Conductivities of 1-Alkyl-3-methylimidazolium Chloride Ionic Liquids in

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Conductivities of 1‑Alkyl-3-methylimidazolium Chloride Ionic Liquids in Disaccharide + Water Solutions at 298.15 K Yujuan Chen, Mengyao Fang, Guangyue Bai, Kelei Zhuo,* and Changling Yan Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China S Supporting Information *

ABSTRACT: Conductivities for ionic liquids (ILs) 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl, n = 4, 6, 8, 10) + sucrose + water solutions and [C4mim]Cl + maltose + water solutions were measured at 298.15 K. Meanwhile, densities, viscosities, and relative permittivities for water + disaccharide mixtures were also measured. The Lee−Wheaton conductivity equation was used to acquire the limiting molar conductivities (Λ0). The Walden products (Λ0η0) were also calculated. The interaction of ILs with disaccharide was discussed in terms of the structure of disaccharides and ILs. Furthermore, values of Λ0 for inorganic salts (ordinary electrolyte, such as NaCl/KCl) and ILs (special electrolyte) were compared, indicating that they have approximate limiting molar conductivities, namely, they have not too much difference in electrical conductivity.

1. INTRODUCTION Carbohydrates, which are a large natural resource, have long interested researchers because of their important roles in biological and industrial applications.1 They can be used as the basic component of tissue cells, the main source of energy, and as information molecules, but also as drugs,2 health products,3 and cosmetics.4 As an energy vector for plants,5 sucrose is produced from photosynthesis, and it is the main form of storage and transportation carbohydrates for plants. Maltose, which is obtained from the degradation of starch under the catalysis of amylases,6 is the predominant form of carbon exported from the chloroplast.7 What’s more, a mass of sucrose and maltose is produced and used worldwide.8,9 Ionic liquids (ILs) have attracted extensive attention in recent years because of their unique physical and chemical properties.10−12 Furthermore, their application for specific purpose can be achieved by selecting or designing the structures of cations and anions.13 Also, they are regarded as ideal substitutes for traditional volatile solvents. Therefore, ILs have been applied widely in many fields.14−26 ILs and carbohydrates can be used to form aqueous biphasic systems for separating different kinds of molecules.27 ILs can also be used as catalysts for the hydrolysis of cellulose.28 However, the mechanisms of the interaction between ILs and carbohydrates are not very clear so far. Therefore, it is essential to carry out systematic study on their physicochemical properties. In addition, with extensive studies of ILs, people gradually realize that ILs have potential toxicity and thus are not really “green”. The low biodegradability of ILs may lead to their accumulation in organisms.29 Consequently, it is © XXXX American Chemical Society

important to explore the interaction of ILs and biomass (carbohydrates). In previous work, we have reported on the interactions of ILs with monosaccharides30 and provided some original data and fundamental inference for interpreting the accumulation and potential toxicity of ILs in organisms. For the aims of deeply exploring the dependences of the interactions on the structures of disaccharides and ILs, we measured the conductivities for ILs + disaccharides + water solutions at 298.15 K. The Lee− Wheaton conductivity equation was used to acquire the limiting molar conductivities (Λ0). The interactions of ILs with disaccharides in water have been discussed in terms of the structures of ILs and saccharides. Furthermore, Λ0 values for ILs (special electrolyte) were also compared with those for inorganic salts (ordinary electrolyte, such as NaCl/KCl).

2. EXPERIMENTAL METHODS 2.1. Chemicals. The chemicals used in this work are shown in Table 1. Sucrose and D-maltose monohydrate were dried under vacuum at room temperature to constant weight. [Cnmim]Cl (n = 4, 6, 8, 10) were used without further purification. The nature of ∼1% impurity of the ILs is to show that they are not likely to affect the results. There, dried reagents were stored over P2O5 in desiccators. Pure distilled deionized water was used with a conductivity of 1.0−1.2 μS· cm−1 at 298.15 K. Received: April 27, 2016 Accepted: October 10, 2016

A

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3. RESULTS AND DISCUSSION Densities (d0), viscosities (η0), relative permittivities (D), and experimental conductivities (σ) for the water + disaccharide (sucrose/maltose) mixtures were measured, and their values are shown in Table 2. The values of the density and viscosity for the water + sucrose mixtures, and the values of density for the water + maltose mixtures were compared with literature values (see Figures S1−S3). It can be seen that the measured densities and viscosities are consistent with the literature data.39−41 The molar conductivities for [C4mim]Cl in water and water + disaccharide (sucrose/maltose) mixtures and those for [Cnmim]Cl (n = 6, 8, 10) in water and water + sucrose mixtures were measured, and resulting values are collected in Tables 3 and 4, respectively. The experimental conductivities (σ) for the [C4mim]Cl + water and [C4mim]Cl + water + disaccharide (sucrose and maltose) mixtures are collected in Tables S1 and S2, respectively. The experimental data were analyzed using the Lee− Wheaton conductivity equation42,43 in the form suggested by Pethybridge and Taba44

Table 1. Sample Purities in Mass Basis chemical

CAS

source

purity

sucrose D-maltose monohydrate 1-butyl-3-methylimidazolium chloride 1-hexyl-3-methylimidazolium chloride 1-octyl-3-methylimidazolium chloride 1-decyl-3-methylimidazolium chloride

57-50-1 6363-53-7 79917-90-1 171058-17-6 64697-40-1 171058-18-7

Sigma Sigma Alfa Alfa Alfa Alfa

>0.995 >0.99 >0.99 >0.99 >0.98 >0.99

2.2. Measurements of Densities, Viscosities, Relative Permittivities, and Conductivities.30−38 A vibrating-tube digital densimeter (model DMA60/602, Anton Paar, Graz, Austria) was used for measurements of solution densities (d).31−33 The instrument was calibrated with air and pure distilled deionized water (0.997046 g·cm−3).34,35 A suspended level Ubbelohde viscometer was used for measurements of solution viscosities (η). The viscometer was calibrated with pure distilled deionized water with a flow time of about 200 s for water at 298.15 K. The viscosity for water at 298.15 K (0.8904 mPa·s) was taken from the literature.36 Flow time measurements were performed by a Schott AVS310 photoelectric time unit with a resolution of 0.01s.37 A CT-1450 temperature controller and a CK-100 ultracryostat were employed to maintain the bath temperature at 298.15 ± 0.01 K. A dielectric constant meter (model Bl-870, Brookhaven Instrument Co., USA) was used for measurements of solution relative permittivity (D), which was calibrated with pure distilled deionized water.38 The temperature was controlled at 298.15 ± 0.01 K with a temperature thermostat HAAKE V26 (Thermo Electron, Germany). The (relative) uncertainties of density, viscosity, and relative permittivity were estimated to be 1.5 × 10−4 g·cm−3, 0.003, and 0.02, respectively. The experimental details have been described elsewhere.31−38 A conductivity meter (model 145A+, Thermo Orion) and a conductivity cell (model 013016D, Thermo Orion) were used for measurements of conductivities on AC power. The temperature was controlled at 298.15 ± 0.01 K with a temperature thermostat HAAKE V26 (Thermo Electron, Germany). The cell was calibrated with potassium chloride solution successively. All data were corrected at 298.15 K with the conductivities of the corresponding solvents. The relative uncertainty of the conductivity was 0.005. The experimental details have been described elsewhere.30

⎧ Λ = γ ⎨Λ 0[1 + C1(βκ ) + C2(βκ )2 + C3(βκ )3 ] ⎩ −

KA =

⎡ ρκ κR ⎤⎫ ⎬ 1 + C4(βκ ) + C5(βκ )2 + ⎢ ⎣ (1 + κR ) 12 ⎦⎥⎭

(1)

1−γ cγ 2f±2

(2)

f±2 = exp[− (κβ)/(1 + κR )]

(3)

β = z 2e 2 /(DkT )

(4)

⎛ 8πNe 2 |z|2 γc ⎞1/2 κ=⎜ ⎟ ⎝ 1000DkT ⎠

(5)

ρ = |z|Fe/(299.79 × 3πη)

(6)

where Λ, Λ0, and γ are the molar conductivity, limiting molar conductivity, and degree of dissociation, respectively. KA, c, and f± are association constant, concentration, and ionic activity coefficient, respectively. D, z, and e are the relative permittivity, ion charge, and electronic charge. R, β, κ, ρ, and C1−C5 are parameters. c (mol·L−1) was converted from the molality (mol· kg−1) using density data. The lower limit of the association integral a is

Table 2. Values of Densities (d0), Viscosities (η0), Relative Permittivities (D), and Experimental Conductivities (σ) for Water + Disaccharide (Sucrose and Maltose) Mixtures at 298.15 K and 101.325 kPaa mS/mol·kg−1

d0/g·cm−3

η0/mPa·s

D

σ/ μs·cm−1

Water + Sucrose 0 0.2000 0.4000 0.6000 0.8000 1.0000 1.2000

0.997046b 1.02212 1.04520 1.06645 1.08602 1.10417 1.12102

0.8904c 1.064 1.271 1.523 1.826 2.196 2.638

78.54d 77.3 76.2 75.1 74.0 72.9 71.8

1.0 2.5 2.6 2.7 2.6 2.7 2.6

mS/mol·kg−1

d0/g·cm−3

η0/mPa·s

D

σ/μs·cm−1

1.02249 1.04520 1.06723 1.08705 1.10541 1.12246

1.072 1.294 1.565 1.894 2.289 2.775

76.2 74.9 73.6 72.3 71.0 69.7

1.0 3.1 3.2 3.5 4.8 4.8 4.8

Water + Maltose 0.2000 0.4000 0.6000 0.8000 1.0000 1.2000

a The (relative) standard uncertainties u (ur) are u(T) = 0.01 K, u(p) = 5 kPa, ur(mS) = 0.0005, u(d0) = 1.5 × 10−4 g·cm−3, ur(η0) = 0.003, ur(σ) = 0.005, and ur(D) = 0.02. bTaken from refs 34 and 35. cTaken from ref 36. dTaken from ref 38.

B

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Table 3. Molar Conductivity (Λ) of [C4mim]Cl in Water and Water + Disaccharide (Sucrose and Maltose) Mixtures at 298.15 K and 101.325 kPaa 104mIL/mol·kg−1

Λ/S·cm2·mol−1

[C4mim]Cl in Water 4.833 6.706 8.243 9.474 [C4mim]Cl in Sucrose + Water mS = 0.2 mol·kg−1 5.711 7.473 9.229 10.85 12.56 14.12 15.76 17.27 18.73 mS = 0.8 mol·kg−1 4.582 7.033 9.322 11.70 13.98 16.10 18.15 20.25 22.31 24.30 26.44 [C4mim]Cl in Maltose + Water mS = 0.2 mol·kg−1 2.756 4.372 6.159 7.903 9.591 11.36 12.90 14.47 16.09 17.67 19.18 mS = 0.8 mol·kg−1 4.612 6.993 9.238 11.60 14.21 16.70 19.29 21.91 24.46

107.08 106.71 106.33 106.05

94.16 93.65 93.24 92.91 92.69 92.43 92.23 92.03 91.87

62.66 62.02 61.66 61.30 61.03 60.86 60.66 60.54 60.36 60.26 60.10

93.10 92.28 91.76 91.42 91.05 90.69 90.55 90.32 90.10 89.91 89.77 61.37 60.71 60.42 60.09 59.88 59.59 59.41 59.29 59.18

104mIL/mol·kg−1

Λ/S·cm2·mol−1

10.75 12.01 13.30 14.43

105.81 105.53 105.37 105.19

mS = 0.4 mol·kg−1 4.035 5.911 7.824 9.532 11.18 12.71 14.23 15.65 17.09 18.49 mS = 1.0 mol·kg−1 5.208 7.753 9.877 12.15 14.48 17.14 19.50 22.16 24.87 27.55 30.03

83.45 82.48 81.98 81.59 81.30 81.12 80.93 80.75 80.61 80.49 54.73 54.19 53.98 53.70 53.48 53.27 53.07 52.87 52.73 52.59 52.45

mS = 0.4 mol·kg−1 3.858 5.810 7.735 9.752 11.70 13.69 15.80 17.83 20.01 22.04

80.99 80.14 79.68 79.28 78.91 78.76 78.51 78.30 78.14 77.97

mS = 1.0 mol·kg−1 3.013 6.018 9.006 12.00 14.88 17.60 20.46 23.26 26.00 28.49 31.09

54.59 53.61 53.15 52.93 52.64 52.46 52.30 52.15 52.06 51.95 51.82

104mIL/mol·kg−1 15.67 16.81

Λ/S·cm2·mol−1 104.96 104.78

mS = 0.6 mol·kg−1 4.520 11.21 14.14 16.18 18.46 20.47 22.48 24.50 26.40

72.18 71.11 70.78 70.62 70.40 70.23 70.08 69.94 69.83

mS = 1.2 mol·kg−1 5.405 8.314 11.34 14.39 17.46 20.35 23.13 25.81 28.44 30.90 33.41

47.94 47.47 47.07 46.87 46.65 46.45 46.28 46.17 46.03 45.92 45.80

mS = 0.6 mol·kg−1 3.512 5.329 7.259 9.090 11.10 13.31 15.49 17.64 19.91 22.06 24.20 mS = 1.2 mol·kg−1 5.431 8.243 11.24 14.13 17.21 19.98 22.70 25.32 27.78 30.05

71.17 70.54 70.02 69.77 69.41 69.14 68.90 68.69 68.50 68.33 68.14 47.44 46.97 46.62 46.40 46.16 46.00 45.89 45.77 45.69 45.60

a

The (relative) standard uncertainties u (ur) are u(T) = 0.01 K, u(p) = 5 kPa, ur(mS) = 0.0005, ur(m) = 0.0005, and ur(Λ) = 0.005. mIL is the molality of ionic liquid (moles of IL per kilogram of solvent (water or water + saccharide mixture). mS is the molality of saccharide (moles of saccharide per kilogram of water). C

DOI: 10.1021/acs.jced.6b00343 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. Molar Conductivity (Λ) of [Cnmim]Cl (n = 6, 8, 10) in Water and in Water + Sucrose Mixtures at 298.15 K and 101.325 kPaa 104mIL/mol·kg−1 [C6mim]Cl in Water 2.349 3.948 5.604 7.176 8.609 10.19 11.74 13.28 14.72 16.20 [C6mim]Cl in Sucrose + Water mS = 0.2 mol·kg−1 4.193 6.122 7.955 9.718 11.48 13.19 14.87 16.56 18.19 19.70 mS = 0.8 mol·kg−1 4.654 6.813 8.937 10.98 12.82 14.52 16.24 17.92 19.55 21.26 [C8mim]Cl in Sucrose + Water mS = 0.2 mol·kg−1 6.561 8.275 9.916 11.58 13.14 14.78 16.38 17.85 19.35

mS = 0.8 mol·kg−1 4.166 6.219 8.224 10.12 12.01 13.85 15.52 17.16 18.82 20.52

Λ/S·cm2·mol−1 103.36 102.88 102.55 102.19 101.91 101.67 101.53 101.30 101.04 100.92

91.16 90.31 89.81 89.39 89.04 88.78 88.60 88.26 88.06 87.93 60.22 60.03 59.81 59.67 59.48 59.36 59.21 59.13 59.01 58.90

89.23 88.79 88.55 88.22 88.04 87.85 87.66 87.51 87.37

58.85 58.13 57.88 57.68 57.50 57.38 57.27 57.19 57.13 57.06

104mIL/mol·kg−1

Λ/S·cm2·mol−1

[C8mim]Cl in Water 2.751 4.181 5.559 6.918 8.221 9.525 10.82 12.08 13.45 14.80 16.31

100.79 100.51 100.18 99.90 99.74 99.54 99.37 99.28 99.14 99.01 98.90

mS = 0.4 mol·kg−1 3.301 4.973 6.636 8.354 9.970 11.40 12.82 14.33 15.79 17.43 mS = 1.0 mol·kg−1 3.895 5.631 7.307 8.890 10.30 11.84 13.21 14.59 15.75 17.03

79.60 78.82 78.44 77.96 77.83 77.70 77.55 77.40 77.27 77.08 52.83 52.70 52.63 52.48 52.45 52.29 52.26 52.13 52.03 51.98

mS = 0.4 mol·kg−1 6.163 7.978 9.798 11.65 13.46 15.16 16.73 18.29 19.87 21.08

77.53 77.19 76.96 76.71 76.56 76.35 76.25 76.16 76.01 75.92

mS = 1.0 mol·kg−1 3.222 6.322 9.233 11.98 14.61 17.40 19.90 22.56 24.95 27.34

51.79 50.84 50.45 50.31 50.17 49.98 49.85 49.78 49.74 49.71 D

104mL/mol·kg−1 [C10mim]Cl in Water 3.586 5.298 7.002 8.780 10.51 12.21 13.91 15.54 17.16 18.78

mS = 0.6 mol·kg−1 3.716 5.471 7.080 8.704 10.27 11.79 13.27 14.59 16.02 17.33 mS = 1.2 mol·kg−1 4.568 8.155 11.56 15.10 18.06 21.42 24.46 27.20 30.32 33.48 36.61 mS = 0.6 mol·kg−1 3.780 6.027 8.172 10.15 12.11 14.04 15.86 17.58 19.25 20.87 22.42 mS = 1.2 mol·kg−1 4.440 7.942 10.96 13.90 16.73 19.28 21.91 24.37 26.70 28.82

Λ/S·cm2·mol−1 97.80 96.35 95.49 94.79 94.26 93.79 93.49 93.09 93.03 92.74

69.39 68.63 68.20 67.89 67.56 67.33 67.20 67.03 66.86 66.75 47.23 46.59 46.23 45.92 45.70 45.53 45.37 45.21 45.08 44.91 44.81

67.72 66.86 66.31 66.09 65.86 65.68 65.59 65.52 65.46 65.44 65.40 45.45 44.51 44.20 44.04 43.97 43.95 43.86 43.80 43.73 43.64 DOI: 10.1021/acs.jced.6b00343 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. continued 104mIL/mol·kg−1

Λ/S·cm2·mol−1

[C10mim]Cl in Sucrose + Water mS = 0.2 mol·kg−1 3.720 6.056 10.36 12.34 14.41 16.32 18.55 20.55 22.71

84.79 82.93 81.43 81.02 80.35 80.19 79.87 79.60 79.36

mS = 0.8 mol·kg−1 3.737 5.430 7.055 8.778 10.18 11.67 13.19 14.48 15.92 17.20

57.82 57.00 56.40 55.90 55.76 55.41 55.17 55.01 54.84 54.74

104mIL/mol·kg−1

Λ/S·cm2·mol−1

mS = 0.4 mol·kg−1 2.429 4.608 6.855 8.928 10.89 12.88 14.77 16.61 18.36 20.05 mS = 1.0 mol·kg−1 4.429 6.477 8.416 10.18 12.00 13.76 15.30 16.97 18.58

75.97 74.04 73.08 72.45 72.00 71.66 71.37 71.09 70.91 70.71 50.08 49.34 48.81 48.56 48.26 48.02 47.90 47.71 47.61

104mL/mol·kg−1 mS = 0.6 mol·kg−1 4.106 5.952 7.753 9.400 11.11 12.67 14.22 15.56 17.10 18.50 mS = 1.2 mol·kg−1 3.686 5.057 6.588 8.213 9.872 11.26 12.89 14.25 15.49 17.14

Λ/S·cm2·mol−1

65.49 64.70 64.00 63.61 63.00 62.97 62.74 62.59 62.41 62.28 44.43 43.86 43.37 42.98 42.67 42.45 42.25 42.15 42.02 41.87

a

The (relative) standard uncertainties u (ur) are u(T) = 0.01 K, u(p) = 5 kPa, ur(mS) = 0.0005, ur(m) = 0.0005, and ur(Λ) = 0.005. mIL is the molality of ionic liquid (moles of IL per kilogram of solvent (water or water + sucrose mixture). mS is the molality of sucrose (moles of sucrose per kilogram of water).

a = a+ + a−

the hydration numbers of sucrose and maltose are 13.9 and 14.5, respectively,53 which also indicates that the interaction of water with maltose is stronger than that with sucrose. Therefore, at a given molality of disaccharide, the viscosity of the maltose + water solution is larger than that of the sucrose + water solution. This is why the ionic mobility of ILs in the maltose + water solution is smaller than that in the sucrose + water solution. Maltose has two glucose units (see Figure 2); however, the behavior of the maltose solution for a certain molality is different from that of the glucose solution for double that molality. When the molality of glucose is double that of maltose, that is, ms (glucose) = 2ms (maltose), we observe that the viscosities of aqueous saccharide solutions are in the sequence maltose > glucose, and the conductivities of ILs in aqueous saccharide solutions are in the sequence glucose < maltose. Because two glucose molecules have two more −OH groups than a maltose molecule, the interaction of water with two glucose molecules is stronger than that with a maltose molecule. Nevertheless, the volume of a maltose molecule is larger than that of a glucose molecule, leading to the bigger moving resistance of maltose. The viscosity of aqueous saccharide solutions is dominated mainly by the volume of a single molecule; therefore, the viscosity for the maltose + water solutions is larger than that for the glucose + water solutions. On the basis of the group additivity principle,54 the phenomena that conductivities are in the sequence glucose < maltose can be interpreted by two facts: (i) the interaction of water with two glucose molecules is stronger than that with a maltose molecule and (ii) the interaction of ILs (cations and anions) with two glucose molecules is stronger than that with a maltose molecule. Figure 3 shows values of Λ0 for different ILs in sucrose + water solutions. It can be seen that values of Λ0 for ILs in

(7)

where a+ and a− are the van der Waals radii of the cation and anion, respectively. The van der Waals radius for Cl− is 181 pm.45 The cationic radii of [C4mim]+, [C6mim]+, [C8mim]+, and [C10mim]+ were calculated by the following equation according to the literature46−48 ⎧ 3 × [(n − 2) × V −CH − + V[C mim]+] ⎫1/3 2 2 ⎬ a+ = ⎨ 4 π ⎩ ⎭ ⎪

⎪

⎪

⎪

(8)

The values for V−CH2− and V[C2mim]+ are 17 and 116 Å3, respectively, taken from refs 46−48. The resulting a+ values for [C4mim]+, [C6mim]+, [C8mim]+, and [C10mim]+ are 330, 353, 373, and 393 pm, respectively. The following equation was used to obtain the upper limit of association49

R = a + ns

(9)

where s is the length of a OH group (s = dOH = 0.28 nm) and n is an integer. By means of nonlinear least-squares iterations, Λ0 and KA were obtained using a two-parameter fit at R = a + 1s, and their values are listed in Table 5. The Λ0 values for ILs in water are consistent with the literature data.30,50−52 As shown in Table 5, values of Λ0 for certain ILs decrease with increasing mS. The trend is similar to that for ILs in monosaccharide solutions.30 However, values of KA do not show regular change. Figure 1 shows that values of Λ0 for [C4mim]Cl at a certain mS are in the sequence maltose < sucrose. Sucrose and maltose are isomers. Maltose has one more exocyclic-CHOH moiety (exo-CHOH) than sucrose (see Figure 2). Because of the hydration characteristic of exo-CHOH, the interaction of water with maltose is stronger than that with sucrose. Furthermore, E

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Table 5. Limiting Molar Conductivities (Λ0), Association Constants (KA), and Walden Products (Λ0η0) for [Cnmim]Cl (n = 4, 6, 8, 10) in Water and in Water + Sucrose Mixtures and [C4mim]Cl in Water and in Water + Maltose/Glucose Mixtures at 298.15 Ka mS/mol·kg−1

Λ0/S·cm2·mol−1

[C4mim]Cl + Sucrose + Water 0 109.32 ± 0.35 109.47b 108.43c 109.47d 0.2000 96.21 ± 0.09 0.4000 84.72 ± 0.21 0.6000 73.43 ± 0.07 0.8000 63.69 ± 0.11 1.0000 55.71 ± 0.07 1.2000 48.79 ± 0.07 [C8mim]Cl + Sucrose + Water 0 102.14 ± 0.039 102.33c 0.2000 91.27 ± 0.08 0.4000 79.10 ± 0.05 0.6000 68.39 ± 0.21 0.8000 59.59 ± 0.15 1.0000 52.12 ± 0.17 1.2000 45.77 ± 0.19 [C4mim]Cl + Maltose + Water 0.2000 94.21 ± 0.12 0.4000 82.11 ± 0.17 0.6000 72.13 ± 0.12 0.8000 62.35 ± 0.13 1.0000 54.93 ± 0.16 1.2000 48.25 ± 0.09

KA/dm3·mol−1

Λ0η0/S·cm2·mol−1·mPa·s

8.56 ± 0.313 7.92b 4.04c

97.34

± ± ± ± ± ±

0.85 2.21 0.58 1.10 0.76 0.77

102.37 107.68 111.83 116.30 122.34 128.71

1.54 ± 2.51c 6.98 ± 4.93 ± 7.01 ± 6.90 ± 5.86 ± 4.89 ±

0.41

90.95

0.68 0.52 2.32 1.93 2.09 2.37

97.11 100.54 104.16 108.81 114.46 120.74

± ± ± ± ± ±

1.21 1.72 1.24 1.45 1.65 1.08

100.99 106.25 112.88 118.09 125.73 133.89

9.67 14.15 6.63 10.27 9.17 8.70

10.80 10.59 11.46 9.30 8.58 8.17

mS/mol·kg−1

Λ0/S·cm2·mol−1

[C6mim]Cl + Sucrose + Water 0 104.68 ± 0.03 104.01c 106.95e 0.2000 93.37 ± 0.29 0.4000 80.59 ± 0.17 0.6000 70.54 ± 0.15 0.8000 61.35 ± 0.02 1.0000 53.70 ± 0.04 1.2000 47.93 ± 0.09 [C10mim]Cl + Sucrose + Water 0 99.41 ± 0.35 100.55c 0.2000 86.13 ± 0.44 0.4000 76.49 ± 0.39 0.6000 66.85 ± 0.25 0.8000 59.00 ± 0.21 1.0000 51.20 ± 0.18 1.2000 45.37 ± 0.18 [C4mim]Cl + Glucose + Water 0.2000 100.44c 0.4000 93.83c 0.6000 87.16c 0.8000 81.29c 1.0000 75.38c 1.2000 70.89c

KA/dm3·mol−1

Λ0η0/S·cm2·mol−1·mPa·s

4.78 ± 0.345 5.36c

93.21

2.4e 18.66 9.94 17.52 4.00 1.67 8.96

± ± ± ± ± ±

2.94 2.02 2.10 0.25 0.65 0.88

99.35 102.43 107.43 112.03 117.93 126.44

25.86 ± 4.53c 27.15 ± 29.74 ± 26.23 ± 31.73 ± 26.91 ± 35.66 ±

3.62

88.51 89.53 91.64 97.22 101.81 107.73 112.44 119.69

4.25 4.90 3.52 3.61 3.28 3.99

1.33c 3.29c 3.05c 2.58c 0.22c 3.25c

a mS is the molality of saccharide (moles of saccharide per kilogram of water). bTaken from ref 50. cTaken from ref 30. dTaken from ref 51. eTaken from ref 52.

slope, indicating that the methylene group has a negative contribution to the values of Λ0 for ILs. The values of the slopes are −1.61 ± 0.17, −1.62 ± 0.23, −1.31 ± 0.18, −1.09 ± 0.11, −0.79 ± 0.14, −0.76 ± 0.09, and −0.62 ± 0.11 S·cm2· mol−1 at mS(sucrose) = 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 mol· kg−1, respectively. These values decrease gradually with increasing mS except at 0.2 mol·kg−1. This phenomenon is similar to that for ILs in monosaccharide + water solutions.30 To explore the difference in electrical conductivity between inorganic salts (ordinary electrolyte, such as NaCl/KCl) and ILs (special electrolyte), we compare the limiting molar conductivity Λ0 for ILs with NaCl/KCl in aqueous solution. Even though the ionic radii of Na+ and K+ (aNa+ = 102 pm, aK+ = 138 pm) are far smaller than those for [Cnmim]+,56 values of Λ0 for NaCl/KCl (122.73/146.84 S·cm2·mol−1)57 are slightly larger than those for ILs. This is because (i) the hydrophobicity of the alkyl chain of [Cnmim]+ is in favor of moving of ILs in aqueous solutions, even though [Cnmim]+ has a relatively large volume, leading to a large moving resistance, and (ii) Na+/K+ has large hydrated radii (218/212 pm).56 The interaction between water in the hydration shell of Na+/K+ and bulk water is stronger than that of [Cnmim]+ and water, leading to the decrease of ionic mobility for Na+/K+. On the basis of the above analysis, we can understand why they have approximate limiting molar conductivities, although the ionic radii of [Cnmim]+ are far larger than those of Na+/K+. Walden products (Λ0η0) were calculated and are shown in Table 5. The Walden rule58 points out that the values of Λ0η0

Figure 1. Variation in the limiting molar conductivity of [C4mim]Cl with the molalities of saccharides (sucrose and maltose): ■, sucrose; ●, maltose.

aqueous sucrose solutions at a certain mS decrease with increasing alkyl chain length of the IL cations. This trend is similar to that for ILs in some organic solvents55 and monosaccharide + water mixed solvents.30 The relationship between the values of Λ0 for ILs and the number of methylene groups in the long alkyl chain of cations (nCH2) is shown in Figure 4. Every straight line has negative F

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Figure 2. Structure of glucose (a), sucrose (b), and maltose (c).

should remain unchanged for certain ILs, regardless of the kinds of solvents, namely, the radius of the ion remains unchanged. As shown in Table 5, however, the values of Λ0η0 increase with increasing mS, indicating that these ions have different radii in the studied solvents. The change in the hydration of the ILs is ascribed to both the competition by the disaccharide and the interactions between ILs and disaccharide.

4. CONCLUSIONS Conductivities for [Cnmim]Cl (n = 4, 6, 8, 10) in sucrose + water solutions and [C4mim]Cl in maltose + water solutions were measured. The limiting molar conductivities Λ0 of [C4mim]Cl at a certain mS are in the sequence maltose < sucrose, which is attributed to the fact that maltose has one more exo-CHOH moiety than sucrose. When ms(glucose) = 2ms(maltose), viscosities of saccharide + water solutions are in the sequence maltose > glucose and conductivities of ILs in saccharide + water solutions are in the sequence glucose < maltose. These are attributed to the fact that the interaction between ILs/water and two glucose molecules is stronger than that with a maltose molecule. Values of Λ0 decrease with increasing alkyl chain length for the IL cations. The Λ0η0 values increase with increasing mS, which provides beneficial evidence that IL ions interact with disaccharides. It is worth noting that the inorganic salts (ordinary electrolyte) and ILs (special electrolyte) have approximate limiting molar conductivities because the limiting molar conductivity is dominated mainly by the structure and hydration of the electrolyte.

Figure 3. Variation in the limiting molar conductivity of [Cnmim]Cl (n = 4, 6, 8, 10) with the molalities of sucrose: ■, [C4mim]Cl; ●, [C6mim]Cl; ▲, [C8mim]Cl;▼, [C10mim]Cl.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00343. Experimental conductivities (σ) for the [C4mim]Cl + water/water + disaccharide (sucrose and maltose) mixtures and [Cnmim]Cl (n = 6, 8, 10) + water/water + sucrose mixtures at 298.15 K and 101.325 kPa, variation of densities/viscosities for water + sucrose mixtures, and densities for water + maltose mixtures with the molalities of sucrose/maltose at 298.15 K (PDF)

Figure 4. Variation of the limiting molar conductivities with the number nCH2 of methylene groups (CH2) in the long alkyl chain of cations of [Cnmim]Cl (n = 4, 6, 8, 10) in different mixed sucrose + water solvents at 298.15 K: ■, pure water; ●, 0.2; ▲, 0.4;▼,0.6; ⧫, 0.8; ◀, 1.0; ▶ 1.2 mol·kg−1 of sucrose.

G

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 373 3329056. Fax: +86 373 3329056. Funding

Financial support from the National Natural Science Foundation of China (Nos. 21173070, 21303044, 21573058, 21273061), the Program for Innovative Research Team in Science and Technology in University of Henan Province (15IRTSTHN 003), the Program for Innovative Research Team in University of Henan Province (17IRTSTHN001), and the Key scientific research project of Henan province higher education of China (No. 15A150058) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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