Pyrrolidinium-Based Ionic Liquids as Sustainable Media in Heat

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Pyrrolidinium-based ionic liquids as sustainable media in heat transfer processes Ma#gorzata Musia#, Katarzyna Malarz, Anna MrozekWilczkiewicz, Robert Musiol, Edward Zorebski, and Marzena Dzida ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02918 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Sustainable Chemistry & Engineering

Pyrrolidinium-based ionic liquids as sustainable media in heat transfer processes

Małgorzata Musiał,1 Katarzyna Malarz,1,2 Anna Mrozek-Wilczkiewicz,2,3 Robert Musiol,1 Edward Zorębski,1 Marzena Dzida1* 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, Institute of Physics, Uniwersytecka 4, 40-007 Katowice,

Poland Corresponding Author E-mail: [email protected]

ABSTRACT Ionic liquids are viewed as green media for many engineering applications and exhibit exceptional properties including negligible vapor pressure, null flammability, wide liquid range, and high thermal and chemical stabilities. We present new thermophysical properties of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides ([CnC1pyr] [NTf2] with n = 3, 4) for future application them as heat transfer media. The speed of sound was measured at pressures up to 100 MPa and at temperatures from 293 to 318 K. The pρT, pCpT data and derived thermophysical properties were determined using the acoustic method. TGA of [CnC1pyr][NTf2] and cytotoxicity of [CnC1pyr][NTf2] and their imidazolium counterparts ([CnC1im][NTf2]) are investigated. The physicochemical properties of [CnC1pyr][NTf2] are compared with those of [CnC1im][NTf2] and commercial heat transfer fluids (Therminol VP1 ACS Paragon Plus Environment


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1, Therminol 66, Marlotherm SH). [C3C1pyr][NTf2] and [C4C1pyr][NTf2] have a wide liquid range of approximately 480 K and high decomposition onset temperatures of 771 K and 776 K, respectively. [CnC1pyr][NTf2] exhibit high energy storage density of ~1.98 MJ·m-3·K-1, which is slightly dependent on temperature and pressure. The thermal conductivity of [CnC1pyr][NTf2] is comparable to that of commercial heat transfer fluids. [CnC1pyr][NTf2] have lower toxicity for normal human dermal fibroblast cells than [CnC1im][NTf2]. Thus, [CnC1pyr][NTf2] are promising heat transfer fluid candidates. Keywords: pyrrolidinium-based ionic liquids, green media, energy storage density, sustainable heat transfer fluids, cytotoxicity, high pressure

INTRODUCTION The sustainability of our energy supply has become one of the greatest challenges in the present time. The development of a modern energy sector requires the search for alternative energy sources, the optimization of used technologies and the search for innovative working fluids, which would reduce energy consumption. The applications of heat transfer fluids include low-temperature refrigeration systems, as well as solar energy collection and thermal storage at high temperatures.1,2 The currently used heat transfer fluids are primarily organic solvents, such as Therminol 66, Therminol VP-1, and Marlotherm SH, these fluids have a relatively high vapor pressure in the desired temperature range. Moreover, as an example, the decomposition of Therminol VP-1 generates hydrogen, which must be trapped, and thus causes an energy loss.3 ILs are currently the subject of intensive research as environmentfriendly engineering liquids in a variety of industrial applications, including solvents in separation processes and chemical reactions, lubricants, heat transfer fluids and more. Some important features that distinguish ionic liquids (ILs) from other compounds are good chemical and thermal stability, non-flammability, a wide range of liquidity and an 2 ACS Paragon Plus Environment

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exceptionally low saturated vapor pressure at high temperatures, which makes them nonvolatile and non-explosive and especially attractive as heat transfer fluids. To date, studies on ILs as heat transfer fluids were focused on imidazolium-based ILs with the [BF4]-, [PF6]- or [NTf2]- anion.4–9 Tenney et al.10 analysed the possibility of application of 9 different ILs as heat transfer fluids such as imidazolium- and pirydinium-based ILs with the [DEP]-, [C1SO4], [TFO]-, [SCN]- or [NTf2]- anion.

Chernikova et al.11 compared

properties of 25 imidazolium, pyrrolidinium, pyridinium,

the thermophysical

phosphonium, and ammonium

derivatives with anions as above with the properties of some commercial heat transfer fluids, and they found that the substituted imidazolium and pyrrolidinium bis(trifluoromethyl sulfonyl)imides are to be best suited as heat transfer fluids among their studied ILs. Based on the available published data, those investigators found that the most thermally stable ILs contain pyrrolidinium and imidazolium cations and that the highest thermal stability is exhibited by ILs with the bis(trifluoromethylsulfonyl)imide anion. Those investigators also noticed that the thermophysical data of these ILs are similar to those of widely used commercial organic and organosilicon heat transfer fluids. Therefore, we focused on pyrrolidinium bis(trifluoromethylsulfonyl)imides in order to confirm their potential for application

as

heat

transfer

bis(trifluoromethylsulfonyl)imide

media.

We

chose

([C3C1pyr][NTf2])

1-propyl-1-methylpyrrolidinium

and

1-butyl-1-methylpyrrolidinium

bis(trifluoromethylsulfonyl)imide ([C4C1pyr][NTf2]), because the use of a smaller side chain results

in

smaller

viscosity

and

lower

toxicity.12

Moreover,

the

1-butyl-1-

methylpyrrolidinium-based IL was chosen because various ILs with a butyl substituent in the cation have already been studied widely.13,14 In contrast, the 1-propyl-1-methylpyrrolidiniumbased IL was chosen because ILs with a propyl group in the cation have been investigated scarcely.13,14 Recently, interest in pyrrolidinium-based ILs has increased because of their varied physicochemical properties, which are often superior to those of imidazolium-based

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ILs. Pyrrolidinium-based ILs have melting points similar to those of their imidazolium-based analogues and low viscosities in comparison with other ILs, but slightly higher than those of their imidazolium counterparts.15 Cytotoxicity studies of ILs have revealed that these ILs are less toxic than well-known piperidinium- and imidazolium-based ILs, which makes them more environmentfriendly.12 These ILs have gained major attention as solvents for extraction and biological processes. If used in catalytic processes, these ILs promote easier processing and improve the feasibility of recycling. These ILs are also of interest for use as lubricants and solid state dye sensitized solar cells.16 Furthermore, some pyrrolidinium-based ILs have already been investigated in the context of heat transfer as a base fluid for IoNanofluids.17,18 The design of industrial processes and the evaluation of applications of new products can only be achieved if their exact physicochemical properties are known. For the rational consideration of ILs as heat transfer fluids, in addition to their isobaric heat capacity and thermal conductivity, it is important to investigate their thermophysical properties, such as their melting, glass transition and decomposition temperatures, viscosity, density, Prandtl number and energy storage density. Moreover, to project new processes on a larger or an industrial scale, knowledge of the physicochemical properties of these fluids both under ambient conditions and under high pressure is necessary.13,19 The high pressure data help in the development of technologies that require working liquids to be exposed to changing pressure. In that context, we have investigated the speed of sound, density, isobaric and isochoric heat capacities, isentropic and isothermal compressibility coefficients, and isobaric thermal expansion coefficient as a function of pressure and temperature. The pρT, pCpT data and derived properties were obtained using an acoustic method, which is an alternative to direct methods and is regarded as a precise tool for the investigations of the thermodynamic properties of compressed liquids.13 The pρT data obtained from the experimental speed of


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sound are considered to be the most reliable because the speed of sound can be measured accurately over a wide range of temperature and pressure values. To clarify the usefulness of the analyzed ILs as heat transfer media, the thermophysical properties of [CnC1pyr][NTf2] (n = 3, 4) are discussed in comparison with [CnC1im][NTf2] (n = 3, 4) and commercial heat transfer fluids, based on the aromatic compounds Therminol VP-1, Therminol 66, and Marlotherm SH, which have a lower operating temperature, similar to that of [CnC1pyr][NTf2]. Moreover, the cytotoxicity of [CnC1pyr][NTf2] and [CnC1im][NTf2] (n = 3, 4) is studied. This work will lead to the development of a new use of [C3C1pyr][NTf2] and confirm the potential application of [C4C1pyr][NTf2]11 as heat transfer media. EXPERIMENTAL SECTION Materials. The ILs used in this work were purchased from Iolitec. The ILs were stored under argon and the water content was determined using the Karl Fischer method. We have used the same batch of liquids as in previous investigations.20 The samples were dried and degassed in the pressure range of 6 – 10 mbar (Heidolph rotary evaporator combined with the SC 920 vacuum pump system) and at temperatures not exceeding 343 K. Table 1 summarizes the names, acronyms, CAS numbers, molar mass, M, purity, water content and halides20 of the studied ILs, and Figure 1 shows the cation and anion structures. The quality of the chemicals was confirmed by comparing the refractive index values, nD (measured by means of Abbe refractometer RL3 (PZO, Poland)), with the values reported in the literature. The nD values at 298.15 K are 1.42050.0002 and 1.42300.0002

for

[C3C1pyr][NTf2] and [C4C1pyr][NTf2], respectively. The agreement between the nD values obtained in this work and the values available in the literature21–27 is very good and the relative deviations vary from -0.001 to 0.035%. These data are presented in detail in Table S1.



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Table 1 Name, Acronym, CAS Number, Molar Mass, Purity, Water Content, Halides of the Ionic Liquids Studied in This Work M name

1-propyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide a

acronym

CAS number

gmol-1

mass

water

fraction

content

puritya

ppm

halides

[C3C1pyr][NTf2]

223437-05-6

408.38

0.99

100a/46b

 100a

[C4C1pyr][NTf2]

223437-11-4

422.41

0.99

100a/370b

 100a

Declared by supplier. b Coulometric Karl Fisher titration, TitroLine 7500.

Speed of sound measurements. The speed of sound at a frequency of 2 MHz was measured under high pressures using a setup that consists of a single transmitting-receiving ceramic transducer and an acoustic mirror and operates on the principle of the pulse-echo-overlap method. The pressure is stabilized to within ±0.03 MPa and measured with an expanded uncertainty of U(p)=0.0015·p MPa by a digital manometer that consists of a strain gauge Hottinger–Baldwin P3MB, equipped with a Hottinger–Baldwin MC3 signal amplifier, modified in our laboratory, and a digital voltmeter Meratronik V 542.1. The temperature is stabilized within the limits of ±0.01 K by a Haake DC 30 temperature controller and measured using an Ertco Hart 850 platinum resistance thermometer (traceable to a NIST standard) with an expanded uncertainty of U(T)=0.05 K and a resolution of 0.001 K. The combined expanded uncertainty with the interval of confidence of 0.95 of the speed of sound measurements was estimated to be better than U(c)= 2 m·s-1 (uc= 1 m·s-1, k = 2) at high pressures.28 More details about the high pressure device and the method can be found in the previous papers.28–30 TGA. Thermogravimetric analysis (TGA) was performed using a TGA/DSC1 Mettler-Toledo thermal analyzer with a heating rate of 10 K·min−1 in a stream of nitrogen (60 cm3·min−1). 6 ACS Paragon Plus Environment

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Cell culture. The normal human dermal fibroblast cell line (NHDF) was purchased from PromoCell. The cells were grown as monolayer cultures in 75 cm2 flasks (Nunc) in Dulbecco's Modified Eagle Medium (DMEM). The medium was supplemented with 15% fetal bovine serum (Gibco) and 100 mg/L of gentamycin (Gibco). The cell lines were maintained at 310.15 K in a 5% CO2 incubator and passaged every 3-4 days as required. Cytotoxicity assay. The investigated ILs were dissolved in the culture medium to achieve necessary concentrations. The exponentially growing cells were harvested via the trypsinization of sub-confluent cultures. The cells were seeded into 96-well cell culture microtiter plates (Nunc) at concentrations of 4.0·103 cells per well and cultured for 24 h. After this time, the growth medium was exchanged for a medium containing varying concentrations of the ILs. After 72 h of incubation under standard cell culture conditions with the compounds being investigated, the medium was replaced with 100 μL of DMEM without phenol red. The metabolic activity of viable cells was determined by adding 20 μL of CellTiter 96AQueousOne Solutions – MTS (Promega) to each well, followed by a 1 h incubation. The MTS assay is a colorimetric method for determining the number of viable cells. A standard solution containing 100 μL of DMEM without phenol red and 20 μL of MTS solution was used to determine the ”blank” absorbance. The absorbance was measured at 490 nm using a Synergy™4 microplate reader (BioTek). The 50% inhibitory concentration (IC 50) was defined as the concentration of the compound that was able to reduce the cell proliferation to 50% of the untreated control cells. Each individual compound was tested in triplicate in a single experiment, with each experiment repeated four times. The IC50 values were calculated using the GraphPad Prism 5 software.



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RESULTS AND DISCUSSION The thermodynamically useful speed of sound was investigated from 293 to 318 K for [C3C1pyr][NTf2] and from 303 to 318 K for [C4C1pyr][NTf2], under pressures that ranged from 15 to 101 MPa. The experimental values are listed in Table S2. The temperature ranges were chosen carefully because pressure-temperature studies of thermodynamic properties by means of the indirect acoustic method used in this work must be carried out outside of the dispersion regions. In this study, the above-mentioned temperature ranges were selected by taking into account the ultrasound velocity dispersion for each investigated ILs, as determined in a previous work.31 It was found that [C3C1pyr][NTf2] shows ultrasound velocity dispersion beginning in the vicinity of 36 MHz at 293.15 K under atmospheric pressure, whereas for [C4C1pyr][NTf2], this phenomenon appears in the vicinity 27 MHz under the same conditions.31 Thus, the dispersion region in the case of [C4C1pyr][NTf2] occurs at lower frequencies. In addition, the dispersion is stronger. Thus, to avoid a dispersion region during this work, the speed of sound in [C4C1pyr][NTf2] was measured only at temperatures from 303 K, for which ultrasound velocity dispersion begins in the vicinity of 45 MHz.31 In our opinion, high pressure investigations up to 100 MPa from 293 K for [C3C1pyr][NTf2] and from 303 K for [C4C1pyr][NTf2] sufficiently protect speed of sound measurements outside of dispersion regions. Thus, the use of the Newton-Laplace relation to determine thermodynamic properties is well-founded and correct. It is noteworthy that the dispersion issue loses importance with increasing temperature because dispersion significantly decreases with increasing temperature.31,32 The speed of sound data, pressure and temperature were correlated using the equation proposed by Sun et al.: 2

2

p  p0   aij c  c0  T j i

(1)

i 1 j  0


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where aij are the polynomial coefficients calculated using the least squares method, c is the speed of sound at p  0.1 MPa, and c0 is the speed of sound at atmospheric pressure p0. The coefficients aij and the mean deviations from the regression lines are given in Table S3. Under atmospheric pressure, the speed of sound in [C3C1pyr][NTf2] is higher than in [C4C1pyr][NTf2], as reported for their imidazolium counterparts (inset of Figure 2a and Table S2). Under high pressures, the inversion of speed of sound is observed (i.e. for ILs with a longer alkyl chain in the cation, the speed of sound is higher) (Figure 2a). This effect was observed for the first time for the [CnC1im][NTf2] series.34 The results obtained for pyrrolidinium-based ILs confirm this observation, as the pressure better differentiated the speed of sound in [CnC1pyr][NTf2] than in [CnC1im][NTf2] (see Table S2 and refs.32,34). For [CnC1pyr][NTf2], the inversion point occurs at a lower pressure (91 MPa) than observed for adequate imidazolium homologs (282 MPa). The crossing point is independent of temperature. Derived Properties. Using the speed of sound under high pressures, as measured in this work, together with the speed of sound, density, ρ, and molar isobaric heat capacity, Cp, at atmospheric pressure, as reported by Zorębski et al.,20 the pρT and pCpT data were calculated for pressures up to 100 MPa at temperatures from 293.15 to 318.15 K for [C3C1pyr][NTf2] and from 303.15 to 318.15 K for [C4C1pyr][NTf2]. In the calculations, the acoustic method was applied, using a slightly modified numerical procedure proposed by Sun et al.37 The change of liquid density, Δ, caused by the change of pressure from p1 to p2 at constant temperature T is given by the following formula:



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p2  1  p2T   p2T 1      2  dp   2 dp  p , c C p  cp c p1  p1

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p2

(2)

where αp is the thermal expansion coefficient, cp is the specific isobaric heat capacity and Δp = p1 - p2. For integration the initial values of c (T), (T), and cp (T) at a reference pressure of p0=0.101325 MPa were used. The approximate relationship (2) is sufficiently accurate, provided that Δp is small, because the heat capacity depends only slightly on pressure. Moreover, the first term of the right hand side of eq. (2) is significantly larger than the second term, since the latter term results from the difference between the adiabatic and isothermal compressibilities, which is rather small. The specific isobaric heat capacity at p2 is given by:

c p ( p2 )  c p ( p1 )  (T /  ){ p2  ( p / T ) p }p ,

(3)

where cp( p1) is the specific isobaric heat capacity at p1. The respective uncertainties estimated using the perturbation method are ± 0.02% and ± 0.3% for the density and specific isobaric heat capacity. The specific isobaric heat capacity was recalculated into the molar isobaric heat capacity. The expanded uncertainties have been estimated to be better than U()=5·10-4 kg·m-3 and U(Cp)=1·10-2Cp J·mol-1·K-1 for the density and the molar isobaric heat capacity, respectively. The calculated density and molar isobaric heat capacity at high pressures are reported in Tables S4 and S5, respectively. The pρT data were correlated using the Tait equation:

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

(4)

where ρ0 is the density at temperature T and at atmospheric pressure p0 = 0.101325 MPa, C is the temperature independent coefficient, and B(T) is the temperature dependent coefficient:

B(T )  A1  A2 (T / 100)  A3 (T / 100) 2 ,

(5)

where A1, A2 and A3 are adjustable coefficients. The respective coefficients and mean deviations for density are presented in Table S6. 10 ACS Paragon Plus Environment

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The agreement of the density values calculated in this work with those obtained experimentally by means of a high pressure densimeter is presented and discussed in the 2 1 Supporting Information (see also Figure S1). The isentropic compressibility,  S  (   c )

was determined based on the experimental speed of sound and the calculated density. The results are given in Table S7. With an increasing number of methylene groups in [CnC1pyr][NTf2] and [CnC1im][NTf2], the density decreases (Figure 2b), while the isentropic compressibility increases (Figure 2c). The differences between the densities of [CnC1pyr][NTf2] and [CnC1im][NTf2] can be compensated by changing the pressure (Figure 2b, see also Table S4 and ref.34). In contrast, the isentropic compressibility values, being the product of density and the speed of sound, are similar for [C3C1pyr][NTf2] and [C3C1im][NTf2] as reported for [C4C1pyr][NTf2] and [C4C1im][NTf2] over the whole pressure range (Figure 2c). The differences between the compressibility of [CnC1pyr][NTf2] and [CnC1im][NTf2] do not exceed 4.5%, at atmospheric pressure, and with increasing pressure, the differences decrease to 3.5% at 100 MPa, independent of temperature. The isobaric thermal expansion coefficient, αp, the isothermal compressibility coefficient, κT, and the isochoric thermal pressure coefficient, βV, form a set of three important fundamental material constants (three so-called thermophysical coefficients). The p was calculated by definition

 p  1   T  p . The results are collected in Table S8 and compared with the data available in the literature in the Supporting Information (see also Figure S2). The αp value is nearly the same for [C3C1pyr][NTf2] and [C4C1pyr][NTf2] and nearly independent of temperature, with values of 6.30⋅10-4 - 6.35⋅10-4 K–1 in the temperature range of 288.15 363.15 K under atmospheric pressure.20 In contrast, the αp values of both ILs clearly decrease with increasing pressure (~20% in the pressure range 0.1-100 MPa) (Table S8). The isothermal compressibility coefficient was calculated using the well-known relationship

T   1  c 2   p2  V  T  C p1 . The results are collected in Table S9 and compared with data 11 ACS Paragon Plus Environment


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available in the literature in the Supporting Information (see also Figure S3). κT lies in the range from 4.85⋅10-10 to 5.61⋅10-10 Pa–1 at temperatures of 293.15 - 323.15 K under atmospheric pressure,20 which is typical for ILs. As a result of the high pressure of 100 MPa, the values decrease by ~40% (Table S9). Usually αp and κT are reported, whereas the isochoric thermal pressure coefficient V  p T V (related to isothermal change entropy per unit volume S V T ) can be easily obtained as the product  p2 T1 . Knowledge of αp and κT also allows to calculate the molar isochoric heat capacity CV from Cp (Table S10), because 2 1 both quantities differ only by the term  p  Vm  T   T . For two homologues of the 2 1 [CnC1pyr][NTf2] series, the term  p  Vm  T   T is almost constant in the investigated

temperature range and decreases slightly with increasing pressure. The overall expanded uncertainties of s, T, p and CV are estimated to be U(s)=1.5·10-3s Pa-1, U(T)=5·10-3T Pa-1, U(p)=1·10-2p K-1, and U(CV)=2·10-2CV J·mol-1·K-1, respectively. Thermal properties of ILs in comparison with commercial heat transfer fluids. Thermal stability is one of the most important requirements in the selection of a fluid for work under certain heat transfer conditions. Industrial heat exchangers often operate using very high temperature heat transfer fluids based on aromatic compounds. The most popular fluid is the high temperature heat transfer liquid medium Therminol 66 (hydrogenated terphenyl).38 This fluid operates in the temperature range from 273 to 618 K.11 Therminol VP-1, which is an eutectic mixture of diphenyl and diphenyl ether, is an equally popular heat transfer fluid. This fluid operates in the temperature range from 285 to 673 K.2 The third well-known heat transfer fluid is Marlotherm SH, which is a mixture of isomeric dibenzylbenzenes and operates in the temperature range from 268 to 623 K.11 The ILs can operate at temperatures higher than their melting points and lower than the temperature of significant weight losses (< 5%). For [C3C1pyr][NTf2], the melting point Tm equals 282.83 K.39 In the case of


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[C4C1pyr][NTf2], the glass transition Tg equals 188.6 K39 and the melting point Tm equals 265.6 K39. Taking into account the results from TGA, it was found that pyrrolidinium-based ILs demonstrate very good thermal stability without significant weight losses (< 5%) below 700 K for both compounds. This finding implies that no thermal decomposition occurred below this temperature. The decomposition onset temperature is 771 K for [C3C1pyr][NTf2] and 776 K for [C4C1pyr][NTf2]. The TGA thermograms of the investigated compounds are given in Figure 3. Compared with [CnC1im][NTf2]40 and the abovementioned commercial heat transfer fluids, the thermal stability of the analyzed liquids is generally higher. [CnC1pyr][NTf2] is characterized by a wide liquidity range of nearly 480 K, while the operating range is approximately 400 K, which is higher than the values observed for [CnC1im][NTf2] (~2%) and heat transfer fluids (Therminol VP-1 ~3%; Therminol 66 ~14%; Marlotherm SH ~12%). The molar isobaric heat capacities of the selected ILs are also higher than those of typical organic solvents and comparable with those of other protic and aprotic ionic liquids.20,41 The heat capacity is needed to estimate heating and cooling requirements as well as energy storage density. The energy storage density, which is also called the volumic heat capacity and denoted Cp/Vm, can be easily calculated as the product of the isobaric specific heat capacity and the density C p Vm  c p    . The energy storage density under atmospheric pressure was calculated for the imidazolium- and pyrrolidinium-based ILs from the previously published molar isobaric heat capacity20 and density20,34, while the Cp/Vm of the commercial heat transfer fluids was calculated from other previously reported data.42–44 The Cp/Vm is the most critical design parameter for thermal fluids because a higher energy storage density value will result in a lower requirement for the thermal fluid volume. Typically, the energy storage density of the liquids is in the range of 1.5–2.0 MJ·m-3·K-1.45 The obtained results show that [CnC1pyr][NTf2] is characterized by a slightly higher energy storage density (for [C3C1pyr][NTf2], 1.99 MJ·m-3·K-1, and for [C4C1pyr][NTf2], 1.97 MJ·m-3·K-1, at 313.15 13 ACS Paragon Plus Environment


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K) than [CnC1im][NTf2] (for [C3C1im][NTf2], 1.93 MJ·m-3·K-1, and for [C4C1im][NTf2], 1.94 MJ·m-3·K-1, at 313.15 K) (Figure 4). More importantly, the Cp/Vm values of ILs are significantly higher than those of Therminol VP-1, Therminol 66, and Marlotherm SH, with Cp/Vm values of 1.68 MJ·m-3·K-1, 1.62 MJ·m-3·K-1, and 1.67 MJ·m-3·K-1 at 313.15 K, respectively. Moreover, the effect of temperature on the energy storage density is more visible for the currently used heat transfer fluids than for ILs (Figure 4). An additional advantage is the small effect of pressure on the Cp/Vm of pyrrolidinium-based and imidazolium-based ILs (Figure 5). The Cp/Vm increases less than 4.5% in the pressure range from 0.1 to 100 MPa. The Cp/Vm values of the pyrrolidinium-based ILs under investigation may suggest that these fluids have better heat adsorption characteristics than commercial thermal fluids. It is worth noting that the specific heat capacity and density of the ILs and heat transfer fluids vary in opposite senses (i.e. the least dense liquid has the highest value for the specific heat capacity and the densest liquid exhibits the lowest specific heat capacity). However, the studied ILs are denser than the heat transfer fluids, and they are characterized by a lower specific heat capacity. The above-described effect resulted in similar values of Cp/Vm, separately for the investigated ILs and the heat transfer fluids. Since the Cp/Vm values of the pyrrolidiniumbased ILs under study are higher than those of their imidazolium counterparts and commercial heat transfer fluids, it is essential to compare other properties in the context of their use as heat transfer media such as thermal conductivity, . Little experimental data is available concerning the thermal conductivity of ILs.20 Their thermal conductivities vary from 0.2137 W·m-1·K-1 at 273.15 K for 1-ethyl-3-methylimidazolium acetate ([C2C1im][CH3COO])46 to 0.105 W·m-1·K-1 at 353 K for 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluoro phosphate ([C4C1pyr][FAP]).47 The  of the analyzed ILs is characterized by rather small values in comparison with other ILs (for [C3C1pyr][NTf2],  equals 0.1181 W·m-1·K-1 at 313.15 K20 and for [C4C1pyr][NTf2],  equals 0.124 W·m-1·K-1 at 313.15 K47). However, 14 ACS Paragon Plus Environment

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these values are similar to those of commercial heat transfer fluids (Therminol 66 (0.117 W·m-1·K-1),42 Therminol VP-1 (0.134 W·m-1·K-1),43 and Marlotherm SH (0.128 W·m-1·K-1)44) (Figure 6). Moreover, the slope of the (T) dependence of Therminol 66 is comparable to the slope of the (T) dependence of ILs. This similarity is undoubtedly a beneficial feature of the analyzed ILs in the context of future use as working fluids. Another important parameter for characterizing heat transfer fluids is viscosity. The viscosity of ILs varies over a broad range from less than 30 mPa⋅s (for example 1-ethyl-3-methylimidazolium thiocyanate51) to higher than 40000 mPa⋅s (for example 1-butyl-3-methylimidazolium chloride52). Imidazolium-based ILs have slightly lower viscosity than ILs that contain the pyrrolidynium cation, but both homologues have relatively low viscosity (< 100 mPa·s) in comparison to other ILs.15,32,53,54 The viscosity values of the investigated ILs are similar to those of commercial thermal fluids, except for Therminol VP-1 at lower temperatures (Figure 7). In particular, the viscosity of Marlotherm SH is similar to the viscosity of the investigated ILs. Moreover, the effect of temperature on the viscosity of the studied ILs is most similar to that observed for Marlotherm SH. Increasing the temperature from 293.15 to 323.15 K decreases the viscosity of the investigated ILs in the range of 65–70%, but in the case of Marlotherm SH the change is approximately 80%. The viscosity is also needed to calculate the Prandtl number, the Nusselt number and the convective heat transfer coefficient.55,56 The Prandtl number, Pr, is the ratio of the heat capacity multiplied by the viscosity to the thermal conductivity, Pr  C p   . Variations between the different ILs in the range of two orders of magnitude

have been observed, and variations in range of one order of magnitude with temperature changes from 273 to 343 K have been reported for a given IL.52 Nearly all of the variations in the Prandtl number are determined by the effect of temperature on viscosity. According to the data available in the literature, the Prandtl number for ILs can vary from 600  40000 at lower temperatures (273.15 K) to 60  500 at higher temperatures (343.15 K).10 These values are 15 ACS Paragon Plus Environment


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typical for oils, and indicate that the convective mechanism of heat transfer is dominant.10 The Prandtl number values on the order of 1140-300 are observed in the investigated temperature range from 293.15 to 323.15 K for [CnC1pyr][NTf2] (Figure 8) and these values suggest that heat transfer with [CnC1pyr][NTf2] will be dominated by convective effects. The Prandtl number values of [CnC1pyr][NTf2] and [CnC1im][NTf2] are the most similar to Therminol 66 at temperatures higher than 303.15 K, while the temperature dependence is the most similar to Marlotherm SH, as in the case of viscosity. The Prandtl number values of Therminol VP-1 deviate from those of the other systems (Figure 8). Working fluids should be characterized by both low thermal expansion and the small changes of αp with temperature. This characteristics translates into a higher power efficiency. The αp values of the ILs under study are close to the αp values of Therminol 66 and Marlotherm SH only at lower temperatures (Figure 9). The αp values of the ILs are almost constant over the whole temperature range from 288.15 to 363.15 K under atmospheric pressure, in contrast to the αp values of heat transfer fluids, which increase with temperature. Moreover, the temperature dependence of the αp of [CnC1pyr][NTf2] is changed only slightly by pressure. This stability is a great advantage of ILs in comparison to commercial heat transfer fluids. Cytotoxicity. The cytotoxicity of the studied compounds was measured using the MTS assay. In this test, the metabolic activity (mitochondrial dehydrogenase) of the cell is determined using a colorimetric method. We chose normal human dermal fibroblasts (NHDF) cells for the experiments. This test is an attractive alternative to expensive in vivo experiments and has been suggested as a good screening system for toxicity.57 Our modification used normal cell lines derived from the skin which yield better insights than poorly differentiated cancer cells. In general these results may reflect the overall toxicity to humans especially the contact risk. After 72 h incubation with [C3C1pyr][NTf2] and [C4C1pyr][NTf2], the absorbance of the red formazan was measured and the number of the proliferating cells was calculated. The IC50 16 ACS Paragon Plus Environment

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values for the [C3C1pyr][NTf2] and [C4C1pyr][NTf2] were 6.24 and 7.29 mM, respectively (Table S11), which show moderate toxicity. These data are comparable to the results available in the literature concerning the toxicity of other ILs.57–59 In comparison to their imidazolium counterparts (2.85 mM for [C3C1im][NTf2] and 5.25 mM for [C4C1im][NTf2], respectively), the pyrrolidinium-based ILs appeared to be approximately 2 times less harmful. Interestingly, the ILs with longer C4 side chain are slightly less toxic than the ILs with C3 side chain for both cations. The mechanism of toxicity of ILs is connected to the formation of reactive oxygen species (ROS)60 and the disruption of the cell membrane, which leads to swelling and cell death.61 The latter mechanism generally prefers longer alkyl chains on ILs, which act as a typical surfactant.60 On the other hand ROS formation can be effective only when the compound freely penetrates the cytosol and organelles, which requires smaller and less lipophilic compounds. Thus, our results may suggest that pyrrolidinium-based ILs damage the antioxidant potential of the cell. Alternatively such organic cations can interact with DNA, leading to intercalation and cell cycle arrest. Nevertheless, the studied ILs can be regarded as moderately toxic, particularly in comparison to those that contain fluorinated anions, such as [NTf2]-, which are generally described as more toxic.62 Conclusions. For two pyrrolidinium-based ILs, namely [C3C1pyr][NTf2] and [C4C1pyr][NTf2] the speed of sound as a function of temperature and pressure, the refractive index, the decomposition temperature and cytotoxicity were measured. The temperature and pressure dependence of the density, the isobaric and isochoric heat capacities, the energy storage density, the isentropic and isothermal compressibility coefficients and the isobaric thermal expansion coefficient were determined using the acoustic method, and the Prandtl number was calculated as a function of temperature. All of the results were compared with those obtained for imidazolium-based ILs, [CnC1im][NTf2] (n = 3, 4), and commercial heat transfer



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fluids based on aromatic compounds such as Therminol VP-1, Therminol 66, and Marlotherm SH. The obtained results support the conclusion that 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides are best suited as heat transfer fluids. The thermophysical parameters, such as the thermal conductivity, viscosity, and Prandtl number of these ILs are similar to or better than those of the widely studied [CnC1im][NTf2] and widely used heat transfer fluids. The Prandtl number values on the order of 1140-300, which were observed at temperatures of 293.15-323.15 K, suggest that heat transfer with [CnC1pyr][NTf2] will be dominated by convective effects. The advantage of the analyzed pyrrolidinium-based ILs is their outstanding thermal stability. Moreover, [CnC1pyr][NTf2] have a high energy storage density of ~1.98 MJ·m-3·K-1, which is higher than those of [CnC1im][NTf2] and heat transfer fluids, and which is nearly independent of temperature and pressure. These ILs exhibit low thermal expansion, with values lower than those observed for [CnC1im][NTf2] and heat transfer fluids, and this parameter is nearly independent of temperature. The above properties translate into higher power efficiency. [CnC1pyr][NTf2] have relatively low toxicity for the normal human dermal fibroblast cells, with values lower than those of its counterparts that contain the imidazolium cation. The obtained results confirmed the recommendation of Chernikova et al.11 that pyrrolidinium bis(trifluoromethylsulfonyl)imides are some of the best candidates for heat transfer media. In sum, all of the thermal and cytotoxic properties of [C3C1pyr][NTf2] and [C4C1pyr][NTf2] are excellent, making them to be sustainable media for heat transfer processes. Moreover: “The recent progress in ionic liquids in both applications and cost-effective synthesis may lead to exciting breakthroughs in future environmentally friendly industrial processes”.63


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Supporting Information contains 11 Tables: # Refractive index, speed of sound, coefficients aij, density, isobaric heat capacity, coefficients of the Tait equation, isentropic compressibility coefficient, isobaric thermal expansion coefficient, isothermal compressibility coefficient, isochoric heat capacity and IC50 are tabulated in Tables S1- S11. 3 Figures: # Percent relative deviations between the density obtained in this work and literature values, percent relative deviations between the isobaric thermal expansion coefficient obtained in this work with the literature data, percent relative deviations between the isothermal compressibility coefficient obtained in this work with the literature data, thermogravimetric curves of [C3C1pyr][NTf2] and [C4C1pyr][NTf2], are shown in Figures S1-S3. #

High-pressure

density,

isobaric

thermal

expansion

coefficient

and

isothermal

compressibility coefficient of [C3C1pyr][NTf2] and [C4C1pyr][NTf2] in comparison with the literature values. ACKNOWLEDGMENTS The authors are profoundly indebted to dr S. Maślanka for TGA measurements, Monika Żarska MSc for participation in speed of sound measurements in [C3C1pyr][NTf2], and Janusz Śliwa MSc for critical reading of the manuscript. 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. AMW and KM thank for National Centre for Science NCN grant 2014/13/D/NZ7/00322.



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Figure 1. Structures of (a) 1-alkyl-1-methylpyrrolidinium cation, [CnC1pyr]+, where R=C3H7 or R=C4H9 and (b) bis(trifluoromethylsulfonyl)imide anion, [NTf2]-, studied in this work.



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a)

b)

c)

Figure 2. Comparison of the effect of pressure on a) speed of sound; b) density; c) isentropic compressibility of , [C3C1pyr][NTf2]; , [C4C1pyr][NTf2] (red open symbols) and ■, [C3C1im][NTf2];34 , [C4C1im][NTf2],34 (blue filled symbols) at 308.15 K. Insets illustrate the effect of the alkyl chain length, n, in the cation on the properties given above at 0.1 and 100 MPa: [CnC1pyr][NTf2], red symbols ( ,■); [CnC1im][NTf2] black symbols (,♦). The ρ and u values for the [CnC1im][NTf2] series taken from: n = 2,35 n = 3–6,32 n = 7,20 n = 8.36 Line arbitrary – for the eye guide.


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Figure 3. Thermogravimetric curves of investigated ILs: [C3C1pyr][NTf2] (black line); [C4C1pyr][NTf2] (red line).

Figure 4. Comparison of the effect of temperature on the energy storage density, Cp/Vm of , [C3C1pyr][NTf2]; , [C4C1pyr][NTf2] (red open symbols), ■, [C3C1im][NTf2]; , [C4C1im][NTf2] (blue symbols), and commercial heat transfer fluids: , Therminol 66; ●, Therminol VP-1; , Marlotherm SH under atmospheric pressure. Line arbitrary – for the eye guide. Cp/Vm of ILs was calculated from previously published the molar isobaric heat capacity20 and the density,20,34 while Cp/Vm of commercial heat transfer fluids was calculated from data reported in.42–44



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Figure 5. Comparison of effect of pressure on energy storage density, Cp/Vm of , [C3C1pyr][NTf2]; , [C4C1pyr][NTf2] (red open symbols) and ■, [C3C1im][NTf2]; , [C4C1im][NTf2] (blue symbols) at 313.15 K. Line arbitrary – for the eye guide. Cp/Vm of [CnC1im][NTf2] was calculated from previously published data.34

Figure 6. Comparison of thermal conductivity,  of [C3C1pyr][NTf2]20 and [C4C1pyr][NTf2]47 at 313.15 K under atmospheric pressure (red points) with thermal conductivity of selected ILs: [C2C1im][OAc],46 [C2C1im][N(CN)2],46 [C2C1im][MeOHPO2],46 [C2C1im][C(CN)3],46 [C2C1im][EtSO4],46 [C2C1im][OcSO4],46 [C4C4im][NTf2],46 [C4C1im][NTf2],47 [C4C1im] [OTf],47 [P6,6,6,14][Cl],47 [C4C1pyr][FAP],47 [P6,6,6,14][Phosph],48 [P4,4,4,1][MeSO4],48 [C4C1im][BF4],49 [C4C1im][PF6],50 [C3C1im][NTf2],20 and commercial heat transfer fluids.42–44


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Figure 7. Comparison of the effect of temperature on the kinematic viscosity, ν of , [C3C1pyr][NTf2];53 , [C4C1pyr][NTf2]54 (red open symbols), ■, [C3C1im][NTf2];32 , [C4C1im][NTf2]32 (blue symbols) with commercial heat transfer fluids: , Therminol 66;42 ●, Therminol VP-1;43 , Marlotherm SH.44

Figure 8. Comparison of the effect of temperature on Prandtl number, Pr of , [C3C1pyr][NTf2]; , [C4C1pyr][NTf2] (red open symbols), ■, [C3C1im][NTf2]; , [C4C1im][NTf2]; (blue symbols) with commercial heat transfer fluids: , Therminol 66; ●, Therminol VP-1; , Marlotherm SH. Line arbitrary – for the eye guide. Pr of ILs was calculated from the literature molar isobaric heat capacity,20 thermal conductivity,20,47 and viscosity,32,53,54 while Pr of commercial heat transfer fluids was calculated from reported data .42–44



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Figure 9. Comparison of the temperature dependence of isobaric thermal expansion coefficient, αp of , [C3C1pyr][NTf2]; , [C4C1pyr][NTf2] (red open symbols), ■, [C3C1im][NTf2]; , [C4C1im][NTf2] with commercial heat transfer fluids: , Therminol 66; ●, Therminol VP-1; , Marlotherm SH under atmospheric pressure, and +, [C3C1pyr][NTf2] and ▲, [C4C1pyr][NTf2] at 100 MPa obtained in this work. Line arbitrary – for the eye guide. αp of ILs under atmospheric pressure was calculated from the previously published density,20,34 while αp of commercial heat transfer fluids was calculated from other previously reported data.42-44


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Synopsis Pyrrolidinium-based ionic liquids exhibit high energy storage density, outstanding thermal stability, relatively low toxicity, which predestine them to be sustainable heat transfer media.

Abstract graphic


Pyrrolidinium-Based Ionic Liquids as Sustainable Media in Heat

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