Aggregation Behavior of Aqueous Solutions of 1-Dodecyl-3

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Aggregation Behavior of Aqueous Solutions of 1‑Dodecyl-3methylimidazolium Salts with Different Halide Anions Mingqi Ao and Doseok Kim* Department of Physics, Sogang University, Seoul 121-742, Korea ABSTRACT: The interfacial and bulk properties of the aqueous solutions of imidazolium-based ionic liquids with different halide anions ([C12mim]Cl, [C12mim]Br, [C12mim]I) were investigated by surface tension and electrical conductivity measurements. The lowest surface tension (γcmc) and maximum surface excess concentration (Γmax) values from the surface tension measurements showed that [C12mim]I had the highest surface activity. The thermodynamic pontentials of micellization (ΔG0m, ΔH0m, ΔS0m) indicated that the micellization of [C12mim]Cl and [C12mim]Br was entropy-driven, while that of [C12mim]I was enthalpy-driven at 15 °C but entropy-driven above 20 °C. This distinct behavior for [C12mim]I was thought to be due to the higher binding affinity of I− to the micellar interface.

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INTRODUCTION Ionic liquids are salts existing in the liquid state at around room temperature.1 Owing to their unique physicochemical properties, such as nonvolatility, nonflammability, high thermal stability, and a wide electrochemical window, they have attracted much interest for applications in chemical synthesis,1−3 electrochemistry,4,5 biocatalytic transformations,6,7 and analytical and separation sciences.8,9 As a protype ionic liquid, ionic liquids with a 1-alkyl-3-methylimidazolium cation, [Cnmim]+, have received much attention and have been extensively studied.10−12 It has been found by some researchers that the aqueous solutions of these ionic liquids can form aggregates, especially as the alkyl group attached to the imidazolium core becomes longer. For example, the measurement of surface tension and conductivity have found that 1-octyl3-methylimidazolium chloride ([C8mim]Cl) and 1-octyl-3-methylimidazolium iodide ([C8mim]I) behave as surfactants and form aggregates when their molalities are above 0.12 mol·kg−1 and 0.10 mol·kg−1, respectively.13 1-Decyl-3-methylimidazolium bromide ([C10mim]Br) has a different aggregation morphology with different concentrations in water, which forms aggregates at low concentrations and then forms lyotropic mesophases at higher concentrations.14−16 A similar behavior of [C16mim]Cl and [C16mim]BF4 was also observed by surface tension measurements both in aqueous solution and in the ionic liquid of ethylammonium nitrate.17 A few articles investigated the other physicochemical properties of long chain ionic liquids ([Cnmim]Cl, n = 8, 10, 12), including their partition coefficients between n-octanol and water18 and solubility of [C12mim]Cl in several alcohols19 and dipolar aprotic solvents.20 Although previous studies have revealed some interesting behaviors of ionic liquids, the aggregation of ionic liquids in aqueous solutions has not been studied systematically yet. From both fundamental and applied viewpoints, particular interests are the molecular structure effect on both the aggregation behavior and the physicochemical properties of the aggregation process. © 2013 American Chemical Society

However, information on this aspect is very limited. This lack of information, together with their potential application has motivated us to investigate the properties of long-chain ionic liquids with the systematic change of anions, such as halide series of [C12mim]Cl, [C12mim]Br, and [C12mim]I. The adsorption at the air−water interface and aggregation in aqueous solutions were investigated by measuring surface tension and conductivity. The data obtained were employed to deduce the following interfacial properties: minimum area per molecule at air/liquid interface (Amin), surface excess concentration (Γmax), and adsorption efficiency (pC20). Thermodynamic potentials (ΔG0m, ΔH0m, ΔS0m) for the micelle formation were also deduced from the temperature dependence of critical micelle concentration (cmc) and the change in the slope of conductivity. These results were used to understand the effect of anion on the aggregation behavior of ionic liquids and the properties of ionic liquid aqueous solutions.

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EXPERIMENTAL SECTION Materials. The 1-dodecyl-3-methylimidazolium chloride ([C12mim]Cl), bromide ([C12mim]Br), and iodine ([C12mim]I) were purchased in FutureChem (Korea) and used without further purification. The most abundant impurity was water, and its impurity level was less than 50 ppm, which would not affect the measurement of aggregation behavior aqueous solution. The purities of these ionic liquids are higher than 99 %. These materials showed thermal hysteresis and melting points varied depending on the literature around 30 °C to 40 °C, while transition points between smectic to isotropic phase are in the range of 80 °C to 100 °C.19,21−25 Surface Tension Measurement. Surface tension was measured by a Wilhelmy plate using a filter paper. We calibrated Received: September 26, 2012 Accepted: April 25, 2013 Published: May 7, 2013 1529

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the surface tension by the value of pure water (72 mN·m−1) at room temperature (25 °C). Electrical Conductivity Measurement. Conductivity measurements were performed by CyberScan CON1500 bench conductivity meter at four different temperatures. The temperature was kept constant within ± 0.1 °C of the desired temperature by using a refrigerated bath circulator. KCl aqueous solutions at different concentrations were used to calibrate the conductivity cell, from which a cell constant of 1.0568 cm−1 was determined. The measurements of each sample were repeated twice at each temperature.

surface activity parameters can be obtained from the surface tension isotherms by using eqs 1 and 2 and are listed in Table 2. From Figure 1, the ionic liquid with iodine anion has the lowest γcmc of 31.7 mN·m−1, while those with chloride and bromide anions are 38.4 mN·m−1 and 36.8 mN·m−1 , respectively, indicating the surface activity of [C12mim]I is higher than the other two. γcmc values in the present study are somewhat lower than the previous reports, in which γcmc is about 39 mN·m−1 for both [C12mim]Cl (measured by using drop shape analysis tensiometer)26 and [C12mim]Br (measured by using DuNuoy tensiometer).27 The maximum surface excess concentration Γmax at the air− aqueous interface is calculated using the following Gibbs adsorption isotherm equation:28

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RESULTS AND DISCUSSION Surface Activity of Long Chain Ionic Liquids. The surface activity of ionic liquids was measured by surface tension. The surface tension isotherms of [C12mim]Cl, [C12mim]Br, and [C12mim]I are shown in Figure 1 (numerical values listed

Γmax = −

dγ 1 · nRT d ln c

(1) −1

where γ is the surface tension in mN·m , Γmax is the maximum surface excess concentration in μmol·m−2, R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature, and c is the ionic liquid concentration. (dγ/d lnc) is the slope of the surface tension isotherm near the cmc, the concentration at which the surface tension isotherm shows a sharp break. The value n is taken as 2.29 The minimum area Amin occupied by each molecule at the air/aqueous interface should reflect the packing density of surfactant molecules at the interface, and can be obtained from the maximum surface excess concentration Γmax,28 A min =

1 NA Γmax

(2)

where NA is Avogadro’s number and Amin is in nm2. From Γmax and Amin values listed in Table 2, it is seen that the Amin values are 0.57 nm2, 0.55 nm2, and 0.37 nm2 for [C12mim]Cl, [C12mim]Br, and [C12mim]I, respectively, showing [C12mim]I has the higher packing density at the air−aqueous solution interface than [C12mim]Cl and [C12mim]Br. A possible explanation is that I− has the largest ionic radius and is more polarizable. The polarizabilities of Cl−, Br− and I− are 4.0 Å3, 4.53 Å3, and 6.90 Å3, respectively.30 Due to its larger polarizability, I− is difficult

Figure 1. Surface tension isotherms of aqueous solutions of [C12mim]Cl, [C12mim]Br, and [C12mim]I. The lines are guides to the eye.

in Table 1). The break points apparent in the isotherms correspond to the critical micelle concentrations (cmc). The

Table 1. Experimental Surface Tension (γ) of Aqueous Solutions of [C12mim]Cl, [C12mim]Br, and [C12mim]I at Different Concentrations (x) at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa [C12mim]Cl −1

a

[C12mim]Br −1

−1

[C12mim]I −1

−1

x/(mol·kg )

γ/(mN·m )

x/(mol·kg )

γ/(mN·m )

x/(mol·kg )

γ/(mN·m−1)

0.001 0.002 0.004 0.006 0.01 0.014 0.018 0.02 0.025 0.03 0.04 0.05

69.37 63.46 58.36 53.85 47.71 43.05 38.43 38.26 38.38 38.61 38.35 38.55

0.0005 0.001 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.025 0.03 0.04 0.05

68.38 62.44 58.12 51.39 43.7 40.35 36.83 36.51 36.56 36.65 36.6 36.57 36.54 36.48 36.67 36.67

0.0001 0.0005 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.009 0.012 0.016 0.02

69.81 65.15 56.7 46.92 38.08 33.49 31.45 31.48 31.39 31.5 31.6 31.57 31.66

The relative standard uncertainty for concentrations is 0.5 %, and the relative standard uncertainty for surface pressure is 0.1 mN·m−1. 1530

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Table 2. Surface Properties (Critical Micelle Concentrations (cmc), Lowest Surface Tension (γcmc), Maximum Surface Excess Concentration (Γmax), Minimum Area Per Molecule (Amin), and Adsorption Efficiency (pC20)) of Aqueous Solutions of [C12mim]Cl, [C12mim]Br, and [C12mim]I at the Temperature T = 298.2 K and Pressure p = 0.1 MPaa IL

cmc/(mol·kg−1)

γcmcb/( mN·m−1)

Γmax/(μmol·m−2)

Amin/(nm2)

pC20

[C12mim]Cl

0.0168 0.0132c 0.0106 0.0109e 0.0046

38.4 ± 0.3 39d 36.8 ± 0.3 39.4e 31.7 ± 0.3

2.91 ± 0.03 2.31c 3.03 ± 0.03 1.91e 4.47 ± 0.03

0.57 ± 0.1 0.72c 0.55 ± 0.1 0.87e 0.37 ± 0.1

2.16 ± 0.01 2.35c 2.38 ± 0.01 2.67e 2.80 ± 0.01

[C12mim]Br [C12mim]I a

Standard uncertainties u are u(T) = 0.1 K and u(p) = 5 KPa. bSurface tension was measured by NIMA PS4 pressure sensor having uncertainty of 0.3 mN·m−1, and the uncertainties of the other physical properties were calculated. cData reported in ref 21 with a Lauda TE1C digital ring tensiometer used. dData reported in ref 25 with a drop shape analysis tensiometer used. eData reported in ref 26 with a DuNuoy tensiometer used.

to hydrolyze in bulk water; instead it is more “surface active” than the other two.29,30 As it is easier to go to the surface, it is more effective in neutralizing the charges of cationic headgroups at the interface. This picture can also explain the observed decrease of the surface tension for [C12mim]I. Simultaneously, electrostatic repulsion decreases between charged headgroups in the surface layer, allowing a more compact monolayer. Another important surface activity parameter is the adsorption efficiency, pC20, which is defined as28,31 pC20 = −log C20

which assures that the cmc values obtained from surface tension measurements are reliable. The slope of conductivity decreases abruptly above cmc due to the binding of counterions to micelles, which reduces the effective charge. The degree of counterion binding to micelle (β) can be obtained from the ratio of the slopes above and below the cmc (α), as β = (1 − α).33,34 As can be seen in Table 4, the β values of the halide anions at the same temperature are different, with β(Cl−) < β(Br−) < β(I−). The largest β value for I− may also be ascribed to its largest polarizability and cavitational force,32 giving it the tendency to accumulate at the micellar interface. Thermodynamic Parameters. The change of the standard Gibbs free energy in the micellization process can be derived by the phase separation model and can be calculated from the following equation:35 cmc ΔG 0 m = (1 + β)RT ln Xcmc = (1 + β )RT ln (4) 55.4

(3)

where C20 is the ionic liquid concentration that lowers the surface tension of pure water by 20 mN·m−1. It is the minimum concentration needed for the saturation of the surface adsorption. Ionic liquids with larger pC20 values will have higher adsorption efficiency at the air−aqueous interface. From the values listed in Table 2, it can be seen that pC20 values increase in the order of [C12mim]Cl < [C12mim]Br < [C12mim]I, which indicates [C12mim]Cl has the lowest adsorption efficiency (pC20), while that of [C12mim]I is superior to the other two ionic liquids. This phenomenon ascribes to the different adsorption capabilities of halide anions at the air−water interface. Levin32 found that Cl− is strongly hydrated and is repelled from the air/water interface. By contrast, I− tends to lose its hydration sheath easily as a result of its larger polarizability and cavitational force and then is adsorbed to the surface. The surface active parameters found above are similar to from those obtained by other groups as listed in Table 2.21,25,26 While the small difference to the literature may be ascribed to the different measuring methods, the tendency of surface activity change between [C12mim]Cl, [C12mim]Br, and [C12mim]I is clear with the same method used for the samples in this study. Degree of Counterion Binding to Micelles (β). To investigate the aggregation behavior of ionic liquids in aqueous solutions, the degree of counterion binding to micelle (β) was studied by electrical conductivity (κ) measurement. The conductivity values as a function of concentration are shown in Figure 2 (numerical values are listed in Table 3) at four different temperatures. Taking [C12mim]Cl at 15 °C (Figure 2a) as an example, its κ value is 78.7 μS·cm−1 at 0.001 mol·kg−1; then it increases linearly with concentration up to 1023.5 μS·cm−1 at 0.0145 mol·kg−1. After that, the slope of the line decreases, and the break point in the plot is indicative of cmc. The cmc values of [C12mim]Cl, [C12mim]Br, and [C12mim]I from the electrical conductivity plots at different temperatures are listed in Table 4. These cmc values measured at 25 °C are in fair agreement with those obtained from the surface tension measurements in Table 2,

where cmc is in mol·kg−1, Xcmc = cmc/55.4 is the cmc in terms of the molar fraction (55.4 comes from 1 L of water corresponding to 55.4 mol of water at 25 °C), and β describes the degree of counterion binding to micelles. The variation of the standard enthalpy, ΔH0m, can be obtained from the Gibbs− Helmholtz equation for aqueous solutions.35 ⎡ ∂(ΔG 0 /T ) ⎤ m ⎥ ΔH 0 m = ⎢ ⎣ ∂(1/T ) ⎦

(5)

ΔH 0 m = −(1 + β)RT 2d ln Xcmc /dT

(6)

Finally, the standard entropy change during the formation of micelles, ΔS0m, is calculated by the following equation: ΔS 0 m =

ΔH 0 m − ΔG 0 m T

(7)

Table 4 shows the calculated thermodynamic parameters for [C12mim]Cl, [C12mim]Br, and [C12mim]I at different temperatures. It is clear that the formation of micelles is a spontaneous process, since all of the ΔG0m’s have negative values. Moreover, ΔG0m decreases with the size increase of the halide anion, indicating the increased tendency to form micelles for ionic liquids with larger anions. As for ΔH0m, it always has a negative value, implying that the micelle formation is an exothermic process. According to the previous study by Nusselder and Engberts,36 the dispersion force (hydrophobic interaction) is the main driving factor for the micelle formation with a negative ΔH0m. Similarly, the observed exothermicity here may be ascribed to the hydrophobic interaction between the surfactant and solvent. Additionally, the values of ΔH0m do 1531

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Table 3. Experimental Electrical Conductivity (κ) of Aqueous Solutions of [C12mim]Cl, [C12mim]Br, and [C12mim]I at Different Concentrations and at Four Different Temperatures (15 °C, 20 °C, 25 °C, and 30 °C) and Pressure p = 0.1 MPaa κ ([C12mim]Cl)

concentration (10−3 mol·kg−1) 0.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 a

15 °C 78.7 148.9 219.3 289.3 355.4 433 491.5 562.1 632.9 688.2 763.6 831.6 886.3 950.5 1002 1050 1073 1112 1150 1172 1197 1222 1254 1278 1308

20 °C 87.72 165.6 245.5 324.2 400.7 480.8 548 636.9 710.7 778.3 861.1 934.1 990.6 1061 1123 1175 1205 1248 1286 1315 1345 1374 1406 1436 1466

25 °C 99.17 186.6 273.5 366.6 448.2 539.8 617 710.2 795.2 871.1 963.7 1052 1121 1185 1258 1312 1355 1400 1436 1462 1505 1544 1584 1624 1656

κ ([C12mim]Br) 30 °C 109.5 206.9 304 407.6 503.3 598.4 686.8 786.7 884.5 961.2 1072 1167 1244 1323 1399 1451 1505 1549 1599 1645 1689 1734 1780 1812 1853

15 °C

20 °C

76.88 156.6 223.9 302.2 379.1 445.4 520.3 588.6 656.8 716.5 757.6 776.2 799.4 814.3 837 854.4 873.6 882 893.4 912.7 922.6 934.8 948 964 984.8

89.85 180.9 260.7 345 430.5 504.1 588.6 664 739.7 813.7 853.2 875.7 906.8 922.3 946.6 965.8 986.1 998.1 1013 1043 1056 1070 1087 1106 1119

κ ([C12mim]I)

25 °C

30 °C

100.1 198.5 290.2 385.7 480.1 562.9 655.4 740.7 823.3 909.3 963.1 986.4 1024 1040 1068 1098 1124 1132 1152 1183 1200 1225 1249 1268 1285

111.5 223.4 320.3 426.9 518.9 626.1 727.6 824.3 916.5 1008 1073 1110 1159 1180 1219 1238 1269 1290 1313 1342 1364 1389 1414 1440 1464

15 °C

20 °C

25 °C

30 °C

42.26 82.83 150.6 226.6 292.1 334.1 359.6 368 377.6 392.5 400.3 412 420.5 424.8 440.9 447.9 458.3 470.5 483.1 488.6 496.1

49.68 96.08 169.6 254.5 332 387.7 418.3 432.1 443.9 459 470.1 484.9 501.7 508.2 518.6 535 542.5 555.9 568.1 574 582.4

56.5 108 190.8 286.7 373.4 451.6 482.9 500.5 516.3 531.9 543.4 558 576 588.7 599.7 616.1 630.7 644.9 660.5 667 677.4

61.84 117.9 211.1 315.7 412.5 506.1 551.9 579.2 594.7 614.5 633.6 652.9 672 686.8 698.8 711.9 727.8 739.3 758.1 773.7 783.6

The relative standard uncertainty for concentrations is 0.5 %, and the relative standard uncertainty for electrical conductivity is 0.1 μS·cm−1.

Table 4. Thermodynamic Parameters of Micellization for [C12mim]Cl, [C12mim]Br, and [C12mim]I vs Temperature at Pressure p = 0.1 MPaa ionic liquids

temperature/°C

cmc/(mol·kg−1)

β

ΔG0m/(kJ·mol−1)

ΔH0m/(kJ·mol−1)

TΔS0m/(kJ·mol−1)

[C12mim]Cl

15 20 25 30 15 20 25 30 15 20 25 30

0.0146 0.0147 0.0151 0.0152 0.0102 0.0104 0.0106 0.0108 0.0046 0.0049 0.0052 0.0054

0.59 0.58 0.56 0.53 0.79 0.77 0.75 0.74 0.87 0.86 0.85 0.84

−27.85 −28.46 −29.31 −30.39 −36.86 −37.00 −37.12 −37.45 −42.07 −42.29 −42.51 −42.81

−4.19 −2.73 −1.17 0.44 −4.84 −4.86 −4.88 −4.92 −22.12 −19.45 −16.60 −13.55

23.66 25.73 28.14 30.83 32.02 32.14 32.24 32.53 19.95 22.84 25.91 29.26

[C12mim]Br

[C12mim]I

Standard uncertainties u are u(T) = 0.1 K and u(p) = 5 KPa. The uncertainties of cmc, β, ΔG0m, ΔH0m, and TΔS0m are 0.0001 mol·kg−1, 0.01, 0.01 kJ·mol−1, 0.01 kJ·mol−1, and 0.01 kJ·mol−1, respectively.

a

temperature. At 15 °C, the enthalpy term has a larger contribution to the negative ΔG0m, while above ∼20 °C the entropy term (−TΔS0m) plays the dominant role. So, the micellization is enthalpy-driven at 15 °C and entropy-driven above 20 °C. It is enthalpy-driven probably due to the higher binding affinity of I− to the micelle at low temperature, which leads to the release of more heat. With increasing temperature, the binding between water and the molecules of ionic liquids is disturbed more, which will increase the hydrophobic interaction of molecules in the micelle, resulting in the increased contribution of TΔS0m in ΔG0m.

not show an obvious dependence on temperature, which means that the environment of the hydrophobic chain of the ionic liquid is not significantly changed with the increase in the temperature. For [C12mim]Cl and [C12mim]Br, the negative ΔG0m is mostly contributed by TΔS0m, as can be seen from Table 4. So the micellization is an entropy-driven process, which means that the tendency of the hydrophobic group to transfer from the solvent to the micelle inside provides the primary driving force for the micelle formation. The situation is changed in the case of [C12mim]I. Both ΔH0m and TΔS0m increase their values with the increase of 1532

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[C12mim]Br is entropy-driven, whereas aggregation of [C12mim]I is enthalpy-driven at 15 °C and entropy-driven above 20 °C.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MEST) No. 2011-0017435 and No. 2011-0031496 and by the Sogang University Research grant No. SRF-20124003. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 2. Plots of electrical conductivity, κ, against concentration of (a) [C12mim]Cl, (b) [C12mim]Br, and (c) [C12mim]I at different temperatures.

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CONCLUSIONS In this work, the surface activities and bulk conductivities of aqueous solutions of long chain ionic liquids ([C12mim]Cl, [C12mim]Br, [C12mim]I) are investigated, and thermodynamic parameters to assess the surface activity of these ionic liquids are obtained. It is found that the surface activity of [C12mim]I is higher than those of [C12mim]Cl and [C12mim]Br, which may be ascribed to the higher polarizability and cavitational force of I− anion. The thermodynamic parameters of micellization (ΔG0m, ΔH0m, ΔS0m) are calculated from conductivity measurement, which indicates that the micellization of [C12mim]Cl and 1533

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dx.doi.org/10.1021/je301147k | J. Chem. Eng. Data 2013, 58, 1529−1534