Activity Coefficients of [Cnmim]Br (

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Activity Coefficients of [Cnmim]Br (n = 3 to 8) Ionic Liquids in Aqueous Fructose Solution at T = 298.15 K Kelei Zhuo,* Hao Ren, Yujing Wei, Yujuan Chen, and Jingjing Ma School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China S Supporting Information *

ABSTRACT: Activity coefficients of the 1-alkyl-3-methylimidazolium bromide [Cnmim]Br (n = 3 to 8) ionic liquids (ILs) in fructose + water mixed solvents at 298.15 K were determined by cell potential measurements. The molalities of [Cnmim]Br ranged from (0.005 to 0.1) mol·kg−1 and those of fructose from (0.2 to 0.8) mol·kg−1. Gibbs free energy interaction parameters were also obtained together with salt constants. The interactions between [Cnmim]Br and fructose are mainly controlled by electrostatic interactions. Gibbs free energy interaction parameters (gES) and salting constants (kS) are negative for the ILs (n = 3 to 6), indicating fructose are salted-in by the ILs in water, whereas fructose are salted-out by the ILs (n = 7 and 8).

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Zafarani-Moattar and Sarmad38 determined activity coefficients of ILs in alcohols and aqueous solutions of potassium salts by isopiestic measurements, respectively. However, to the best of our knowledge, the activity coefficients of ILs in aqueous saccharide solutions have been rarely reported in the literature. Hence, there are no available activity coefficient data for comparison. In our previous work, the interactions of ionic liquids with some monosaccharides in water were studied by volumetry, viscosity, conductivity, and NMR.39 As a continuation, we report here the activity coefficients for the 1-alkyl-3methylimidazolium bromide ([Cnmim]Br, n = 3 to 8) + fructose + water systems at 298.15 K. The interactions between the ILs and fructose in water are discussed.

INTRODUCTION Carbohydrates and their derivatives are important biomolecules and play an important role in maintaining the life of animals and plants.1 Because of their complex three-dimensional structure, sugar molecules are the most important informative macromolecules and exert a great influence on molecular recognition.2 Their biological features are now becoming obvious but are far from being fully understood.3,4 Fructose (Fru) is the sweetest of all naturally occurring and available sweeteners or sugars,5 its sources include sugar cane, sugar beets, fruits, and some vegetables.5−10 Ionic liquids (ILs) are a kind of organic salts that are composed completely of ions and are liquid at or close to room temperature. They are considered to be “green” alternatives to organic solvents in many processes due to their unique physicochemical properties, such as near-zero vapor pressure, high ionic conductivity, good thermal stability, and solvent miscibility.11−13 In addition, their physicochemical properties can be regulated and controlled by simply selecting different combinations of cations and anions or attaching substituents.14 Thus, ILs have found wide applications in extraction,15 inorganic synthesis,16 organic synthesis and catalysis,17,18 nanomaterial synthesis,19 separation,20 electrochemistry,21 and biocatalysis.22−27 As solvents or catalysts, ILs may be released into the environment and accumulate in the ecological environment or organism. Therefore, it is important to study the physicochemical properties of ionic liquids in aqueous solution and their interaction with the biomolecules including saccharides. Recently, saccharides in aqueous ionic liquid solutions have gained a great deal of attention.28 Several authors have reported activity coefficients of some organic solutes (pure substance as the standard state) in ILs by gas−liquid chromatography.29−36 Sardroodi et al.37 and © 2014 American Chemical Society

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EXPERIMENTAL SECTION Reagents and Instruments. The chemicals used in this work are described in Table 1. Poly(vinyl chloride) (PVC) of high molecular weight and other reagents was all dried under vacuum at room temperature to constant weight and then stored over P2O5 in desiccators.40 Tetrahydrofuran (THF) was dried with sodium and redistilled before use. The imidazolium cation selective electrodes [Cnmim-ISE (n = 3 to 8)] were prepared (see the Supporting Information). A bromide ion-selective electrode (Br-ISE, model 302) was procured from Jiangsu Electronical Instrument Co. “Cell potentials were measured by a PH/ISE meter (model 920A+, Orion) with a resolution of 0.1 mV”.41 “The cell was equipped with a water-circulating jacket, and the temperature of all test solutions was controlled at T = 298.15 ± 0.05 K with a Received: May 30, 2013 Accepted: January 22, 2014 Published: January 30, 2014 640

dx.doi.org/10.1021/je4005195 | J. Chem. Eng. Data 2014, 59, 640−648

Journal of Chemical & Engineering Data

Article

Table 1. Purities and Sources of the Samples Used in This Work chemical

source

[Cnmim]Br (n = 3 to 8)a D-fructose NaTPBb DBPc THFd

Lanzhou Greenchem. Co. LICP Sigma Aladdin Aladdin Tianjin Chem. Co.

Cnmim−ISE(n = 3 to 8)|[Cnmim]Br(mr ), H 2O; fructose(mS)|Br−ISE

purity/mass % > > > > >

99.0 99.0 99.0 98.5 99.0

“where m and mr are respectively the molalities (mol·kg−1) of ILs in the working and reference solutions, defined as the number of moles of ILs per kilogram of the fructose + water mixed-solvent, and mS is the molality of fructose in pure water.”45 The prepared imidazolium cation selective membrane electrodes provide a fast, stable, and good Nernstian response over a wide [Cnmim]+ (n = 3 to 8) concentration range with a low detection limit, and a response time lower than 20 s (see Figures S1 and S2). Two solutions were prepared carefully to make sure that the molalities of fructose in the two solutions are exactly identical with each other: a ternary system (ionic liquid + fructose + water) and a binary system (fructose + water).46 “Each set of experiments was performed at a fixed molality of fructose. The molality of fructose ranged from 0.2 to 0.8 mol·kg−1.”43 The concentration of ionic liquids were increased by the addition of the ternary system into the binary system step by step and controlled under their critical aggregation concentration (CAC)47 ranging from about (0.005 to 0.1) mol·kg−1. The resulting values of cell potentials are listed in Tables 2 to 7.

% % % % %

a [Cnmim]Br = 1-alkyl-3-methylimidazolium bromide. bNaTPB = sodium tetraphenylborate. cDBP = dibutyl phthalate. dTHF = tetrahydrofuran.

low-temperature thermostat (model DC-2006, Shanghai Hengping Instrument Factory). Pure distilled deionized water was used with a conductivity of 1.0 × 10−4 to 1.2 × 10−4 S·m−1 at 298.15 K. In order to minimize the concentration gradients in the cell, all the solutions were stirred continuously using a magnetic stirrer.”42,43 Measurements of Cell Potentials. “The following electrochemical cells were set up to measure cell potentials for the ternary systems”44 Cnmim−ISE(n = 3 to 8)|[Cnmim]Br(m), H 2O; fructose(mS)|Br−ISE

(II)

(I)

Table 2. Cell Potentials (E) and Mean Ionic Activity Coefficients (γ) for [C3mim]Br in Water and Fructose + Water Mixtures at T = 298.15 K at P = 1 atm ma/mol·kg−1

γ

E/mV −1

mS = 0.0000 mol·kg 0.004900 −135.7 0.009880 −102.0 0.01504 −82.2 0.02002 −68.7 0.02514 −58.0 0.03011b −49.6 0.03000c 0.04014 −36.3 0.05005 −26.1 0.06011 −17.7 0.07000 −10.8 0.08008 −4.8 0.09012 0.5 0.09895 4.7 mS = 0.6000 mol·kg−1 0.004520 −132.7 0.009189 −98.6 0.01388 −78.9 0.01865 −64.8 0.02313 −54.8 0.02758b −46.6 0.03000c 0.03665 −33.4 0.04554 −23.4 0.05451 −15.2 0.06351 −8.3 0.07251 −2.3 0.08130 2.7 0.09001 7.3

m/mol·kg−1

γ

E/mV −1

0.909 0.868 0.839 0.820 0.804 0.790 0.790 0.768 0.751 0.736 0.723 0.711 0.700 0.692 0.913 0.872 0.847 0.830 0.813 0.800 0.793 0.778 0.761 0.745 0.732 0.720 0.708 0.699

mS = 0.2000 mol·kg 0.004603 −135.8 0.009219 −102.5 0.01393 −82.5 0.01873 −68.5 0.02337 −58.1 0.02804b −49.5 0.03000c 0.03737 −36.2 0.04653 −26.1 0.05582 −17.7 0.06503 −10.8 0.07436 −4.8 0.08371 0.5 0.09272 5.1 mS = 0.8000 mol·kg−1 0.004363 −131.0 0.008707 −97.7 0.01293 −78.8 0.01733 −65.0 0.02144 −54.9 0.02582b −46.4 0.03000c 0.03444 −33.1 0.04314 −22.5 0.05175 −14.3 0.06031 −7.4 0.06886 −1.5 0.07750 3.8 0.08650 8.5

0.917 0.876 0.855 0.836 0.820 0.808 0.801 0.785 0.768 0.753 0.740 0.727 0.716 0.707

m/mol·kg−1

E/mV

mS = 0.4000 mol·kg−1 0.005071 −130.4 0.009710 −99.2 0.01501 −78.4 0.01971 −65.3 0.02467 −54.8 0.02962b −46.1 0.03000c 0.03920 −33.3 0.04879 −23.1 0.05834 −14.9 0.06797 −7.9 0.07766 −1.9 0.08716 3.3 0.09554 7.6

γ 0.914 0.876 0.849 0.835 0.818 0.807 0.803 0.782 0.766 0.752 0.740 0.727 0.717 0.711

0.911 0.872 0.849 0.828 0.815 0.798 0.788 0.775 0.761 0.744 0.730 0.717 0.707 0.694

a m is the molality of ionic liquids in aqueous fructose solution. Standard uncertainties u are u(T) = 0.05 K, u(p) = 5 kPa, u(m) % = 0.2 %, u(E) = 0.1 mV and combined expanded uncertainly Uc(k = 2) is Uc(r) = 0.006. bReference molality. cGiven molality for comparison, the γ values were calculated by eq 3.

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Table 3. Cell Potentials (E) and Mean Ionic Activity Coefficients (γ) for [C4mim]Br in Water and Fructose + Water Mixtures at T = 298.15 K at P = 1 atm m/mol·kg−1

γ

E/mV −1

mS = 0.0000 mol·kg 0.004950 −138.5 0.009950 −104.8 0.01492 −85.3 0.02001 −71.3 0.02498 −60.7 0.02982a −52.4 0.03000b 0.03974 −38.9 0.04964 −28.7 0.05970 −20.2 0.06961 −13.2 0.07957 −7.1 0.08955 −1.9 0.1016 3.7 mS = 0.6000 mol·kg−1 0.004474 −137.9 0.008505 −106.4 0.01281 −86.6 0.01727 −72.3 0.02187 −61.2 0.02631a −52.4 0.03000b 0.03547 −38.5 0.04446 −28.2 0.05378 −19.5 0.06289 −12.5 0.07198 −6.4 0.08108 −1.1 0.08861 2.8 a

m/mol·kg−1

m/mol·kg−1

γ

E/mV −1

0.918 0.879 0.857 0.839 0.826 0.814 0.812 0.794 0.775 0.760 0.747 0.736 0.724 0.711 0.922 0.896 0.875 0.856 0.840 0.828 0.818 0.805 0.785 0.769 0.753 0.741 0.729 0.720

mS = 0.2000 mol·kg 0.005044 −136.9 0.009905 −103.9 0.01474 −84.6 0.01955 −71.2 0.02428 −60.9 0.02924a −52.2 0.03000b 0.03886 −38.9 0.04836 −28.9 0.05799 −20.6 0.06758 −13.7 0.07717 −7.8 0.08689 −2.5 0.09196 0.0 mS = 0.8000 mol·kg−1 0.004279 −137.6 0.008762 −102.6 0.01322 −82.8 0.01755 −69.4 0.02191 −58.9 0.02616a −50.4 0.03000b 0.03477 −37.2 0.04355 −26.9 0.05228 −18.6 0.06105 −11.7 0.06983 −5.7 0.07851 −0.5 0.08735 4.1

E/mV

mS = 0.4000 mol·kg−1 0.004658 −138.5 0.009248 −104.9 0.01396 −85.0 0.01857 −71.4 0.02327 −60.6 0.02792a −52.0 0.03000b 0.03734 −38.4 0.04648 −28.3 0.05579 −19.9 0.06512 −12.9 0.07441 −7.0 0.08387 −1.8 0.09337 3.4

0.921 0.891 0.872 0.854 0.840 0.826 0.825 0.805 0.786 0.770 0.756 0.743 0.731 0.725

γ 0.924 0.895 0.873 0.856 0.842 0.830 0.825 0.809 0.791 0.776 0.762 0.748 0.734 0.730

0.920 0.888 0.865 0.846 0.831 0.822 0.810 0.799 0.780 0.763 0.748 0.735 0.723 0.711

Reference molality. bGiven molality for comparison, the γ values were calculated by eq 3.

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RESULTS AND DISCUSSION Activity Coefficients. In this work, the ILs were considered as electrolytes in dilute solution. According to Nernst− Nikolsky,48−50 the cell potential for 1:1 electrolyte ([Cnmim]Br, n = 3 to 8) can be expressed as E = E 0 + 2S ln(γ±m /m0)

where

(1)

“where E is the cell potential, E0 is the apparent standard cell potential, which depends on the activity of the ions in the internal reference solution and the types of the two ion selective electrodes, m0 is the standard molality (1 mol·kg−1), S is the slope of the electrode response, and γ± (simplified as γ below) is the mean ionic activity coefficient of the electrolyte ([Cnmim]Br, n = 3 − 8).”41,44 Then, the difference in cell potential between cells (I) and (II) can be represented by ΔE = E(ΙΙ) − E(Ι) = 2S ln(γrmr /γm)

(4)

Br = 2β 0 + 2β1y

(5)

x = m1/2 /(1 + bm1/2) + (2/b) ln(1 + bm1/2)

(6)

y = [1 − exp( − am1/2)(1 + am1/2 − 0.5a2m)]/(a2m)

(7)

In the equations above, the symbols have their usual meanings described in the literature.51,52 “The values of a and b were taken as 2.0 mol−1/2·kg1/2 and 1.2 mol−1/2·kg1/2, respectively”.51 The AΦ values were calculated by41,52 A Φ /(mol−1/2·kg1/2) = 1.4006· 106[d /(g·cm−3)]1/2 /(εrT /K)3/2 (8)

where the symbols also have their usual meanings (see SI). The resulting values are listed in Table S1. Inserting eq 3 into eq 2, values of γr, β0, β1, and Cr were obtained by the least-squares for the systems studied. The resulting values are listed in Table S2. Similar with our previous work,44 the results obtained by the fit with Cr are regarded as experimental values. The γr values were used to calculate the γ values for [Cnmim] Br (n = 3 to 8) in aqueous fructose solution studied using eq 2, and the results are also summarized in Tables 2 to 7.

(2)

The activity coefficients of [Cnmim]Br in pure water have not been reported yet. So, the slope S in this work was taken from the theoretical value (RT/F = 25.69 mV at 298.15 K). Activity coefficients of electrolytes ([Cnmim]Br, n = 3 to 8) in water and in fructose + water mixed solvents can be evaluated from the Pitzer equation51 ln γ = fr + Br m + Crm2

fr = −A Φx

(3) 642



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γ

E/mV −1

mS = 0.0000 mol·kg 0.004950 −134.4 0.009890 −100.7 0.01493 −80.7 0.01988 −67.4 0.02485 −56.9 0.02996a −48.3 0.03000b 0.03984 −34.9 0.04948 −25.0 0.05957 −16.8 0.06967 −9.7 0.07961 −3.9 0.08958 1.3 0.09915 5.5 mS = 0.6000 mol·kg−1 0.004572 −129.4 0.008976 −96.0 0.01334 −77.4 0.01758 −64.0 0.02219 −52.8 0.02656a −44.2 0.03000b 0.03580 −29.9 0.04495 −19.6 0.05402 −11.2 0.06324 −4.3 0.07237 1.6 0.08174 6.9 0.09107 11.6 a

m/mol·kg−1

γ

E/mV −1

0.916 0.884 0.864 0.841 0.825 0.809 0.811 0.790 0.771 0.751 0.737 0.722 0.710 0.697 0.922 0.899 0.869 0.856 0.843 0.833 0.824 0.816 0.794 0.778 0.760 0.745 0.731 0.719

mS = 0.2000 mol·kg 0.004854 −134.0 0.009589 −101.0 0.01420 −82.2 0.01876 −69.0 0.02360 −58.2 0.02869a −48.5 0.03000b 0.03812 −35.8 0.04793 −25.3 0.05752 −17.1 0.06708 −10.4 0.07682 −4.4 0.08645 0.3 0.09816 6.1 mS = 0.8000 mol·kg−1 0.004872 −122.3 0.009315 −90.7 0.01345 −73.0 0.01795 −59.4 0.02211 −49.6 0.02658a −41.1 0.03000b 0.03538 −28.0 0.04408 −18.0 0.05283 −9.9 0.06136 −3.3 0.07034 2.6 0.07930 7.8 0.08710 11.8

0.918 0.884 0.860 0.842 0.826 0.820 0.820 0.791 0.771 0.754 0.737 0.723 0.704 0.694

m/mol·kg−1

E/mV

mS = 0.4000 mol·kg−1 0.004172 −139.3 0.008674 −103.5 0.01332 −82.6 0.01817 −67.7 0.02291 −56.8 0.02757a −47.8 0.03000b 0.03665 −34.5 0.04538 −24.8 0.05375 −17.2 0.06236 −10.4 0.07067 −4.8 0.07910 0.3 0.08981 6.5

γ 0.933 0.901 0.881 0.863 0.847 0.838 0.830 0.817 0.797 0.780 0.767 0.755 0.745 0.740

0.915 0.885 0.865 0.844 0.830 0.814 0.805 0.789 0.770 0.752 0.736 0.720 0.707 0.696


the activity coefficients of those ILs decrease with increasing mS. For [Cnmim]+ (n = 3 to 6), the change in γIL is controlled by the second factor when mS is small (< 0.4 mol·kg−1). Conversely, when mS is high (> 0.4 mol·kg−1), the first factor is predominant. Consequently, γIL first increases and then decreases for these ILs. Standard Transfer Gibbs Free Energies. According to the McMillan−Mayer theory, the standard transfer Gibbs free energy ΔtG0m(W) (on the water based molality scale55) of E an electrolyte from water to aqueous fructose solutions can be expressed as54

From Tables 2 to 7, it can be seen that the variation tendencies of activity coefficients with increasing mS for [C3mim]Br, [C4mim]Br, [C5mim]Br, and [C6mim]Br are the same with each other, whereas those for [C7mim]Br and [C8mim]Br are the same with each other. Figure 1, as an example, shows that the activity coefficients of [C3mim]Br, [C4mim]Br, [C5mim]Br, and [C6mim]Br first increase and then decrease with increasing molality of fructose. For [C7mim] Br and [C8mim]Br, their activity coefficients in aqueous fructose solution decrease with increasing mS. These variation tendencies can be interpreted on the basis of two factors: relative permittivity (εr) and volume of fructose molecule.53 (1) The addition of fructose into the solvent (water/water + fructose) increases the electrostatic attraction between [Cnmim]+ and Br− in the solvent (medium) due to the decrease in relative permittivity of the mixed solvent (see Table S1) (according to the Coulomb’s law, electrostatic attraction is inversely proportional to the εr). This will contribute a negative value to γIL. (2) The presence of fructose molecules in the solvent will hinder the interaction of [Cnmim]+ with Br− and thus contributes a positive value to γIL, because fructose molecule is much larger in size than water molecule. This factor appears to make more influence on the interaction between cations and anions for smaller cations ([Cnmim]+, n = 3 to 6). For [Cnmim]+ (n = 7, 8), the contribution from the change in relative permittivity (factor 1) is predominant, and therefore

Δt G E0m(W)(W → W + S) = μ0 (mE , mS) − μ0 (mE) = 2vgESmS + 3vgESSmS2 + ...

(9)

“where gES and gESS are pair and triple Gibbs free energy interaction parameters, μ0(mE, mS) and μ0(mE) are the standard chemical potentials of the electrolyte in (saccharide + water) mixture and pure water, respectively.”44 E and S stand for electrolyte ([Cnmim]Br) and sacccharide (fructose), respectively, and therefore mE is the molality of [Cnmim]Br defined as moles of [Cnmim]Br per kilogram of fructose + water mixed solvent. In this work, a Cnmim-ISE (n = 3 to 8) was used as an indicator electrode, and Br-ISE was used as a reference electrode, so the standard Gibbs free energy of transfer ΔtG0m(M) (on the E mixed-solvent molality scale55) can be written as44 643



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γ

E/mV −1

mS = 0.0000 mol·kg 0.005300 −138.0 0.01034 −105.4 0.01521 −86.8 0.02013 −73.5 0.02524 −62.9 0.03011a −54.6 0.03000b 0.03996 −41.6 0.05013 −31.3 0.06017 −23.1 0.07013 −16.3 0.08003 −10.6 0.09006 −5.5 0.09912 −1.4 mS = 0.6000 mol·kg−1 0.004505 −138.1 0.009081 −103.8 0.01353 −84.5 0.01786 −71.3 0.02244 −60.6 0.02697a −52.0 0.03000b 0.03589 −38.8 0.04486 −28.7 0.05393 −20.5 0.06282 −13.8 0.07196 −8.0 0.08099 −2.9 0.08955 1.3 a

m/mol·kg−1

γ

E/mV −1

0.918 0.888 0.867 0.848 0.831 0.819 0.819 0.795 0.774 0.757 0.741 0.726 0.712 0.701 0.922 0.892 0.871 0.853 0.837 0.823 0.815 0.800 0.779 0.760 0.743 0.726 0.713 0.699

mS = 0.2000 mol·kg 0.005382 −138.1 0.01033 −106.3 0.01576 −86.0 0.02093 −72.5 0.02617 −61.9 0.03144a −53.4 0.03000b 0.04202 −40.0 0.05246 −29.8 0.06267 −21.9 0.07282 −15.2 0.08284 −9.6 0.09165 −5.2 0.09619 −3.1 mS = 0.8000 mol·kg−1 0.004990 −129.6 0.009557 −98.0 0.01410 −79.5 0.01877 −66.0 0.02314 −56.3 0.02740a −48.5 0.03000b 0.03598 −36.1 0.04486 −26.2 0.05346 −18.4 0.06203 −11.9 0.07083 −6.2 0.07965 −1.2 0.09152 4.5

0.917 0.887 0.863 0.845 0.831 0.816 0.820 0.793 0.774 0.756 0.741 0.726 0.715 0.710

m/mol·kg−1

E/mV

mS = 0.4000 mol·kg−1 0.005363 −131.7 0.01024 −100.0 0.01457 −83.2 0.01960 −69.0 0.02436 −58.6 0.02893a −50.4 0.03000b 0.03859 −37.3 0.04805 −27.4 0.05749 −19.5 0.06672 −13.0 0.07598 −7.4 0.08508 −2.5 0.09751 3.2

γ 0.921 0.894 0.872 0.854 0.842 0.831 0.825 0.804 0.783 0.763 0.746 0.731 0.718 0.700

0.910 0.879 0.854 0.834 0.817 0.803 0.795 0.779 0.757 0.739 0.723 0.708 0.694 0.675

Reference molality. bGiven molality for comparison, the γ values were calculated by eq 3. 0 Δt G E0m(M) = nF(ES0 − E W )

(10)

where the symbols have their usual meanings as the same as the literature.44 The values of E0S and E0W were obtained from the least-squares fit and given Table S3, together with values of ΔtG0m(M) . E Figure 2 shows that the values of standard transfer Gibbs free energy ΔtG0m(M) for [Cnmim]Br (n = 3 to 8) become more E positive with increasing molalities of fructose, showing that these ILs’ thermodynamic stabilization is smaller in the mixed solvents than in water. Pair Gibbs Free Energy Interaction Parameters and Salting Constants. For evaluating the free energy interaction parameters, the standard Gibbs free energies of transfer for [Cnmim]Br (n = 3 to 8) ΔtG0m(M) were first converted into E those ΔtG0m(W) through the use of the expression55 E Δt G E0m(W) = Δt G E0m(M) − vRT ln(1 + 0.001mSMS)

(11)

Figure 1. Plots of the activity coefficients γ of [Cnmim]Br (n = 3 to 8) versus the molality mS of fructose at the given molality mIL = 0.03000 mol·kg−1.

where mS and MS are the molality and molecular weight of fructose, respectively. The resulting values are also given in Table S3. Then values of gES were obtained by least-squares fitting these converted data to eq 9 and listed in Table 8. On the basis of the models of structural and electrostatic interactions,44 the value of gES is primarily controlled by the electrostatic interaction. The anion−O (O stands for −OH, CO, −O−) interaction is mainly electrostatic (repulsive) and contributes a positive value to gES, whereas the interaction of

cation with O is mainly electrostatic (attractive) and contributes a negative value to gES. Table 8 and Figure 3 show that the values of gES are negative for the ILs (n = 3 to 6) and consequently controlled by the cation−O interaction. However, the values of gES are positive when n = 7 and 8, so the interaction is mainly controlled by the anion−O. Recently, Shekaari et al.56,57 644



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γ

E/mV −1

mS = 0.0000 mol·kg 0.005140 −125.1 0.01017 −90.9 0.01558 −69.5 0.02033 −56.6 0.02519 −46.1 0.03001a −37.8 0.03000b 0.04031 −24.1 0.05038 −13.9 0.06018 −5.9 0.07050 0.8 0.08020 6.3 0.09006 11.3 0.09954 15.4 mS = 0.6000 mol·kg−1 0.004642 −121.4 0.009012 −88.6 0.01351 −68.8 0.01812 −54.6 0.02263 −44.0 0.02706a −35.7 0.03000b 0.03619 −22.3 0.04523 −12.4 0.05429 −4.4 0.06328 2.0 0.07231 7.5 0.08128 12.3 0.09044 16.5 a

m/mol·kg−1

γ

E/mV −1

0.947 0.930 0.921 0.907 0.898 0.886 0.886 0.861 0.840 0.822 0.799 0.782 0.768 0.752 0.933 0.910 0.893 0.877 0.864 0.849 0.841 0.824 0.799 0.778 0.756 0.737 0.719 0.702

mS = 0.2000 mol·kg 0.004614 −127.8 0.009589 −91.3 0.01450 −70.7 0.01924 −56.8 0.02398 −46.3 0.02891a −37.4 0.03000b 0.03830 −24.4 0.04799 −14.2 0.05752 −6.2 0.06705 0.4 0.07662 5.9 0.08633 10.8 0.09406 14.2 mS = 0.8000 mol·kg−1 0.003932 −125.5 0.008201 −88.9 0.01199 −70.4 0.01608 −56.2 0.02021 −45.3 0.02431a −36.6 0.03000b 0.03275 −23.1 0.04094 −13.1 0.04948 −4.9 0.05777 1.6 0.06596 7.1 0.07426 11.9 0.08087 15.2

0.944 0.924 0.912 0.901 0.887 0.875 0.876 0.851 0.828 0.807 0.787 0.767 0.749 0.734

m/mol·kg−1

E/mV

mS = 0.4000 mol·kg−1 0.004443 −126.9 0.009044 −91.9 0.01399 −70.3 0.01868 −56.2 0.02340 −45.4 0.02809a −36.7 0.03000b 0.03740 −23.6 0.04661 −13.6 0.05612 −5.4 0.06533 1.0 0.07465 6.7 0.08399 11.3 0.09355 16.0

γ 0.941 0.913 0.899 0.886 0.873 0.861 0.854 0.835 0.813 0.793 0.771 0.754 0.733 0.721

0.937 0.916 0.898 0.883 0.868 0.855 0.835 0.825 0.802 0.778 0.757 0.738 0.719 0.704


Figure 2. Variation of ΔtG0m(M) for [Cnmim]Br (n = 3 to 8) as E functions mS of the molalities of fructose.

Figure 3. Plots of 2vgES versus the number nCH2 of methylene groups in the alkyl side chain of the cation for [Cnmim]Br (n = 3 to 8) in fructose + water mixtures.

reported the values of association constant (Ka) and limiting molar conductivity (Λ0) of [Hmim]Br in aqueous D-fructose solutions and show that Ka and Λ0 decrease with increasing the concentration of D-fructose. It also indicates that there was reciprocity interaction between [Cnmim]Br and hydrophilic −OH and −O− groups of D-fructose. On the other hand, the values of gES first decrease and then increase with increasing the

length of carbon chains; the minimum value appears at n = 5. It indicates that the electrostatic attractions are the strongest between [C5mim]+ and fructose among these studied ILs. Based on the definition of Friedman,58 the salting constant (kS) can be calculated by

RTk S = 2vgES 645

(12) dx.doi.org/10.1021/je4005195 | J. Chem. Eng. Data 2014, 59, 640−648


Article


γ

E/mV −1

mS = 0.0000 mol·kg 0.005160 −127.8 0.01023 −93.9 0.01496 −75.2 0.02004 −61.1 0.02493 −50.6 0.02998a −42.0 0.03000b 0.03979 −28.9 0.04981 −18.9 0.05992 −10.8 0.07003 −4.1 0.08005 1.3 0.08991 5.9 0.1015 10.5 mS = 0.6000 mol·kg−1 0.004450 −125.4 0.009121 −90.4 0.01359 −71.3 0.01796 −58.1 0.02243 −47.7 0.02691a −39.3 0.03000b 0.03580 −26.6 0.04490 −16.7 0.05426 −8.7 0.06354 −2.2 0.07241 3.0 0.08148 7.5 0.08893 10.8 a

m/mol·kg−1

γ

E/mV −1

0.933 0.910 0.896 0.880 0.868 0.853 0.853 0.829 0.805 0.783 0.763 0.742 0.722 0.700 0.923 0.890 0.866 0.847 0.831 0.815 0.804 0.785 0.759 0.733 0.711 0.690 0.670 0.654

mS = 0.2000 mol·kg 0.004775 −128.2 0.009431 −94.7 0.01423 −74.8 0.01927 −60.2 0.02422 −49.2 0.02900a −40.8 0.03000b 0.03859 −27.7 0.04833 −17.7 0.05791 −9.8 0.06749 −3.4 0.07718 2.0 0.08685 6.7 0.09115 8.6 mS = 0.8000 mol·kg−1 0.004077 −125.2 0.008181 −91.0 0.01235 −71.1 0.01665 −57.0 0.02082 −46.8 0.02502a −38.4 0.03000b 0.03349 −25.4 0.04162 −15.8 0.04975 −8.1 0.05822 −1.4 0.06665 3.9 0.07494 8.4 0.08639 13.6

0.930 0.904 0.882 0.865 0.853 0.839 0.835 0.813 0.789 0.768 0.746 0.725 0.706 0.698

m/mol·kg−1

E/mV

mS = 0.4000 mol·kg−1 0.004743 −126.0 0.009499 −91.8 0.01422 −72.2 0.01874 −58.9 0.02342 −48.4 0.02804a −39.9 0.03000b 0.03754 −26.7 0.04700 −16.7 0.05639 −8.9 0.06565 −2.5 0.07497 2.8 0.08439 7.4 0.09308 11.1

γ 0.930 0.903 0.883 0.868 0.852 0.840 0.833 0.811 0.787 0.764 0.743 0.721 0.701 0.683

0.919 0.891 0.870 0.849 0.828 0.811 0.795 0.781 0.757 0.736 0.716 0.694 0.674 0.646


attractive and fructose is salted-in by the ILs in water, whereas fructose are salted-out by [C7mim]Br and [C8mim]Br in water.

Table 8. Gibbs Free Energy Interaction Parameters gES for Fructose−IL Pairs and Salting Constants kS in Water at T = 298.15 K fructose−IL

2vgES/J·kg·mol−2

kS/kg·mol−1

fructose−[C3mim]Br fructose−[C4mim]Br fructose−[C5mim]Br fructose−[C6mim]Br fructose−[C7mim]Br fructose−[C8mim]Br

−227.4 −631.0 −683.2 −433.3 349.2 482.8

−0.09 −0.25 −0.28 −0.17 0.14 0.19

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ASSOCIATED CONTENT

S Supporting Information *

Electrodes preparation, performance of electrode (Figures S1 and S2), values of Debye−Hückel constant AΦ at different molalities of mixed solvents (Table S1), values of all parameters of the Pitzer equation for [Cnmim]Br (n = 3 to 8) (Table S2), and values of standard cell potentials E0 and standard Gibbs free energy of transfer (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

The calculated values are also summarized in Table 8. The kS values are negative when n = 3 to 6, showing that fructose is salted-in by [Cnmim]Br in water, whereas the kS values are positive when n = 7 and 8, indicating that fructose is salted-out by these two ILs in water.

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AUTHOR INFORMATION

Corresponding Author

■

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

CONCLUSIONS The self-prepared imidazolium-based ionic liquid cation selective electrodes and a bromide ion-selective electrode were applied to determine accurately the activity coefficients of [Cnmim]Br (n = 3 to 8) in the fructose + water system at 298.15 K as well as Gibbs free energy interaction parameter gES between ILs and fructose in water. The values of gES for the ILs (n = 3 to 6) are negative and show that the interactions between these four ILs and fructose are thermodynamically

Funding

Financial support from the National Natural Science Foundation of China (nos. 20973055, 21173070) and the Plan for Scientific Innovation Talent of Henan Province (no. 124200510014) is gratefully acknowledged. Notes

The authors declare no competing financial interest. 646



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Activity Coefficients of [Cnmim]Br (

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