Synthesis and Physicochemical Properties of Amino Acid Ionic

Article pubs.acs.org/jced

Synthesis and Physicochemical Properties of Amino Acid Ionic Liquid 1‑Butyl-3-methylimidazolium Aspartate and Binary Mixture with Methanol Ying Wei,†,‡ Yi Jin,§ Zhi-Jing Wu,‡ Yang Yang,‡ Qing-Guo Zhang,*,‡ and Zhen-Hui Kang*,† †

Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, China ‡ Department of Chemistry, Bohai University, Jinzhou 121013, China § China Criminal Police University, Shenyang 110035, China S Supporting Information *

ABSTRACT: An amino acid based ionic liquid [Bmim][Asp], 1-butyl-3methylimidazolium aspartate, and a binary mixture system composed of [Bmim][Asp] and methanol were synthesized and prepared, respectively. The density, surface tension, and electrical conductivity of the pure IL and the binary system were measured or estimated by the extrapolation method at different temperatures. The physicochemical properties of the IL or the binary system like the molecular volume, standard molar entropy, lattice energy, parachor, speed of sound, molar enthalpy of vaporization, interstice volume, thermal expansion coefficient, excess volume, and so forth were estimated by empirical and semiempirical equations. The glass transition temperature was determined by the differential scanning calorimetry (DSC). The status of intermolecular interaction based on the excess volume change of the IL mixture was discussed at 298.15 K. By Vogel−Fulcher−Tamman (VFT) and Castell− Amis (CA) equations, the relationship between the electrical conductivity and temperature or concentration of the IL and binary system were described, respectively. The results indicate that the VFT and CA equations are fit for the IL and the mixture, respectively, and the change of methanol concentration can obviously influence the electrical conductivity of the binary IL mixture.

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INTRODUCTION As a family of “greener solvents”, amino acid ionic liquids (AAILs) derived from natural amino acids1−3 have attracted considerable attention. In terms of the environment-friendly background, now AAILs are expected to be a kind of “taskspecific” liquid that can exhibit desirable physicochemical properties or certain specific functions.4−10 Several research groups are focusing on the synthesis, properties, and application of AAILs or their derivatives.2−4,7 Because of the strong hydrogen bonding ability of the AAILs, these “greener solvents” have shown interesting characteristics in many fields, such as the excellent ability to dissolve biomaterials like DNA, cellulose, or carbohydrates, a specific viscous property, unique chirality, the particular hybrid capability as a surfactant, or accessory ingredients in functional material synthesis.2−5,11,12 They also can be applied in the research of peptide syntheses of intermediates, chiral solvents, separation processes, and biodegradable ionic liquids. Furthermore, the mixtures of AAIL and certain solvents also may have great potential to be a class of low-cost working fluids with adjustable properties. However, the physicochemical properties of AAILs or mixed binary systems of AAILs + solvents still have not been studied systematically, which may limit the use of AAILs in the © XXXX American Chemical Society

industrial community. Through our continuous interest in the AAILs,13−16 and since aspartate is one of the 20 basic proteinogenic amino acids, in this work, a new AAIL 1-butyl3-methylimidazolium aspartate ([Bmim][Asp]) was synthesized by a neutralization method and characterized by 1H NMR and differential scanning calorimetry (DSC). Also a binary mixture system composed of [Bmim][Asp] and one of the most common solvents, methanol, was prepared. The density, surface tension, and electrical conductivity of pure ionic liquid and the binary mixture were determined at different temperatures from (278.15 to 343.15 ± 0.05) K, respectively. By specific empirical or semiempirical methods,17−21 the physicochemical properties of the [Bmim][Asp] like molecular volume Vm, standard molar entropy S0, the lattice energy UPOT, parachor P, the speed of sound μ, thermal expansion coefficients α, interstice parameters, molar enthalpy of vaporization, and so on were estimated or predicted. For the binary system of IL+ methanol, the excess volumes, VE, at 298.15 K were calculated and the intermolecular interaction Received: August 25, 2012 Accepted: December 20, 2012

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dx.doi.org/10.1021/je300932n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Water Content. The water content of the pure IL measured by Karl Fischer moisture titrator (ZSD-2 type) was 350 ppm before the determination of properties and approximately 800 ppm after the determination. The uncertainty of the water content measurement is ± 10 ppm. Measurement of Density and Surface Tension. The densities of the pure [Bmim][Asp] were measured by a Westphal balance from (303.15 to 348.15 ± 0.05) K with a solid thermostat. Each value of the density is the average of three measurements. The surface tension of [Bmim][Asp] was measured using the tensiometer (DP-AW type produced by Sang Li Electronic Co.) by the forced bubble method from (303.15 to 348.15 ± 0.05) K. Each value of the surface tension is the average of five measurements. The values of the density and surface tension of the IL are listed in Table 1. The Westphal balance and the tensiometer were calibrated by degassed water, and the values are in good agreement with the literature.29 Furthermore, the densities and surface tension of the binary system [Bmim][Asp]/methanol samples (molar ratio of methanol from 0 to 1) were also measured by the same method at temperatures of (298.15, 303.15, 308.15, and 318.15 ± 0.05) K. The uncertainties of the measurement are ± 0.001 g·cm−3 and near ± 1.5 mJ·m−2, respectively. Measurement of Electrical Conductivity. The SG3 conductivity meter (Mettler−Toledo, DC 6V) is operated with the Inlab 738 conductivity electrode (nominal 0.57 cm−1 ± 20 % cell constant) under dry argon from (303.15 to 348.15 ± 0.05) K, and the cell was calibrated with the standard aqueous KCl solution before use. When it starts measuring, the sensor input signal must not deviate by more than 0.4 % from the measured average conductivity of the probe in 6 s. The electrical conductivities of the binary system [Bmim][Asp]− methanol over the entire concentration range were measured at the temperatures of (298.15, 303.15, 308.15, and 318.15 ± 0.05) K by the same method. Each value of the electrical conductivity is the average of three determinations. The uncertainty of measurement is ± 5 %, and the values are listed in Table 1.

between the IL and the solvent molecules was also discussed according to the change of VE. The relationships of the temperature and the electrical conductivity were described by the VFT equation.22−25 The changes of the properties from the pure IL to the binary system were compared and discussed, particularly the conductivity. Furthermore, the empirical Castell−Amis equation26−28 was applied to express the concentration dependence of the electrical conductivity of the binary IL solution.

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EXPERIMENTAL SECTION Chemicals and Materials. Deionized water was distilled in a quartz still before use. Aspartic acid (AR grade Sinopharm Chemical Reagent Co., Ltd.) was recrystallized and dried under reduced pressure. 1-Methylimidazole (AR, J&K Chemica Co.) was distilled under reduced pressure prior to being used. Butyl bromide, methanol, ethyl acetate, and acetonitrile were distilled and then stored over molecular sieves of type 4 Å, respectively. Preparation of Amino Acid Ionic Liquid [Bmim][Asp] and the Binary System[Bmim][Asp]−Methanol. The intermediate [Bmim][Br] and [Bmim][OH] was synthesized according to our previously reported method.14−16 A neutralization method was employed to prepare the IL [Bmim][Asp]. The viscous IL product [Bmim][Asp] was characterized by 1H NMR and DSC (see the Table A and Figure B of the Supporting Information). The structure of the [Bmim] cation and [Asp] anion is shown in Figure 1. The

Figure 1. Chemical structures of the cation and anion of IL [Bmim][Asp].

residual Br− was below the detection limit of approximately 50 ppm under a test of AgNO3/HNO3 solution. Then the IL [Bmim][Asp] was further dried for 36 h at 353.15 K via vacuum before use. The final purity of the IL was estimated to be better than 99 % in mass. The reactions route followed Scheme 1.

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RESULTS AND DISCUSSION As shown in Table 1, the data of the density, surface tension, and electrical conductivity of the pure IL [Bmim][Asp] and the binary system [Bmim][Asp]−methanol were determined, respectively. Volumetric and Surface Properties of the IL [Bmim][Asp]. The temperature dependence on the density and surface tension of the pure IL [Bmim][Asp] at the temperature range from (303.15 to 348.15 ± 0.05) K can be fitted by the following equation:

Scheme 1. Synthesis Route of IL [Bmim][Asp], Where 1 Is the Intermediate [Bmim]Br, 2 Is the [Bmim][OH], and 3 Is the IL [Bmim][Asp]

Y = A + BT + CT 2

(1)

where Y is the density or surface tension; A, B, and C are adjustable parameters. The fitting curve can be obtained (see Figures 2 and 3). The best fitting values and the correlation coefficient are listed in Table 2. Furthermore, according to the following empirical equation:

A series of [Bmim][Asp]−CH3OH binary mixtures were prepared by weighing. The mole fractions of IL in the mixture are from 0.0099 to 0.8999. DSC and TG. The glass transition temperature Tg was determined by DSC (Mettler−Toledo) at a heating rate of 10 K·min−1 from (165 to 370) K (see the Figure B of the Supporting Information). Then, Tg = 236.5 ± 0.1 K was obtained. The uncertainty of the heat flow is ± 0.1 mW.

ln ρ /g·cm−3 = b − αT /K

(2)

where b is an empirical constant and the negative value of the slope, α = −(∂ ln ρ/∂T)p, is the thermal expansion coefficient of the sample, a straight line can be obtained (see Figuer 4), B



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Table 1. Density, Surface Tension, Electrical Conductivity, and Molar Conductivity of IL [Bmim][Asp] and Binary System [Bmim][Asp]−Methanol, Respectively [Bmim][Asp] ρ/g·cm−3 1.1826 1.1803 1.1757 1.1719 1.1678 1.1608 1.1559 1.1519 1.1485 1.1454 1.1408

γ/mJ·m−2

σ/mS·cm−1

Λ/S·cm2·mol−1

54.8 0.75 0.17 53.8 0.82 0.19 53.2 0.98 0.24 52.2 1.23 0.30 51.1 1.60 0.37 50.2 1.97 0.46 49.7 2.34 0.55 48.7 2.78 0.66 47.5 3.26 0.78 47 3.68 0.90 46.2 4.51 1.07 Binary System [Bmim][Asp]−Methanol ρ/g·cm

T/K 298.15a 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

Figure 2. Fitting of density against the temperature following the Y = A + BT + CT2.

−3

xIL

298.15 K

303.15 K

308.15 K

318.15 K

0.1012 0.2043 0.3031 0.4051 0.4985 0.6020 0.7001 0.8001 0.8999

0.9706 1.0333 1.0754 1.1450 1.1503 1.1585 1.1657 1.1731 1.1739

0.9657 1.0291 1.0712 1.1400 1.1479 1.1530 1.1589 1.1657 1.1700

0.9624 1.0259 1.0703 1.1373 1.1440 1.1493 1.1562 1.1617 1.1634

0.9602 1.0221 1.0653 1.1300 1.1379 1.1450 1.1487 1.1519 1.1568

γ/mJ·m−2 xIL

298.15 K

0.1012 0.2043 0.3031 0.4051 0.4985 0.6020 0.7001 0.8001 0.8999

27.17 34.58 37.3 40.5 42.1 43.6 44.8 51.1 52.1

xIL

298.15 K

303.15 K

308.15 K

318.15 K

0.0099 0.0250 0.0499 0.0786 0.1012 0.2043 0.3031 0.4051 0.4985 0.6020 0.7001 0.8001 0.8999

8.24 12.28 13.58 12.72 11.31 6.36 3.14 2.18 1.61 1.25 1.01 0.93 0.87

8.87 12.47 13.87 13.17 12.19 7.54 3.77 2.41 2.31 1.85 1.48 1.08 1.01

9.98 13.56 14.42 13.94 12.83 8.31 4.96 3.36 2.63 2.18 2.02 1.55 1.24

10.29 13.85 15.72 14.84 14.16 9.78 6.35 4.29 3.78 2.6 2.33 2.12 2.05

303.15 K

308.15 K

26.8 26.55 34.02 32.95 36.1 35.4 39.4 38.5 41.7 40.2 43 41.5 44.4 43.6 50.3 49.7 50.9 50.6 σ/mS·cm−1

318.15 K 25.46 31.33 33.42 36.9 38.8 39.7 42.7 48.7 49.6

Figure 3. Fitting of surface against the temperature following the Y = A + BT + CT2.

Table 2. Fitted Values of Temperature Dependence on Density and Surface Tension by Equation Y = A + BT + CT2, and Estimated Physicochemical Property Parameters of [Bmim][Asp] at 298.15 Ka 1

R2

A

B

C

1.8117

−0.0031

3.41·10−6 −4

2

169.5747

−0.5579

Vm/nm3 S0/J·K−1·mol−1 106 Sa/J·K−1·m−2 Ea/J ·m2 UPOT/kJ·mol−1 107k/K−1 Tc/K T b/K μ/m·s−1

0.5323 693 173.3 106.4 427 5.6 664 398 1755

P ΔlgHm0/kJ·mol−1 1024 v/cm3 ∑v/cm3 V/cm−3·mol−1 ∑v/V 104 α(exp)/K−1 104 α (cal)/K−1

5.88·10

0.994 0.995

624 195 20.58 24.80 230.9 0.11 7.7 5.4

a

Estimated values by the extrapolation method. The uncertainty of the density is ± 0.001g·cm−3; the uncertainty of the surface tension is about ± 1.5 mJ·m−2; the uncertainty of the electrical conductivity is ± 5 %. The experimental pressure is 1 atm.

a 1: the fitted parameters for density; 2: the fitted parameters for surface tension. Tc: the fitting value of the Eötvös equation.

and the fitted equation is ln ρ/g·cm−3 = 0.3991 − 7.7·10−4 T/K, R2 = 0.997. Then the experimental thermal expansion

coefficient of [Bmim][Asp] is 7.7·10−4 K−1 and listed in Table 2. C



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at 298.15 K, 54.8 mJ·m−2, can be obtained from the linear extrapolation. From the slope of the linear fitting, (∂γ/∂T)p, the surface excess entropy at T = 298.15 K, Sa = −(∂γ/∂T)p = 173.3·10−6 J·K−1·m−2, is obtained; meanwhile, the surface excess energy at T = 298.15 K, Ea = γ − T(∂γ/∂T)p =106.4 J·m2 and are listed in Table 2, respectively. From the experimental data, the surface tension and surface energy are larger than those of other AAILs which may result from the stronger intermolecular interactions of anion [Asp] and cation [Bmim] resulted from the carboxyl group. The surface tension of binary system [Bmim][Asp]−methanol decreases with the increase of the methanol ratio (see Figure D of the Supporting Information), implying that the addition of the methanol can diminish the interaction between the cations and the anions. For the surface tension, γ, there is another relationship with temperature in terms of the Eötvös equation: γV 2/3 = k(Tc − T )

Figure 4. Plot of ln ρ versus T for the IL [Bmim][Asp].

where V is the molar volume of the liquid, Tc is a critical temperature value fitted by the equation, and k is an empirical constant. A straight line was obtained (see Figure 5). From the

From the values of density, the molecular volume, Vm, of pure [Bmim][Asp] can be calculated by following equation:

Vm = M /(N ·ρ)

(6)

(3)

where M is average molar mass of [Bmim][Asp] = 271.1 g·mol−1, N is Avogadro number, the value of ρ at 298.15 K, 1.1826 g.cm−3, was obtained by linear extrapolation from the experimental densities. The calculated value of Vm for [Bmim][Asp] is 0.5323 nm3. According to the previous work,15,16 the volume of the AAILs EMIGly and [Bmim][Glu] are 0.2653 nm3 and 0.3972 nm3, respectively, less than that of [Bmim][Asp]. It should be attributed to the bigger size of the cation [Bmim] or the anion [Asp], and the more irregularly or puffed packing involving the cation and anion. Further, the volume of the anion [Asp], 0.2534 nm3, can be obtained from the volume 0.2751 nm3 of the cation.30 In terms of Glasser’s theory,17 standard molar entropy, S0/ J·K−1·mol−1, and the lattice energy, UPOT/kJ·mol−1 of the [Bmim][Asp] at 298.15 K can be estimated by following equations:

Figure 5. The Eötvös equation plot of IL [Bmim][Asp].

S 0/J·K−1·mol−1 = 1246.5(Vm/nm 3 per formula) + 29.5 (4)

linear fit, the empirical constant k, critical temperature Tc, and the correlation coefficient R = 0.997 can be obtained and listed in Table 2. Commonly, a fit with a correlation coefficient larger than 0.995 is considered a good fit. Rebole et al. have reported that the normal boiling point, Tb, is approximately 0.6 Tc for ILs.19 Herein, the normal boiling point, Tb, can be calculated, and the value is Tb = 398 K. The thermostability of the AAIL is lower than that of some hydrophobic ILs like [Cnpy][NTf2].22 For the majority of organic liquids, k ≈ 2.13·10−7 J·K−1,31 it indicates that the polarity of the AAIL [Bmim][Asp] is larger than that of organic liquids in terms of the value of k (see Table 2). Excess Volume of the Binary System [Bmim][Asp]− Methanol. For the binary system, the density and surface tension are determined by the same methods as the pure IL and listed in Table 1. Then we can obtain the excess volumes of the system with different molar ratios according to the following equation:

and UPOT(298.15 K)/kJ·mol−1 = 1981.2(ρ /M )1/3 + 103.8 (5)

The calculated standard molar entropy of [Bmim][Asp] by eqs 4, S0(298.2 K)/J·K−1·mol−1 = 693, and the calculated lattice energy of [Bmim][Asp] by eqs 5, Upot(298.15 K)/kJ·mol−1 = 427, are listed in Table 2. Compared to fused salts, the lattice energy of [Bmim][Asp] is much lower, like, UPOT = 613 kJ·mol−1 for fused CsI at 298.15 K29 which is the smallest crystal energy of alkali halides. The low crystal energy is one of the underlying reasons of forming amino acid ionic liquid at room temperature according to the opinion of Krossing and Slattery.8 Generally, surface tension, γ, of many liquids almost linearly decreases, while temperature increases. The surface tension of pure [Bmim][Asp] is plotted versus T and a good straight line (see Figure C of the Supporting Information). The correlation coefficient is larger than 0.99, and then the surface tension of IL

V E = (x1M1 + x 2M 2)/ρ12 − x1M1/ρ1 − x 2M 2 /ρ2 D

(7)



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the entire concentration range indicate that the relative small methanol molecules fit into the free volume which is derived from the big size and irregularity packing of the cation [Bmim] and anion [Asp] upon the dilution and the negative extremum at about xIL = 0.4. It is also interesting to find that the VE increases when the xmethanol ≥ 0.6, which implies that the intermolecular interactions among the methanol molecules are becoming stronger with the increasing methanol concentration. Parachors, Speed of Sound, and Molar Vaporization Enthalpy of the IL. The parachor, P, of the pure IL was estimated from the following equation:18

where 1 is the ionic liquid and 2 is the solvent methanol. The excess volumes VE of the binary system at 298.15 K are graphically presented in Figure 6 and listed in Table 3. The

P(298.15 K) = (M ·γ 1/4)/ρ

(8)

where M is molar mass, ρ is density, and γ is surface tension. The value of molar enthalpy of vaporization was estimated in terms of Kabo’s empirical equation:20 Δl g Hm 0(298 K) = A(γV 2/3N1/3) + B

where V is molar volume of IL, N is Avogadro’s constant, γ is the surface tension at 298.15 K of [Bmim][Asp], and A and B are empirical parameters; their values are A = 0.01121 and B = 2.4 kJ·mol−1.20 The molar enthalpy of vaporization for IL [Bmim][Asp] was calculated from eq 9, and the result is 195 kJ·mol−1. The values of parachor and molar enthalpy of vaporization are listed in Table 2. The speed of sound of the IL can be calculated by the Auerbach relation35 from the density and surface tension by the following equation:

Figure 6. Plot of excess volumes, VE, of the binary system [Bmim][Asp]−methanol in the entire concentration range at temperatures of (298.15, 303.15, 308.15, and 318.15) K; ▼, 298.15 K; ▲, 303.15 K; ●, 308.15 K; ■, 318.15 K.

Table 3. Excess Volume, VE, of the Binary System [Bmim][Asp]−Methanol in the Entire Concentration Range at 298.15 Ka xIL

ρ/(g·cm−3)

VE/cm3·mol−1

0 0.1012 0.2043 0.3031 0.4051 0.4985 0.602 0.7001 0.8001 0.8999 1

0.7717 0.9706 1.0333 1.0754 1.1450 1.1503 1.1585 1.1657 1.1731 1.1739 1.1742

0 −2.7460 −1.9319 −1.7420 −5.6675 −4.4617 −3.6336 −3.02990 −2.6667 −1.2273 0

(9)

u/m·s−1 = (γ /(0.00063ρ))2/3

(10)

where γ is surface tension; ρ is density; and u is speed of sound. The estimated value at 298.15 K is listed in Table 2. Interstice Model Theory. From previous works,15,16,21,30 the interstice model is good at explaining the thermoexpansion and molar volume change of the pure ILs. Derived from the classical statistical mechanics, the interstice volume, v, was calculated by the following equation: v = 0.6791{k bT /γ }3/2

(11)

where kb is Boltzmann constant, T the thermodynamic temperature, and γ the surface tension of ionic liquid. According to eq 11, the average volume of the interstices of ionic liquid [Bmim][Asp] can be obtained, v, 20.58·10−24 cm3, from the surface tension of [Bmim][Asp] at 298.15 K, γ = 54.8 mJ·m−2, obtained by extrapolation. Then, the molar volume of the interstice is ∑v = 2Nv = 24.80 cm3. The volume fraction of interstice, ∑v/V, is about 0.11 for [Bmim][Asp], and close to the (10 to 15) % volume expansion exhibiting in process from the solid to liquid state of most materials. The molar volume of ionic liquid, V, is composed of inherent volume, Vi, and total volume of the all interstices, ∑v = 2Nv, that is;

a The methanol density at 298.15 K (t = 25 °C) was calculated following the equation ρt20 = ρt + 0.0008(t − 20), and ρt20(methanol) = 0.7917 g·cm−3.

excess volumes of the mixture are all negative, and the plot exhibits a more complicated change of the mixed volume of this binary system based on AAIL [Bmim][Asp] than those of other classic ILs.32−34 According to the literature,32−34 the VE of the binary system is mainly concerned with the weak chemical interactions, the physical and structural characteristics of the system components. If the chemical or nonchemical interaction of the same components is the dominant way of the interaction, the excess volume of the mixture should be positive. If the molecules of the components have a more effective packing in the mixture than that in the pure component, that is, the attracting interaction between the molecules of two components is the dominant way, the VE of the binary system should be negative. From Figure 6, the plot indicates that with the increasing molar ratio of methanol, the intermolecular interaction between the cations and anions of the IL become weaker, and the negative values of the molar excess volume in

V = Vi + 2Nv

(12)

If the expansion of IL volume only results from the expansion of the interstices when temperature increases, then calculation expression of α was derived from the interstice model: α = (1/V )(∂V /∂T )p = 3Nv /VT

(13)

The α of [Bmim][Asp] calculated by eq 8, α(cal) = 5.4·10−4 K−1, is listed in Table 2 and compared to the experimental values α(exp) = 7.7·10−4 K−1; both values are in good E



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at high temperature, the Casteel−Amis (CA) equation is employed to express the relationship between the conductivity of the binary system and the molar ratio of the IL at different proper temperatures like (298.15, 303.15, 308.15, and 318.15 ± 0.05) K.

agreement within the error extent. That means the interstice model is reasonable for the AAIL [Bmim][Asp]. The parameters about the interstice model are all listed in Table 2. The bigger values than those of [Bmim][Glu] maybe also result from the bigger size of the anion [Asp] and more irregularity packing of cations and anions. Electrical Conductivity Properties of IL and Binary System. First, the molar electrical conductivity of the IL [Bmim][Asp] can be calculated according to the following equation:

Λ = σ · M · ρ− 1

k = k max(x /xmax )a exp[b(x − xmax )2 − a /xmax(x − xmax )] (17)

where kmax is the maximum value of the electrical conductivity at the mole fraction scale xmax and a and b are the empirical parameters. The fitted values, kmax, xmax, a, and b, and the standard deviation, s, are listed in Table 5. From the standard

(14)

where Λ is the molar conductivity, σ is the electrical conductivity, M is the molar mass, and ρ is the density. The values of the molar conductivity are listed in Table 1. Then, the relation of temperature and electrical conductivity or molar electrical conductivity of IL [Bmim][Asp] can be fitted by the Vogel−Fulcher−Tamman (VFT) equation24,25 as the following, respectively: D = D0 exp(−B /(T − T0))

Table 5. Fitted Values of the Empirical Parameters, kmax, xmax, a, b, and s, of the [Bmim][Asp]−Methanol System According to the Empirical Casteel−Amis Equation at (298.15, 303.15, 308.15, and 318.15) K

(15)

where D is electrical conductivity or molar electrical conductivity; D0, B, and T0 are adjustable parameters. The best fitted parameters of D0, B, T0, and the correlation coefficient, R2, are listed in Table 4. From Table 4, the

T/K

kmax/ms·cm−1

xmax

a

b

s/ms·cm−1

298.15 303.15 308.15 318.15

12.6089 12.8403 13.6183 14.9269

0.4401 0.3622 0.3004 0.3691

0.0479 0.0482 0.0471 0.0562

5.2320 3.7859 2.9363 3.4056

0.48 0.59 0.52 0.33

deviation, the four-parameter empirical equation shows good agreement with the experimental results of the binary IL solutions. The concentration dependence on experimental values, calculated values, and deviations of ILs with methanol are presented in Figure 7. The increasing and decreasing parts

Table 4. Fitted Values of Electrical Conductivity and Molar Electrical Conductivity of σ0, Λ0, B, Eσ, T0, and Correlation Coefficients (R) VFT equation σ0/S·cm−1 1.11 Λ0/S·cm2·mol−1 3.04

Eσ/eV 9.06·10−2

B/K 1050.2

T0/K 157.6

R2 0.9996

1082.1

156.8

0.9996

correlation coefficient, R2 = 0.9996, implies that the VFT equation should be suitable for evaluating the temperature dependence of the electrical conductivity of [Bmim][Asp]. Moreover, the temperature dependence on electrical conductivity also can be discussed by the traditional Arrhenius equation by fitting ln σ against T−1. In the fitting process, it is found that the correlation coefficient, R2, is 0.998, and the curve is not straight line. That implies the values of the experimental do not follow the Arrhenius behavior well by the traditional Arrhenius equation compared to the VFT equation. Cabeza et al.36 and Liu et al.37 have related the fitting of the VFT equation with the Arrhenius equation, and the final version is: σ = σ∞ − exp( −Eσ /(k b(T − T0))

Figure 7. Concentration dependence on the electrical conductivity of binary system [Bmim][Asp]−methanol at temperatures of (298.15, 303.15, 308.15, and 318.15) K; ■, 298.15 K; ●, 303.15 K; ▲, 308.15 K; ▼, 318.15 K; , calculated by the C−A equation.

(16,)

that is, D0 = σ∞ and B = Eσ/kB, where σ∞ is the maximum electrical conductivity, Eσ is the activation energy for electrical conduction which indicates the energy needed for an ion to hop to a free hole, and kb is the Boltzmann constant. Then, the calculated values are also listed in Table 4. For the binary system of [Bmim][Asp]−methanol, there is a interesting result that the conductivity of the mixture show a dramatic increase with the increased molar ratio of the methanol (see Table 1), which implies that the addition of methanol can significantly influence the conductivity of the binary fluid system composed of IL and molecular solvent and may provide us a method to adjust the conductivity of the mixed fluid in demand. In consideration of the unstable mixture

of the curves depend on the increasing ratio of the ILs. The electrical conductivity of the system increases with the increase of the IL concentration at low IL concentrations. However, the increments do not show a linear behavior. The slowly increased electrical conductivity follows the increasing concentration of the IL until the maximum values show at about xIL = 0.4. From the previous works,28 there should be two factors of the results: (1) the mobility of the charge carriers is reduced with increasing viscosity; (2) the number of the charge carriers is reduced due to the aggregate formation. The aggregation formation may become the dominant factor for the increasing F



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conductivity of the IL solutions.38−40 Hence, the curve at the low IL concentrations indicates the aggregation formation. At the high IL concentrations, the electrical conductivity decreases with the increasing IL concentration. The relationship between the electrical conductivity and the ion mobility or number of charge carriers41,42 can explain that the decrease of the organic solvents enhances the interaction force among ions and weakens the movement of ions in the IL solution system.

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CONCLUSION The density, surface tension, and electrical conductivity of amino acid ionic liquid [Bmim][Asp] and binary system [Bmim][Asp]−methanol were determined and calculated at different temperatures, and the temperature dependences were illustrated. The physicochemical properties of the IL or the binary system were estimated by the empirical and semiempirical equations. The excess volume change of the IL binary system indicates that, with the increasing molar ratio of methanol, the intermolecular interaction between the cations and anions of the IL are becoming weaker. The Vogel− Fulcher−Tamman equation was used to express the relationship between the electrical conductivity and temperature. By Casteel−Amis equations, the relationship between the electrical conductivity and concentration of IL for the binary system were described, and the results indicated that the decrease of the methanol enhanced the interaction force among ions and weakened the movement of ions in the IL solution system.

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ASSOCIATED CONTENT

S Supporting Information *

H NMR spectral data, DSC curve, and linear fit of the surface tension versus temperature for [Bmim][Asp] and surface tension for the whole concentration interval of the binary system [Bmim][Asp]−methanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 18604968816. Fax: +86 416 3400292. E-mail: [email protected] (Q.-G.Z.) and [email protected]. cn (Z.-H.K.). Funding

This work was financially supported by the National Nature Science Foundation of China under Grant: NSFC No. 21003081; the National Basic Research Program of China (973 Program) (No. 2010CB934500). Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis and Physicochemical Properties of Amino Acid Ionic

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