The Polar Solvent Effect - American Chemical Society

Article pubs.acs.org/jced

Ionic Liquid-Based Aqueous Biphasic Systems with Controlled Hydrophobicity: The Polar Solvent Effect Jing Gao,† Li Chen,† Yun Xin,‡ and Zongcheng Yan*,† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong P. R. China ‡ School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong P. R. China S Supporting Information *

ABSTRACT: Recovery of ionic liquids (IL) is necessary and urgent because of increasing production costs and potential environmental pollution. Dipolar aprotic solvents (dimethyl sulfoxide, DMSO; N,N-dimethylformamide, DMF; and N,Ndimethylacetamide, DMCA) and polar protic solvents (methanol, ethanol, and nbutanol) were used as cosolvents and antisolvents, respectively. These compounds have crucial effects on the separation and purification of ILs. We determined the effect of different polar solvents on the formation of aqueous biphasic systems (ABSs) using K3PO4 to control the hydrophobicity of 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl). Phase equilibria of the ABSs comprising IL + K3PO4 + water in the presence of the polar solvents were obtained at 298 K and at atmospheric pressure. In general, the IL aptitude that induced the formation of ABS in the presence of the polar solvents increased with decreasing hydrogen bond basicity and polarizability of the polar solvents in the following order: none < methanol < ethanol ≈ DMSO < nbutanol ≈ DMF < DMCA. Densities, pH value, conductivities, and surface tensions of both aqueous phases were experimentally measured.

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polar solutes,10,20,28 biocatalysis products,29−33 and enzymes.34−36 For applications in engineering, the applied physical and chemical parameters of fluids are essential. Solute−solvent and solvent−solvent interactions can vary with the amount and nature of cosolvents.34 Previous research on the addition of ILs to molecular solvents shows that strong synergetic effects are present in IL−methanol binary mixtures.34 Mellein et al.35 systematically studied the effects of organic solvents, cations, and anions on the solvent strength of IL/organic mixtures. The polarity of ILs is largely unaffected by the organic solvent, but more specific solvation forces can be affected indirectly by the strength of the anion/cation interaction. However, a study that used steady state and picosecond time-resolved fluorescence spectroscopy showed that gradual addition of cosolvents increased the polarity of IL/water, IL/methanol, and IL/ acetonitrile mixtures.36 However, surface tension is an important measure of cohesive forces between liquid molecules at the surface and represents the quantification of force per unit length of free energy per unit area.37 Unfortunately, most studies have focused on the effects on the properties of pure ILs, and the solutions in such studies comprise a relatively small quantity of data. In IL processing of biomass, using mixtures of IL and cosolvent instead of pure IL as solvent can effectively break the

INTRODUCTION

Separation and purification associated with recovery and recycling of solvents are crucial issues in integrated chemical processes because of the significant cost benefits.1 At present, ionic liquid (IL) use has led to numerous processing options with conventional organic solvents. However, IL removal from aqueous streams can be challenging.2 Several processes for the separation of ILs have been developed, including magnetic method, membrane separation, phase separation, distillation, adsorption, and supercritical CO2.3−9 However, high energy consumption, long processing duration, high cost, and complex operations limit the application of these processes. Aqueous biphasic system (ABS) is an alternative phase separation method for recovering ILs from aqueous effluents. IL-based ABSs are becoming popular because of their benefits, such as environmental safety, enhanced solvent recovery, low viscosity, rapid separation of the coexisting phases, and vast applicability in controlling phase polarity.10−12 Gutowski et al.13 were the first to show that a mixture of an aqueous solution of 1-N-butyl-3-methylimidazolium chloride ([C4mim]Cl) and K3PO4 could produce ABS with an upper IL-rich phase and a lower K3PO4-rich phase. Studies involving IL/kosmotropic salt,14,15 IL/biodegradable salt,16 IL/saccharide,17−20 IL/polypropylene glycol,21 IL/poly(ethylene glycol),22 and IL/ inorganic salt/polymer23 ABSs have been reported. Recovery efficiencies of 60 % to 100 % have been obtained for different ILs by using ABS method.24,25 IL-based ABSs are mainly applied in the extraction of biomolecules,12,26 antibiotics,27 © XXXX American Chemical Society

Received: September 4, 2013 Accepted: May 12, 2014

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dx.doi.org/10.1021/je400794m | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Phase diagrams for [C8mim]Cl and K3PO4 with selected polar solvents (20 wt %) at 298.2 K and 0.1 MPa: ■, none; ●, methanol; ethanol; ▼, n-butanol; ★, DMSO; *, DMF; +, DMAC.

Experimental Liquid−Liquid Equilibria Procedure. The determination of the binodal curves was performed by the cloud point titration method at 298 ± 1 K and at atmospheric pressure.40 Aqueous solutions of K3PO4 with variable mass fractions were used at 40 to 50 wt %, and aqueous solutions of dipolar aprotic solvent and IL with mass ratios between 0 and 20 wt % were prepared. The salt solution was added dropwise into the solution of dipolar aprotic solvents and ILs, and the mixture was shaken until turbid or cloudy. Water was added dropwise into the tube to obtain a clear one-phase system, and salt solution was further added to form ABS. This process was continued until sufficient points were available to create a binodal curve. All measurements were in mass fractions. The experimental binodal curves were correlated according to the following equation proposed by Merchuk et al.41

microscopic hydrogen bond networks and the aggregation of IL, which can significantly reduce the viscosity of IL and improve the mixing and transfer process in the IL medium. The effect of pretreatment with a mixture of IL and cosolvent on the changes in lignocellulosic composition, structure, and biogas production of water hyacinth was reported in our previous study.38 Cellulose or starch dissolved in ILs can be precipitated easily from the solutions by adding antisolvents, such as water, ethanol, and methanol.39 Nevertheless, a systematic and comprehensive analysis of IL recovery and recycling in the presence of a polar solvent by forming ABSs during biomass processing is required. This study focuses on the effect of the polar solvents used as cosolvents and antisolvents of IL in biomass processing on the formation of an IL-based ABS comprising K3PO4. To evaluate the effects of polar solvents on hydrophobicity, systematic Kamlet−Taft values of the ILs and polar solvents were studied. Moreover, the physicochemical properties, including density, pH value, conductivity, and surface tensions of both aqueous phases were also investigated.

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▲,

Y = a exp(bX 0.5 − cX 0.3)

(1)

where X and Y are the mass fractions of IL and salt, respectively. The constants a, b, and c were obtained by leastsquares regression. Tie-lines (TLs) characterize the compositions of the two phases in equilibrium. For the determination of each TL, a ternary or quaternary mixture was prepared by mixing water, IL, salt or dipolar aprotic solvent of designed concentrations with the biphasic region. The mixtures were vigorously agitated. Subsequently, the mixtures were allowed to equilibrate in a thermotank at 298.2 ± 0.1 K overnight. The concentrations of IL and salt in the upper and lower phases, respectively, were determined by ion chromatography (Basic IC 792, Methohm, Switzerland). Karl−Fisher titration was used to measure the water content in all samples. The weight fractions of dipolar aprotic solvents were calculated by the material balance method.38 The TL lengths (TLLs) at different compositions were calculated by the following equation:

EXPERIMENTAL SECTION

Materials. The ILs used in this work included the following: 1-ethyl-3-methylimidazolium chloride, [C2mim]Cl; 1-butyl-3methylimidazolium chloride, [C4mim]Cl; 1-hexyl-3methylimidazolium chloride, [C6mim]Cl; and 1-octyl-3-methylimidazolium chloride, [C8mim]Cl. All ILs were supplied by the Center of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics (purity > 99.0 wt %), and were dried at 70 °C in a vacuum oven before use. The polar solvents used were dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), methanol, ethanol, and nbutanol. The above solvents were acquired from Sigma-Aldrich. The inorganic salt K3PO4·3H2O (purity > 99.0 wt %) was purchased from Sinopharm Chemical Reagent Co., Ltd. The solvatochromic dyes used were Reichardt’s dye 33, 4-nitroaniline, and N,N-diethyl-4-nitroaniline ≥ 99.5 wt %, and all dyes were purchased from Fluorochem. Ultrapure water was doubledistilled, passed through a reverse osmosis system, and further treated with a Milli-Q plus 185 water purification apparatus. All material purities in mass fraction are presented in the Supporting Information, Table S1.

TLL = [(YT − YB)2 + (X T − XB)2 ]1/2

(2)

where Y and X are the mass fractions (w/w) of IL and salt, respectively. The subscripts T and B are the points representing the top and the bottom phases, respectively. Density, pH Value, Conductivity, and Surface Tension. The solutions from the monophasic and biphasic regions that show increasing mass ratio of K3PO4 to [Cnmim]Cl in the B



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Figure 2. Phase diagrams for K3PO4 and selected ILs with different concentrations of DMSO at 298.2 K and 0.1 MPa: ■, [C2mim]Cl; ○, [C2mim] Cl + 5 % DMSO; +, [C2mim]Cl+20 %DMSO; ●, [C4mim]Cl; ☆, [C4mim]Cl + 5 % DMSO; □, [C4mim]Cl + 20 % DMSO; ◆, [C6mim]Cl; ◊, [C6mim]Cl + 5 % DMSO; *, [C6mim]Cl + 20 % DMSO; ★, [C8mim]Cl; Δ, [C8mim]Cl + 5 % DMSO; |, [C8mim]Cl + 20 % DMSO.

Table 1. Correlation Parameters (a, b, and c) Obtained by the Regression of the Experimental Binodal Data through the Application of eq 1 (and Respective Standard Deviations, σ) for the IL + K3PO4 + Polar Solvent + Water Systems at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa ionic liquid

polar solvent

[C2mim]Cl

none 5 % DMSO 20 % DMSO none 5 % DMSO 20 % DMSO none 5 % DMSO 20 % DMSO none 20 % methanol 20 % ethanol 20 % n-butanol 5 % DMSO 20 % DMSO 20 % DMF 20 % DMAC

[C4mim]Cl

[C6mim]Cl

[C8mim]Cl

a

a 0.59 0.60 0.53 0.71 0.67 0.49 0.76 0.78 0.59 0.89 0.65 0.60 0.57 0.73 0.52 0.61 0.70

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

b 0.06 0.01 0.02 0.01 0.01 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.01 0.01 0.01 0.01

−2.38 −2.60 −2.61 −3.12 −3.14 −2.33 −3.25 −3.31 −3.02 −3.27 −2.69 −2.74 −2.59 −3.20 −2.25 −2.79 −3.34

c

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.31 0.07 0.11 0.05 0.06 0.13 0.08 0.09 0.10 0.03 0.12 0.05 0.21 0.03 0.07 0.05 0.04

42.22 40.01 37.51 43.92 43.71 42.99 55.38 58.49 60.24 52.43 51.63 59.59 65.48 55.65 61.35 63.96 56.69

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.47 0.45 0.93 0.93 0.38 0.09 0.20 0.27 0.52 0.23 0.13 0.25 0.55 0.90 0.63 0.58

R2

106σ

0.9966 0.9992 0.9992 0.9994 0.9994 0.9962 0.9994 0.9994 0.9994 0.9998 0.9989 0.9993 0.9989 0.9998 0.9970 0.9998 0.9998

20.0 10.0 4.01 5.47 3.56 40.0 3.08 3.55 2.12 1.71 10.0 3.50 4.96 1.43 20.0 0.83 1.16

Standard uncertainties u are u(T) = 0.1 K, u(p) = 10 kPa.

removed by drying under vacuum at 40 °C for 12 h. The IL containing the dye was added into a quartz cell in a drybox, and the cell was capped and sealed. The visible spectra of the solvent mixed with solvatochromic dye were recorded on an UV-2450 UV−visible spectrophotometer at 25 °C. The wavelength at the maximum absorption (λmax) was determined, and α, β, and π* values were calculated by the following equations:

presence of various dipolar aprotic solvents were prepared. The density (g·mL−1) of each phase was measured using an automated DES PHOTIME, which functions in accordance with the Archimedes principle, namely, high static pressure of the liquid column is proportional to the density of the liquid. The pH value was tested by a PHS-25 pH meter equipment. Conductivity measurements were performed by a DDS-11A conductometer. The pendant drop method was performed by a Dataphysics OCA40 Micro and was used to measure the surface tension (σ) of the IL solutions. All the above measurements were performed at 298.2 ± 0.1 K. Kamlet−Taft Parameter Measurements. The Kamlet− Taft parameters of ILs and dipolar aprotic solvents were determined according to the procedure reported by Ohno et al.42 In a drybox, a given amount of the dried IL and a concentrated solution of dye in dry methanol were added into a vial and mixed until homogeneous. The methanol was carefully C

ν(dye) = 1/(λmax(dye) × 10−4)

(3)

ET (30) = 0.9986(28.592/λmax(dye33)) − 8.6878

(4)

β = (1.035ν(NA) + 2.64 − ν(DENA))/2.80

(5)

π* = 0.314(27.52 − v(DENA))

(6)



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α = 0.0649E T(30) − 2.03 − 0.72π *

with long cation side alkyl chain ([C8mim]Cl), the ability to form ABS decreased.8,46−48 However, the addition of polar solvent could improve the ability of [C8mim]Cl to form ABS. As shown in Figure 2, at 5 % mass ratio of DMSO to [C8mim]Cl, the ability of the liquid−liquid demixing for [C 8 mim]Cl was comparable with that of [C 6 mim]Cl. Increasing the concentration of DMSO to 20 % improved ABS formation ability. The effect of DMSO on IL-based ABS formation was also observed in [C2mim]Cl, [C4mim]Cl, and [C6mim]Cl. The cations of ILs can be preferentially solvated by the dipolar aprotic solvent through ion−dipole interaction49 and the entropy of IL increased in water. Thus, results presented in Figure 2 indicate that the ability of IL to form ABS improved with the addition of polar solvents at different concentrations. The experimental results for the TLs and TLLs are presented in Table 3 and Figure 3. X and Y were the IL and salt content, whereas M, T, and B represent the mixture, top phase, and bottom phase, respectively. In accordance with the phase diagrams and the TLs in Table 3, the phase separation is improved by the addition of the polar solvents. The highest recovery ratio of 98.55 % for [C6mim]Cl was obtained at [C6mim]Cl, K3PO4, and DMSO concentrations of 30.21 %, 12.10 %, and 6.42 %, respectively. When concentrations of methanol, DMSO, ethanol, DMF, n-butanol, and DMAC were at 20 %, the recovery ratios of [C4mim]Cl reached 93.24 %, 93.77 %, 94.16 %, 94.25 %, 94.27 %, and 94.41 %, respectively. The trend of the recovery ratio was in accordance with the ABS creation ability in the presence of these polar solvents. However, only 86.32 % and 83.44 % of [C6mim]Cl and [C4mim]Cl, respectively, were recovered in the absence of a polar solvent. Such low IL recovery efficiencies demonstrate that the polar solvents contribute to the recovery of hydrophilic ILs. The following correlating equations proposed by Othmer− Tobias (eq 8) and Bancroft (eq 9) have been used to correlate the TL compositions:

(7)

RESULTS AND DISCUSSION The phase diagrams presented in Figures 1 and 2 illustrate the effect of different polar solvents during ABS formation. The binodal data of these systems (in mass fraction units) are presented in the Supporting Information, Tables S2 and S3. The binodal curves of all systems were correlated with eq 1, and the respective parameters a, b, and c, along with their corresponding standard deviations, are listed in Table 1. Given the correlation coefficients, we conclude that eq 1 provides a good description of the experimental data. Figure 1 reports the ABSs comprising [C8mim]Cl and K3PO4 in the presence of different dipolar aprotic solvents (DMSO, DMF, and DMAC) and polar protic solvents (methanol, ethanol, and buthanol). The concentration of the polar solvents in the systems was in definite proportion with [C8mim]Cl. A large biphasic region improves polar solvent ability to induce ABS formation. The addition of polar solvents increased the area of the biphasic region, in the following order: none < methanol < ethanol ≈ DMSO < n-butanol ≈ DMF < DMAC. This trend of ABS induction by dipolar aprotic solvents is in agreement with our previous results for ABS formed with [C4mim]Cl and K3PO4 in the presence of DMSO, DMF, or DMAC.43 This close agreement suggests that the formation of ABS is not only dependent on the IL characteristics, but also on the nature of polar solvents. The ability to create ABS was based on the hydrophobicity of IL.43,44 The hydrogen bond basicity is a measure of the ability of a solvent to accept a proton in a solute−solvent hydrogen bond. In the IL-based ABS, low hydrogen bond basicity of an IL results in high ABS promotion activity.45 To evaluate the effect of the polar solvents on the hydrophobicity of IL, the Kamlet−Taft values of the polar solvents were measured, and the results are presented in Table 2. The Kamlet−Taft linear Table 2. Kamlet−Taft Values for a Series of Dipolar Aprotic Solvents and Polar Protic Solvents at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa α methanol ethanol n-butanol DMSO DMF DMAC a

1.02 0.89 0.56 0.30 0.34 0.38

± ± ± ± ± ±

β 0.03 0.04 0.02 0.02 0.04 0.02

0.76 0.75 0.70 0.82 0.74 0.69

± ± ± ± ± ±

π* 0.07 0.06 0.00 0.02 0.05 0.03

0.71 0.62 0.57 0.90 0.89 0.88

± ± ± ± ± ±

0.01 0.01 0.00 0.03 0.03 0.03

⎛ 1 − w b ⎞n ⎛ 1 − w1t ⎞ 2 ⎜⎜ ⎟⎟ K = ⎟ ⎜ t b ⎝ w1 ⎠ w ⎝ ⎠ 2

(8)

⎛ wb ⎞ ⎛ w t ⎞r ⎜⎜ 3b ⎟⎟ = K1⎜ 3t ⎟ ⎝ w1 ⎠ ⎝ w2 ⎠

(9)

where wt1 is the mass fraction of ILs in the top phase; wb2 is the mass fraction of K3PO4 in the bottom phase; wb3 and wt3 are the mass fractions of water in the bottom and top phases, respectively; and K, n, K1, and r are the fit parameters. Equations 8 and 9 are linearized by obtaining the logarithm on both sides of the equations to determine the fit parameters. The values of the parameters are presented in Table 4. The corresponding correlation coefficient values (R2) and standard deviations (σa) shown in Table 4 indicate that eqs 8 and 9 can be satisfactorily used to correlate the TL data of the investigated systems. Physicochemical properties of the upper and lower phases in different systems are required for the design and scale up of separation processes. Besides the salting-out ability of inorganic salt, the pH of the aqueous solution plays a crucial function in the formation of IL-based ABSs.45 Moreover, in separation and extraction, the pH value of ABSs has an important effect on the activity of the extracted materials, especially on pH-sensitive

Standard uncertainties u are u(T) = 0.1 K, u(p) = 10 kPa.

solvation energy relationship classifies solvent strength into dipolarity and polarizability (π*), hydrogen bond donating acidity (α), and hydrogen bond accepting basicity (β).35 As shown in Table 2, the β values of dipolar aprotic solvents and polar protic solvents followed the order DMSO > DMF > DMAC, and methanol > ethanol > n-butanol, respectively. A close relationship was observed between ABS promotion and the ability of each polar solvent to increase the hydrophobicity of IL. Therefore, for dipolar aprotic solvents or polar protic solvents, a low β value can increase the efficiency of IL in creating ABS in the presence of the polar solvent. Increasing the length of the aliphatic chain from C2 to C6 alkyl chains increased the ability of ILs to form ABS; for ILs D



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Table 3. Mass Fraction Compositions for the Coexisting Phases of [C8mim]Cl (Y) + K3PO4 (X) + H2O (Z) + Polar Solvent systems at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa, and Respective Values of TLL mass fraction/(w/w) polar solvent

XM

YM

ZM

XT

YT

ZM

XB

YB

ZM

TLL

methanol

0.1200 0.1186 0.1223 0.1180 0.1214 0.1220 0.1228 0.1213 0.1220 0.1190 0.1200 0.1190 0.1227 0.1167 0.1190 0.1254 0.1200 0.1220

0.2549 0.3143 0.3540 0.2617 0.3200 0.3486 0.2500 0.3200 0.3506 0.2520 0.3210 0.3590 0.2520 0.3156 0.3500 0.2511 0.3102 0.3526

0.5741 0.5042 0.4529 0.5679 0.4946 0.4596 0.5772 0.4947 0.4572 0.5786 0.4948 0.4502 0.5749 0.5045 0.4610 0.5732 0.5077 0.4548

0.0631 0.0414 0.0278 0.0580 0.0352 0.0252 0.0536 0.0315 0.0243 0.0632 0.0297 0.0218 0.0576 0.0356 0.0212 0.0516 0.0344 0.0206

0.3219 0.4052 0.4899 0.3289 0.4251 0.4755 0.3342 0.4357 0.4840 0.3233 0.4355 0.4869 0.3243 0.4123 0.4880 0.3348 0.4031 0.4899

0.6049 0.5467 0.4778 0.6038 0.5340 0.4952 0.6036 0.5277 0.4872 0.6038 0.5308 0.4872 0.6084 0.5464 0.4878 0.6054 0.5566 0.4862

0.3120 0.3487 0.3609 0.3207 0.3488 0.3675 0.3252 0.3448 0.3590 0.3325 0.3443 0.3825 0.3320 0.3567 0.3749 0.3472 0.3621 0.3845

0.0283 0.0184 0.0136 0.0181 0.0120 0.0100 0.0180 0.0132 0.0112 0.0190 0.0156 0.0125 0.0152 0.0133 0.0105 0.0132 0.0113 0.0089

0.6585 0.6321 0.6249 0.6604 0.6387 0.6221 0.6560 0.6414 0.6293 0.6477 0.6394 0.6045 0.6521 0.6294 0.6141 0.6390 0.6261 0.6062

0.3849 0.4940 0.5812 0.4069 0.5186 0.5778 0.4168 0.5260 0.5793 0.4064 0.5247 0.5960 0.4133 0.5122 0.5942 0.4368 0.5108 0.6031

ethanol

n-butanol

DMSO

DMF

DMAC

a

Standard uncertainties u are u(T) = 0.1 K, u(X) = 0.0005, u(Y) = 0.0005, u(Z) = 0.0005, u(p) = 10 kPa.

Figure 3. Phase diagrams for [C8mim]Cl and K3PO4 in the presence of 20 % polar solvent at 298.2 K: □, binodal curve data; ■, TL data.

biomacromolecules. The experimental lg[OH−] for the different systems is presented in Table 5. All systems were alkaline, and an increase in lg[OH−] with the increase of K3PO4 was also

observed. The pH value of the [C8mim]Cl-rich phases was higher than the pH observed in the K3PO4-rich phase. The pH values of IL-rich phases are strongly dependent on IL E



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Table 4. Values of the Parameters of eqs 8 and 9 polar solvent

K

n

R2

σa

K1

r

R2

σa

methanol ethanol n-butanol DMSO DMF DMAC

0.2071 0.2145 0.0906 0.2768 0.1645 0.1579

2.9655 2.9883 4.2092 2.6943 3.6397 3.991

0.9826 0.9954 0.9866 0.9776 0.9989 0.9967

0.036 0.022 0.023 0.041 0.010 0.015

1.6077 1.5809 1.6402 1.5421 1.5279 1.4730

0.2949 0.3336 0.2297 0.2936 0.2826 0.2540

0.9731 0.9941 0.9858 0.9756 0.9977 0.9976

0.043 0.020 0.027 0.045 0.014 0.015

Table 5. Experimental density (ρ), lg[OH−], and conductivity (κ) for the [C8mim]Cl-rich (T) Phase and K3PO4-rich (B) Phase as a function of mass ratio of K3PO4 to [C8mim]Cl at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa K3PO4/[C8mim]Cl polar solvent none

methanol

DMSO

DMAC

a

ρT

ρB −1

w/w

g·mL

1.67 1.80 2.00 2.13 1.67 1.80 2.00 2.13 1.67 1.80 2.00 2.13 1.67 1.80 2.00 2.13

1.06 1.02 1.02 1.01 1.05 1.04 1.03 1.01 1.05 1.05 1.03 1.01 1.03 1.02 1.01 1.01

κT −1

−

g·mL

1.25 1.30 1.33 1.35 1.32 1.33 1.35 1.39 1.32 1.36 1.37 1.39 1.32 1.34 1.35 1.41

−

lg [OH ]T

lg [OH ]B

−0.02 0.15 0.35 0.46 0.02 0.13 0.21 0.56 0.24 0.08 0.65 0.79 0.20 0.36 0.56 0.69

−0.17 −0.07 0.20 0.22 0.00 0.04 0.15 0.23 1.98 0.65 0.16 0.26 −0.08 −0.02 0.10 0.27

κB −1

ms·cm 28.2 31.2 32.5 27.2 34.3 29.8 25.5 23.3 22.6 22.7 21.0 20.6 20.6 21.9 20.7 19.1

ms·cm−1 31.5 45.2 44.2 39.9 44.7 41.2 41.0 40.7 30.2 28.4 28.3 27.0 38.9 37.6 30.4 28.5

Standard uncertainties u are u(T) = 0.1 K, u(ρ) = 0.01, u(pH) = 0.01, u(κ) = 0.1, u(p) = 10 kPa.

where z represents the valence of the charge carrier, e0 is the elementary charge, r is the diffusion coefficient of a model spherical species of an effective radius, and V is the volume.50 The viscosities of the IL-rich phase were reported to be higher than those of the K3PO4-rich phase.10 Thus, the conductivities of the [C8mim]Cl-rich phase are lower than those of the K3PO4-phase. The correlation results shown in Figure 4, Table 6, and eq 11 demonstrated that the conductivity value of mixtures

concentration and the alkyl side chain length of the cation. Increasing the alkyl side chain length of the cation increased the pH values.45 A high concentration of [C8mim]Cl in the top phase leads to a high pH value in the [C8mim]Cl-rich phase. The densities of different systems were evaluated, and the data showed that the difference of the densities between the bottom and the top phases increased with increasing mass ratio of K3PO4 to [C8mim]Cl, and that the addition of the polar solvents could improve the density gradient between the two phases. This finding is in agreement with the effect of these polar solvents on phase demixing because large density differences between the fluids result in a high density gradient force. ABSs comprising polar solvents are satisfactory because such systems have high phase-separation rates, which improve their use in industrial applications. Conductivity is an important physical property of ILs considering their possible application as electrolytes in electrochemical processes and devices. In the present work, the changes in conductivity depend on salt concentration, presence or absence of a polar solvent, and the mass transfer between the two phases. The results of conductivity (κ) experiments performed on the two phases of ABSs are shown in Table 5. According to eq 8, which is a combination of the Nernst−Einstein and the Stokes−Einstein equations, the conductivity of a classical electrolyte solution is proportional to the number of charge carrier N and inversely proportional to the medium viscosity η, as follows: κ=

z 2e0 2 N 6Vπr η

Figure 4. Correlation of the determined conductivity (κ) with the density (ρ) for the mixture of [C8mim]Cl + K3PO4 + H2O + polar solvent.

(10) F



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Table 6. Correlation Parameters (A, B, and C) Obtained by the Regression of the Experimental Data from Figure 4 through the Application of eq 11 (and Respective Standard Deviations, σa) for the [C8mim]Cl + K3PO4 + Polar Solvent + Water Systems at the Temperature T = 298.2 K and Pressure p = 0.1 MPa polar solvent

A

B

C

R2

σa

none methanol DMSO DMAC

93.39 62.19 48.31 44.88

−11.23 −5.65 −5.25 −6.94

−1.00 −0.99 −0.98 −0.95

0.9627 0.9668 0.9990 0.9899

4.85 2.04 0.07 0.17

CONCLUSIONS To determine the effect of dipolar aprotic solvents and polar protic solvents on the [Cnmim]Cl-based ABSs, phase behavior, physicochemical properties, and Kamlet−Taft parameters of the solvents were analyzed. The addition of methanol, ethanol, n-butanol, DMSO, DMF, and DMAC improved phase separation ability and the ability to create ABSs for [Cnmim] Cl with different lengths of alkyl side chains, and these processes can be controlled by adding various amounts of the polar solvent. Moreover, the trends in the density gradient of the two phases, the surface tension, the conductivity, and the dipolarity/polarizability of the mixtures are dependent on the changes in hydrophobicity of ILs caused by the addition of polar solvents.

comprising [C8mim]Cl, K3PO4, H2O, and polar solvent fit into the density value. The mass ratio of K3PO4 to [C8mim]Cl in each system was in the range 0 to 1.4, and ABS was not formed. In general, the conductivity value shows logarithmic growth with increasing density value until the two phases are created. κ = A − B ln(ρ + C)

Article

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ASSOCIATED CONTENT

S Supporting Information *

Experimental binodal mass fraction data for the ABSs composed by each system. This material is available free of charge via the Internet at http://pubs.acs.org.

(11)

The surface energy of a liquid is the amount of energy required to create the unit area of a new surface. Based on a large set of experimental values of IL surface tension, Kolbeck et al.51 concluded that the surface composition is determined by the interplay of cohesive energy and surface orientation. Nevertheless, the surface tension of IL solutions is not easily correlated with the surface tension of the pure ILs because this parameter is determined by chemical composition.52 The surface tension data corresponding to mixtures of [C8mim]Cl, K3PO4, water, and polar solvents are presented in Figure 5. The

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86 20 87111109. Fax: + 86 20 87111109. E-mail: [email protected]. Funding

This research was supported by the National Natural Science Foundation of China (21376088), the Project of Production, Education and Research, Guangdong Province and Ministry of Education (2012B09100063, 2012A090300015), and the Key Social Development Program of Guangdong Province (2011A030600011). The authors would also gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 5. Surface tension at 298.2 K of the systems of [C8mim]Cl + K3PO4 + H2O + polar solvents: ○, none; □, methanol; Δ, DMSO; ▽, DMAC.

surface tension decreased rapidly until a mass ratio of 1.33 for K3 PO 4 to [C 8 mim]Cl was reached. When high K 3 PO 4 concentration was added, the surface tension increased remarkably with increasing [C8mim]Cl concentration in the top phase. Minimum surface tension value was found at the mass ratio of K3PO4 to [C8mim]Cl, above which the IL-ABS started to form. Both Malham et al.53 and Sung et al.54 found that the surface tension decreased in the water-rich phase, whereas the surface tension increased slightly until the value for pure IL was reached. G



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