Aqueous-phase green synthesis of formate-based ionic liquids and their thermophysical properties

Accepted Manuscript Aqueous-phase green synthesis of formate-based ionic liquids and their thermophysical properties

Zhengjian Chen, Zuopeng Li, Xiaoyun Ma, Lin Xu, Yu Wang, Shiguo Zhang PII: DOI: Reference:

S0167-7322(18)34503-3 https://doi.org/10.1016/j.molliq.2019.01.141 MOLLIQ 10372

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

1 September 2018 18 January 2019 27 January 2019

Please cite this article as: Z. Chen, Z. Li, X. Ma, et al., Aqueous-phase green synthesis of formate-based ionic liquids and their thermophysical properties, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.01.141

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Aqueous-phase green synthesis of formate-based ionic liquids and their thermophysical properties Zhengjian Chen a, *, Zuopeng Li b, Xiaoyun Ma a, Lin Xu a, Yu Wang a, Shiguo Zhang c Key Laboratory of Clean Energy Materials and Devices, Guizhou Education University, Guiyang

PT

a

550018, China.

Department of Chemistry and Environmental Engineering, Shanxi Datong University, Datong

RI

b

037009, China.

College of Materials Science and Engineering, Hunan University, Changsha 410082, China.

SC

c

NU

* Corresponding author.

MA

E-mail address: [email protected] (Z. Chen)

Abstract:

D

In this study, the anion exchange reaction of an organic cation chloride with excess

PT E

KHCO2 to synthesize formate-based ionic liquids (ILs) was performed in water, and the water-miscible IL products were easily obtained via salting-out phase separation caused

CE

by the excess KHCO2. According to the synthesis results, a set of heuristic rules could

AC

be generated as follows. First of all, the more “hydrophobic” ILs required less amount of excess KHCO2 for salting-out phase separation. Secondly, the amphiphilic ILs with a long cationic alkyl chain (e.g., octyl) are likely to form reverse micelles to encapsulate inorganic salt (i.e., KCl), resulting in relatively high residual contents of K+ and Cl−. Lastly, cooling would exert a positive effect on the salting-out phase separation and thus expand the application range of this method. For example, [C2-MIm]HCO2 was prepared at −18 °C, because it has to be phase-separated out by cooling. The basic 1

ACCEPTED MANUSCRIPT

properties of these ILs were also thoroughly characterized and analyzed, such as melting point, thermal decomposition temperature, density, refractive index, surface tension, viscosity and conductivity etc.

PT

Keywords: formate-based ionic liquids; concentrated aqueous solution; salting-out

AC

CE

PT E

D

MA

NU

SC

RI

phase separation; thermophysical properties.

2

ACCEPTED MANUSCRIPT

1. Introduction Ionic liquids (ILs) are a special class of molten salts usually composed of organic cations and inorganic or organic anions with melting points below 100 °C. The entire

PT

ionic composition endows ILs with the characteristic properties of high-temperature molten salts, such as negligible volatility, non-flammability and inherent high ionic

RI

conductivity and so on.[1, 2] Owing to their low melting points below 100 °C and

SC

even below room temperature relative to high-temperature molten salts, ILs allow a

NU

much wider application potential and much easier operation, and ILs have been extensively used as solvents, electrolytes and functional materials in various fields,

MA

especially their potential as an environmentally friendly alternative to conventional volatile organic solvents.[3, 4] The environmental friendliness of ILs is primarily

PT E

D

concerned with their negligible volatility, which can minimize the emission into the atmosphere and the risk of exposure.[5, 6]

CE

However, recent studies reveal that ILs are not as “environmentally friendly” as expected, mostly because of both their own eco-toxicity and their hazardous synthesis

AC

processes.[6-12] On the one hand, most ILs are toxic more or less to a wide variety of biological targets.[6, 10] The toxic effects of ILs were strongly and positively correlated with their lipophilicity, since the lipophilic components can interact with lipidic cell membranes by hydrophobic interactions.[13] Accordingly, hydrophobic ILs are usually more toxic than hydrophilic ones.[13, 14] On the other hand, the synthesis of ILs in most cases is time and energy consuming and involves the use of various volatile organic solvents or noble metal salts (e.g., AgNO3) or other hazardous 3

ACCEPTED MANUSCRIPT chemicals.[12, 15] There are generally two synthetic routes to ILs, denoted as one-step and two-step respectively.[12] The one-step synthesis is usually achieved by quaternization reaction of a Lewis base (e.g., amine, phosphine or sulfide) with an alkylating agent (e.g., haloalkane or dialkyl sulfate), and the cation and anion are

PT

formed simultaneously.[12] Although the one-step route is facile and efficient, it is

RI

only suitable for the synthesis of certain types of ILs with strongly coordinating

SC

anions, such as Cl− and CH3SO4−, and the resulting products cannot act as inert media.[16] In comparison, the two-step route is more applicable and widely employed

NU

for synthesis of various ILs.[12, 17] The two-step route generally begins with the

MA

formation of the desired cation in a similar manner performed in the one-step route, followed by anion exchange to produce the final product.[12, 15] Between the two

D

steps, the second anion exchange step is the current research focus related to the large

PT E

scale synthesis and application of ILs.[12] The practical strategy for anion exchange is dependent on the hydrophilic/hydrophobic character of the product and the kinds of

CE

starting materials.[12, 18, 19] The anion exchange reaction for synthesis hydrophobic

AC

ILs can proceed in water and subsequently the product can be easily obtained by liquid/liquid phase separation.[12, 18, 20] However, the anion exchange for synthesis of hydrophilic ILs is mainly carried out either using an organic solvent as reaction medium[21, 22] or using a silver salt as sacrificial reagent[23] or through ion-exchange

resin,[24]

accompanied

by

complex

operation

and

more

environmentally harm.[25] Formate anion (HCO2−) is of relatively low toxicity and good biocompatibility and 4

ACCEPTED MANUSCRIPT readily available.[26, 27] However, there are only a few reports concentrating on HCO2-based ILs and most of them are protic, primarily because HCO2-based aprotic ILs are difficult to be synthesized.[27-34] HCO2-based protic ILs are commonly synthesized by neutralization reaction of an organic Lewis base with formic acid, as

PT

similar as other protic ILs.[27, 31-34] While the synthesis of HCO2-based aprotic ILs

RI

via anion exchange cannot be carried out in organic solvent media or by using silver

SC

formate as sacrificial reagent as usual, since HCO2-based products have very similar solubility characteristics with their halide-based precursors,[35] and silver formate is

NU

unstable and decomposes spontaneously.[36] Therefore, the primary method applied

MA

for anion exchange to prepare HCO2-based aprotic ILs is ion-exchange resin, where the halide ion in precursor was converted into OH− and then neutralized with formic

D

acid to generate product, although over a long period of time.[28, 29, 37, 38] Besides,

PT E

some methods were specifically developed towards HCO2-based aprotic ILs, for example, acidolysis of the intermediates obtained from alkylation of an amine with a

CE

dialkyl carbonate,[39, 40] successive anion exchange from halide (e.g. [P4444]Br) to

AC

BF4− ([P4444]BF4) and finally to HCO2 ([P4444]HCO2),[41, 42] and equivalent reaction of formic acid, an alcohol and an organic amine at an elevated temperature of around 165 °C.[43, 44] HCO2-based ILs have recently been applied as a sorbent for CO2 separation,[28, 41, 42] as reducer and stabilizer for synthesis of noble metal nanoparticles,[27, 30] and particularly as solvents for dissolution and transformation of biopolymers,[31-34, 38, 39, 43, 44] and so on. Methodological innovation for the synthesis of ILs is critical for their production 5

ACCEPTED MANUSCRIPT cost reduction and large-scale application in industry.[20, 25] Our recent study showed a green and efficient method to prepare water-miscible ILs, which were easily obtained through a spontaneous liquid/liquid phase separation by salting-out effect after anion exchange performed in saturated aqueous solution.[45] In the present study,

PT

the method based on salting-out phase separation was further developed and applied

RI

to prepare a series of HCO2-based ILs. All The ILs were prepared at room temperature,

SC

except for [C2-MIm]HCO2, which could not be phase separated out by salting-out effect at room temperature, and had to been prepared at a low temperature of −18 °C.

NU

This finding supports our previous suggestion that cooling would enhance the

MA

salting-out effect on phase separation and expand its application range for IL synthesis. The contents of residual K+ and Cl− were determined. The properties of the obtained

D

ILs were also thoroughly characterized and analyzed, such as melting point, thermal

PT E

decomposition temperature, density, refractive index, surface tension, viscosity and

CE

conductivity.

2. Results and discussion

AC

2.1 Synthesis and purity analysis HCO2− based ILs were synthesized through anion exchange from Cl− to HCO2− in aqueous solution by using KHCO2 as both anion precursor and salting-out agent. The structures and abbreviations of ILs used in this study are shown in Fig. 1. Actually, HCO2-based ILs are highly water soluble,[35] thus a great amount of excess KHCO2 is required as salting-out agent to phase-separate the IL products from aqueous reaction medium, thanks to the extremely high water solubility of KHCO2 (~331 6

ACCEPTED MANUSCRIPT g/100 mL at 20 °C). These ILs except for [C2-MIm]HCO2 were synthesized at room temperature (~25 °C) by following the methods described in our previous study.[45] Most probably because the shorter cationic ethyl chain makes [C2-MIm]HCO2 less “hydrophobic” than other IL products, no liquid/liquid phase separation could be

PT

observed in the aqueous [C2-MIm]Cl solution with the continuous addition of KHCO2

RI

until saturation at room temperature. As a result, [C2-MIm]HCO2 was attempted to be

SC

prepared at a relatively low temperature, as shown in Fig. 2. First of all, the homogeneous solution consisting of 0.10 mol 14.7 g [C2-MIm]Cl, 45.0 g KHCO2 and

NU

15.0 mL ultrapure water in a glass tempering beaker was cooled under stirring, until

MA

down to around −11 °C liquid/liquid phase separation took place. The volume of the upper product-rich phase increased with the decrease of temperature and then nearly

D

kept constant until lower than −18 °C. Consequently, the operating temperature was

PT E

set at −18 °C. Secondly, in order to identify the optimum dosage of KHCO2, KHCO2 was added under vigorous stirring in batches of ~1 g every 10 min to the solution of

CE

14.7 g [C2-MIm]Cl and 15.0 mL ultrapure water at −18 °C. Until 36.9 g KHCO2 was

AC

added, phase separation started to occur and the product-rich phase contained the Cl− residue up to 1.28wt%. With the further addition of KHCO2, the Cl− content decreased and then maintained at around 0.95wt% with the dosage of KHCO2 larger than 40.8 g. Accordingly, [C2-MIm]HCO2 was prepared by reaction of 14.7 g [C2-MIm]Cl with 40.8 g KHCO2 in 15.0 mL ultrapure water at −18 °C with vigorous stirring for 20 min, and the upper product-rich solution was collected after spontaneous phase separation. The product-rich solution was washed with a solution of 6.5 g KHCO2 in 2.0 mL 7

ACCEPTED MANUSCRIPT water three times to decrease the content of the Cl− residue, and the washing solution was kept for reuse. The washed solution was vacuum dried at 70 °C to remove water, leaving a solid-liquid mixture. After “aging” for 2 h, the mixture was vacuum filtered by membrane filter (0.22 μm), and rinsed by 3×30 mL CHCl3 to completely collect

PT

the remnants left in the pores of the filter membrane and on the surface of the solid

RI

particles. The solid filter residue was kept for reuse. The filter solution was collected

SC

and vacuum concentrated at 70 °C to afford [C2-MIm]HCO2 (10.5 g with a yield of 67.3%). The resultant bottom aqueous solution, washing solution and filter residue

NU

were mixed in a glass tempering beaker for reuse test, and proper amounts of

MA

[C2-MIm]Cl, KHCO2 and water were added to offset their loss in the initial synthesis process. The reuse test was repeated for two times by following the above-described

D

synthesis process, resulting in yields > 92% referring to the addition amount of

PT E

[C2-MIm]Cl. Table 1 summarized the synthesis and reuse results of [C2-MIm]HCO2, as well as the results of these IL products synthesized at room temperature.

CE

In theory, the anion exchange from Cl− to HCO2− is an equimolar reaction between

AC

[Cation]Cl and KHCO2 (e.g., 0.10 mol/8.4 g). However, a great deal of excess KHCO2 is required to serve as salting-out agent to cause liquid/liquid phase separation. The dosage of KHCO2 for its reaction with 0.10 mol each [Cation]Cl in 15.0 mL ultrapure water decreased in the order of [C1OC2-MIm]HCO2 (48.6 g) > [C4-MIm]HCO2 (46.0 g) > [C4-MPyr]HCO2 (42.7 g) > [C6-MIm]HCO2 (41.4 g) > [C8-MIm]HCO2 (38.5 g), which are all far greater than the theoretical equimolar amount of 8.4 g. This order implies that the KHCO2 dosage is related with the product 8

ACCEPTED MANUSCRIPT “hydrophobicity” and the more hydrophobic IL products are easier to be phase separated by even less KHCO2 dosage for salting out. The synthesis yields of ILs varied in the range from 66.8% to 91.2%, recorded as Run1 in Table 1. Generally, the synthesis

yields

obtained

at

room

temperature

increased

as

following:

PT

[C1OC2-MIm]HCO2 (66.8%) < [C4-MPyr]HCO2 (89.2%) < [C4-MIm]HCO2 (90.5%)

RI

< [C6-MIm]HCO2 (90.8%) < [C8-MIm]HCO2 (91.2%), roughly in accordance with

SC

the “hydrophobicity” of IL products. This result strengthens that the more “hydrophobic” IL products can be more thoroughly phase-separated out, resulting in a

NU

relatively higher yield. The reuse results were recorded in columns Run2/Run3 in

MA

Table 1. The yields in columns Run2/Run3 are all larger than 90%, indicating that the resultant aqueous KHCO2 brine, washing solution and filter residue could be

D

efficiently recycled and reused.

PT E

The possible impurities in IL products introduced in this synthesis route should be only Cl− and K+, according to the used reactants ([Cation]Cl and KHCO2) and solvent

CE

(water, which can be vacuum evaporated). The residual Cl− and K+ contents in each

AC

product were determined and summarized in Table 1. The K+ contents in IL products range from 0.020wt% to 0.86wt%. It appears that the residual K+ content is relevant with the cationic alkyl chain length, according to the increasing order of K+ content: [C4-MIm]HCO2 (~0.039wt%) < [C6-MIm]HCO2 (~0.27wt%) < [C8-MIm]HCO2 (~0.83wt%), which were all prepared at room temperature. The amphiphilic ILs with longer hydrocarbon chains are able to form reverse micelles to dissolve polar solutes (such as KCl) in the polar cores.[46-48] This should be the primary reason for the 9

ACCEPTED MANUSCRIPT above increasing order of K+ content. The Cl− contents in IL products are overall relatively high (~1.5wt%), perhaps because of the similar polarity of Cl− and HCO2− rendering low efficiency of their metathesis via salting-out phase separation. 1H-NMR spectra were also recorded and analyzed, as shown in Fig. S4-S9 (Supplementary

PT

Materials). Accordingly, no significant impurity signals were detected, except the

RI

CDCl3 solvent peak (~7.3 ppm) and water peak (within 4~6 ppm). NMR purity was

SC

estimated by the integration of compound-related signals and unrelated signals (except the solvent and water peaks) in 1H-NMR spectra. The residual water contents

NU

in ILs were determined using a coulometric Karl Fischer titrator instead of NMR, in

MA

consideration of moisture absorption during NMR sample preparation in the air. The final purity (> 95%) of ILs was assessed by using 1H-NMR spectra, the residual water,

D

Cl− and K+ contents, as presented in Table 2.

PT E

In order to evaluate the effect of the residual K+ and Cl− on properties, HCO2-based ILs with purity > 98% (denoted as IL-98% in the following) were prepared through

CE

silver-salt metathesis reaction and subsequent CO32− decomposition with formic acid,

AC

as shown in Fig. S1 (Supplementary Materials), and their physicochemical properties were also investigated at 298.15 K and summarized in Table S1 (Supplementary Materials).

2.2 Thermophysical properties The fundamental physicochemical properties, such as melting point (Tm), thermal decomposition temperature (Td), density (ρ), refractive index (n), surface tension (γ), viscosity (η) and conductivity (σ), were also thoroughly studied by characterizing the 10

ACCEPTED MANUSCRIPT products obtained in the initial synthesis process. The properties were determined after vacuum dehydration at 60 ~ 70 °C for 24 h, and then the residual water contents in ILs were determined to be about 1000 ppm, as listed in Table 2, indicating that

as PF6−-based ILs with residual water content < 30 ppm.[49]

PT

HCO2-based ILs are much more hydrophilic than conventional fluorinated ILs, such

RI

Four of the prepared ILs are solid and another two are liquid at room temperature.

SC

Melting points (Tm, °C) of the solid substances were measured using a Pyrex tube at a heating rate of 2 °C min−1 and recorded as the temperature at which the thermometer

NU

can be inserted into the resultant solid-liquid mixture. The obtained melting points fell

MA

in a range from 27.9 to 64.4 °C, as listed in Table 2. The validity of this method was confirmed by further determination of the melting points of water and [C2-MIm]BF4,

D

which were found to melt at 0.2 and 14.7 °C respectively, in fine accordance with the

PT E

theoretical value (0 °C for water) and literature value (15 °C for [C2-MIm]BF4).[50] The two room temperature ILs ([C4-MIm]HCO2 and [C6-MIm]HCO2) were cooled

CE

and kept even at −80 °C for crystallization. However, glass transition always took

AC

place instead of crystallization, and thus no melting was observed. Generally, the presence of impurity will cause a decrease in the melting point of a substance. For example, [C2-MIm]HCO2-98% (Tm: 50.8 °C) has a higher Tm than its counterpart (48.6 °C) bearing K+ and Cl− impurities. However, [C8-MIm]HCO2-98%, [C1OC2-MIm]HCO2-98% and [C4-MPyr]HCO2-98% have Tm values at 50.2, 23.6, 55.9 °C, respectively, lower than their counterparts (61.2, 27.9, 64.4 °C, respectively) obtained from salting-out method, perhaps due to the presence of the supramolecular 11

ACCEPTED MANUSCRIPT interactions (such as hydrogen bonding and electrostatic forces) caused by the K+ and Cl− impurities. Thermal decomposition temperatures (Td, °C) of these ILs were listed in Table 2. Generally, HCO2-based ILs are relatively low temperature resistant with Td of only

PT

around 200 °C. For [Cn-MIm]HCO2 series (n = 2, 4, 6, 8), thermal resistance

RI

generally decreases with lengthening the alkyl side chain, coincident with the

SC

previous findings from those analogues based on other anions.[51, 52] By comparison of [C4-MIm]HCO2 (207.6 °C) with [C1OC2-MIm]HCO2 (198.1 °C), the replacement

NU

of “-CH2-” by “-O-” in side chain exerts a small negative impact on thermal stability,

MA

which is most likely because the strong electron withdrawing effect of the “-O-” unit increases the electron deficiency of the part of the side chain linked to the

D

imidazolium ring and its reactivity with the nucleophilic HCO2− anion.[24, 50, 53]

PT E

Besides, the K+ and Cl− residues have no significant effect on the thermal stability and mass loss pattern of ILs, as shown in Fig. S2 and S3 (Supplementary Materials).

CE

There is only one mass loss stage for these ILs, except [C8-MIm]HCO2, which has

AC

two mass loss stages.

Temperature dependences of density, refractive index, surface tension, viscosity and conductivity of the three ILs that are liquid ([C4-MIm]HCO2 and [C6-MIm]HCO2) or supercooled liquid ([C1OC4-MIm]HCO2) at room temperature were investigated and summarized in Table 3. Density of ILs is evidently affected by the strength of cation-anion interaction and molecular packing, so density is a critical property that is useful for a more in-depth 12

ACCEPTED MANUSCRIPT understanding of ILs and other properties.[54] The densities (ρ, g·cm−3) were measured between 283.15 and 353.15 K at atmospheric pressure, and the temperature dependence of density was described in Fig. 3. It is evident that density increases in the order of [C6-MIm]HCO2 < [C4-MIm]HCO2 < [C1OC2-MIm]HCO2, for example

PT

they are 1.0469, 1.0770, 1.1762 g·cm−3 respectively at 298.15 K. This order is

RI

coincident with the finding that density decreases with lengthening the side alkyl

SC

chain (i.e. from C4- to C6-) and increases with the replacement of “-CH2-” by “-O-” (i.e. from C4- to C1OC2-)[24, 50]. The change of density with temperature over the

(1)

MA

NU

measurement range could be expressed by a linear equation:

where a, b are the fitting parameters and T is the temperature in Kelvin. Based on the

D

slopes (∂ρ/∂T) (i.e. b in the above linear equation) in Fig. 3, thermal expansion

PT E

coefficient defined as equation:

(2)

CE

was calculated and shown in Table 4. The obtained αp data at 298.15 K vary in a

AC

narrow range of 5.40 ~ 5.89×10−4 K−1, which are close to other ILs.[55] Refractive index is a dimensionless number that describes how much the path of a light wave is bent when entering a medium. In theory, refractive index is positive with the electric polarizability (α, primarily the atoms’ outer electrons) of the medium that creates a disturbance to the transmitted light.[56, 57] Based on Lorenz−Lorentz equation: (3) 13

ACCEPTED MANUSCRIPT where ρ is the density, N is a constant, α is polarizability and M is the molar weight, respectively, refractive index also exhibits a positive correlation with density. The temperature dependence of refractive index (n) between 283.15 and 353.15 K was described in Fig. 4. It is clear that refractive index decreases almost linearly with

PT

temperature increasing, as usual.[57, 58] Generally, refractive index varies in the

RI

order of [C6-MIm]HCO2 < [C4-MIm]HCO2 < [C1OC2-MIm]HCO2, which could be

SC

primarily ascribed to their density difference, according to the above Lorenz−Lorentz equation.

NU

Surface tensions (γ, mN·m−1) measured from 283.15 and 353.15 K were graphically

MA

shown in Fig. 5. As excepted, [C1OC2-MIm]HCO2 (47.97 mN·m−1) exhibits a highest surface tension at 298.15 K, followed by [C4-MIm]HCO2 (46.99 mN·m−1) and

D

[C6-MIm]HCO2 (41.23 mN·m−1). This trend is roughly coincident with the principle

PT E

that longer alkyl side chains are readily oriented away from the bulk and thus result in lower surface tensions.[59] As seen in Fig. 5, the surface tensions decrease almost

CE

linearly with increasing temperature in the set measuring range,[60] which can be

AC

fitted to the following equation: (4)

where γ is surface tension, T is Kelvin temperature, ES and SS represent the surface energy and surface entropy, respectively.[61] The best-fitting ES and SS are shown in Table 4, which are close to that of protic ILs.[61] In comparison to ILs-98% (Table S1, Supplementary Materials), ILs prepared by salting-out method are of very similar density, refractive index and surface tension, 14

ACCEPTED MANUSCRIPT indicating that the effect of the K+ and Cl− impurities on the properties is very limited. For example, ρ, n and γ of [C6-MIm]HCO2-98% was determined to be 1.0400 g·cm−3, 1.4914 and 42.63 mN·m−1 at 298.15 K respectively, which are close to 1.0469 g·cm−3, 1.4922 and 41.23 mN·m−1 of [C6-MIm]HCO2 having 0.28wt% of K+ and 1.42wt%

PT

of Cl− impurities.

RI

High viscosity of ILs is one of main bottlenecks for their industrial application, and

SC

hence the in-depth study of viscosity and its relationship with structure remains a key issue for IL research. As can be seen from the viscosity values (η, mPa·s) in Table 3,

NU

[C4-MIm]HCO2 is evidently less viscous than [C6-MIm]HCO2 and a bit more viscous

MA

than [C1OC2-MIm]HCO2, for example at 298.15 K, they are 169.1, 332.2 and 150.3 mPa·s respectively. This result is roughly consistent with the previous studies

D

showing that lengthening alkyl side chain leads to higher viscosity and the

PT E

replacement of “-CH2-” by “-O-” in alkyl chain exerts an opposite effect.[24, 50] The temperature dependence of viscosity in the range from 283.15 and 353.15 K was

AC

equation:

CE

shown in Fig. 6. The viscosity profiles are well fit to Vogel-Tammann-Fulcher (VTF)

(5)

where η0 is the high temperature limit of the viscosity, and B is a fitting parameter controlling the curvature, T0 is Vogel temperature that typically lies a few tens of degrees below Tg. The best fitting parameters for viscosity are summarized in the Table 4, together with the correlation coefficient R2 (> 0.99) for the fit. Besides, the temperature dependence of viscosity of these ILs could be also well described by 15

ACCEPTED MANUSCRIPT Arrhenius equation (Fig. 7): (6) where A is a constant, Eη (kJ mol−1) is the activation energy that can be regarded as the energy barrier to be overcome by mass transport in viscous flow. Generally, the

PT

more viscous a fluid is, the higher its Eη value should be. Consequently, Eη value

SC

kJ·mol−1) > [C1OC2-MIm]HCO2 (36.53 kJ·mol−1).

RI

decreases in the order of [C6-MIm]HCO2 (41.39 kJ·mol−1) > [C4-MIm]HCO2 (37.83

Generally, the conductivity of an IL is mainly governed by its viscosity, molar mass,

NU

density and ion size.[62] According to the conductivity values (σ, mS·cm−1, in Table 3)

MA

measured from 283.15 and 353.15 K, the conductivity of [C4-MIm]HCO2 (1.84 mS·cm−1, at 298.15 K) is a bit less than that of [C1OC2-MIm]HCO2 (2.03 mS·cm−1),

D

while much more than that of [C6-MIm]HCO2 (0.31 mS·cm−1), primarily because

PT E

[C4-MIm]HCO2 is a bit more viscous than [C1OC2-MIm]HCO2 and much less viscous than [C6-MIm]HCO2, as above stated. The molar conductivity and viscosity of an IL

CE

are often combined into what is termed Walden’s rule: Λη = constant

AC

(7)

where Λ is molar conductivity of the IL.[63] Λ (S·cm2·mol−1) is given by the Equation:

Λ = σM/ρ

(8)

where M is molar mass and ρ is density. The Walden plot of log(Λ, S·cm2·mol−1) against log(η−1, Poise−1) was shown in Fig. 8, in which the solid line is the ideal Walden plot for aqueous 0.01 M KCl solution. Deviation below the ideal line in the 16

ACCEPTED MANUSCRIPT Walden plot is presumably a result of ion aggregation originating from inter-ion interaction such as electrostatic force, and has been used as an indicative to evaluate ionicity of ILs.[64] Generally, the larger magnitude of the deviation is, the lower ionicity the IL has. As evident in Fig. 8, the ionicity of [C6-MIm]HCO2 is lower than

PT

[C4-MIm]HCO2 and [C1OC2-MIm]HCO2, which should be attributed to that

RI

increasing cation size leads to higher Van der Waals’ force and lower conductivity.

SC

According to the relatively lower viscosity and higher conductivity of ILs-98% at 298.15 K (Table S1, Supplementary Materials), the K+ and Cl− impurities in ILs from

NU

salting-out method exert a negative effect on mass transfer. For [C4-MIm]HCO2 and

MA

[C1OC2-MIm]HCO2 from salting-out method, the Cl− impurity dominates (1.68wt% and 1.35wt%, respectively), and their viscosity and conductivity are about 1.14 ~

D

1.17 times and 0.79 ~ 0.73 times that of the corresponding ILs-98%, respectively.

PT E

While for [C6-MIm]HCO2 from salting-out method, the K+ impurity (0.81wt%) rises to over half the Cl− impurity (1.42wt%), and its viscosity and conductivity are 1.37

CE

and 0.67 times respectively that of [C6-MIm]HCO2-98%.

AC

3. Conclusions

In conclusion, the anion exchange reaction of [Cation]Cl with KHCO2 to prepare HCO2-based ILs was performed in water, and the water-miscible IL products could be readily obtained through salting-out phase separation caused by the excess KHCO2, vacuum concentration and filtration. All the ILs were prepared at room temperature, except for [C2-MIm]HCO2, which was prepared at −18 °C, because the corresponding aqueous reaction solution has to be cooled to induce phase separation. The dosage of 17

ACCEPTED MANUSCRIPT KHCO2 depended on the cation structure, and the less amount of excess KHCO2 was required for salting out of an IL with a longer hydrophobic alkyl chain. Nevertheless, the long alkyl chains (such as octyl) are likely to form reverse micelles to encapsulate inorganic salt (KCl), resulting in relatively high residual contents of K+ and Cl−. The

PT

basic properties of these ILs were also investigated and analyzed, including melting

RI

point and thermal decomposition temperature, and temperature dependence of density,

SC

refractive index, surface tension, viscosity and conductivity from 283.15 and 353.15

NU

K.

4. Experimental

MA

Chloride-based cation precursors ([C1OC2-MIm]Cl, [C4-MPyr]Cl and [Cn-MIm]Cl, n = 2, 4, 6 or 8) with purity of 99% were obtained from Lanzhou Institute of Chemical

PT E

D

Physics, Chinese Academy of Sciences. KHCO2 (ReagentPlus grade, 99%) and LiNO3 (99.99% trace metals basis) were purchased from Sigma-Aldrich. All

CE

chemicals were used as received without purification. Ultrapure water (18.2 MΩ·cm−1) was produced using a Millipore Milli-Q laboratory water system.

AC

The content of ions (Cl− and K+) was determined on a LeiCi PXSJ-216F ion meter equipped with ion-selective electrodes (ISE), which were calibrated by standard KCl solutions (1×10−2, 1×10−3, 1×10−4 and 1×10−5 mol/L) containing 0.10 mol/L LiNO3 for keeping the ionic strength constant. The solutions to be measured were prepared by dissolution of ~0.1 g sample and 0.6895 g LiNO3 with ultrapure water in a 100.0 mL volumetric flask. Before property measurement, ILs were all vacuum-dried for 24 h at 60 ~ 70 °C, and the residual water contents in ILs were measured using a 18

ACCEPTED MANUSCRIPT WA-3000 coulometric Karl Fischer titrator. 1H-NMR spectra were recorded on a JEOL ECX-500 spectrometer using CDCl3 as solvent and TMS as internal standard. Melting point was determined on a ground Pyrex tube equipped with a glass-stem thermometer with a range of −20 ~ 100 °C in 1 °C resolution. In a typical procedure,

PT

around 5 g sample was vacuum-dried at 60 ~ 70 °C for 24 h in a 25 × 200 mm Pyrex

RI

test tube, which was then sealed by a rubber stopper with a thermometer and kept at 0

SC

or −40 °C until it crystallized. The test tube containing solid sample was heated by a thermostatic water bath at a rate of 2 °C min−1, and melting point was recorded as the

NU

temperature at which the thermometer could be inserted into the resultant solid-liquid

MA

mixture. Each melting point was determined in triplicate with deviation less than 2 °C. Thermal stability was assessed by using a Pyris Diamond PerkinElmer TG/DTA at a

D

rate of 10 °C min−1 under N2 atmosphere, and the thermal decomposition temperature

PT E

was defined as the temperature at 5% mass loss. Density was measured by a DMA 35 portable density meter at 25 °C or a 10 mL pycnometer as a function of temperature

CE

between 10 and 80 °C. The two densitometers were calibrated with ultrapure water.

AC

Refractive index was determined on an Atago NAR-1T liquid Abbe Refractometer. Surface tension was investigated by using a QBZY-2 automatic surface tension meter. Viscosity was recorded on a Haake RS6000 rheometer/viscometer with a cone-plate sensor (C60/1° Ti L). Conductivity was measured by using a Mettler Toledo FE30K conductivity meter. The measurement uncertainties of density, refractive index, surface tension, viscosity and conductivity are less than ±0.0005 g·cm−3, ±0.0002, ±0.04 mN·m−1, ±1% and ±1% respectively. The measurement temperature was 19

ACCEPTED MANUSCRIPT controlled to within ±0.01 °C by means of an external controller (Thermo Scientific Haake A40 Waterbath). Acknowledgments The authors gratefully acknowledge the financial support of the Natural Science

PT

Foundation of China (No. 21503050 and 21464005), the Construction Project of Key

RI

Laboratories of Guizhou Provincial Education Department (No. QJHKY[2015]329),

SC

the Natural Science Foundation of Guizhou Province (No. QKHJC[2016]1111 and QKHJC[2017]1137), the Natural Science Foundation of Shanxi (No. 201701D121016)

NU

and the start-up fund from Guizhou Education University.

MA

References

[1] K. Dong, X.M. Liu, H.F. Dong, X.P. Zhang, S.J. Zhang, Chem. Rev., 117 (2017) 6636-6695. [2] K.R.J. Lovelock, Roy. Soc. Open Sci., 4 (2017) 171223-171255.

D

[3] K.D. Clark, M.N. Emaus, M. Varona, A.N. Bowers, J.L. Anderson, J. Sep. Sci., 41 (2018)

PT E

209-235.

[4] M.J. Salar-Garcia, V.M. Ortiz-Martinez, F.J. Hernandez-Fernandez, A.P. de los Rios, J.

CE

Quesada-Medina, J. Hazard. Mater., 321 (2017) 484-499. [5] M. Amde, J.F. Liu, L. Pang, Environ. Sci. Technol., 49 (2015) 12611-12627.

AC

[6] S.P.F. Costa, A.M.O. Azevedo, P. Pinto, M. Saraiva, ChemSusChem, 10 (2017) 2321-2347. [7] M.C. Bubalo, K. Radosevic, I.R. Redovnikovic, J. Halambek, V.G. Srcek, Ecotox. Environ. Safe, 99 (2014) 1-12. [8] G. Cevasco, C. Chiappe, Green Chem., 16 (2014) 2375-2385. [9] M. Petkovic, K.R. Seddon, L.P.N. Rebelo, C.S. Pereira, Chem. Soc. Rev., 40 (2011) 1383-1403. [10] P.T.P. Thi, C.W. Cho, Y.S. Yun, Water Res., 44 (2010) 352-372. [11] S. Zhu, R. Chen, Y. Wu, Q. Chen, X. Zhang, Z. Yu, Chem. Biochem. Eng. Q., 23 (2009) 207-211. 20

ACCEPTED MANUSCRIPT [12] M. Deetlefs, K.R. Seddon, Green Chem., 12 (2010) 17-30. [13] K. Roy, R.N. Das, P.L.A. Popelier, Chemosphere, 112 (2014) 120-127. [14] S. Viboud, N. Papaiconomou, A. Cortesi, G. Chatel, M. Draye, D. Fontvieille, J. Hazard. Mater., 215 (2012) 40-48. [15] B.R. Clare, A. Sirwardana, D.R. Macfarlane, Top. Curr. Chem., 290 (2009) 1-40.

PT

[16] M. Schmeisser, P. Illner, R. Puchta, A. Zahl, R. van Eldik, Chem. Eur. J., 18 (2012) 10969-10982.

RI

[17] E. Garcia-Verdugo, B. Altava, M.I. Burguete, P. Lozano, S.V. Luis, Green Chem., 17 (2015) 2693-2713.

SC

[18] K.R. Seddon, A. Stark, M. Torres, Pure Appl. Chem., 72 (2000) 2275-2287. [19] A.K. Burrell, R.E.D. Sesto, S.N. Baker, T.M. Mccleskey, G.A. Baker, Green Chem., 9 (2007)

NU

449-454.

[20] S. Passerini, G.B. Appetecchi, MRS Bull., 38 (2013) 540-547.

MA

[21] S.A. Dharaskar, K.L. Wasewar, M.N. Varma, D.Z. Shende, C. Yoo, Arab. J. Chem. , 9 (2016) 578-587.

D

[22] L. Cammarata, S.G. Kazarian, P.A. Salter, T. Welton, Phys. Chem. Chem. Phys., 3 (2001)

PT E

5192-5200.

[23] S. Eisele, M. Schwarz, B. Speiser, C. Tittel, Electrochim. Acta, 51 (2006) 5304-5306. [24] Z. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J., 11 (2005) 752-766.

455-468.

CE

[25] F. Guo, S.J. Zhang, J.J. Wang, B.T. Teng, T.Y. Zhang, M.H. Fan, Curr. Org. Chem., 19 (2015)

AC

[26] C. Brigouleix, M. Anouti, J. Jacquemin, M. Caillon-Caravanier, H. Galiano, D. Lemordant, J. Phys. Chem. B, 114 (2010) 1757-1766. [27] K. Richter, T. Backer, A.V. Mudring, Chem. Commun., (2009) 301-303. [28] Y.J. Xie, D.G. Raut, R. Samikannu, J.P. Mikkola, X.Y. Ji, Energy Technol., 5 (2017) 1464-1471. [29] F. Lu, L.J. Wang, X.J. Ji, B.W. Cheng, J. Song, X.R. Gou, Carbohydr. Polym., 99 (2014) 132-139. [30] S. Okazoe, Y. Yasaka, M. Ueno, Y. Kimura, Chem. Lett., 46 (2017) 1344-1346. [31] J.P. Mann, A. McCluskey, R. Atkin, Green Chem., 11 (2009) 785-792. 21

ACCEPTED MANUSCRIPT [32] H.M. Choi, I. Kwon, Ind. Eng. Chem. Res., 50 (2011) 2452-2454. [33] W.J. Wei, N.D. Danielson, Biomacromol., 12 (2011) 290-297. [34] N. Bicak, J. Mol. Liq., 116 (2005) 15-18. [35] H. Wang, G. Gurau, R.D. Rogers, Chem. Soc. Rev., 41 (2012) 1519-1537. [36] A.N. Puzan, V.N. Baumer, P.V. Mateychenko, Journal Of Solid State Chemistry, 246 (2017)

PT

264-268. [37] A.R. Xu, Y.J. Zhang, Z.Y. Li, J.J. Wang, J. Chem. Eng. Data, 57 (2012) 3102-3108.

RI

[38] Y. Fukaya, A. Sugimoto, H. Ohno, Biomacromol., 7 (2006) 3295-3297. [39] B. Zhao, L. Greiner, W. Leitner, RSC Adv., 2 (2012) 2476-2479.

SC

[40] C. Rijksen, R.D. Rogers, J. Org. Chem., 73 (2008) 5582-5584.

[41] Y. Yasaka, M. Ueno, Y. Kimura, Chem. Lett., 43 (2014) 626-628.

NU

[42] Y. Yasaka, Y. Kimura, J. Chem. Eng. Data, 61 (2016) 837-845.

Biotechnol., 88 (2013) 2209-2215.

MA

[43] V.P. Soudham, J. Grasvik, B. Alriksson, J.P. Mikkola, L.J. Jonsson, J. Chem. Technol.

[44] J. Grasvik, B. Eliasson, J.P. Mikkola, J. Mol. Struct., 1028 (2012) 156-163.

D

[45] Z.J. Chen, Z.P. Li, X.Y. Ma, P.F. Long, Y. Zhou, L. Xu, S.G. Zhang, Green Chem., 19 (2017)

PT E

1303-1307.

[46] M. Moniruzzaman, K. Noriho, K. Nakashima, M. Goto, ChemPhysChem, 9 (2008) 689-692. [47] J.H. Liu, S.Q. Cheng, J.L. Zhang, X.Y. Feng, X.G. Fu, B.X. Han, Angew. Chem. Int. Ed., 46

CE

(2007) 3313-3315.

[48] O. Zech, W. Kunz, Soft Matter, 7 (2011) 5507-5513.

AC

[49] E. Vanecht, K. Binnemans, S. Patskovsky, M. Meunier, J.W. Seo, L. Stappers, J. Fransaer, Phys. Chem. Chem. Phys., 14 (2012) 5662-5671. [50] Z.B. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J., 10 (2004) 6581-6591. [51] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Green Chem., 3 (2001) 156-164. [52] C. Maton, N. De Vos, C.V. Stevens, Chem. Soc. Rev., 42 (2013) 5963-5977. [53] Z.J. Chen, S.M. Liu, Z.P. Li, Q.H. Zhang, Y.Q. Deng, New J. Chem., 35 (2011) 1596-1606. [54] Z.J. Chen, Y.A. Huo, P.F. Long, H.J. Shen, J.M. Lee, Phys. Chem. Chem. Phys., 19 (2017) 5389-5395. 22

ACCEPTED MANUSCRIPT [55] M.S. AlTuwaim, K. Alkhaldi, A.S. Al-Jimaz, A.A. Mohammad, J. Chem. Eng. Data, 59 (2014) 1955-1963. [56] J. Mitroy, M.S. Safronova, C.W. Clark, J. Phys. B: At. Mol. Opt. Phys., 43 (2010) 202001-202038. [57] S. Seki, S. Tsuzuki, K. Hayamizu, Y. Umebayashi, N. Serizawa, K. Takei, H. Miyashiro, J.

[58] X.J. Kang, Y.S. Zhao, J.J. Li, J. Mol. Liq., 250 (2018) 44-49.

PT

Chem. Eng. Data, 57 (2012) 2211-2216.

RI

[59] M. Tariq, M.G. Freire, B. Saramago, J.A.P. Coutinho, J.N.C. Lopes, L.P.N. Rebelo, Chem. Soc. Rev., 41 (2012) 829-868.

SC

[60] T.M. Koller, C. Steininger, M.H. Rausch, A.P. Froba, Int. J. Thermophys., 38 (2017) 167-186. [61] P.K. Chhotaray, R.L. Gardas, J. Chem. Eng. Data, 60 (2015) 1868-1877.

NU

[62] S.K. Tang, G.A. Baker, H. Zhao, Chem. Soc. Rev., 41 (2012) 4030-4066. [63] Q.G. Zhang, Y. Wei, S.S. Sun, C. Wang, M. Yang, Q.S. Liu, Y.A. Gao, J. Chem. Eng. Data, 57

MA

(2012) 2185-2190.

[64] A.R. Neale, P. Goodrich, T.L. Hughes, C. Hardacre, S.C. Ball, J. Jacquemin, J. Electrochem.

AC

CE

PT E

D

Soc., 164 (2017) H5124-H5134.

23

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Fig. 1. The structures and abbreviations of ILs used in this study.

24

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Fig. 2. Schematic synthesis of [C2-MIm]HCO2 based on salting-out phase separation at −18 °C.

25

SC

RI

PT

ACCEPTED MANUSCRIPT

NU

Fig. 3. Temperature-dependent density profiles of three ILs. The solid lines are the fitted curves

AC

CE

PT E

D

MA

using Eq. (1).

26

SC

RI

PT

ACCEPTED MANUSCRIPT

NU

Fig. 4. Temperature-dependent refractive index profiles of three ILs. The solid lines are linear fits

AC

CE

PT E

D

MA

of refractive index versus temperature.

27

SC

RI

PT

ACCEPTED MANUSCRIPT

NU

Fig. 5. Temperature-dependent surface tension profiles of three ILs. The solid lines are the fitted

AC

CE

PT E

D

MA

curves using Eq. (4).

28

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 6. Vogel-Tammann-Fulcher (VTF) plots of viscosity for ILs. The solid lines are the fitted

AC

CE

PT E

D

MA

NU

curves using Eq. (5).

29

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

Fig. 7. Arrhenius plots of viscosity for ILs. The solid lines are the fitted curves using Eq. (6).

30

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

Fig. 8. Walden plot of temperature-dependent conductivities and viscosities for ILs.

31

ACCEPTED MANUSCRIPT

Table 1. Synthesis results of ILs. Cl− contente (wt%)

Dosage of

Yieldc (%)

KHCO2b

Run1

Run2

Run3

Run1

Run2

Run3

Run1

Run2

Run3

40.8

67.3

92.8

94.0

0.062

0.068

0.071

1.38

1.41

1.37

[C4-MIm]HCO2

46.0

90.5

97.4

97.2

0.041

0.039

0.038

1.68

187

1.51

[C6-MIm]HCO2

41.4

90.8

97.5

97.6

0.28

0.27

0.27

1.42

1.45

1.50

[C8-MIm]HCO2

38.5

91.2

97.8

97.7

0.81

0.86

1.48

1.53

1.44

[C1OC2-MIm]HCO2

48.6

66.8

93.2

90.2

0.025

0.020

1.35

1.42

1.40

[C4-MPyr]HCO2

42.7

89.2

98.1

97.5

0.029

0.041

1.41

1.54

1.37

Sample

[C2-MIm]HCO2

a

K+ contentd (wt%)

A M

U N 0.021 0.031

a

Synthesized at −18 °C.

b

Including the use of excess KHCO2 as salting out agent.

c

Referring to the addition amount of [Cation]Cl for complementing the consumption in the previous run.

d

Residual K content in final products.

e

Residual Cl− content in final products.

D E

T P E

+

C C

A

32

I R

SC 0.82

T P

ACCEPTED MANUSCRIPT Table 2. Residual water content, estimated purity, melting point (Tm), and thermal decomposition temperature (Td) of ILs after vacuum drying for 24 h at 60 ~ 70 °C.a Sample

Water content (ppm)

Purity (%)b

Tm (°C)

Td (°C)

[C2-MIm]HCO2

1150

96.5

48.6c

214.7d

[C4-MIm]HCO2

953

96.8

207.6

[C6-MIm]HCO2

879

97.3

187.0

[C8-MIm]HCO2

855

95.6

[C1OC2-MIm]HCO2

1188

97.1

[C4-MPyr]HCO2

897

97.0

PT

61.2

186.6

27.9

198.1

64.4

196.6

Standard uncertainties of water content and temperature are 20 ppm and 1 °C respectively.

b

Estimated by 1H-NMR spectra (Fig. S4-S9) and the residual water, K+ and Cl− contents.

c

Literature value 52.[38]

d

Literature value 212.[38]

AC

CE

PT E

D

MA

NU

SC

RI

a

33

ACCEPTED MANUSCRIPT Table 3. Experimental density (ρ), refractive index (n), surface tension (γ) , viscosity (η), conductivity (σ) and molar conductivity (Λ) of three room temperature ILs ranging from 283.15 to 353.15 K at atmospheric pressure (888 ~ 897 mbar).a Sample

[C4-MIm]HCO2

[C4-MIm]HCO2

[C4-MIm]HCO2

−1

[C4-MIm]HCO2

[C4-MIm]HCO2

[C4-MIm]HCO2

Property

ρ (g·mL )

283.15 K

1.0866 ± 0.0005

1.0573 ± 0.0005

1.1865 ± 0.0005

1.5019 ± 0.0002

1.4969 ± 0.0002

1.5088 ± 0.0002

293.15 K

1.0801 ± 0.0005

1.0510 ± 0.0005

1.1797 ± 0.0005

1.4990 ± 0.0002

1.4937 ± 0.0002

1.5056 ± 0.0002

298.15 K

1.0770 ± 0.0005

1.0469 ± 0.0005

1.1762 ± 0.0005

1.4975 ± 0.0002

1.4922 ± 0.0002

1.5043 ± 0.0002

303.15 K

1.0738 ± 0.0005

1.0435 ± 0.0005

1.1729 ± 0.0005

1.4959 ± 0.0002

1.4905 ± 0.0002

1.5027 ± 0.0002

313.15 K

1.0673 ± 0.0005

1.0376 ± 0.0005

1.1667 ± 0.0005

1.4930 ± 0.0002

1.4871 ± 0.0002

1.4994 ± 0.0002

323.15 K

1.0615 ± 0.0005

1.0309 ± 0.0005

1.1603 ± 0.0005

1.4905 ± 0.0002

1.4841 ± 0.0002

1.4963 ± 0.0002

333.15 K

1.0552 ± 0.0005

1.0254 ± 0.0005

1.1540 ± 0.0005

1.4872 ± 0.0002

1.4807 ± 0.0002

1.4933 ± 0.0002

343.15 K

1.0492 ± 0.0005

1.0196 ± 0.0005

1.1479 ± 0.0005

1.4843 ± 0.0002

1.4773 ± 0.0002

1.4902 ± 0.0002

353.15 K

1.0440 ± 0.0005

1.0143 ± 0.0005

1.1419 ± 0.0005

1.4814 ± 0.0002

1.4741 ± 0.0002

1.4871 ± 0.0002

Property

γ (mN·m )

283.15 K

48.55 ± 0.04

43.24 ± 0.04

50.14 ± 0.04

419.6 ± 4.5

890.8 ± 9.6

358.0 ± 3.8

293.15 K

47.39 ± 0.04

41.91 ± 0.04

48.70 ± 0.04

233.7 ± 2.5

462.9 ± 4.9

203.0 ± 2.1

298.15 K

46.99 ± 0.04

41.23 ± 0.04

47.97 ± 0.04

332.2 ± 3.5

150.3 ± 1.6

121.5 ± 1.3

230.1 ± 2.4

115.5 ± 1.2

82.3 ± 0.9

149.5 ± 1.6

79.8 ± 0.8

NU

SC

η (mPa·s)

RI

−1

PT

n

169.1 ± 1.8 b

46.45 ± 0.04

40.40 ± 0.04

47.46 ± 0.04

313.15 K

45.36 ± 0.04

39.39 ± 0.04

46.34 ± 0.04

323.15 K

44.26 ± 0.04

38.13 ± 0.04

45.05 ± 0.04

51.3 ± 0.5

88.0 ± 0.9

49.1 ± 0.5

333.15 K

43.52 ± 0.04

36.38 ± 0.04

44.04 ± 0.04

35.6 ± 0.4

59.9 ± 0.6

33.6 ± 0.4

343.15 K

42.34 ± 0.04

35.24 ± 0.04

43.09 ± 0.04

22.5 ± 0.2

37.7 ± 0.4

21.9 ± 0.2

353.15 K

41.60 ± 0.04

33.96 ± 0.04

42.06 ± 0.04

26.7 ± 0.3

16.4 ± 0.2

17.5 ± 0.2

Property

σ (mS·cm )

D

MA

303.15 K

283.15 K

0.69 ± 0.01

0.18 ± 0.01

0.73 ± 0.01

0.12

0.036

0.11

293.15 K

1.36 ± 0.01

0.35 ± 0.01

1.41 ± 0.01

0.23

0.071

0.22

298.15 K

1.84 ± 0.02

0.51 ± 0.01

2.03 ± 0.02

0.31

0.10

0.32

0.76 ± 0.01

2.72 ± 0.03

0.42

0.15

0.43

CE

c

2

−1

Λ (S·cm ·mol )

PT E

−1

2.47 ± 0.02

313.15 K

4.17 ± 0.04

1.32 ± 0.01

4.58 ± 0.05

0.72

0.27

0.73

323.15 K

7.13 ± 0.07

2.21 ± 0.02

7.74 ± 0.08

1.24

0.46

1.24

333.15 K

11.12 ± 0.11

3.60 ± 0.04

12.24 ± 0.12

1.94

0.75

1.98

343.15 K

16.15 ± 0.17

5.69 ± 0.07

18.13 ± 0.19

2.84

1.18

2.94

353.15 K

22.50 ± 0.24

8.39 ± 0.08

25.99 ± 0.27

3.97

1.76

4.24

AC

303.15 K

a

The analysis of the experimental uncertainties were described in Supplementary Materials.

b

Literature value 138.5 mPa·s.[37]

c

Literature value 1.909.[37]

34

ACCEPTED MANUSCRIPT

Table 4. Equation fitting parameters of ILs. Equation

(1)

Parameter

αp×104

Sample

(K−1)

[C4-MIm]HCO2

5.69

[C6-MIm]HCO2 [C1OC2-MIm]HCO2

R2

(5)

(4) ES

SS×103

R2

(mN·m−1)

(mN·m−1·K−1)

0.9995

76.8

-100.0

5.89

0.9992

80.8

5.40

0.9974

82.0

(6)

η0

B

T0

(10−3·mPa·s)

(K)

(K)

0.9976

7.33

1831.0

116.1

-132.9

0.9980

5.92

1976.3

-113.8

0.9953

12.31

1711.8

D E

A M

T P E

C C

A

35

R2

A

Eη

(10−3·mPa·s)

(kJ·mol−1)

0.9986

0.041

37.83

0.9965

117.4

0.9991

0.019

41.39

0.9972

116.6

0.9991

0.062

36.53

0.9985

I R

SC

U N

T P

R2

ACCEPTED MANUSCRIPT Highlights: Formate-based ionic liquids were prepared in water via salting-out phase separation. [C2-MIm]HCO2 was prepared at −18 °C via low temperature phase separation.

PT

KHCO2 was used as both reactant and salting-out agent. The KHCO2 dosage and reaction yield depend on the hydrophobicity of the product.

AC

CE

PT E

D

MA

NU

SC

RI

Thermophysical properties of ionic liquids were characterized and analyzed.

36