Micellization and Thermodynamic Study of 1-Alkyl-3

Article pubs.acs.org/jced

Micellization and Thermodynamic Study of 1‑Alkyl-3methylimidazolium Tetrafluoroborate Ionic Liquids in Aqueous Solution Ying Wei, Fang Wang,* Zhiqing Zhang, Chengcheng Ren, and Yan Lin College of Science, China University of Petroleum, 66 West Changjiang Road, Qingdao, Shandong 266580, P. R. China S Supporting Information *

ABSTRACT: Three surfactant-like ionic liquids, 1-dodecyl-3-methylimidazolium tetrafluoroborate (C12mimBF4), 1-tetradecyl-3-methylimidazolium tetrafluoroborate (C14mimBF4), and 1-hexadecyl-3-methylimidazolium tetrafluoroborate (C16mimBF4) had been systematically studied by conductivity measurement, surface tension, steady-state fluorescence measurement and ultraviolet (UV) absorption spectra at 298.15 K. Micellization of the three ILs were certified by the above methods and aggregation number of micelles (Nagg) were determined by pyrene fluorescence quenching method. A comparison of CnmimBF4 with different alkyl chain lengths shows that that the critical micelle concentration (cmc) decreased remarkably with the increase of alkyl chain length, but the surface tensions at cmc (γcmc) were approximately the same, except for that of C16mimBF4. The effects of temperature, inorganic salt, and organic alcohols on the cmc of CnmimBF4 (n = 12, 14, 16) aqueous solution were also investigated. The results showed that the cmc values assumed a trend of increase along with the increase of temperature, and decreased remarkably with the presence of inorganic salt. For alcohol−water systems at lower temperature, the cmc values increased with the presence of short chain alcohols (ethanol, 1-propanol), but decreased or were invariable with long chain alcohols (1-butanol, 1-pentanol). Finally, the micellization thermodynamic parameters (ΔGm°, ΔHm°, ΔSm°) of CnmimBF4 (n = 12, 14, 16) showed that the micelle formation process of C12mimBF4 was entropy-driven, and the micellization behavior of C14mimBF4 and C16mimBF4 was found to be an enthalpy-driven process in the investigated temperature range. (SAILs),14−23 combining properties of ILs and surfactants.14,15 The SAILs can self-assemble in aqueous media to form micelles, vesicles, lyotropic liquid crystals, and gels, etc.24 Since the hydrophobicity of SAILs can be tuned by changing the alkyl chain length, the type of head-groups and the nature of anions,16,25,26 also can change the structure of aggregates formed by the SAILs. The self-assembly of amphiphilic ILs plays an important role in the application of many chemical and biotechnological processes. ILs with cations with hydrophobic chains, for example, 1alkyl-3-methylimidazolium cation ([Cnmim]+), have been extensively studied. Results revealed that this amphiphilic ILs can form various aggregates in aqueous solutions.2,25−30 The micelle formation of CnmimBr with a long-hydrocarbon chain in aqueous solution had been investigated by different research groups.30,31 The surface activity of CnmimBr is superior to that of conventional cationic surfactants with the same hydrocarbon chain, and the ILs molecules have higher aggregation tendency. Wang et al. observed that the nature of anions and the ring type of cations weakly influenced the morphology of the aggregates,

1. INTRODUCTION Ionic liquids (ILs) are kind of organic molten salts at or close to room temperature, composed of organic cations and inorganic anions.1,2 They have incomparable advantages, such as nonvolatility, nonflammability, high ion-conductivity, and outstanding catalytic properties.3 These favorable properties had driven ILs to be widely applied as electrolytes,4 homogeneous catalysts,5,6 and solvent media for reactions and used in separation techniques.5−8 The great advantage of ILs is their structural designability. The physical and chemical properties of ILs, which have been discussed in previous studies,9−13 can be tailored by an appropriate selection of cation, anion, or substituent. An interesting characteristic of ILs is that cations with long alkyl chains show obvious amphiphilic character, as shown in Figure 1. This kind of ILs is called surface active ionic liquids

Received: April 30, 2013 Accepted: March 25, 2014 Published: April 2, 2014

Figure 1. Chemical structure of 1-alkyl-3-methylimidazolium tetrafluoroborate, CnmimBF4, n = 12, 14, and 16. © 2014 American Chemical Society

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dx.doi.org/10.1021/je400861g | J. Chem. Eng. Data 2014, 59, 1120−1129

Journal of Chemical & Engineering Data

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but highly affected the aggregates size.32 Bowers and coworkers studied the micellization behavior of C4mimBF4, C8mimCl, and C8mimI in aqueous solution.2 The surface tension values of C4mimBF4 and C8mimI were similar despite the difference in chain length and were slightly larger than that of C8mimCl. This indicated that the surface tension of ILs was relative to the nature of anions. Dorbrita et al. discussed the aggregates formation of C4mimBF4 in water, methanol, 2propanol, and ethyl acetate by means of electrospray ionization mass spectrometry and conductivity measurements. They concluded that the size of the formed aggregates decreased with an increase in the polarity of solvent and a decrease of the concentration of ILs.33 Zheng et al. investigated the micellization behavior of C12mimBF4 and C12mimBr in ethylammonium nitrate (EAN) by a surface tension method and 1H NMR spectroscopy.16 The values of cmc and minimum surface area per molecule (Amim) decreased when replacing Br− with BF4−. Kunz et al. also reported the micelle aggregation of C16mimBF4 and C16mimCl in EAN and observed that the cmc and surface tension of the solution at the cmc (γcmc) of C16mimBF4 were lower than that of C16mimCl.34 The previous reports showed that CnmimBF4 has many physicochemical properties superior to that of CnmimX (X = Br, Cl), but the research was mainly foused on the short alkyl chain length (n = 4, 6, 8). Therefore, in our work, we studied the aggregation behavior and physicochemical properties of CnmimBF4 (n = 12, 14, 16) in aqueous solution by conductivity, surface tension, fluorescence probe techniques, and UV absorption spectra. In addition, it is known to all that counterion and alcohol have strong influence on cmc, Nagg, size, and shape of the aggregates of the micelle system.35−37 So, in this paper, we also discussed the effects of the additives (inorganic salt and organic alcohol) on the micellization behavior of CnmimBF4 aqueous solution. The thermodynamic behavior of the ILs provides detailed understanding of the driving force behind micelle aggregation.24,38−41 As an extension of the previous study, we also analyzed the micellization mechanism of CnmimBF4 aqueous solution through thermodynamic parameters, which were calculated from the temperature dependence of cmc and the degree of counterion association to the aggregates (β) obtained by electrical conductivity measurements. This work will add to the knowledge of the micellization behavior of long alkyl chain imidazolium ILs.

tubes and dried with nitrogen. Then the ILs solution was added. The deionized water was used in all the solutions preparation. Conductivity Measurements. Electrical conductivities of ILs solutions were measured using a DDS-11A conductometer (Shanghai Leici Experimental Instrument Factory). The temperature of the conductance cell was controlled to ± 0.10 K by circulating water from a ZH-1C constant-temperature bath (Nanjing Duozhu Instrument Company). Each conductivity was recorded after allowing sufficient time for equilibration, Conductivity stability was better than 1 %. The conductivity measurements were carried out at four different temperatures (298.15 to 313.15) K for each IL system. The cmc and β were determined from the curves of conductivity versus concentration. Surface Tension Measurements. Surface tensions were carried out on a JK99B tensiometer (Shanghai Zhongchen Instrument Co., Ltd., accuracy ± 0.05 mN·m−1) using the wilhelmy plate method. The temperature was controlled at (298.15 ± 0.10) K using a thermostatic bath. All measurements were repeated until the experiment values were reproducible. From surface tension curves (γ−C), the values of cmc, γcmc, and the effectiveness of surface tension reduction (Πcmc) were determined.42 Steady-State Fluorescence Measurements. Steady-state fluorescence measurements were taken using F97pro spectrofluorophotometer; 335 nm was selected as the excitation wavelength, and the emission spectra were scanned from (350 to 500) nm. Pyrene and benzophenone were used as fluorescence probe and quencher, respectively. Pyrene exhibits five peaks in (370 to 400) nm regions of the steady-state fluorescence emission spectra. The first and the third vibronic peaks appear at (373 and 383) nm, respectively. The ratio of I1/ I3 shows the extreme dependence on solvent, and hence, it can be used to probe the micropolarity of the surfactant aggregates and also to determine the cmc of the surfactants aqueous solution.43 The concentration of pyrene was kept constant at 1· 10−6 mol·L−1. The measurement temperature was conducted at 298.15 K. The steady-state fluorescence quenching technique was used to determine Nagg. The pyrene/benzophenone pair assures that the residence time of quencher into the aggregates is longer than the fluorescence lifetime of the probe.44 Ultraviolet Absorption Spectra. UV absorption spectra were measured on a TU-1810 ultraviolet−visible spectrophotometer, made by Beijing purkinje general Instrument Co. Ltd. The wavelength was scanned from (220 to 500) nm. Pyrene was used as an UV absorbent, and the absorption intensity is extremely dependent on the polarity of the microenvironment. The cmc values can be obtained from the curves of absorption intensity versus concentration of CnmimBF4 solutions.

2. EXPERIMENTAL SECTION Materials. The SAILs 1-dodecyl-3-methylimidazolium tetrafluoroborate (C12mimBF4, > 99 %), 1-tetradecyl-3methylimidazolium tetrafluoroborate (C14mimBF4, > 99 %), 1-hexadecyl-3-methylimidazolium tetrafluoroborate (C16mimBF4, > 99 %) were purchased from Centre for Green Chemistry and Catalysis, LICP, CAS. The chemical structure was displayed in Figure 1. Benzophenone was obtained from Sinopharm Chemical Reagent Co., Ltd. Pyrene was from Acros Organics. Ethanol, 1-propanol, 1-butanol, and 1-pentanol, obtained from Sinopharm Chemical Reagent Co., Ltd., were all analytical reagents. Potassium tetrafluoroborate (KBF4, analytical reagents, ≥ 99 %) was purchased from the Tianjin Damao Chemical Reagent Factory, China. Dilute stock solutions of pyrene (1·10−3 mol·L−1) were prepared by dissolving pyrene in methanol in a volumetric flask. Samples for fluorescence and UV were prepared as follows: equivalent amounts of probe stock solutions were transferred into test

3. RESULTS AND DISCUSSION The Critical Micelle Concentration of CnmimBF4. The cmc of surfactants is usually determined as a break point on the plot of a given property (conductivity, surface tension, microenvironment, etc.). Experimental conductivities (κ) of the three CnmimBF4 aqueous solutions with different chain lengths at 298.15 K were shown in Figure 2 as a function of ILs concentration (the detailed dates of conductivity were listed in Table 1). It was obvious that two distinguishable straight lines appeared in the curves with the increase of CnmimBF4 concentration. In the low concentration domains, the growing 1121



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Figure 2. Conductivity dependence of concentration for CnmimBF4 in aqueous solution at 298.15 K.

Figure 3. Variation of surface tensions as a function of concentration for CnmimBF4 with different chain length at 298.15 K.

Table 1. Experimental Values of Conductivities (κ) for the Three CnmimBF4 (n = 12, 14, 16) in Solutions of Water at Temperature T = 298.15 K, and at Pressure p = 0.1 MPa.a

obviously indicates the formation of aggregates, since pyrene preferentially locates in a more hydrophobic microenvironment of the aggregates relative to water.43 Figure 4 represented the variation of I1/I3 ratio with various concentration of CnmimBF4, which was utilized to estimate the cmc value.

CC12mimBF4 mmol·kg 0.30 0.60 1.00 1.80 3.00 6.00 8.00 10.0 18.0 30.0 40.0 60.0

−1

κ

κ

CC14mimBF4 −1

mS·cm

mmol·kg

0.324 0.346 0.379 0.455 0.553 0.629 0.640 0.653 0.712 0.809 0.910 1.098

0.10 0.18 0.30 0.60 1.00 1.80 3.00 6.00 10.0 18.0 30.0 40.0 60.0

−1

mS·cm

CC16mimBF4 −1

0.307 0.311 0.320 0.344 0.360 0.368 0.376 0.393 0.423 0.487 0.543 0.633 0.732

−1

mmol·kg 0.003 0.005 0.007 0.010 0.020 0.030 0.050 0.070 0.10 0.20 0.30 0.50 0.70 1.00

10κ mS·cm−1 0.022 0.022 0.024 0.032 0.040 0.052 0.070 0.103 0.142 0.203 0.265 0.372 0.462 0.593

a Standard uncertainty u are u(C) = 0.002 mmol·kg−1, u(T) = 0.01 K, u(p) = 10 kPa, and the combined expanded uncertainty Uc is Uc(κ) = 0.010 mS·cm−1 with 0.95 level of confidence (k = 2).

Figure 4. I1/I3 ratio of pyrene as a function of concentration for CnmimBF4 aqueous solution (n = 12, 14, 16) at 298.15 K.

BF4−

number of free Cnmim and ions led to the rise of κ, whereas the abrupt change in curves originated from the start of micelle formation. The concentrations at which the two linear fragments intersected are assigned to cmc, as shown in Figure 2. Surface tension measurements were performed to determine the surface activity and cmc of CnmimBF4 (the detailed values of surface tension were listed in Table 3). The results were shown in Figure 3. Below the cmc, the surfactant molecules absorb at the interface and loosely arrange as monomers. With increased concentration, the CnmimBF4 molecules are closely arranged, which leads to a rapid decrease in surface tension. When the concentration reaches the region of cmc, the surfactant molecules begin to build up their own structure, and the aggregates are formed, and correspondingly, the surface tension becomes almost constant. Thus the cmc is determined from the intersection of the two straight lines in the low- and high-concentrations regions in the surface tension curves. The steady-state fluorescence probe technique that involved a pyrene probe is always used to determine the cmc of surfactant aqueous solutions. The abrupt decrease in I1/I3 +

The UV absorption spectra are also served as a simple and accurate method to determine the cmc of many surfactants, especially for the mixed systems.45 The UV absorption spectra were measured with various concentrations of CnmimBF4 solutions containing pyrene as the UV absorbent. In polar media, the intensity of the peaks enhanced slowly, while the absorption intensity enhanced dramatically when the microenvironment changed. Thus, the abrupt change on the curve of absorption intensity vs concentration signified that the aggregates of ILs were formed. Figure 5 represented the plots of absorption intensity versus concentration of CnmimBF4 (n = 12, 14, 16) at a fixed wavelength. The cmc values of CnmimBF4 with different chain length obtained from the four methods mentioned are listed in Table 2. Effect of Hydrocarbon Chain Length and the Type of Counterion on cmc. From Table 2, we can conclude that the cmc values of CnmimBF4 decreased with increasing alkyl chain length. In addition, the cmc of other ILs with different chain 1122



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According to the literature, the cmc was also dependent on the size of hydrophilic groups and the type of anions. From Table 4, a comparison of CnmimX with the same cation and hydrocarbon chain length but different anions shows that the values of cmc increased in the sequence CnmimBF4 < CnmimBr < CnmimCl. In other words, the ability of these anions to promote micellization of CnmimX followed the order BF4− > Br− > Cl−. The effect of anions on micellization is considered to be relative to its hydrated radius, polarizability, hydrophobicity, and size. A decreased hydrated radius, in other words, a high hydrophobicity of anions (BF4− > Br− > Cl−), could enhance binding at the aggregates surface, which leads to the decrease of electrostatic repulsion between head groups, thereby increasing the tendency to aggregate and lowering values of cmc. Effect of Hydrocarbon Chain Length and the Type of Counterion on Surface Tension. The surface tension can also investigate the interface behavior and surface activity of CnmimBF4 aqueous solutions with different chain length. The value of γcmc is a measure of the efficacy of the surfactant to populate the air/water interface in the form of a monolayer prior to the micellization. Πcmc indicates the maximum reduction of surface tension at cmc, so γcmc and Πcmc can be used as measures to evaluate the effectiveness of surfactant to lower the surface tension of solvent.31,42 The γcmc and Πcmc were obtained from Figure 3 and were summarized in Table 4 together with that of CnmimCl and CnmimBr. Increasing concentration resulted in the orderly gathering of CnmimBF4 molecules at the air/water interface to avoid energetically unfavorable contact of hydrophobic hydrocarbon chains and water. The adsorption of surfactant molecules at the air/water interface led to the decrease of surface energy. Therefore, the surface tension decreased sharply until becoming almost constant with further increased in the concentration of CnmimBF4 in the solutions, and the distinct point in the γ-C curves corresponding to the γcmc. The γcmc values were found to be (28.4, 28.1, and 35.4) mN·m−1 for C12mimBF4, C14mimBF4, and C16mimBF4, respectively. It was evident that the γcmc values did not change significantly with the increase of alkyl chain length except for the C16mimBF4. This change was consistent with that of CnmimCl and CnmimBr (Table 4), and this was reasonable because they were homologues with little difference in the hydrocarbon chain length. In general, the hydrocarbon chain length weakly influences the γcmc.42 On the other hand, by comparing the dates of CnmimX with same cation and alkyl chain length, but different anions, it was found that γcmc changed significantly and γcmc of CnmimBF4 aqueous solutions were smaller than that of CnmimCl and CnmimBr. In general, the surfactant with more hydrophobic anion exhibits a slightly lower surface tension (at the same temperature).48 Since BF4− is less hydrated than Br− and Cl−, the BF4− were more attracted with ILs molecules, which effectively decreased the electrostatic repulsion between the imidazolium head groups. As a result, Cnmim+ ions were adsorbed at the water/air interface more densely, and accordingly, the γcmc value became smaller for CnmimBF4. These results indicated that the effect of the hydrocarbon chain length and the counterions on the γcmc were quite different, the latter being greater. From Table 4, it can be seen that the values of Πcmc (Πcmc = γ0 − γcmc) increased following the order of CnmimCl, CnmimBr, and CnmimBF4, which indicated that the surface activity of CnmimBF4 was superior to that of CnmimBr and CnminCl.

Figure 5. The plots of absorption intensity vs concentration of CnmimBF4 at a fixed wavelength.

Table 2. The cmc Values Determined by Conductivity Measurement, Surface Tension, Steady-State Fluorescence Measurement and UV Absorption Spectra, for the CnmimBF4 with Different Chain Length in Solutions of Water at 298.15 K cmc/mmol·kg−1 ILs

conductivity measurement

surface tense

fluorescence measurement

UV

C12mimBF4 C14mimBF4 C16mimBF4

3.58 1.03 0.12

3.47 1.24 0.075

3.61 0.858 0.073

3.14 1.02 0.11

Table 3. Surface Tension Experimental Values for CnmimBF4 (n = 12, 14, 16) in Solutions of Water at Temperature T = 298.15 K, and at Pressure p = 0.1 MPab CC12mimBF4

γ

CC14mimBF4

γ

CC16mimBF4

γ

mmol·kg−1

mN·m−1

mmol·kg−1

mN·m−1

mmol·kg−1

mN·m−1

0.30 0.60 1.00 1.80 3.00 6.00 8.00 10.0 18.0 30.0 60.0

56.4 50.4 43.1 34.3 29.6 28.2 28.3 28.2 28.3 28.0 27.7

0.10 0.18 0.30 0.60 1.00 1.80 3.00 6.00 10.0 18.0

47.9 41.9 39.0 33.6 29.9 28.4 27.8 27.6 27.3 27.0

0.010 0.020 0.030 0.050 0.070 0.10 0.20 0.30 0.50 0.70

52.9 47.9 43.8 38.4 34.9 36.7 37.0 36.4 35.2 35.2

Standard uncertainty u are u(C) = 0.002 mmol·kg−1, u(T) = 0.01 K, u(p) = 10 kPa, and the combined expanded uncertainty Uc is Uc(γ) = 0.3 mN·m−1 with 0.95 level of confidence (k = 2). b

length and anions were collected in Table 4. As reported,14,46,47 a remarkable decrease in cmc values was also found for CnmimCl and CnmimBr going from C12mimX to C16mimX. Such a decrease could be due to the stronger hydrophobic interactions with longer alkyl chain length; hence, the micelles can form easily. 1123



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Table 4. Comparison of cmc (from Electrical Conductivity Measurement); γcmc and Πcmc of CnmimCl, CnmimBr and CnmimBF4 as a Function of Alkyl Chain Length (n = 12, 14, 16) in Solutions of Water at 298.15 K CnmimCl

a

CnmimBF4f

CnmimBr

cmc

γcmc

cmc

γcmc

cmc

γcmc

Πcmc

n

mmol·L−1

mmol·L−1

mN·m−1

mmol·L−1

mN·m−1

mN·m−1

mmol·kg−1

γmN·m−1

mN·m−1

12 14 16

15a 4a 0.87b

38.7 37.5 37.0

33.6 34.3 34.8

9.68 2.69 0.51

39.4 39.2 39.1

33.6 33.8 33.9

3.58 1.03 0.12

28.4 28.1 35.4

43.0 43.8 36.8

c

Πcmcc

d

Πcmc

e

e

Reference 47. bReference 13. cReferences 38 and 41. dReference 46. eReferences 30 and 31. fCnmimBF4 were determined by our experiments.

Table 5. The cmc Values for the System CnmimBF4 (n = 12, 14, 16) in Solutions of Water + Inorganic Salt KBF4 (the Molarity of KBF4 in the Solutions of Water is 0.10 mmol·kg−1) or Water + Organic Alcohols (the Mass Fraction of Alcohol is 1 %) at Different Temperatures T cmc/mmol·kg−1 ILs

T/K

no additive

KBF4

C2H5OH

C3H7OH

C4H9OH

C5H11OH

C12mimBF4

298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

3.58 3.75 4.22 4.39 1.03 1.27 1.81 2.22 0.121 0.156 0.176 0.290

3.11 3.35 3.50 4.02 0.828 1.01 1.23 1.40 0.093 0.131 0.150 0.212

3.86 4.04 4.31 4.65 1.16 1.34 1.85 2.01 0.137 0.194 0.235 0.304

3.63 3.97 4.31 4.55 1.10 1.41 1.84 2.09 0.151 0.203 0.255 0.333

3.53 3.87 3.97 4.37 0.901 1.07 1.41 1.51 0.116 0.167 0.178 0.273

3.46 3.70 4.04 4.30 0.874 0.903 1.34 1.44 0.120 0.165 0.184 0.253

C14mimBF4

C16mimBF4

Effect of Additives on Micellization of CnmimBF4. The micellization behavior of CnmimBF4 exhibited lower cmc and γcmc values when compared with traditional cationic surfactants with the same hydrophobic alkyl chain. As is well known, the self-assemble of micelles in water can be affected by the nature of additives presented in solution.49,50 Figures S1 to S8 (in Supporting Information) showed the electrical conductivities plots affected by KBF4 and different organic alcohols in varying concentrations of CnmimBF4 (n = 12, 14, 16) aqueous solution, respectively (the detailed dates of conductivity affected by KBF4 and different organic alcohols at different temperatures were listed in Supporting Information, Tables S1 to S5). The cmc values determined from Figures S1 to S8 were listed in Table 5. The effect of KBF4 and organic alcohols on the cmc of CnmimBF4 with different chain length showed the same trend. It was obvious that the cmc decreased when KBF4 was presented in CnmimBF4 aqueous solution. This phenomenon is caused by the counterion effect. With an increase in the amount of counterions, enhancing the binding of counterions at the surface of micelles and also decreasing the electrostatic repulsion between imidazolium head-groups of CnmimBF4 accordingly increases the tendency to aggregate and lowers the cmc. It can be seen from Figure S5 to S8 that cmc of CnmimBF4 tended to increase with ethanol and 1-propanol added, and slightly decreased or did not change when 1-butanol and 1pentanol were added. In general, the polar organic molecular affects the properties of surfactant by protecting or destroying the structure of water surrounding the hydrophobic chains. The short chain alcohol can destroy the “iceberg structure” surrounding the hydrophobic chains of surfactant molecules, which can weaken the hydrophobic effect and the ability of

micellization of CnmimBF4, thereby the cmc tended to increase. This maybe the reason that ethanol and 1-propanol caused cmc values to increase. But cmc values were almost invariable or slightly decreased when 1-butanol and 1-pentanol was added. With the chain length of alcohol increasing, the solubility of alcohol in water was lower, producing the “iceberg structure” around the long chain alcohol molecules. This effect was favorable for the long chain alcohols participating in the process of micellization. In general, the effect caused by long chain alcohols corresponded to that of cosurfactants. In this way some long chain alcohol molecules can insert into the “palisade layer” of micelle aggregates formed by CnmimBF4 molecules, which can decrease the electrostatic repulsion forces of ionic head-groups in the micelle. Both effects justified the lower cmc values when the additives were 1-butanol and 1-pentanol. Aggregation Number of CnmimBF4. The steady-state fluorescence probe technique was also used to determine microenvironment and Nagg of CnmimBF4 aqueous solutions. As seen in Figure 4, I1/I3 values were nearly the same above cmc. The low I1/I3 values indicated that pyrene was solubilized in the palisade layer of the micelle.43 The I1/I3 values for C12mimBF4, C14mimBF4, and C16mimBF4 were 1.34, 1.30, and 1.35, respectively. It can be seen that I1/I3 values of CnmimBF4 micelles were decreased with the increase of chain length, and except for that of C16mimBF4, this behavior was very similar to that of CnmimBr (Table 6). It was well-known that I1/I3 values of CnmimBr were larger than those in CnTAB micelles with the same hydrocarbon chain length.51 This suggested that the palisade layer of CnmimBr micelles packed less tightly than that of CnTAB. This might be attributed to the hydrophobicity of longer chain length which led to the molecules pack more tightly in micelles. Furthermore, a comparison of CnmimBF4 1124



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Table 6. I1/I3 Values of Pyrene Solubilized in Micelles, and Nagg Determined by Steady-State Fluorescence Measurements for ILs in Solutions of Water at 298.15 K

a

ILsa

I1/I3

C12mimBr C14mimBr C16mimBr C12mimBF4 C14mimBF4 C16mimBF4

1.28 1.25 1.24 1.34 1.30 1.35

smaller than that of CnmimBr with the same chain length. This may be caused by (1) the small aggregate size in aqueous solution, this result being accordant with the trend of β and also (2) in our former work, we had found that the I1/I3 value of CnmimBF4 was larger than that of CnmimBr. This maybe due to the loose packing of CnmimBF4 molecules in their micelles, which causes the water molecules to penetrate into the micelles, and may lead to the bending of the long rigid hydrophobic alkyl chain.26 Thermodynamic Analysis of Micellization. The thermodynamic parameters of the micellization process provided valuable information about the driving force of micelle formation. These parameters were calculated through the cmc and β, which were measured from the electrical conductivities curves of CnmimBF4 aqueous solution at different temperatures (the detailed dates of conductivity at different temperatures were listed in Supporting Information, Table S6). The β can be obtained by the slope ratio of dκ/dC from the two straight lines above and below the cmc (Supporting Information, Figure S9).54 The size of the micelle, charge density, and hydrophobicity of the counterion can influence the β value. It is reported that more hydrophobicity counterions are attracted at the surface of larger micelles than that of the smaller ones.55,56 The β values of CnmimBF4 with various chain length at different temperatures are listed in Table 7. We can see that the β values decreased as the alkyl chain length increased from dodecyl- to hexadecyl- at the same temperature. This may be due to the larger size of micelles formed by the highly ordered and compact aggregate structures with C12mimBF4 molecules.41 This result also can be verified by Nagg. Also, the β values of three CnmimBF4 solutions decreased with the increase of temperature. The thermal motion of ions was the major factor for the decrease of β. We have also found that the cmc values of CnmimBF4 with different chain length all increased as the temperature increased, and the increasing trend fit quadratic curves. In general, the effects of temperature on ionic surfactants mainly include the following aspects:57−59 The degree of hydration of hydrophilic ionic head-groups reduces at higher temperature, which facilitates micelle formation. Whereas, a higher temperature enhances the solubility of hydrophobic chains, the thermal agitation from high temperature disrupts the structured water surrounding the hydrophobic alkyl chains. In other words, the “iceberg structure” surrounding the hydrophobic chains is destroyed as temperature increase, which hinders micelle formation. It was found that cmc of CnmimBF4 increased with an increase in temperature in our work, so the latter effect played a crucial role in micellization. According to the mass-action model of micellization, the standard Gibbs free energy of micellization, ΔGm°, for 1:1-type ionic surfactants is related to cmc and β according to the following equation.39,60−62

Nagg 37 48 64 58 39 25

(at (at (at (at (at (at

20 mmol·L−1) 10 mmol·L−1) 4 mmol·L−1) 20 mmol·kg−1) 10 mmol·kg−1) 3 mmol·kg−1)

Note: CnmimBr, ref 31.

with CnmimBr shows that the I1/I3 values of CnmimBF4 micelles were larger than those of CnmimBr micelles. This occurs because the steric hindrance between the imidazolium headgroup Cnmim+ and counterion BF4− was stronger, since the size of BF4− is larger than Br−; in this way CnmimBF4 molecules packed loosely in micelles. Additionally, the Nagg was determined by steady-state fluorescence quenching technique according to the Turro-Yekta method.52

Nagg × CQ ⎛I ⎞ ln⎜ 0 ⎟ = ⎝ I ⎠ C ILS − cmc

(1)

where I0 and I represent the fluorescence intensities of pyrene at 373 nm in the absence and in the presence of quencher (benzophenone in the present case), CQ and CILs are the molar concentration of quencher and ILs, respectively. The Nagg can be calculated from the slope of ln(I0/I) vs CQ plot at a fixed CILs. Figure 6 displalys the ln(I0/I) as a function of CQ in

Figure 6. ln(I0/I) of pyrene as a function of concentration of the quencher benzophenone in CnmimBF4 aqueous solution at 298.15 K.

CnmimBF4 aqueous solutions, and the good linear correlations are seen. The relevant Nagg value was obtained by applying eq 1, and these values are listed in Table 6. It can be seen that the Nagg of CnmimBr increased on going from dodecyl- to hexadecyl- as reported in ref 31. The reported Nagg of CnTAB micelles were 55 (C12TAB), 70 (C14TAB), and 89 (C16TAB).53 The Nagg of CnmimBr were much less than that of CnTAB with the same hydrocarbon chain length. It may be attributed to the different hydrophobicity of chain length and the different cationic headgroup size between CnmimBr and CnTAB. However, it was found that the Nagg were (58, 39, and 25) for C12mimBF4, C14mimBF4, and C16mimBF4, respectively, and

ΔGmo = (1 + β)RT ln Xcmc = (1 + β)RT ln

cmc 55.4

(2)

where Xcmc is the cmc in mole fraction, cmc is that in mol·L−1, and 55.4 indicates that 1 L of water corresponds to 55.4 mol of water at 298.15 K.39 The ΔGm° values of CnmimBF4 were calculated according to eq 2 using the values of cmc and β listed in Table 7. The standard enthalpy of micellization (ΔHm°) can be derived by the Gibbs−Helmholtz equation: 1125



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Table 7. The Values of cmc and β, and Thermodynamic Parameters for CnmimBF4 in Solutions of Water at Different Temperatures T T

cmc

ΔGm°

ΔHm°

−TΔSm°

IL

K

mmol·kg−1

β

kJ·mol−1

kJ·mol−1

kJ·mol−1

C12mimBF4

298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

3.58 3.75 4.22 4.39 1.03 1.27 1.81 2.22 0.121 0.156 0.176 0.290

0.89 0.89 0.88 0.86 0.89 0.87 0.79 0.76 0.60 0.55 0.55 0.54

−45.4 −45.7 −45.8 −45.8 −51.2 −50.5 −47.4 −46.3 −51.7 −49.8 −50.2 −48.9

−21.9 −21.6 −21.2 −20.6 −75.5 −76.4 −76.7 −74.9 −46.3 −58.9 −74.2 −90.1

−23.5 −24.1 −24.6 −25.2 24.3 25.9 27.3 28.6 −5.43 9.17 23.9 41.1

C14mimBF4

C16mimBF4

Figure 7. Thermodynamic parameters of aggregate formation as a function of temperatures for CnmimBF4 with different chain length: a, C12mimBF4; b, C14mimBF4; c, C16mimBF4.

ΔHmo

⎡ ΔGmo ⎢∂ T =⎢ 1 ⎢⎣ ∂ T

( ) ⎤⎥ = −T ∂( ) ()

⎥ ⎥⎦

2

entropy of micellization (ΔSm°) was obtained through the following relation:

ΔGmo T

∂T

cmc ⎡ ∂ln 55 . 4 2⎢ = −(1 + β)RT ⎢ ⎢⎣ ∂T

ΔSmo =

( ) ⎤⎥ ⎥ ⎥⎦

ΔHmo − ΔGmo T

(4)

The thermodynamic parameters of micelle formation process at different temperatures were calculated and shown in Table 7. The plots of ΔGm°, ΔHm°, and −TΔSm° vs temperatures were shown in Figure 7. The values of ΔGm° were all negative for CnmimBF4 (n = 12, 14, 16) in the range of temperatures investigated, which indicated a spontaneous process of

(3)

The variation of ln(cmc/55.4) as a function of T was approximated to a quadratic equation. Then, the standard 1126



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micellization in water. ΔHm° were negative for CnmimBF4, indicating that the micellization process was exothermic at the temperatures investigated. It is known that the enthalpy change of micelle formation in aqueous solution is mainly the result of hydrophobic effect and electrostatic interactions.59,63 During micellization in aqueous solution, the hydrocarbon chains of CnmimBF4 molecules transfer from water to the micelle core, and the solvating water molecules are released from the alkyl chain simultaneously; this process is expected to be exothermic.64 Electrostatic interactions can be divided into two aspects. One is the self-repulsion of head-groups and counterions, which is an exothermic function. The other is the attraction between headgroup and counterion, which is an endothermic function.65,66 In this case, there is a strong interaction among the imidazole cations of CnmimBF4, that suggests that the self-repulsion is stronger than the attraction. So, the contribution of electrostatic interactions should also be exothermic. The micellization of a surfactant can be described by two processes:46 (a) the dehydration of the hydrocarbon (ΔHm° > 0, ΔSm° > 0); (b) aggregation of the hydrocarbon chain to form micelles (ΔHm° < 0, ΔSm° < 0). Obviously, ΔGm° values are dependent on the relative contribution of enthalpy and entropy during the two processes in the systems. According to the Gibbs−Helmholtz equation, ΔHm° and ΔSm° have an opposite effect on ΔGm°. Figure 7a showed that the entropy term −TΔSm° played the dominant role in the negative ΔGm°; in other words, the micellizaton of C12mimBF4 was entropy-driven. But for C14mimBF4 and C16mimBF4 systems (Figure 7b,c), the negative ΔGm° values were mainly a result of ΔHm °, therefore, the micellization of C14mimBF4 and C16mimBF4 in aqueous solutions was enthalpy-driven in the range of temperatures investigated.

Article

ASSOCIATED CONTENT

S Supporting Information *

The electrical conductivities plots affected by KBF4 and different alcohols in varying concentrations of CnmimBF4 (n = 12, 14, 16) solutions at different temperatures (293.15 to 313.15) K are provided in Figures S1 to S8 (the detailed dates of conductivity affected by KBF4 and different organic alcohols at different temperatures are listed in Tables S1 to S5). The electrical conductivities curves of CnmimBF4 aqueous solution at different temperatures are also shown in Figure S9 (the detailed dates of conductivity at different temperatures are listed in Table S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-532-86984568. Funding

This work was supported by the National Natural Science Foundation of China (NSFC, No. 51103179) and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2011BL017), and the Fundamental Research Funds for the Central Universities (12CX02015A, 13CX06003A, 14CX02007A) and the scholarship of China Scholarship Council (CSC 201206455009). Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS We have studied systematically the micellization of the longchain imidazolium ionic liquid CnmimBF4 with different chain length (n = 12, 14, 16) in aqueous solution. Various physical and chemical parameters for characterization of CnmimBF4 aggregates had been obtained from the results of conductivity, surface tension, fluorescence measurement, and UV absorption spectra. These results indicated that the micellization behavior of CnmimBF4 were affected by the hydrocarbon chain length and the type of anions, that is, the longer the hydrocarbon chain is, the easier is the formation of aggregates. Also, the surface activity of CnmimBF4 was superior to that of CnmimBr and C nminCl with the same alkyl chain length. The fluorescence pyrene probe sensed higher micropolarity of the palisade layer for CnmimBF4 micelles than that of CnmimBr and C n TAB, owing to greater loose packing of C n mimBF 4 molecules in micellization and the small size of micelles caused by the interaction between Cnmim+ and BF4−. Moreover, the cmc of CnmimBF4 aqueous solutions decreased remarkably with the presence of inorganic salt. For an alcohol−water system, the cmc values increased with the addition of ethanol and 1-propanol but slightly decreased or were invariable with the addition of 1-butanol and 1-pentanol. The detailed analysis of thermodynamic parameters revealed that the micellization of C14mimBF4 and C16mimBF4 was an enthalpy-driven process, whereas the micellization of C12mimBF4 was an entropy-driven process in the investigated temperature range. 1127



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