaric thermal expansion are presented under pressures of 0.1 MPa, 50 MPa and 100 MPa at 313.15 K for [C2C1im][EtSO4], [C2C2im][EtSO4], other ILs also proposed as hydraulic fluid and hydraulic oils (MIN-H02, SYN-2T, BIO-H). For [C1C1im][(C1O)2PO2], [C2C1im][EtSO4] and [C2C2im][EtSO4], the weakest influence of pressure on the isobaric thermal expansion and isothermal compressibility was observed, especially in comparison to references hydraulic oils (Figure 4). The increase in the pressure from 0.1 to 50 MPa causes a decrease in the  T values by 24% – 29% and p values by 15% – 18% for selected oils (MIN-H02, SYN-2T, BIO-H) in the temperature range from 298.15 to 323.15 K, respectively. For [C2C1im][EtSO4] and [C2C2im][EtSO4] in the temperature range 14 ACS Paragon Plus Environment

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from 293.15 to 318.15 K, an increase in the pressure from 0.1 to 50 MPa causes a decrease in the κT values by 14% – 17% and p values by 8% – 10%, respectively, which translates into a higher energy efficiency for these compounds. Surface tension, contact angle, and viscosity of the ILs and Specol hydraulic oils. The surface tension of [C2C1im][EtSO4], [C2C2im][EtSO4] and the Specol oils was measured at temperatures from 293.2 to 323.2 K. The results are summarized in Table S13 (see also Figure 5a). The surface tension of the ILs studied here ranged from 45.3·10-3 N·m-1 and 47.3·10-3 N·m-1 (293.2 K) to 40.6·10-3 N·m-1 and 42.4·10-3 N·m-1 (323.2 K) for [C2C1im][EtSO4] and [C2C2im][EtSO4], respectively. Thus, the γ values obtained are in the middle range for the surface tension of typical ILs.59 The surface tension value is affected by cations and anions because they are present in the interface layer. The surface tension will grow as the ion diameter decreases. The lower surface tension of [C2C2im][EtSO4] relative to its lower homologue is therefore preferable. Specol oils exhibit a much lower surface tension that ranges between 25.7·10-3 N·m-1 and 30.7·10-3 N·m-1 because of their very good lubricity properties. Comparing the surface entropy, Sa and surface enthalpy, Ha calculated from linear γ(T) dependencies is visible that both values are smaller for [C2C2im][EtSO4] than those for [C2C1im][EtSO4]. In relation to the Specol oils, both ILs have clearly smaller the Sa values, whereas the Ha values are nearing (Table S14). The results of the  measurements at 298 K are illustrated in Figure 5b. The stainless steel is the most important due to applications as a construction material in pumps and compressors. Interestingly, that the contact angle on the stainless steel of [C2C2im][EtSO4] ( = 60.6°) is significantly lower than that on the stainless steel of [C2C1im][EtSO4] ( = 78.4°). The shapes of the drop on stainless steel are shown in Figure S8. On the glass, the value of the contact angle decreases in the row [C2C1im][EtSO4] > [C2C2im][EtSO4] > Specol L-HL 150 > Specol L-HV 46 > Specol L-HV 32 > Specol L-HV 22 (see Figure 5b). The same trend is observed in 15 ACS Paragon Plus Environment


the case of contact angle on the stainless steel. Nonetheless, the  values of the investigated [C2C2im][EtSO4] on stainless steel and glass, even if they are lower than those of [C2C1im][EtSO4], are relatively high versus the  values of the Specol oils (for all  < 30°). Note also that the better wettability of [C2C2im][EtSO4] than that of [C2C1im][EtSO4] on stainless steel correlates very well with the ability of abovementioned IL to be used in hydraulic systems. The Specol oils have also a lower density than the analyzed ILs. [C2C1im][EtSO4] has the highest density, while Specol L-HV 32 has the lowest density (Figure 5c). As the temperature rises from 293.15 to 363.15 K, the density decreases by 4.9%–5.2% for the Specol oils and by 3.8% for [C2C1im][EtSO4] and [C2C1im][EtSO4]. The viscosities of the [C2Cnim][EtSO4] and Specol oils were measured at temperatures from 293.15 to 343.16 K and from 293.2 to 323.2 K, respectively. Notably, [C2C2im][EtSO4] has a lower viscosity than that of their homologue [C2C1im][EtSO4] (see Table S13 and Figure 5d). Generally, the viscosity of both homologues is lower than the other ILs.60,61 The comparison between the viscosity of [C2C1im][EtSO4] measured in this work and the literature values is presented in Figure S9. As is known, in all industrial and technological processes, the viscosity is one of the most important parameters of the working fluids. The currently used mineral hydraulic fluids have viscosity index (VI) values of approximately 90–110.3 Generally, the higher the VI, the better the stability of viscosity of fluid. A high VI of fluids leads to a good start up and a minimal loss in performance at low temperatures.3 The effect of temperature on the viscosity of Specol oils is higher than the results obtained for [C2C2im][EtSO4] and [C2C1im][EtSO4]. The VI values (determined using the ASTM D227004 standard) equals 179 and 194 for [C2C1im][EtSO4] and [C2C2im][EtSO4], respectively. [C2C2im][EtSO4] has a 25% higher viscosity index (VI) than that of the Specol oils (14553,54), 29% higher than SYN-2T (13855), 39% higher than SYN-2T (11855) and similar to BIO-H 16 ACS Paragon Plus Environment

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(19655). Besides all abovementioned properties, ILs should be characterized by optimal energy transmission and heat removal. In the case of both studied ILs, the heat transfer is dominated by convective effects because the calculated Prandtl numbers Pr (Table S15) vary from (1124 and 941) at 293.2 K to (387 and 342) at 318.2 K for [C2C1im][EtSO4] and [C2C2im][EtSO4], respectively, and these values are similar to those reported previously for pyrrolidinium-based imides.38 Toxicity of the ILs. There is a growing realization that ILs can be just as toxic as any other class of chemicals. It is therefore important for new ILs to be tested for toxicity before considering their use in industrial processes. Imidazolium-based ILs are poorly biodegradable 62

but compared to the water, or mineral oil-based fluid, ILs can often be recycled and reused

while retaining their volume (non-volatile) and hence can reduce the processing time as well as the maintenance cost.63 Because the toxicity of ILs is strictly related to the length of the carbon chain and [C4C1im][EtSO4] is more harmful than its bicarbonate homologue;11 thus, the toxicity level of 1,3-diethylimidazolium IL should be checked. In our approach we used tetrazolium\formazan based colorimetric test MTS which is known for simplicity and reliable results64. The cytotoxicity was measured on normal fibroblasts derived from human new-born skin (NHDF) as good contact toxicity assessment particularly suitable for such materials.38,43 Results are presented as IC50 values in Table 2 in comparison with other imidazolium and pyrrolidinium analogs [CnC1C1im][NTf2] (n = 2 and 4), [CnC1im][NTf2] and [CnC1pyr][NTf2] (n = 3 and 4).



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Table 2. Anti-proliferative activity (IC50 values) of the ILs against NHDF cells.

IL

a

IC50 (mM)

[C2C1im][EtSO4]

20.82 ± 0.89

[C2C2im][EtSO4]

20.44 ± 1.44

[C3C1pyr][NTf2]a

6.24 ± 0.21

[C4C1pyr][NTf2]a

7.29 ± 0.34

[C3C1im][NTf2]a

2.85 ± 0.15

[C4C1im][NTf2]a

5.25 ± 1.03

[C2C1C1im][NTf2]b

1.87 ± 0.39

[C4C1C1im][NTf2]b

0.85 ± 0.33

38 b

data reported in ; data reported in 43

Noticeably, ethyl sulfate-based ILs are remarkably less toxic than their bis(trifluoromethyl sulfonyl)imide analogs. For example, [C4C1C1im][NTf2] is roughly twenty times more toxic in our experiments. These findings agree with literature when compounds with fluorinated anions such as [NTf2]- are generally described as more toxic.65 However it should be noted, that most of the ILs used in this comparison have longer side chains in the cation than ethyl sulfate compounds. Thus, the length of the side chains in the cation and the corresponding lipophilicity may affect the mechanism of toxicity independently from the anion. The toxicity of ILs is explained by the formation of ROS

66

or a less specific

damage in the cell membrane.67 These two mechanisms are facilitated by different structural features. The longer chains and higher lipophilicity are, the more pronounced the membrane– based toxicity is.67 Particularly for long alkyl chain containing more than six carbon atoms toxicity relay on their insertion into lipid bilayer following by swelling and disintegration as reported recently by Jing et al.68 On the other hand, ROS formation can be effective only when the compound freely penetrates the cytosol and organelles which is generally better in 18 ACS Paragon Plus Environment

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smaller and less lipophilic compounds. Thus, our results may suggest that the tested ILs may increase the ROS and damage the antioxidant potential of the cell. To evaluate this hypothesis, we performed additional experiments measuring the overall ROS level in the cells treated by the tested ILs. The results are presented in Figure 6. [C2C2im][EtSO4] exhibits a considerably increased ROS level in a dose-dependent manner, which is higher than the hydrogen peroxide used as a positive control. Generally, it may be assumed that imidazolium ILs exert their cytotoxic effect by increasing the reactive oxygen species. Conclusions The speeds of sound in [C2C1im][EtSO4] and [C2C2im][EtSO4] were measured at temperatures from 293 K to 318 K and at pressures from 0.1 MPa to 101 MPa. We have analyzed the suitability of [C2C2im][EtSO4] as hydraulic liquids. Thus, appropriate thermodynamic properties (obtained via speed of sound) such as compressibility and expansibility were studied. Additionally, analyses of viscosity, surface tension, and wettability, as well as cytotoxicity were performed. Our primary comparative base was [C2C1im][EtSO4], which is already used as a hydraulic fluid, but also other ILs (proposed earlier as hydraulic fluids), a reference and commercial hydraulic oils were utilized. We have found that [C2C2im][EtSO4] has very similar properties to [C2C1im][EtSO4], the coefficients of compressibility and thermal expansion in both cases are small, which is an advantage when considering its potential use as a hydraulic fluid. Notably, [C2C2im][EtSO4] has a lower viscosity, a lower surface tension and better wetted stainless steel than [C2C1im][EtSO4]. Despite the fact that the obtained values of the surface tension and contact angles are smaller than [C2C1im][EtSO4], but they are still larger than those of the commercial Specol oils. Moreover, in spite of the longer carbon chain, the cytotoxicity of [C2C2im][EtSO4] is maintained at the same level as [C2C1im][EtSO4]. Both investigated ILs have a three to twenty times lower toxicity for normal human dermal fibroblast cells than disubstituted and



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trisubstituted imidazolium-based and pyrrolidinium–based ILs with bis(trifluoromethyl sulfonyl)imide anions. The presented results along with the data reported previously

20

show

that [C2C2im][EtSO4] can potentially be used as a hydraulic fluid similar to [C2C1im][EtSO4]. We believe that the studied ILs will be used in industry in a large scale. We believe also that in the age of advanced technology development, any problems with the presence of water can be and will be eliminated, considering the fact that the properties of ethyl sulfate-based ILs are very promised. However, because the data on the effect of studied imidazolium-based ethyl sulfates on environment are to date still very scarce (see e.g., safety data sheet published by supplier), the relevant parameters (i.e., toxicity, biodegradability and bioaccumulation) should be detailed studied. Especially biodegradability seems crucial because this parameter is in some contradiction to required thermal, electro and chemical stability of ILs.

ACKNOWLEDGMENTS The authors are profoundly indebted to Anton Paar Poland for sharing the Anton Paar Lovis 2000 ME microviscometer and Anton Paar Abbemat 550 Automatic Refractometer and for donating the ISO 17025 / ISO Guide 34 viscosity and density reference standard. The authors are profoundly indebted to Prof. I. Piwoński from the Department of Materials Technology and Chemistry, University of Lódź for providing the content and photograph of the surface of stainless steel (SEM), and Dr. S. Maślanka and D. Kwapulińska MSc for the NMR measurements. The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Institute of Chemistry of University of Silesia. JS thanks the „DoktoRIS -Scholarship program for innovative Silesia” for funding. AMW and KM thank the National Center for Science NCN grant 2014/13/D/NZ7/00322 for funding.


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Supporting Information contains: 15 Tables and 9 Figures. Corresponding Author * Marzena Dzida, University of Silesia in Katowice, Institute of Chemistry, Szkolna 9, 40-006 Katowice, Poland, E-mail: [email protected]

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49. Matkowska, D.; Golden, A.; Hofman, T. Densities, Excess Volumes, Isobaric Expansivities, and Isothermal Compressibilities of the 1–Ethyl–3–methylimidazolium Ethylsulfate + Ethanol System at Temperatures (283.15 to 343.15) K and Pressures from (0.1 to 35) MPa. J. Chem. Eng. Data 2010, 55, 685–693. 50. Matkowska, D.; Hofman, T. High–pressure volumetric properties of ionic liquids: 1–butyl–3– methylimidazolium tetrafluoroborate, [C4mim][BF4], 1–butyl–3–methylimidazolium methylsulfate [C4mim][MeSO4] and 1–ethyl–3–methylimidazolium ethylsulfate, [C2mim][EtSO4]. J. Mol. Liq. 2012, 165, 161–167. 51. Regueira, T.; Lugo, L.; Fernandez, J. High pressure volumetric properties of 1–ethyl–3– methylimidazolium ethylsulfate and 1–(2–methoxyethyl)–1–methyl–pyrrolidinium bis(trifluoromethylsulfonyl)imide. J. Chem. Thermodyn. 2012, 48, 213–220. 52. Hofman, T.; Goldon, A.; Nevines, A.; Letcher, T. M. Densities, excess volumes, isobaric expansivity, and isothermal compressibility of the (1–ethyl–3–methylimidazolium ethylsulfate + methanol) system at temperatures (283.15 to 333.15) K and pressures from (0.1 to 35) MPa. J. Chem. Thermodyn. 2008, 40, 580–591. 53. Technical Data Sheet Specol L-HV, http://www.specol.com.pl/wp-content/uploads/2017/ 06/IT_-Specol-L-HV-1.pdf 54. Technical Data Sheet Specol L-HL, http://www.specol.com.pl/wp-content/uploads/2017/ 05/IT_-Specol-L-HL.pdf 55. Regueira, T.; Lugo, L.; Fandiño, O.; López, E. R.; Fernández J. Compressibilities and viscosities of reference and vegetable oils for their use as hydraulic fluids and lubricants. Green Chem. 2011, 13, 1293–1302. 56. Gacino, F. M.; Regueira, T.; Bolotov, A. V.; Sharipov, A.; Lugo, L.; Comunas, M. J. P.; Fernandez, J. Volumetric behaviour of six ionic liquids from T = (278 to 398) K and up to 120 MPa. J. Chem. Thermodyn. 2016, 93, 24–33. 57. Gaciño, F. M.; Regueira, T.; Comunas, M. J. P.; Lugo, L.; Fernandez J. Density and isothermal compressibility for two trialkylimidazolium-based ionic liquids at temperatures from (278 to 398) K and up to 120 MPa. J. Chem. Thermodyn. 2015, 81, 124–130. 58. Regueira, T.; Lugo, L; Fernández, J. Influence of the pressure, temperature, cation and anion on the volumetric properties of ionic liquids: new experimental values for two salts. J. Chem. Thermodyn. 2013, 58, 440–448. 59. Tariq, M.; Freire, M.G.; Saramago, B.; Coutinho, J.A.P.; Canongia Lopes, J.N.; Rebelo, L.P.N. Surface tension of ionic liquids and ionic liquid solutions. Chem. Soc. Rev. 2012, 41, 829-868. 60. Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties. J. Phys. Chem. B 2006, 110, 19593−19600.



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Figure 1. Structures of the cations: a) [C2C1im]+; b)[C2C2im]+ and anion c) [EtSO4]- studied in this work.


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a)

b)

Figure 2. Temperature dependence of a) isobaric thermal expansion, αp, b) isentropic compressibility, S of: (blue point) ◆, [C2C1im][EtSO4]; (red points) ▲, [C2C2im][EtSO4]; (green point) □, Specol L-HV 22; (orange point) ◇, Specol L-HV 32 and (purple point) △, Specol L-HV 46; (black point) ○, Specol L-HL 150.

Figure 3. The isothermal compressibility κT vs isobaric thermal expansion, αp under atmospheric pressure: (red symbols) ●, [C2C1im][EtSO4]; ○, [C2C2im][EtSO4]; (blue symbols) □, MIN–H0255; +, SYN-2T55; ✱, BIO–H55; (black symbols) △, [P6,6,6,14][(C2F5)3PF3]56; ◆, [C1OC2C1pyr][NTf2]51; □, [C4C1pyr][B(CN)4]56; ◇, [C2C1im][C6SO4]56; ■, [C4C1C1im][(C2F5)3PF3]57; ▲, [C4C1pyr][NTf2]38; +, [C2C1C1im][NTf2]43; ▼, [C2C1im][SCN]20; ●, [C1C1im][(C1O)2PO2]56.



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a)

b) Figure 4. a) Isothermal compressibility, κT and b) isobaric thermal expansion, ap: ○, △ and ● under pressures of 0.1 MPa, 50 MPa and 100 MPa, respectively, of: [C2C1im][EtSO4], [C2C2im][EtSO4],

MIN–H0255,

[C1OC2C1pyr][NTf2]51, [(C2F5)3PF3]58,

SYN-2T55,

[C4C1pyr][B(CN)4]56,

[C4C1C1im][(C2F5)3PF3]57,

BIO–H55;

[P6,6,6,14][(C2F5)3PF3]56,

[C2C1im][C6SO4]56, [C4C1pyr][NTf2]38,

[C1C1im][(C1O)2PO2]56 at 313.15 K.


[C1OC2C1pyr]

[C2C1C1im][NTf2]43,

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a)

b)

c)

d)

Figure 5. a) Temperature dependence of the surface tension, ; b) contact angle, ; c) temperature dependence of the density, ρ; d) temperature dependence of the kinematic viscosity, υ, of: (blue point) ◆, [C2C1im][EtSO4]; (red points) ▲, [C2C2im][EtSO4]; (green point) □, Specol L-HV 22; (orange point) ◇, Specol L-HV 32; (purple point) △, Specol LHV 46; and (black point) ○, Specol L-HL 150.



Figure 6. Level of reactive oxygen species of the NHDF cell untreated (control) and treated with two concentrations of ILs (IC50 and 1.5IC50). H2O2 used as the positive control, while the known antioxidant resveratrol provided the results for the negative control.


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Synopsis 1,3-diethylimidazolium ethyl sulfate have low compressibility and expansibility which predestine him to use as sustainable and efficient hydraulic fluid Graphical Abstract


Comparative Study of the High Pressure Thermophysical Properties of

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