J. Phys. Chem. B 2007, 111, 6181-6188
6181
Conductivities, Volumes, Fluorescence, and Aggregation Behavior of Ionic Liquids [C4mim][BF4] and [Cnmim]Br (n ) 4, 6, 8, 10, 12) in Aqueous Solutions Jianji Wang,* Huiyong Wang, Sheli Zhang, Hucheng Zhang, and Yang Zhao School of Chemical and EnVironmental Sciences, Henan Key Laboratory of EnVironmental Pollution Control, Henan Normal UniVersity, Xinxiang, Henan 453007, People’s Republic of China ReceiVed: December 21, 2006; In Final Form: April 1, 2007
Densities, conductivities, and polarity indexes of pyrene for aqueous solutions of a series of ionic liquids [Cnmim]Br (n ) 4, 6, 8, 10, 12 ) and [C4mim][BF4] have been determined at 298.15 K as a function of ionic liquid concentrations. It was shown that possible aggregation appeared for the ionic liquids in aqueous solutions except for [C4mim]Br. The critical aggregation concentration (CAC) of the ionic liquids, the ionization degree of aggregates (β), the standard Gibbs energy of aggregation (∆G°m), the limiting molar conductivity (Λ°m), and the standard partial molar volume (V°m) for the ionic liquids were derived from the experimental data. The dependence of the CAC, ∆G°m, Λ°m, and V°m on the length of the alkyl chain of the cations was examined. It was further suggested from volumetric data that a micelle was formed for [C8mim]Br, [C10mim]Br, and [C12mim]Br in aqueous solutions. Their apparent molar volumes at the critical micelle concentration (VΦ,CMC), apparent molar volumes in the micelle phase (Vmic Φ ), and the change of their apparent molar volume upon micellization (∆VΦ,m) were calculated by application of the pseudophase model of micellization. In addition, the average aggregation number of [Cnmim]Br (n ) 8, 10, 12 ) has been determined by the steady-state fluorescence quenching technique, and predicted from a simple geometrical mode. It is found that the experimental values are in good agreement with the predicted ones.
Introduction Room temperature ionic liquids (ILs) are a special class of molten salts composed of organic cations and inorganic or organic anions, which melted at temperatures below 373 K. As a class of potential greener solvents, ILs exhibit many unique physicochemical properties, such as negligible volatility and nonflammability under ambient conditions, large liquid range, high thermal stability, wide electrochemical window, and strong ability to dissolve many chemicals. Thus, ILs have found wide applications in chemical synthesis,1-4 biocatalytic transformations,5,6 electrochemistry,7,8 and analytical and separation science.9,10 In recent years, ILs based on 1-alkyl-3-methylimidazolium cation ([Cnmim]+) have received much attention and have been the most studied.11 An interesting aspect of such ILs is that the [Cnmim]+ cations possess inherent amphiphilic character when their alkyl group is a longer hydrocarbon chain. Thus, 1-decyl3-methylimidazolium bromide ([C10mim]Br) forms aggregates in water at low concentrations12,13 and then self-assembles to form lyotropic mesophases at higher concentrations.14 However, recent studies15,16 have shown that this character is also sometimes exhibited by short-chain cations. Measurements of the surface tension and conductivity of aqueous solutions of 1-butyl3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-octyl3-methylimidazolium chloride ([C8mim]Cl), and 1-octyl-3methylimidazolium iodide ([C8mim]I) have found that these salts behave as surfactants and form aggregates when their molalities are higher than 0.81, 0.12, and 0.10 mol L-1, respectively.15 In a recent thermodynamic study of water-[C4mim][BF4] mixtures, excess chemical potentials, enthalpies, and entropies * To whom correspondence should be addressed. E-mail: Jwang@ henannu.edu.cn.
were measured by Katayanagi and co-workers.17 Their results showed that some organizations took place among the IL at a molar fraction (χ) of 0.015 of the salt. Malham et al.18 investigated the surface thermal coefficients for the mixtures of water with two imidazolium salts, [C4mim][BF4] and 1-butyl2,3-dimethylimidazolium tetrafluoroborate ([C4dmim][BF4]). They also found that the two ILs showed a clear discontinuity at χ ) 0.016 for [C4mim][BF4] and χ ) 0.004 for [C4dmim][BF4]. This discontinuity can be attributed to aggregation of these ILs. Dorbritz et al.19 studied the aggregation formation of [C4mim][BF4] in water, methanol, 2-propanol, and ethyl acetate by electrospray ionization mass spectrometry. From the spectral signals in regular distances corresponding to the mass-to-charge ratio of the IL, they observed the formation of aggregates [C4mim]n[BF4]n-1 and [C4mim]n[BF4]n+1 and found that, with increasing polarity of the solvent and decreasing concentration of the IL, the size of the formed aggregates decreased. On the basis of their conductivity and turbidity measurements, Miskolczy et al.20 reported that [C4mim][C8SO4] can form aggregates above 0.031 mol L-1 in aqueous solution, whereas [C8mim]Cl did not micellize but formed an inhomogeneous solution of larger aggregates. During the review process of this manuscript, Goodchild et al.21 published their studies on the surface, phase, and aggregation behavior of a mixture of [Cnmim]Br with water investigated by using conductivity, surface, and small-angle neutron scattering. Interesting results have been reported. Although previous studies have revealed some interesting behaviors of ILs, the aggregation of ILs in aqueous solutions has not been studied systematically. This is particularly surprising because ionic liquid type salts may constitute a new class of surfactants with special properties. From both fundamental and applied viewpoints, particular interests are the molecular
10.1021/jp068798h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/11/2007
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design of ILs and their structure effect on both the aggregation behavior and the physicochemical properties of the aggregation process. Indeed, information on this aspect is very limited. In the present work, we prepared a series of ILs with a fixed cationic backbone and anionic structure but varied length of the alkyl chain of cations, i.e., 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br; n ) 4, 6, 8, 10, 12). The aggregation behaviors of these ILs in aqueous solutions and their physicochemical properties have been investigated by conductivity, volume, and fluorescence probe techniques. The results have been used to understand (i) the effect of the alkyl chain length of the cation on the aggregation behavior of these ILs and (ii) the dependence of the alkyl chain length of the cation on some physicochemical properties of aqueous IL solutions. Experimental Section Materials. 1-Methylimidazole (99%) and sodium tetrafluoroborate (NaBF4, 99%) were purchased from Shanghai Chem. Co.; 1-bromobutane (99%), 1-bromohexane (99%), 1-bromooctane (99%), 1-bromodecane (99%), and 1-bromododecane (99%) were from Acros Organic. Pyrene (99%) and benzophenone (99%) were from Fluka and Cezchoslovakia, respectively, and were used as received. The deionized water was doubly distilled over KMnO4. The water sample with a conductivity of 1.2 × 10-6 S cm-1 was used throughout the experiments. Dilute stock solutions of pyrene were prepared by dissolving the pyrene in methanol in an amber-glass vial and stored in a refrigerator at 277 K. Samples for fluorescence study were prepared in the following procedure: appropriate aliquots of probe stock solutions were transferred into vials, evaporated under ultrahigh purity hydrogen, and then the IL was added. Synthesis of the Ionic Liquids. [Cnmim]Br (n ) 4, 6, 8, 10, 12) and [C4mim][BF4] were prepared and purified by using the procedure previously described in the literature.22,23 Briefly, the reactions of 1-methylimidazole with excess 1-bromoalkane were performed in 1,1,1-trichloroethane under reflux at ca. 343 K for 48 h. The products [C4mim]Br and [C12mim]Br were recrystallized three times from ethyl acetate and ethyl acetate/ acetonitrile (3:2 by volume), respectively, to remove any unreacted reagents. [C6mim]Br, [C8mim]Br, and [C10mim]Br were washed with 1,1,1-trichloroethane. The residual solvents were removed by heating at 343 K under a vacuum. Then, the [C4mim]Br was reacted with excess sodium tetrafluoroborate in aqueous solution; the resulting ionic liquid [C4mim][BF4] was extracted by dichloromethane, and deionized water was then added. After vigorous stirring, the water layer was replaced with fresh deionized water. This procedure was repeated at least 15 times until no precipitation of AgBr occurred in the aqueous phase on the addition of a concentrated AgNO3 solution. The dichloromethane was then removed from the IL by rotary evaporation. All of the ILs were dried under a vacuum at 343 K for 2-3 days in the presence of P2O5. The melting points for [C4mim][BF4], [C4mim]Br, [C6mim]Br, [C8mim]Br, [C10mim]Br, and [C12mim]Br were 192.2, 349.2, 218.3, 211.3, 283.6, and 306.5 K, respectively, as determined by a Mettler Toledo 822e differential scanning calorimeter. The 1H NMR spectra of these imidazolium salts were determined by a Bruck AV400 spectrometer, and they were found to be in good agreement with those reported in the literature.22,23 The water content in the ionic liquids was determined by Karl Fischer titration. It was found that less than 0.02 wt % of water still remained in [Cnmim]Br (n ) 4, 6, 8, 10, 12) and [C4mim][BF4]. The content of bromide and sodium was, respectively, determined by a bromide-selective electrode and a Z-500 polarized Zeeman
Figure 1. Concentration dependence of the specific conductivity for [C6mim]Br in aqueous solutions.
atomic absorption spectrophotometer. It was shown that 0.018 mol kg-1 of Br- and 0.023 mol kg-1 of Na+ were contained in [C4mim][BF4]. Experimental Methods. Mixtures were prepared by mass on the molality concentration scale. Solution densities were determined by an Anton Paar DMA 60/602 vibrating-tube digital densimeter with a resolution of 1 × 10-6 g cm-3. The temperature around the density meter cell was controlled by circulating water from a constant-temperature bath (Schott, Germany). A CT-1450 temperature controller and a CK-100 ultracryostat were employed to maintain the bath temperature at 298.15 ( 0.005 K. The densimeter was calibrated with known densities of pure water and dry air every day. The uncertainty in density was estimated to be (3 × 10-6 g cm-3. Conductivity measurements were performed at 298.15 K with a DDS-12A conductometer (XiaoShan Experimental Instrument Factory) at a fixed frequency of 1100 Hz with an uncertainty of 0.3%. The conductance cell was equipped with a water circulating jacket, and the temperature was controlled within (0.03 K with a DC-2006 low temperature thermostat (Shanghai, Hengping Instrument Factory). The cell was calibrated with aqueous KCl solutions at different concentrations, and a cell constant of 1.0474 cm-1 was determined. The steady-state fluorescence quenching technique was used to investigate the aggregation behavior of the ILs in aqueous solutions. Pyrene and benzophenone were used as a fluorescence probe and a quencher, respectively. The pair pyrene/benzophenone assured that the residence time of the quencher into the aggregates is longer than the fluorescence lifetime of the probe.24,25 The concentration of pyrene was kept constant at 1 × 10-6 mol L-1, while the quencher concentration was varied from 0 to 1.5 × 10-3 mol L-1, assuring a Poisson distribution.26,27 Steady-state fluorescence measurements were carried out with a Shimadzu RF-5310(PC)S spectrofluorophotometer. Both excitation and emission band slits were fixed at 3 nm. The excitation wavelength was selected at 335 nm, while the emission spectra were scanned from 350 to 450 nm. The first and third vibronic peaks of pyrene appear at 373 and 383 nm, respectively. The measurements were conducted at 298 ( 1 K. At an excitation wavelength of 335 nm, the effect of fluorescence of IL on the first and third vibronic peaks of pyrene was
Ionic Liquids [C4mim][BF4] and [Cnmim]Br
J. Phys. Chem. B, Vol. 111, No. 22, 2007 6183 is in excellent agreement with 0.043, 0.040, and 0.039 mol kg-1 determined from surface tension,12 electromotive force,13 and small-angle neutron scattering measurements.21 The CAC values of [C6mim]Br and [C8mim]Br reported in this work are in reasonable agreement, respectively, with the values 0.56 and 0.16 mol kg-1 reported by Goodchild and co-workers.21 For the other bromide salts, no data could be found in the literature for comparison. Examination of the CAC data for [Cnmim]Br (n ) 6, 8, 10, 12) shows that the values of the CAC decrease by a factor of 2.23 with an increase of the alkyl chain length by a methyl group. This factor is comparable to that established for ionic surfactants, such as alkylpyridinum bromides,30,31 alkyltrimethylammonium chlorides and alkylpyridinum chlorides,32 and alkyltrimethylammonium bromides.30 It suggests that the effect of the alkyl chain length of the cation on the CAC does not depend markedly on the structure of the cations and the type of counterions. It is interesting to note that the following linear correlation:
Figure 2. Concentration dependence of the specific conductivity for [C8mim]Br in aqueous solutions.
log(CAC) ) 1.845 - 0.325nc
exists between log(CAC) and the number of carbon atoms (nc) in the alkyl chain with a correlation coefficient of 0.9999. The values of the ionization degree (β) of the aggregates are found to decrease with increasing alkyl chain length of the ILs from C8 to C12. This phenomenon is similar to that of some surfactants. It shows that the counterion is easier to bind with aggregates as its alkyl chain becomes longer. According to the pseudophase model of micellization, the standard Gibbs energy of aggregation (∆G°m) can be calculated from the following equation33
∆G°m ) (2 - β)RT ln χCAC
Figure 3. Concentration dependence of the specific conductivity for [C10mim]Br and [C12mim]Br in aqueous solutions.
small. All fluorescence spectra were background subtracted using an appropriate blank.28,29 Results and Discussion The Critical Aggregation Concentrations and Gibbs Energies of Aggregation of the ILs. Experimental conductivities (κ) for aqueous solutions of [Cnmim]Br (n ) 6, 8, 10, 12) at 298.15 K are shown in Figures 1-3 as a function of IL concentration. A characteristic shape of the curves has been observed. They exhibit typical behavior with two linear fragments, and the concentration at which the two linear fragments intersect is assigned to the critical aggregation concentration (CAC). The ratio of the slope of the linear fragments above and below the CAC gives an estimate of the degree of ionization (β) of the aggregates.30 The values of the CAC and β obtained by a least-squares analysis are presented in Table 1. The CAC value for [C4mim][BF4] reported by Bowers and co-workers15 from conductivity measurements is also included in this table. As can be seen, our average value of the CAC for [C10mim]Br
(1)
(2)
where χCAC is the critical aggregation concentration expressed in mole fraction scale. The values of ∆G°m calculated for the ILs are included in Table 1. It can be seen that the longer the alkyl chain of ILs, the more negative the standard Gibbs energy of aggregation, indicating that the aggregation comes more easily with the increase of the alkyl chain length of ILs. The addition of a -CH2 unit to the cationic alkyl chain causes a decrease in the electrostatic attraction interactions between cation and anion. On the other hand, an increase in the hydrocarbon unit enhances the van der Waals interactions by means of the alkyl chainion inductive force (dielectric polarization) and the hydrocarbonhydrocarbon interactions.34 The balance between these two opposite factors determines the aggregation character of the system. Therefore, the observed increased negative values of ∆G°m with the increase of the alkyl chain length suggest that the aggregation is driven by alkyl chain-ion inductive and hydrocarbon-hydrocarbon interactions. The Gibbs energy of aggregation contains contributions from transfer of the IL segments from bulk water to the aggregates, and bears, therefore, on the relative importance of ionic liquid hydrophilic and hydrophobic moieties to its aggregation. Therefore, the ∆G°m values for the ILs can be divided into contributions from the terminal CH3 group of the alkyl chain (∆G°m,CH3), the methylene group of the alkyl chain (nCH2∆G°m,CH2), and the headgroup (∆G°m,headgroup), i.e.,
∆G°m ) ∆G°m,headgroup + ∆G°m,CH3 + nCH2∆G°m,CH2
(3)
A plot of ∆G°m versus nCH2 gives a straight line with intercept ) ∆G°m,headgroup + ∆G°m,CH3 ) -3.7 kJ mol-1 and slope )
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TABLE 1: Critical Aggregation Concentration (CAC), Ionization Degree of Aggregates (β), and Standard Gibbs Energies of Aggregation (∆G°m) for the ILs in Aqueous Solutions at 298.15 K CAC (mol kg-1) IL
conductivity volume fluorescence
[C12mim]Br [C10mim]Br [C8mim]Br [C6mim]Br [C4mim][BF4] a
0.009 0.039 0.16 0.77 0.97a
0.012 0.042 0.18 0.80 0.71
0.010 0.046 0.19 0.88 0.96
β
∆G°m (kJ mol-1)
0.22 0.34 0.37 0.15
-37.2 -29.6 -23.5 -19.5
Reference 15.
TABLE 2: Limiting Molar Conductivities, the Coefficient r of eq 5, and the Cationic Transport Number for [Cnmim]Br (n ) 4, 6, 8, 10, 12) at 298.15 K IL
Λ°m (S cm2 mol-1)
R
Λ°m (S cm2 mol-1)
t+
[C12mim]Br [C10mim]Br [C8mim]Br [C6mim]Br [C4mim]Br
119.6 123.6 140.9 147.2 164.4
618 866 1679 1744 1974
41.5 45.5 62.8 69.1 86.3
0.35 0.37 0.45 0.47 0.52
∆G°m,CH2 ) -3.0 kJ mol-1. These values are comparable to those of alkylbenzyldimethylammonium chloride in D2O which amount to -3.3 and -2.7 kJ mol-1, respectively.35 As it is expected, ∆G°m,CH2 is similar in value for both homologous salts, since the transfer of a methylene group from bulk solution to the aggregates is expected to be independent of the cationic structure. The Limiting Molar Conductivities of the ILs and Their Cations. The molar conductivities of the ILs can be calculated by
Λm ) 103κ/c
Figure 4. Variation of molar conductivity with the square root of molarity for [Cnmim]Br (n ) 4, 6, 8, 10, 12) in the low concentration range at 298.15 K.
(4)
where κ and c are the solution conductivity (S cm-1) and molality (mol L-1) of ILs. According to the Debye-Hu¨ckelOnsager equation, molar conductivity for 1:1 electrolytes in a dilute solution is often related to its concentration by
Λm ) Λ°m - Rc1/2
(5)
where Λ°m is the limiting molar conductivity of the electrolytes and R is a parameter which reflects the association interactions between cations and anions, and the degree of association increase with the increase of R values.36 Linear plots of the molar conductivity with the square root of molarity for [Cnmim]Br (n ) 4, 6, 8, 10, 12) were shown in Figure 4 in the low concentration range. The values of Λ°m and R were obtained by a least-squares linear regression procedure, and the results are given in Table 2. It can be seen that the values of Λ°m and R decrease with increasing alkyl chain length of the ILs, indicating both the decreased association interactions and the decreased cationic mobility. This phenomenon can be explained by the increased intrinsic size of the cation of the ILs and the steric hindrance effect of the increased alkyl chain length on the association between cation and anion, which was formed by the hydrogen bonding interactions of 2-H of the imidazolium cation with Br- anion.36 The limiting molar conductivity of bromide anion was reported to be 78.1 S cm2 mol-1 in water at 298.15 K.37 From this value, data of the limiting molar conductivity for the cations can be obtained (Table 2). As it is expected, the values of Λ°m+ decreased with increasing alkyl chain length of the cation of
Figure 5. Variation of apparent molar volumes with the reverse of molality for [C8mim]Br at 298.15 K.
the ILs. From Λ°m and Λ°m+ values, the transport number for the cation of the ILs in infinite-dilution solution has been calculated by t+ ) Λ°m+/Λ°m; the results are represented in Table 2. It is evident that, despite a significant difference in the cationic and anionic radii, the ionic transport number of [C4mim]+ is estimated to be higher than 0.5. This indicates that the cation can move faster than the anion, even if the cationic radius is much larger than that of the anion. This is similar to the behavior of the cation in the [C4mim]-based ionic liquids with different anionic structure.38 Ionic mobilities in solution are known to depend on the ionic radius and degree of ionic solvation. Therefore, we can conclude that Branion was highly hydrated as compared with the [Cnmim]+ cations. In addition, the transport number of the cations is observed to decrease with increasing number of carbon atoms in the alkyl chain, indicating the effect of the cationic size on their mobility.
Ionic Liquids [C4mim][BF4] and [Cnmim]Br
J. Phys. Chem. B, Vol. 111, No. 22, 2007 6185
Figure 6. Variation of apparent molar volumes with the reverse of molality for [Cnmim]Br (n ) 10, 12) at 298.15 K.
Figure 8. Variation of I1/I3 with the concentration of [C10mim]Br at 298 K.
respectively. Assuming the pseudophase model of micellization,39 the apparent molar volume may be written in the following form:
VΦ )
CAC m - CAC mic V°Φ + VΦ m m
(7)
where V°Φ is the infinite-dilution apparent molar volume of ILs in monomer form and Vmic Φ is the apparent molar volume of ILs in the aggregation phase. From the viewpoint of thermodynamics, the former is equal in value to the standard partial molar volume of monomer ILs. In fact, eq 7 can be rewritten as
VΦ ) Vmic Φ +
Figure 7. Variation of apparent molar volumes with the reverse of molality for [C4mim][BF4] at 298.15 K.
Analysis of the data in Table 2 reveals a linear relation between the limiting molar conductivity of [Cnmim]Br (n ) 4, 6, 8, 10, 12) and the number of carbon atoms in the alkyl chain of cations. The slope of this linear relation presents the contribution per methylene group to the limiting molar conductivity of ILs. By using a linear least-squares analysis, Λ°CH2 ) -5.7 ( 0.6 S cm2 mol-1 was derived. This information is important for the molecular design of ILs. Apparent Molar Volumes of the Ionic Liquids in the Aggregation Phase and Their Standard Partial Molar Volumes in Water. The apparent molar volumes (VΦ) for [Cnmim]Br (n ) 4, 6, 8, 10, 12) and [C4mim][BF4] were calculated from density data by using the equation 3 M 10 (F0 - F) VΦ ) + F mF0F
(6)
where M and m are the molar mass and the molality of IL and F and F0 are the densities of solutions and pure water,
CAC (V°Φ - Vmic Φ ) m
(8)
The plots of apparent molar volumes against the reciprocal of molality (m-1) for [Cnmim]Br (n ) 8, 10, 12) are shown in Figures 5 and 6. It is clear that there are two linear regions in each of these plots. The intercept of the line at concentrations above the CAC gives the value of Vmic Φ , and that at concentrations below the CAC gives the value for V°Φ. The concentration and the apparent molar volume at which the two lines intersect correspond to the critical aggregation concentration of the ILs and their apparent molar volume at CAC (VΦ,CAC), respectively. Thus, obtained values for the CAC, V°Φ, Vmic Φ , and VΦ,CAC are given in Tables 1 and 3, respectively. From values of the apparent molar volume in the aggregation phase and the apparent molar volume at the CAC, change in the apparent molar volume upon aggregation (∆VΦ,m) can be calculated by
∆VΦ,m ) Vmic Φ - VΦ,CAC
(9)
The values of ∆VΦ,m for [Cnmim]Br (n ) 8, 10, 12) are found to be positive and increase with increasing alkyl chain length of cations (Table 3). A similar observation was reported earlier for some typical surfactants.40-42 It is generally assumed that this effect is mainly due to the release of structure water in the hydration shell of the monomers when the aggregations are formed. It should be stressed that the volumetric data of [C6mim]Br and [C4mim]BF4 cannot be described by the pseudophase model
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TABLE 3: Apparent Molar Volume at Infinite Dilution (V°Φ), Apparent Molar Volume at the Critical Aggregation Concentration (VΦ,CAC), Apparent Molar Volume in the Aggregation Phase (Vmic Φ ), the Change of Apparent Molar Volume upon Aggregation (∆VΦ,m) for the Ionic Liquids, and the Coefficient Bv of eq 10 IL
V°Φ (cm3 mol-1)
VΦ,CAC (cm3 mol-1)
3 -1 Vmic Φ (cm mol )
∆VΦ,m (cm3 mol-1)
Bv (cm3 kg mol-1)
298.9 (299.0)a 263.2 (263.3)a 231.5 (231.6)a 200.9 188.1
297.3
304.9
7.7
-10.0
263.7
269.4
5.7
-5.9
231.4
235.2
3.8
-3.1
[C12mim]Br [C10mim]Br [C8mim]Br [C6mim]Br [C4mim][BF4] a
-3.3 -0.21
Determined by eq 8.
of micellization. This suggests that [C6mim]Br and [C4mim]BF4 aggregated but no micelle was formed because of their shorter alkyl chain and the weaker hydrophobic and van der Waals interactions. For these ILs, their CAC values were determined from the intersection of two straight lines in the plot of apparent molar volumes against the reciprocal of molality, as shown in Figure 7 for [C4mim]BF4. The standard partial molar volumes of ILs can also be represented with
VΦ ) V°Φ + Avm1/2 + Bvm
(10)
by assuming that solutions of ILs behave like those of 1:1 electrolytes in the preaggregation region, where Av is the Debye-Hu¨ckel limiting law coefficient which amounts to 1.865 cm3 kg1/2 mol-3/2 for 1:1 electrolytes in water at 298.15 K and Bv is an adjustable parameter related to the pair interactions and equivalent to the second virial coefficient, which indicates the deviation from the limiting law due to the nonelectrostatic solute-solute interactions.43 Bv is generally negative except for hydrogen bonding interactions.43 Values of V°Φ and Bv obtained by a least-squares analysis for [Cnmim]Br (n ) 4, 6, 8, 10, 12) and [C4mim][BF4] were included in Table 3. It can be seen that V°Φ values determined in this way are in good agreement with those calculated from the application of the pseudophase model. In addition, the observed increasing negative Bv values from C4-C12 salts indicate the increased nonelectrostatic interactions of the ILs. This supports the conclusion drawn from the CAC data. A linear relation is also observed between V°Φ and the number of carbon atoms in the alkyl chain of the imidazolium cation:
V°Φ ) 105.6 + 15.9nc
(11)
with a correlation coefficient of 0.998. In this equation, the slope of 15.9 ( 0.3 cm3 mol-1 represents the contribution per methylene group in the alkyl chain of the imidazolium cation to the standard partial molar volume of the ILs. It is interesting to note that the V°CH2 value derived in this study is only slightly higher than the values 15.5 cm3 mol-1 derived from homologous alkyldimethylbenzylammonium chloride30 and 15.4 cm3 mol-1 derived from a series of alkylpyridinum bromides,31 and is very close to the values 15.9 cm3 mol-1 derived from RCOONa44 and 15.8 cm3 mol-1 derived from amino acids.45 This confirms the group additivity within experimental errors. Aggregation Number of the Ionic Liquids Determined by the Fluorescence Probe Method. The fluorescence probe technique was used to determine the CAC and aggregation number of ILs in aqueous solutions. For this purpose, the polarity index (I1/I3) of pyrene was measured as a function of
the IL concentrations, where I1/I3 stands for the ratio of the intensities of the first and third vibronic peaks for pyrene. Representative results are displayed in Figure 8 for [C10mim]Br, illustrating the I1/I3 dependence on IL concentrations. The abrupt sigmoidal decrease in I1/I3 clearly indicates that the aggregation of ILs was formed, and pyrene preferentially resides in a more hydrophobic microenvironment of the aggregates relative to water. The CAC of ILs can be taken as the concentration that corresponds to the intersection between the linear extrapolation of the rapidly varied portion of the curve and of the relative stabilization portion at higher concentrations. The CAC values obtained by this method were also included in Table 1. Clearly, those values derived from conductivity, apparent molar volume, and polarity index of the fluorescence probe are in good agreement within experimental errors. Our values for [C4mim][BF4] are also close to the results determined from conductivity (0.97 mol kg-1), surface tension (0.95, 0.90 mol kg-1), excess thermodynamic property (0.85 mol kg-1), and surface thermal coefficient (0.90 mol kg-1) measurements.13-16 The aggregation number (N) of the monomers in the aggregates can also be determined by the steady-state fluorescence quenching technique. The equilibrium of the IL between the aqueous and aggregation phases follows the Poisson distribution. The equation to be applied is27,29
ln I ) ln I0 - Cq/Ca ) ln I0 - NCq/(Ct - CAC) (12) where Cq, Ca, and Ct are the concentrations of quencher, aggregate, and total ILs, respectively, while I and I0 are the fluorescence intensities with and without the presence of quencher. Figure 9 illustrates the emission spectra of pyrene in aqueous [C10mim]Br solutions in the presence of different quencher concentrations. Figure 10 shows the linear plot of ln(I/I0) against Cq for this system. Similar plots have been found for pyrene in aqueous [C8mim]Br and [C12mim]Br solutions. According to eq 12, values of the average aggregation number for [Cnmim]Br (n ) 8, 10, 12) have been calculated from the slope of these linear plots and the CAC values already determined. These results are collected in Table 4. It can be seen that although the aggregation number of 53 for [C8mim]Br reported in this work at 0.2517 mol L-1 is much greater than that of 21 determined by small-angle neutron scattering,21 it is very close to the value of 45 for [C8mim]I determined by the same technique15 at 0.250 mol L-1. Also, our aggregation number of 35 for [C10mim]Br is in excellent agreement with that of 38 reported by Goodchild et al.21 This provides support for our aggregation number results derived from the fluorescence probe method. Aggregation Number of Ionic Liquids Predicted by a Simple Geometrical Consideration. Toward this end, the
Ionic Liquids [C4mim][BF4] and [Cnmim]Br
J. Phys. Chem. B, Vol. 111, No. 22, 2007 6187
N)
4πL3 3Vcore
(13)
where L is the length of the alkyl chain embedded in the core. L and Vcore can be calculated by the equations47
L ) (1.5 + 1.265ncore) × 10-8
(14)
Vcore ) (27.4 + 26.9ncore) × 10-24
(15)
and
where ncore is the carbon number in the core. The next problem is how to obtain ncore. The mean volume of the hydrophobic portion of the monomer in a micelle can be described by31
Vh ) Vmic Φ - VΦ,[mim]+ - (1 - β)VΦ,Br-
Figure 9. Variation of the emission spectra intensity of 1 × 10-6 mol L-1 pyrene in aqueous [C10mim]Br solutions as a function of the quencher concentration.
(16)
where VΦ,[mim]+ and VΦ,Br- represent the apparent molar volumes of 3-methylmimdazolium cation and Br-, respectively. It is known48 that V°Φ,Br- ) 24.71 cm3 mol-1. By subtracting this value from the standard partial molar volumes for the ILs, the following linear correlation
V°m,[Cnmim]+ ) 80.9 + 15.9nc
(17)
can be obtained from eq 11, where V°m,[Cnmim]+ stands for the standard partial molar volume of the cations, and the value of 80.9 cm3 mol-1 denotes the standard partial molar volume of 3-methylmimdazolium cation, i.e., V°m,[mim]+ ) 80.9 cm3 mol-1. It was known that, at a concentration at which the distances between ions correspond to the distance of head groups on the aggregate surface (C ≈ 2.2-2.4 mol L-1), the apparent molar volumes of ions exceed their standard partial molar values by about 1.4 cm3 mol-1.47,48 Therefore, at this concentration, VΦ,[mim]+ ) 82.3 cm3 mol-1 and VΦ,Br- ) 26.1 cm3 mol-1. Substitution of these two values and those of β and Vmic Φ collected in Tables 1 and 3 for [Cnmim]Br (n ) 8, 10, 12) into eq 16 gives
Vh ) 5.11 + 16.44nc Figure 10. Linear plot of ln(I/I0) for 1 × 10-6 mol L-1 pyrene in aqueous solutions of [C10mim]Br as a function of the benzophenone concentration.
TABLE 4: Aggregation Number of [Cnmim]Br (n ) 8, 10, 12) in Aqueous Solutions at 298 K IL
C (mol L-1)
N1a
N2b
N3c
[C12mim]Br [C10mim]Br [C8mim]Br
0.03787 0.06958 0.2517
44 35 53
49 33 22
38 21
a Determined by the fluorescence probe technique. b Calculated by simple geometrical consideration. c Reference 21.
method described by Sˇ kerjank and co-workers31 for surfactants has been closely followed. Small-angle neutron scattering studies15 on aqueous solutions of [C4mim][BF4] and [C8mim]I have proved that the aggregate shape of the ILs are spherical and near-spherical. On the basis of the idea of Tartar46 and Tanford,47 it can be assumed that the aggregate contains a hydrocarbon core consisting entirely of the hydrocarbon chain. The volume of the core, 4πL3/3, is thus equal to the product NVcore. Therefore, the aggregation number of ILs can be represented by
(18)
or in cubic centimeters per alkyl chain
Vh ) (7.64 + 27.39nc) × 10-24
(19)
Equating the above equation with eq 15 yields
ncore ≈ nc - 1
(20)
Using this equation together with eqs 13, 14, and 15, the aggregation number for [C8mim]Br, [C10mim]Br, and [C12mim]Br was calculated, and the results were included in Table 4. It can be seen that most of the predicted values are in reasonable agreement with the experimental ones, indicating the important role of this simple geometrical model in estimating the aggregation number of some ILs in aqueous solutions. Conclusions The work reported here has demonstrated that no aggregation occurs for [C4mim]Br in aqueous solutions, [C6mim]Br and [C4mim][BF4] form aggregates above the critical aggregation concentration, and the aggregation of [C8mim]Br, [C10mim]Br, and [C12mim]Br is strong enough to form micelles above the critical micelle concentration. This indicates that the alkyl chain
6188 J. Phys. Chem. B, Vol. 111, No. 22, 2007 length of a cation can be tailored to switch the aggregation behavior of ILs. Therefore, these ILs act as short- or moderatechain cationic surfactants in aqueous solutions; they are different from classical ionic aggregation of which ion pairs between cation and anion and ion triplets are widely recognized examples. Furthermore, a linear relationship was found between the logarithm of the critical aggregation concentration, the standard Gibbs energy of aggregation, the limiting molar conductivity, the standard partial molar volume, and the number of carbon atoms in the alkyl chain of cations. It is shown that the contribution per -CH2 group in the alkyl chain of cations to these physicochemical properties does not depend markedly on the cationic structure and the anionic type. They are comparable to those derived from similar homologous surfactants. Calculations of the transport number of ions from the limiting molar conductivities of the ILs and the Br- anion suggest that Br- anion was highly hydrated as compared with the 1-alkyl-3-methylimidazolium cations. In addition, the average aggregation numbers of [C8mim]Br, [C10mim]Br, and [C12mim]Br have been determined with the steady-state fluorescence technique and a simple geometrical model. Reasonable agreement was found between the experimental and predicted values. The above findings are expected to be useful for the molecular design of ILs and for the understanding of how ILs participate as components in a mixed solvent system. Acknowledgment. This work was supported financially by the National Natural Science Foundation of China (Grant No. 20573034). Supporting Information Available: Tables showing the data of solution densities, conductivities, apparent molar volumes, and molar conductivities for aqueous [C4mim][BF4] and [Cnmim]Br (n ) 4, 6, 8, 10, 12) solutions at 298.15 K as a function of ionic liquid concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (3) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (4) Wasserschein, P.; Welton, T. Ionic liquids in syntheses; VCHWiley: Weinhein, Germany, 2003. (5) Rantwijk, F.; Lau, R. M.; Sheldon, R. A. Trends Biotechnol. 2003, 21, 131. (6) Jain, N.; Kumar, A.; Chauhan, S.; Chauhan, S. M. S. Tetrahedron 2005, 61, 1015. (7) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106. (8) Endres, F.; Abedin, S. Z. E. Phys. Chem. Chem. Phys. 2006, 8, 2101. (9) Liu, J.; Jonsson. J. A.; Jing, G. Trends Anal. Chem. 2005, 24, 20.
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