Densities at Pressures up to 200 MPa and Atmospheric Pressure

Article pubs.acs.org/jced

Densities at Pressures up to 200 MPa and Atmospheric Pressure Viscosities of Ionic Liquids 1‑Ethyl-3-methylimidazolium Methylphosphate, 1‑Ethyl-3-methylimidazolium Diethylphosphate, 1‑Butyl-3-methylimidazolium Acetate, and 1-Butyl-3methylimidazolium Bis(trifluoromethylsulfonyl)imide Yuya Hiraga,† Aya Kato,† Yoshiyuki Sato,‡ and Richard L. Smith, Jr.*,†,‡ †

Graduate School of Environmental Studies, and ‡Research Center of Supercritical Fluid Technology, Tohoku University, Aramaki Aza Aoba 6-6-11, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *

ABSTRACT: High pressure densities at (10−200) MPa over a range of temperatures and atmospheric pressure viscosities at (293−373) K for three ionic liquids (ILs) that are able to dissolve biomass, 1-ethyl-3-methylimidazolium methylphosphate ([emim][MP]), 1-ethyl-3-methylimidazolium diethylphosphate ([emim][DEP]), and 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) are reported. Densities of the IL 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) were measured at the same conditions to verify the apparatus and to extend the range of present data. The high pressure densities of [emim][MP] and [emim][DEP] are newly measured in this work. Combined expanded uncertainties of densities for [emim][MP], [emim][DEP], [bmim][Ac], and [bmim][Tf2N] were estimated to be 1.5 kg·m−3, 1.4 kg·m−3, 1.4 kg·m−3, and 1.7 kg·m−3, respectively. The Tait equation could correlate the experimental density data to within 0.03 % of average relative deviation. The derivative properties, isobaric expansivity, and isothermal compressibility were calculated using the Tait equation and it was observed that the isobaric expansivity decreased with increasing temperature. The trend of the isobaric expansivity and isothermal compressibility with temperature were in accordance with the theory of corresponding states using methanol for comparison.

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have not been reported yet18−23 and in this work, measurements of [bmim][Tf2N] are used to demonstrate the apparatus and to extend the range to 200 MPa. High pressure densities (P < 200 MPa) of [emim][MP] and [emim][DEP] have not been reported in the literature. The Tait equation was used to correlate the experimental data and to calculate derivative properties. The trends of the isobaric thermal expansivity and isothermal compressibility of the ILs with temperature are compared with the properties of methanol within the framework of the theory of corresponding states. Methanol is considered to be a representative molecular solvent whose properties are well-known, and its critical pressure and polarity are comparable with that of the ILs.

INTRODUCTION Many new types of ionic liquids (ILs) are being proposed for processing cellulose, lignin, and other biomass.1−3 ILs having an allyl substituted alkyl imidazolium cation4−8 and alkylphosphate anion8−14 exhibit low viscosities and so they are more suitable for processing biomass than chloride containing ILs.1 For example, the cellulose solubility in phosphate ILs has been reported to be comparable with that of ILs that have an acetate or chloride anion.9 The density of an IL is considered to be an important fundamental physical property; however, for ILs that dissolve cellulose, density data are scarce. Additionally, the derivative properties, isobaric expansivity, and isothermal compressibility, are important for assessing the physical characteristics of ILs in many mechanical applications.15−17 In this work, high pressure densities and atmospheric pressure densities and viscosities are reported for three ILs that can be used for processing biomass, namely, 1-ethyl-3methylimidazolium methylphosphate ([emim][MP]), 1-ethyl3-methylimidazolium diethylphosphate ([emim][DEP]), and 1-butyl-3-methylimidazolium acetate ([bmim][Ac]). Densities of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) at pressures greater than 140 MPa © XXXX American Chemical Society

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EXPERIMENTAL SECTION Materials and Pretreatment. The ILs used along with their abbreviations, suppliers, purities, and measured water content are given in Table 1. All of the ILs were degassed to remove water and volatile impurities under vacuum at ca. 3 Pa

Received: October 18, 2014 Accepted: February 4, 2015

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DOI: 10.1021/je5009679 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Ionic Liquids Studied in This Worka ionic liquid

water content [ppm] b

wHP,before H2 O

wHP,after H2 O

45 60 180 40

40 50 270 20

full name

abbreviation

supplier

purity [%]

watm H2O

1-ethyl-3-methylimidazolium methylphosphate 1-ethyl-3-methylimidazolium diethylphosphate 1-butyl-3-methylimidazolium acetate 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[emim][MP] [emim][DEP] [bmim][Ac] [bmim][Tf2N]

Kanto Chemical Merck Iolitec Tokyo Chemical Industry

97 98 98 98

45 60 250 60

a

Water content was measured before samples were introduced into the apparatus for atmospheric pressure at 0.1 MPa (atm) measurements. Water content was measured before and after samples were introduced into the apparatus for high pressure (HP) measurements. bManufacturer stated minimum purity. Estimated purity after evacuation and drying pretreatment is >99% by NMR (Figures S1−S4, Supporting Information).

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CORRELATION MODEL FOR DENSITY Tait Equation for Density. The Tait equation29 is an empirical equation for representing liquid densities that has the following form:

and 353 K for at least 24 h, and water content was measured by Karl Fischer titration (MKC-510, Kyoto Electronics Manufacturing) before use of the ILs in each experiment. Density and Viscosity Measurements Method at Atmospheric Pressure. Densities and viscosities at atmospheric pressure were measured with a Stabinger viscometer (Anton Paar, SVM 3000). Values reported are based on the average of at least three measurements. The standard uncertainties of the apparatus were u(T) = 0.02 K, u(ρ) = 0.5 kg·m−3 and ur(η) = 0.35 %. The combined expanded uncertainties with a level of confidence of 0.95 (k = 2) for the ILs were estimated from the standard uncertainties and the standard deviation of the density and viscosity measurements. The stability of the temperature was controlled to within ± 0.005 K during the measurements and density stability was judged by the variation of its value being within 0.3 kg·m−3 over a period of 60 s. Measurements for the ILs were performed at temperatures from 293.15 K to 373.15 K at 10 K intervals. Density Measurement Method at High Pressures. Densities at high pressures were measured with a bellows apparatus.24 The apparatus consisted of a bellows dilatometer, a pressure vessel, multiple temperature control sections, a hand pump, pressure gauges, and a section for displacement measurement. In the measurements, the dilatometer was filled under vacuum (ca. 3 Pa) with 5.1 g to 7.0 g of sample depending on sample density, and then the dilatometer was loaded into the pressure vessel. The change of volume for the IL was obtained from the change in length of the bellows given by a linear variable differential transformer (LVDT) that was calibrated with water and mercury. The combined expanded uncertainties of density with a level of confidence of 0.95 (k = 2) for each IL was estimated, and the detailed estimation method is given in Table S1 (Supporting Information). Further detailed procedures have been described in previous reports.25−28 For each IL, the measurements were performed at temperatures from ca. 311 K to the maximum temperature at 20 K intervals and at pressures from 10 MPa to 200 MPa at 10 MPa intervals. The maximum temperature used for each IL was lower than the measured onset of decomposition temperature as shown in Table S2 (Supporting Information). Additionally, after an isothermal series of measurements at the maximum temperature were completed, measurements at the lowest temperature (ca. 311 K) were repeated to verify the apparatus and the integrity of the sample. The samples before and after the density measurement were analyzed for water by Karl Fischer, and impurities were characterized by 1H and 13C NMR spectra (DRX500, Bruker). Through these methods, it could be concluded that the sample remained chemically unchanged during the measurement conditions.

ρ (P , T ) =

ρ(P0 , T ) 1 − C ln(1 + P /B(T ))

(1)

1 = a0 + a1T + a 2T 2 ρ(P0 , T )

(2)

B(T ) = b0 exp( −b1T )

(3)

where P0 [MPa], P [MPa], and T [K] are atmospheric pressure (0.1 MPa), pressure, and absolute temperature, respectively. The parameters C [-], a0 [m3·kg−1], a1 [m3·kg−1·K−1], a2 [m3· kg−1·K−2], b0 [MPa], and b1 [K−1] are constant for each liquid. The parameters a0, a1, and a2 were determined by fitting the experimental densities at atmospheric pressure. The other parameters C, b0, and b1 were determined by correlation to densities at high pressures. The objective function for density used in the fitting procedures is given by eq 4: ARD % =

1 N

N

∑

ρcalc, i − ρexp, i

i=1

ρexp, i

·100 (4)

where ARD is the average relative deviation and N is the number of data. The Tait equation was used to estimate derivative properties, the isobaric expansivity αP, and the isothermal compressibility βT from the following relationships: a1 + 2a 2T 1 ⎛ ∂ρ ⎞ ⎜ ⎟ = ρ ⎝ ∂T ⎠ P a0 + a1T + a 2T 2 b1CP − (B(T ) + P){1 − C ln(1 + P /B(T ))}

αP = −

βT =

(5)

1 ⎛ ∂ρ ⎞ 1 ⎜ ⎟ = ρ ⎝ ∂P ⎠T (P + B(T )){1/C − ln(1 + P /B(T ))} (6)

where all the parameters have been described previously. In this work, discussion on the parameters is based on the calculated values from these two equations since the deviations from the experimental density data were generally less than 0.03%. Vogel−Fulcher−Tammann Equation for Viscosity. The Vogel−Fulcher−Tammann (VFT) equation was used for viscosity correlation that has the following form: ⎛ d1 ⎞ η = η0 exp⎜ ⎟ ⎝ T + d2 ⎠ B

(7) DOI: 10.1021/je5009679 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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where η0 [mPa·s], d1 [K], and d2 [K] were determined by fitting experimental viscosities at atmospheric pressure. The objective function for viscosity used in the fitting procedures is given by: ARD % =

1 N

N

∑ i=1

ηcalc, i − ηexp, i ηexp, i

·100 (8)

where ARD is the average relative deviation.

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RESULTS AND DISCUSSION Density and Viscosity Measurements at Atmospheric Pressure. Table 2 shows atmospheric pressure densities, and

Figure 1. Relative deviation (RD) plot of densities for [bmim][Tf2N] at atmospheric pressure. Reference values were calculated by eq 2 with parameters determined from correlation to experimental data measured in this work: ▽, Harris et al.;31 △, de Azevedo et al.;18 □, Jacquemin et al.;19 ○, de Castro et al.;20 ◊, Troncoso et al.;32 ×, Salgado et al.33 Dashed lines: combined expanded uncertainty in this work.

Table 2. Experimental Densities of ILs at Atmospheric Pressure (0.1 MPa) ρ [kg·m−3] T/K

[emim][MP]

[emim][DEP]

[bmim][Ac]

[bmim][Tf2N]

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 Uc(ρ)a

1198.3 1191.7 1185.3 1178.8 1172.5 1166.1 1159.9 1153.8 1147.6 1.0

1148.2 1141.2 1134.4 1127.8 1121.2 1114.6 1108.0 1101.5 1094.9 1.0

1053.8 1047.4 1041.4 1035.4 1029.5 1023.7 1017.9 1012.0 1006.1 1.0

1440.9 1431.6 1422.1 1412.6 1403.4 1394.0 1384.8 1375.6 1366.7 1.2

RD% =

ρ − ρref ρref

· 100 (9)

where ρ is the literature value18−20,31−33 and ρref is the reference value calculated by eq 2 with the parameter determined by correlation to densities shown in Table 2. In Figure 1, relative deviations from Harris et al.,31 de Azevedo et al.,18 de Castro et al.,20 Troncoso et al.,32 and Salgado et al.33 were within the experimental uncertainty of this work. The deviations of Jacquemin et al.19 were higher than the uncertainties of this work at temperatures below 350 K even if the uncertainty of that data (0.1 kg/m3 ≃ 0.007 %) was considered. For other ILs, relative deviations of [emim][MP], [emim][DEP], and [bmim][Ac] are shown in Figures S6 to S8 (Supporting Information). There have been some reports on properties that have deviations which are much higher than the uncertainties of this work.34−39 One reason for the deviations is water contamination. An important point in discussing the effect of water on density is that it does not always lead to a decrease in the density of the IL. For example, Huddleston et al.40 reported that the densities of several ILs decreased with increasing water concentration from dried to water-saturated conditions, and the density of [bmim][Tf2N] decreased from (1430 to 1390) kg· m−3 for a water concentration from (474 to 3280) ppm. On the other hand, Ma et al.41 reported that the densities of [bmim][Ac] increased with increasing water concentration from (8610 to 12560) ppm for temperatures from (298 to 338) K and at a pressure of 0.1 MPa. From Figure S8, the deviation of [bmim][Ac] from Almeida et al.35 was higher than the uncertainties of this work, and the deviations were positive. The sample supplier and purity were the same and the apparatus used in the measurements in that work were similar to that used in this work, so that the deviation is most likely caused by

Uc(ρ) is the combined expanded uncertainty [kg·m−3] with a 0.95 level of confidence (k = 2) calculated from the standard uncertainty of temperature, u(T) = 0.02 K, and standard deviation of the measurement and does not include the possible effect of impurities. a

their combined expanded uncertainties with a level of confidence of 0.95 (k = 2) for four ILs. The 1H NMR spectra of pretreated samples (Figures S1 to S4, Supporting Information) did not include any extra peaks and so the NMR purity of the samples is estimated to be > 99 % as stated in Table 1.30 The water content of the sample before the measurement was measured by Karl Fischer titration and is shown in Table 1. It is estimated that the effect of impurities including water on the density values shown are negligible. The parameters for eq 2 were determined by correlation with atmospheric pressure densities and shown in Table 3. The ARDs for all ILs were within 0.01 %. Figure 1 shows deviations for [bmim][Tf2N] at atmospheric pressure for literature values relative to values reported in this work that were calculated as follows:

Table 3. Tait Equation Parameters and Average Relative Deviation (ARD) for Various ILs Studied in This Work Determined by Correlation with Experimental Densities at Pressures up to 200 MPa ρ(P0,T) in eq 2

B(T) in eqs 1 and 3

a0·104

a1·107

a2·1010

b0

b1

ionic liquids

m3·kg−1

m3·kg−1·K−1

m3·kg−1·K−2

MPa

K−1


4.931 7.137 8.079 5.758

4.985 5.445 4.240 3.484

−0.563 −0.275 1.992 1.868

830.0 556.5 601.4 579.8

2.797 3.469 3.126 4.019

C

C

ARD %

0.1120 0.0855 0.0924 0.0893

0.025 0.011 0.011 0.012



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Table 4. Experimental Viscosities of ILs at Atmospheric Pressure (0.1 MPa) η [mPa·s] T/K

[emim][MP]

[emim][DEP]

[bmim][Ac]

[bmim][Tf2N]

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 Uc,r(η)a in %

156.4 89.90 56.11 37.41 26.30 19.32 14.71 11.54 9.289 4.65

664.7 317.3 173.9 103.8 66.37 44.92 31.86 23.50 17.92 4.65

592.3 274.1 142.4 81.88 51.11 33.65 23.51 17.09 12.93 4.63

63.41 41.26 28.46 20.58 15.47 12.00 9.562 7.788 6.469 4.63

a

Uc,r(η) is the combined expanded uncertainty [%] with a 0.95 level of confidence (k = 2) calculated from the standard uncertainty of temperature, u(T) = 0.02 K, standard deviation of the measurement and the estimated impurity effect. Values of the combined expanded uncertainty are for a fictitious low-viscosity impurity (x2 = 0.01) with viscosity of η2(= η1/10) mPa·s and assuming ln η = x1 ln η1 + x2 ln η2.

water contamination. Since the relative deviation was positive, it is possible that the water concentration of that work was higher than that in this work. The deviation from Araujo et al.34 was higher (positive) than the uncertainty in this work and water concentration was also higher than that in this work; however, the deviation cannot be easily explained because the sample purity was lower than others (> 95 %). The acetate containing IL is well-known to be strongly hydrophilic, and water will affect the IL density. It is possible that water influences the deviations of these works. Therefore, careful and stringent removal of water was absolutely essential to making reliable measurements. For [emim][MP] and [emim][DEP], there are some large deviations from literature, namely, Hasse et al.,36 Freire et al.,37 Normazlan et al.,38 and Wang et al.,39 in comparison with this work even if the uncertainties of these works are considered. One possible reason is water contamination in the handling, although the effect of water concentration on the density of many ILs has not been clearly understood yet. The water concentration of the sample in this work was lowest among that in the other studies. Table 4 shows atmospheric pressure viscosities, and their combined expanded uncertainties with a level of confidence of 0.95 (k = 2) for four ILs. The purity of the pretreated IL samples was estimated to be > 99 % from the 1H NMR spectra (Figures S1 to S4, Supporting Information) because no extra peaks could be found for common impurities such as 1methylimidazole. The possible effect of a fictitious impurity (x2 = 0.01) with viscosity of η2 (= η1/10) mPa·s on the combined expanded uncertainty assuming the simple mixing equation ln ηmix = x1 ln η1 + x2 ln η2 is shown in Table 4. The uncertainties of viscosity in the absence of impurity were estimated to be less than 0.9 % for all ILs. The viscosity values at atmospheric pressure were obtained simultaneously with the density measurement. The water content of each sample before the experiments was measured by Karl Fischer titration and is shown in Table 1. The parameters for eq 7 were determined by correlation of the atmospheric pressure viscosities (Table 4) and are shown in Table 5. Figure 2 shows the relative deviation for [bmim][Tf2N] at atmospheric pressure calculated as follows: η − ηref RD% = · 100 ηref (10)

Table 5. Vogel−Fulcher−Tammann (VFT) Equation Parameters and Average Relative Deviation (ARD) for Various ILs Studied in This Work Determined by Correlation with Experimental Viscosities η0

d1

d2

ARD

ionic liquids

mPa·s

K

K

%


0.1557 0.1398 0.1153 0.1608

801.0 912.8 841.5 774.6

−177.1 −185.0 −194.9 −163.5

0.315 0.184 0.429 0.072

Figure 2. Relative deviation (RD) plot of viscosities for [bmim][Tf2N] at atmospheric pressure, (a) RD plotted on (−1.5 to 1.5) % and (b) RD plotted on (−10 to 10) %. Reference values were calculated by eq 7 with parameters determined from correlation to experimental data measured in this work: ▽, Harris et al.;31 △, Tariq et al.;42 □, Jacquemin et al.;43 ○, Vranes et al.;44 ×, Salgado et al.33 Dashed-dotted lines and dashed lines are for combined expanded uncertainty of measurements with and without the effect of possible impurities in this work.

correlation to viscosities shown in Table 4. From Figure 2, the relative deviations from Harris et al.31 and Tariq et al.42 were within the uncertainty of this work when the uncertainties, 2 %,

where η is the literature value31,33,42−44 and ηref is the reference value calculated by eq 7 with the parameter determined by D



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Table 6. Experimental Densities for [emim][MP] at Pressures up to 200 MPaa ρ/kg·m−3 P/MPa

T/K = 311.2

T/K = 330.9

T/K = 351.3

T/K = 371.3

T/K = 391.2

T/K = 411.1

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

1189.6 1193.3 1197.1 1200.8 1204.4 1207.8 1211.3 1214.6 1217.9 1221.0 1224.2 1227.2 1230.3 1233.1 1236.1 1238.9 1241.7 1244.5 1247.2 1249.9

1176.8 1180.9 1184.9 1188.8 1192.5 1196.1 1199.8 1203.3 1206.6 1210.0 1213.3 1216.4 1219.5 1222.5 1225.4 1228.3 1231.2 1234.0 1236.7 1239.6

1164.2 1168.7 1172.8 1176.9 1180.9 1184.7 1188.4 1192.0 1195.5 1198.9 1202.1 1205.4 1208.7 1211.8 1214.8 1217.8 1220.8 1223.7 1226.6 1229.5

1152.1 1156.2 1161.0 1165.2 1169.2 1173.3 1177.2 1180.9 1184.5 1188.2 1191.6 1195.0 1198.3 1201.6 1204.7 1207.9 1210.9 1213.9 1216.8 1219.8

1140.6 1144.9 1149.5 1154.0 1158.2 1162.3 1166.4 1170.4 1174.1 1177.9 1181.4 1184.9 1188.3 1191.6 1194.9 1198.1 1201.2 1204.2 1207.2 1210.2

1128.7 1133.3 1138.1 1142.8 1147.3 1151.7 1155.8 1159.9 1163.9 1167.6 1171.3 1174.9 1178.4 1181.8 1185.3 1188.5 1191.7 1194.8 1197.9 1201.0

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 MPa for 10 ≤ P ≤ 80 MPa and u(P) = 0.25 MPa for 90 ≤ P ≤ 200 MPa, and combined expanded uncertainty with a 0.95 level of confidence (k = 2) is Uc(ρ) = 1.5 kg·m−3.

a

Table 7. Experimental Densities for [emim][DEP] at Pressures up to 200 MPaa ρ/kg·m−3 P/MPa

T/K = 311.4

T/K = 331.3

T/K = 351.4

T/K = 371.6

T/K = 391.6

T/K = 411.7

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

1140.5 1145.4 1150.1 1154.6 1158.9 1163.0 1167.0 1170.9 1174.6 1178.3 1181.8 1185.2 1188.6 1191.8 1195.2 1198.2 1201.4 1204.4 1207.3 1210.3

1127.5 1132.6 1137.6 1142.3 1146.8 1151.2 1155.3 1159.4 1163.2 1167.1 1170.7 1174.4 1177.8 1181.3 1184.8 1187.9 1191.0 1194.1 1197.3 1200.2

1114.6 1120.0 1125.2 1130.2 1134.9 1139.5 1144.0 1148.1 1152.2 1156.1 1159.9 1163.6 1167.2 1170.7 1174.2 1177.5 1180.8 1184.0 1187.1 1190.3

1102.0 1107.7 1113.3 1118.5 1123.4 1128.2 1132.7 1137.2 1141.3 1145.5 1149.4 1153.2 1157.0 1160.6 1164.2 1167.7 1171.0 1174.3 1177.5 1180.8

1089.8 1095.6 1101.5 1107.0 1112.3 1117.2 1122.0 1126.6 1131.0 1135.2 1139.4 1143.4 1147.1 1151.0 1154.6 1158.1 1161.5 1165.0 1168.2 1171.5

1077.9 1084.3 1090.4 1096.2 1101.7 1106.9 1111.9 1116.7 1120.6 1125.1 1129.3 1133.4 1137.4 1141.2 1145.0 1148.7 1152.3 1155.8 1159.2 1162.6

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 MPa for 10 ≤ P ≤ 80 MPa and u(P) = 0.25 MPa for 90 ≤ P ≤ 200 MPa, and combined expanded uncertainty with a 0.95 level of confidence (k = 2) is Uc(ρ) = 1.4 kg·m−3.

a

Almeida et al.35 and Xu et al.46 for the same IL should be caused by higher water concentration than in this work. When the effect of 1 % of a fictitious impurity having a viscosity of η2(= η1/10) mPa·s is considered, much of the literature viscosity data would be in the range of the viscosities reported in this work (Table 4). However, the reason for extremely high deviations (ca. 40 % to 110 %) by Freire et al.37 for [emim][MP] cannot be easily explained and similar high deviations (ca. 1.2 %) for the same literature37 appear in density

of their works were considered. The deviation from Salgado et al.33 was within the uncertainty of this work except at low temperatures. Supporting Information, Figures S9 to S11 show the relative deviations of each IL, namely, [emim][MP], [emim][DEP], and [bmim][Ac]. Some data34,37,45 have deviations that are much higher than the uncertainties of this work. For example, the deviations from Iguchi et al.45 for [bmim][Ac] could be estimated because the purity of that work was lower than the purity in this work, and the deviations from E



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Figure 4 shows relative deviations for [bmim][Tf2N] at pressures up to 140 MPa calculated by eq 9 for the confirmation of the procedure in this work. For the reference value in eq 9, calculated densities with eqs 1 to 3 and parameters shown in Table 4 were used. From Figure 4, relative deviations from de Azevedo et al.,18 Jacquemin et al.,19 Widowati et al.,21 and Curras et al.22 were within the uncertainties in this work over the full range of pressures. In Figure 4, the two straight trends reported by de Castro et al.20 that had large negative deviations cannot be compared since the temperature in this work is limited to 413 K. The deviations from Hamidova et al.23 were higher than the uncertainties of this work as the measurements of those authors were at low temperature and high pressure conditions. The validity of the data in this work was also confirmed by reproducibility of the measurements at ca. 311 K after a series of measurements at the maximum temperature for each IL were made (Table S3, Supporting Information). High pressure densities have been reported for [bmim][Ac] by Stevanovic et al.48 and Safarov et al.49 Supporting Information, Figure S12 shows the relative deviations of those data. From the figure, both deviations were larger than the uncertainties of this work. One of the reasons is estimated to be due to water contamination. For [bmim][Ac], the density increases with increasing water concentration.41 The order of water concentration was Stevanovic et al. (600 ppm)48 > Safarov et al. (300 ppm)49 > this work (180 ppm) and the relative deviation corresponds to this order although it cannot be confirmed whether the water concentration effect on density exists at high pressure compared with that on densities at atmospheric pressure. Especially, [bmim][Ac] is greatly hydrophilic, thus the handling of the IL has to be done with extreme care. The water concentration of the samples in this work was confirmed before and after high pressure density measurement, and the integrity of the sample due to thermal treatment was confirmed by repeated measurement after each series of experiments. Therefore, it can be concluded that the sample integrity and handling were sufficient in this work. Tables S4 to S13 (Supporting Information) summarize the source and purity used by researchers for a given IL or for a given type of measurement along with estimated uncertainties based on information given in the literature report. Not only water content, but also undetected impurities, experimental procedure, and calibration standards affect the variations. Derivative Properties, αP and βT. The derivative properties, isobaric expansivity αP and isothermal compressibility βT, were determined from the Tait equation, eqs 5 and 6, with parameters described in Table 3. Figure 5 shows αP and βT for [emim][MP], and Figures S13 to S15 (Supporting Information) show those for the other ILs. Figure 5 and Figures S13 to S15 show that the βT for all ILs studied in this work decreased with increasing pressure or with decreasing temperature as expected. The trend was in accordance with other IL systems. On the other hand, it can be observed that the αP decreased with increasing temperature for [emim][MP] and [emim][DEP], and was constant or slightly decreased with increasing temperature for [bmim][Ac] and [bmim][Tf2N]. Supporting Information, Figure S16 shows the relative deviations of (a) isobaric expansivity and (b) isothermal compressibility for [bmim][Tf2N] in comparison with the literature. The relative deviations were calculated according to a similar form of eq 9 or 10 and with parameters of the Tait equation shown in Table 3. The isobaric expansivities and isobaric compressibilities of the

measurements (Figure S6, Supporting Information), so that it is likely that the IL measured in Freire et al.37 is different from that studied in this work or that there was an error in the property calibration procedure. Density Measurement at Pressures up to 200 MPa. Tables 6 to 9 shows the densities for [emim][MP], Table 8. Experimental Densities for [bmim][Ac] at Pressures up to 200 MPaa ρ/kg·m−3 P/MPa

T/K = 311.4

T/K = 331.1

T/K = 351.3

T/K = 371.3

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

1046.7 1050.8 1054.8 1058.6 1062.3 1065.8 1069.3 1072.7 1076.0 1079.0 1082.1 1085.1 1088.1 1090.9 1093.7 1096.5 1099.1 1101.9 1104.5 1107.1

1034.7 1039.1 1043.3 1047.3 1051.2 1054.9 1058.5 1062.1 1065.2 1068.7 1071.9 1075.0 1078.2 1081.1 1084.0 1086.8 1089.6 1092.4 1095.1 1097.7

1023.2 1027.7 1032.1 1036.4 1040.4 1044.3 1048.0 1051.6 1055.0 1058.4 1061.7 1065.0 1068.1 1071.2 1074.3 1077.3 1080.2 1083.0 1086.0 1088.5

1011.7 1016.5 1021.2 1025.5 1029.6 1033.8 1037.7 1041.5 1045.0 1048.5 1051.9 1055.3 1058.6 1061.8 1064.9 1067.8 1071.0 1073.9 1076.8 1079.5

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 MPa for 10 ≤ P ≤ 80 MPa, and u(P) = 0.25 MPa for 90 ≤ P ≤ 200 MPa, and combined expanded uncertainty with a 0.95 level of confidence (k = 2) is Uc(ρ) = 1.4 kg·m−3. a

[emim][DEP], [bmim][Ac], and [bmim][Tf2N] measured by the bellows method at pressures up to 200 MPa. The combined expanded uncertainties of density with a level of confidence of 0.95 (k = 2) for each IL are shown in the footnotes (Tables 6 to 9). Figure 3 shows the densities of [emim][MP] for which the smooth density behavior indicates that no phase transitions occurred in the measurement region. The water content of the sample before and after the measurement was measured and is shown in Table 1. The reason for the difference in water content of the samples before and after the measurements, however, is that this amount of water is lower than that reported in previous literature. The [emim][MP] contained 40 ppm after the measurement in this work, whereas it was 284 ppm for Hasse et al.36 and 780 ppm for Freire et al.37 The [bmim][Ac] contained 270 ppm after the measurement for this work, whereas it was 99.1 ppm for Xu et al.,47 850 ppm for Almeida et al.,35 and 1500 ppm for Araujo et al.34 These results show that the vacuum conditions of pressure at 3 Pa and temperature at 353 K for 24 h were sufficient to remove water from the ILs. Parameters for eqs 1 and 3 were determined by correlation with high pressure densities and are shown in Table 3. Correlation results for [emim][MP] are shown as lines in Figure 3. From the ARDs shown in Table 3 and smooth lines shown in Figure 3, it was concluded that the Tait equation provided reliable correlation of the experimental densities over the full range of conditions. F



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Table 9. Experimental Densities for [bmim][Tf2N] at Pressures up to 200 MPaa ρ/kg·m−3 P/MPa

T/K = 311.8

T/K = 331.9

T/K = 352.0

T/K = 372.2

T/K = 392.4

T/K = 412.1

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

1430.7 1437.5 1444.6 1451.2 1457.5 1463.6 1469.5 1475.2 1480.5 1485.7 1490.9 1496.0 1500.9 1505.6 1510.2 1514.8 1519.3 1523.5 1527.7 1531.8

1412.5 1420.1 1427.4 1434.5 1441.1 1447.6 1453.6 1459.5 1464.9 1470.5 1476.0 1481.1 1486.2 1491.2 1495.9 1500.5 1505.2 1509.6 1513.9 1518.3

1394.0 1402.4 1410.0 1417.4 1424.3 1431.2 1437.5 1443.8 1449.5 1455.3 1460.8 1466.3 1471.5 1476.5 1481.4 1486.3 1491.0 1495.8 1500.2 1504.7

1376.1 1384.9 1393.3 1401.1 1408.4 1415.5 1422.1 1428.4 1434.4 1440.3 1446.2 1452.0 1457.4 1462.6 1467.8 1472.5 1477.5 1482.1 1487.0 1491.4

1358.5 1367.8 1376.7 1385.0 1392.8 1400.2 1407.2 1414.0 1420.7 1426.6 1432.6 1438.2 1443.8 1449.2 1454.4 1459.3 1464.5 1469.2 1474.1 1478.8

1341.1 1351.0 1360.4 1369.2 1377.5 1385.1 1392.6 1399.6 1406.1 1412.5 1418.8 1424.8 1430.5 1436.2 1441.4 1446.8 1451.9 1456.9 1461.6 1466.6

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 MPa for 10 ≤ P ≤ 80 MPa, and u(P) = 0.25 MPa for 90 ≤ P ≤ 200 MPa, and combined expanded uncertainty with a 0.95 level of confidence (k = 2) is Uc(ρ) = 1.7 kg·m−3.

a

Figure 3. Density of [emim][MP] versus pressure at various temperatures: ○, 311.2 K; △, 330.9 K; □, 351.3 K; ▽, 371.3 K; ◊, 391.2 K; ▷, 411.1 K; lines, Tait equation.

literature were used as original values reported in the paper without recalculation from experimental density data. There were some large deviations in the calculated αP and βT that are attributed to both deviations in the experimental data and the calculation method used. In general, it is considered that the αP of a molecular solvent increases monotonically with increasing temperature up to its critical temperature; however, different trends have often been reported for ILs.27,50−52 Although there have been some theoretical analyses for the reasons,53 one of the simplest explanations is because the ILs have a high critical temperature. For the critical temperature (Tc) of 1130 ILs estimated by Valderrama54 et al., 75 % of the ILs had Tc greater than 800 K and 84% of them had Tc greater than 700 K of the temperature. The αP of a molten polymer, poly(butylene succinate), which is calculated from PVT data55 with the Tait equation shows the same trends as the ILs in this work (Supporting Information, Figure S17). As shown in Figure S18, the isobaric expansivity decreased with increasing temperature far below the critical temperature at high pressure even for methanol (Tc, 512 K; and Pc, 8.10 MPa56). The trends can be understood by considering the van der Waals theory of corresponding states.

Figure 4. Relative deviation (RD) plot of densities for [bmim][Tf2N] at high pressures up to 150 MPa, (a) low pressure and small RD range and (b) full pressure and RD range. Reference values were calculated by eqs 1 to 3 with parameters determined from correlation to experimental data measured in this work: △, de Azevedo et al.;18 □, Jacquemin et al.;19 ○, de Castro et al.;20 ×, Widowati et al.;21 ◊, Curras et al.;22 +, Hamidova et al.23 Dashed lines, combined expanded uncertainty in this work at 313 K; dashed-dotted lines, combined expanded uncertainty in this work at 413 K.

Figure 6 shows the isobaric expansivity αP and isothermal compressibility βT of ILs and methanol at 100 MPa against the reduced temperature, Tr = T/Tc. Figure 6 includes the αP of [bmim]Cl (1-butyl-3-methylimidazolium chloride) taken from the literature,27 and the estimated critical temperature Tc from the literature.54,56 From Figure 6, the αP of methanol decreased G



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reduced temperature, the trend is probably important for building a predictive model for the density of ILs.

■

CONCLUSIONS In this work, densities of four ILs at pressures up to 200 MPa were measured. Among the ILs, densities for two phosphatecontaining ILs were newly measured at high pressure. The trend of density with pressure or temperature of the ILs studied in this work was in accordance with previous reports on high pressure densities in the literature. The measured densities of this work could be correlated by the Tait equation with low deviation (< 0.03 %). The derivative properties, αP and βT, were calculated using the Tait equation with parameters determined by correlation to the experimental data. The trends of αP and βT of the four studied ILs are the same as those of the molecular solvent methanol in terms of corresponding states using reduced temperature.

■

ASSOCIATED CONTENT

S Supporting Information *

Detail description of the combined expanded uncertainties; tables of available data for TGA analysis of ILs, deviation of experimental densities for four ILs pressures up to 200 MPa at ca. 311 K before and after a series of measurements at maximum temperature were made, and source and purity information on ILs of literature compared in this work; figures for NMR spectra, relative deviation plots of density and viscosity for three ILs, isobaric expansivity and isothermal compressibility of three ILs and molten polymer; relative deviations plots for the isobaric expansivity of methanol. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 5. (a) Isobaric expansivity (αP) temperature at 293 K to 493 K and pressure at (0.1, 10, 20, 30, 40, 50, 70, 100, 150, and 200) MPa and (b) isothermal compressibility (βT) pressure at 0.1 MPa to 200 MPa and temperature at from 293 K to 473 K with 20 K intervals for [emim][MP] calculated with the Tait Equation shown in eqs 5 and 6.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +81-22-795-5863/5864. E-mail: [email protected]. tohoku.ac.jp. Funding

This work was supported by a JSPS Grant in Aid for JSPS Fellows (No. 254392) and a JSPS Grant in Aid Scientific Research (B) research grant, Japan Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 6. (a) Isobaric expansivity (αP) and (b) isothermal compressibility (βT) of various liquids studied in this work pressure at 100 MPa. Lines: solid line, methanol; dashed line, [emim][MP]; dashed-dotted line, [emim][DEP]; dotted line, [bmim][Ac]; dasheddouble dotted line, 1-butyl-3-methylimidazolium chloride.27 Critical temperatures of ILs were obtained from the literature.54

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Densities at Pressures up to 200 MPa and Atmospheric Pressure

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