J Solution Chem (2006) 35:1337–1346DOI 10.1007/s10953-006-9064-7

ORIGINAL PAPER

The Physical Properties of Aqueous Solutionsof the Ionic Liquid [BMIM][BF4]

Weiwei Liu · Tianyu Zhao · Yumei Zhang ·Huaping Wang · Mingfang Yu

Received: 16 January 2006 / Accepted: 28 February 2006 / Published online: 21 September 2006C© Springer Science+Business Media, Inc. 2006

Abstract We report here the systematic study of the effect of concentration on the phys-ical properties of aqueous solutions of the room-temperature ionic liquid, [BMIM][BF4].The measurements of density, ρ, refractive index, �n, viscosity, η, specific conductivity,κ , and surface tension, γ , were made over the whole concentration range. The equivalentconductance �m was calculated. The observed linear variations of density and refractiveindex with the molar concentration are established as those of an ideal solution. The sur-face tension varied most rapidly in the dilute region whereas the viscosity changed muchmore rapidly in the concentrated region. Two regions with different composition depen-dences were found after the analyses of the relationship between the conductivity and theconcentration of [BMIM][BF4]. A proposed model for a structural change in the mixtureswas described. The physical origin of the observed concentration dependence of these prop-erties is discussed. The physical properties of the solutions vary with changes of associationbetween anions and cations and the interaction between [BMIM][BF4] and water.

Keywords Ionic liquids · Aqueous solutions · Physical property · Density · Refractiveindex · Viscosity · Specific conductivity · Surface tension

1. Introduction

In recent years, room temperature ionic liquids (RTILs) have received a lot of attention aspotential “green” and “designable” solvents [1]. Although ionic liquids have been studiedsince the 1950s [2], these were not part of our studies until water-stable and air-stable ionicliquids such as [BMIM][BF4] were found. These liquids need not be handled under an inertatmosphere. Because of their extremely low volatilities, some of the RTILs are promisingenvironment-friendly solvents (instead of volatile organic solvents) for a range of science

W. Liu (�)· T. Zhao · Y. Zhang · H. Wang · M. YuState Key Laboratory for Modification of Fiber Materials, Donghua University, Shanghai 200051,People’s Republic of Chinae-mail: towardsun@mail.dhu.edu.cn

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Table 1 The density, viscosity and surface tension at 25 ◦C of aqueoussolutions of [BMIM][BF4] at different concentrations

Concentration 10–3 × Density Surface tension ViscosityEntry (mol·mL–1) (kg·m–3) (mN·m–1) (cP)

1 0.000152 1.0012 63.50 0.95232 0.000203 1.0016 60.45 0.96943 0.000324 1.0076 57.41 1.01714 0.000540 1.0147 53.34 1.10535 0.000932 1.0272 49.39 1.27086 0.001268 1.0386 48.66 1.45637 0.001452 1.0445 47.90 1.57808 0.002122 1.0668 47.48 2.17479 0.002531 1.0803 47.00 2.6878

10 0.003108 1.0996 46.42 3.695511 0.003690 1.1195 46.48 5.537512 0.004319 1.1389 46.33 10.512413 0.004670 1.1544 46.12 18.347014 0.004842 1.1600 46.53 27.769715 0.004977 37.018816 0.005066 49.793117 0.005130 78.134218 Pure water 0.9970 74.07 0.890419 Pure ILs 1.1711 45.74 153.7804

and technology applications such as the media for organic and inorganic reactions [3, 4],materials processing [5], electrochemistry [6] and chemical separation [7].

At present, most of the available data are focused on bulk physico-chemical properties,such as decomposition temperature, phase transitions, viscosity and density, and the correla-tion between these properties and the molecular structures of the RTILs [8]. Relatively littleis known about the physical properties of the mixtures which consist of these new materialsand other organic or inorganic solvents. Some excellent papers are available on the phasebehavior and thermodynamic properties of the mixtures [9, 10].

Water, beyond doubt, is the most common “green” solvent. With the good solvents ofRTILs for chemical separation and material processing, water often serves as the extractingor curing agent. The presence of water in the RTILs affects the physical properties andapplications of the RTILs [11]. Thus, basic research concerning how water affects the physicalproperties of aqueous solutions of ionic liquids provides necessary information. Therefore,this work is focused on the effect of concentration on the physical properties of density,refractive index, viscosity, conductivity and surface tension over the whole concentrationrange of the [BMIM][BF4]/H2O system (Table 1).

2. Experimental

2.1. Materials

The chlorobutane, 1-methylimidazole, ethyl acetate, acetone and NaBF4 were used as sup-plied. [BMIM]Cl and [BMIM][BF4] were prepared based on reported procedures [12].

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2.2. Synthesis of RTILs

An appropriate amount of 1-methylimidazole and an equimolar amount of chlorobutane wereweighed with an electronic balance and then added to a round-bottomed flask fitted with areflux condenser. They were heated for 72 h at 70 ± 0.1 ◦C. The mixture was then decantedusing ethyl acetate after the reaction was complete. The product, [BMIM]Cl, is slightlyyellow and may be crystalline at room temperature, depending on the amount of water thatis present. Then [BMIM]Cl was transferred to a larger container followed by the addition ofNaBF4 in a 1:1 molar ratio. Then, acetone was added as a solvent and the reaction allowedto proceed for 48 h at 30 ± 0.1 ◦C. The [BMIM][BF4]-sodium chloride-acetone mixture wasfiltered to remove sodium chloride and the filtrate was put into a round-bottomed flask to beroto-evaporated at 75 ± 0.1 ◦C under a vacuum in order to remove any residual acetone. Theproduct, [BMIM][BF4], is colorless and did not undergo any degradation when stored in thedark at 4 ◦C for several weeks.

2.3. Density

The densities of the samples were determined by gravimetric analysis, by measuring theweight of the sample in a 25 mL calibrated density bottle. Each measurement was repeatedthrice at 25 ± 0.1 ◦C and the average values were calculated and reported.

2.4. Refractive index

All measurements were made with a 2WAJ Abbe’s refractive index instrument. Deionizedwater was used as a reference for calibration. All measurements were taken at 25 ± 0.1 ◦Cand repeated thrice. Average values were calculated and reported (Table 2).

2.5. Viscosity

The viscosities of the samples were measured with an Ostwald viscometer. For each com-position, a 10 mL sample was used and the measurements were performed thrice. The tem-perature of the sample was maintained to 25 ± 0.1 ◦C via an external temperature controller.The average values were calculated and reported.

2.6. Surface tension

The surface tension of each sample was measured with an OCA30 from Dataphysics Com-pany. For each measurement, the Pendant Drop Method was used and the surface tension wascalculated using the Young–Laplace equation. All measurements were taken at 25 ± 0.1 ◦Cand repeated thrice; average values were calculated and reported.

2.7. Conductivity

The conductivity measurements were carried out with a DDSJ-308A conductivity instru-ment. Before and after the measurements, the instrument was calibrated with an aqueousKCl solution. The temperature of the sample was maintained at 30 ± 0.1 ◦C during the mea-surements. Each measurement was repeated thrice and the average values were calculatedand reported.

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Table 2 The refractive index ofaqueous solutions at 25 ◦C of[BMIM][BF4] at differentconcentrations

Entry Concentration (mol·mL–1) Refractive Index

1 0.000177 1.33702 0.00140 1.36743 0.00189 1.37894 0.00208 1.38305 0.00233 1.38886 0.00263 1.39687 0.00286 1.40278 0.00312 1.40849 0.00331 1.4133

10 0.00354 1.419011 0.00379 1.425112 0.00408 1.431913 0.00441 1.439914 0.00478 1.448815 0.00500 1.454016 0.00506 1.455317 0.00510 1.458018 0.00512 1.457519 0.00514 1.458020 0.00515 1.459921 0.00517 1.459322 0.00519 1.459723 0.00521 1.458924 0.00523 1.459025 0.00525 1.459526 0.00527 1.460027 0.00529 1.461728 Pure water 1.332429 Pure ILs 1.4626

3. Results and discussion

3.1. The effects of concentration on density and refractive index

Figures 1 and 2 show the curves of the density of solution versus molar fraction and molarconcentration, respectively, of the ionic liquid and show both experimental data and calculatedvalues. The calculated values were obtained by the ideal molar volume equation.

The ideal molar volume equation:

Vm = x1V1 + x2V2

= (x1MW1 + x2MW2)/dm = x1(MW1/d1) + x2(MW2/d2) (1)

can be transformed into the form

dm = ϕ1d1 + ϕ2d2 (2)

where V: molar volume; d: density; MW: molar mass; ϕ: volume fraction. Subscripts m, 1,and 2 denote, respectively, binary mixture, liquid 1, and liquid 2.

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0 20 40 60 80 1000.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

experimental datacalculated data

[Bmim]BF mole fraction(%)

Den

sity

/g*m

l-3

0 1 2 3 4 5 60.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

Concentration of [BMIM][BF ] /mol*L4-1

Den

sity

/g*m

l-3

experimental data calculated data

4(a) (b)

Fig. 1 Concentration dependence {mole fraction (a), molar concentration (b)} of the density for aqueous[BMIM][BF4] solutions

In Fig. 1, the densities of solution are plotted versus the molar fraction and molar con-centration of the ionic liquid and are found to obey the molar volume equation for an idealsolution. Similarly, the linear plot of refractive index versus the molar concentration of[BMIM][BF4] is as expected for an ideal solution (see Fig. 2b), given the vastly differentsizes of water and the ionic liquid.

In Figs. 1a and 2a are shown the non-linear variations of density and refractive indexwith the mole fraction. However, when the mole concentration ratio is considered (seeFigs. 1b and 2b), a linear relationship is clearly established. Wu et al. [13] also obtaineda similar result by a molecular dynamics simulation of mixtures of [BMIM][BF4] andacetonitrile. The linear relationships of the density and refractive index with the concentrationof [BMIM][BF4] are given by: d = 0.9959 + 0.0336C and n = 1.3327 + 0.0244C , where d

(a) (b)

0 20 40 60 80 1001.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

1.48

[Bmim]BF mole fraction (%)4

Ref

ract

ive

inde

x

1.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

1.48

Ref

ract

ive

inde

x

0 1 2 3 4 5 6

Concentration of [BMIM][BF ]/mol*L4-1

Fig. 2 Concentration dependence {mole fraction (a), molar concentration (b)} of the refractive index foraqueous [BMIM][BF4] solutions

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0 20 40 60 80 100-20

0

20

40

60

80

100

120

140

160

[Bmim]BF mole fraction (%)4

Vis

cosi

ty/c

P

-20

0

20

40

60

80

100

120

140

160

Vis

cosi

ty/c

P

0 1 2 3 4 5 6

Concentration of [BMIM][BF ]/mol L4-1

(a) (b)

Fig. 3 Concentration dependence {mole fraction (a), molar concentration (b)} of the viscosity for aqueous[BMIM][BF4] solutions

is the density (g·mL–1), n is the refractive index and C is the molar concentration (mol·L–1)of [BMIM][BF4]. The concentration of water in RTILs can be estimated from these equationsfor the process of chemical separation.

3.2. The effect of concentration on viscosity and surface tension

Other researchers have found that the decrease of the viscosity of ionic liquids caused bythe addition of cosolvents follows a different pattern that depends on the nature of thecosolvents, possibly due to differences in polarities that lead to different interactions withthe ions in the RTILs [14, 15]. In this study, the magnitude of the increase of viscosity withconcentration for the [BMIM][BF4] mixtures is considerable (see Fig. 3a and b). When the[BMIM][BF4] concentration is increased, the viscosity does not increase a great deal inthe dilute solution but it does more rapidly in the concentrated region. The effect of wateron the surface tension of [BMIM][BF4] shows a distinct trend. The surface tension ofthe mixture decreases rapidly in dilute solutions but then becomes almost constant in theconcentrated solution (see Fig. 4a and b). The results show that [BMIM][BF4] mainly actsas a surfactant in its aqueous solution.

3.3. The effects of concentration on conductivity

The plots in Fig. 5 imply that the conductivity of the solutions goes through two differentregions: the water-rich region (the mole fraction of [BMIM][BF4] < 0.1) and the salt-rich re-gion (the mole fraction of [BMIM][BF4] > 0.1). Similar distinct regions in the (Ag/TI)NO3 +water systems were also identified by other researchers [16].

Mixtures of [BMIM][BF4] and water display the classical properties of concentratedsaline solutions, with a maximum conductivity of 47.1 mS·cm–1 at a concentration near2.54 mol·L–1 where the mole fraction of [BMIM][BF4] is 0.097 (see Fig. 5a and b). Theconductivity increases sharply in the water-rich region and decreases linearly in the salt-richregion. The equivalent conductance of [BMIM][BF4] was calculated. The limiting equivalentconductance was obtained by extrapolating the equivalent conductance with the square rootof the concentration at infinite dilution (see Fig. 6a and b). The extrapolated value of �∞

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0 20 40 60 80 100

45

50

55

60

65

70

75S

urfa

cete

nsio

n/m

N*m

-1

[Bmim]BF mole fraction (%)4

0 1 2 3 4 5 6

45

50

55

60

65

70

75

Sur

face

tens

ion

/mN

*m-1

Concentration of [BMIM][BF ]/mol*L4-1

(a) (b)

Fig. 4 Concentration (mole fraction (a), molar concentration (b)) dependence of surface tension for aqueous[BMIM][BF4] solutions

is 0.01044 S·m2·mol–1. Other researchers [17] reported similar results for the system of[BMIM]Br/[DMIM]Br and water.

Figure 7 presents our proposed schematic model to explain the changes of the physicalproperties of an aqueous solution of the ionic liquid, [BMIM][BF4], over the whole con-centration range. In the water-rich region, ionic charges are free. Most of the [BMIM][BF4]molecules added to the dilute solution ionize to form free anions and cations. Therefore,the specific conductance increases a great deal in this region. At higher concentrations of[BMIM][BF4], more association occurs between the anions and cations, thus resulting inmore aggregates of the pure liquid salt dispersed in water. The concentration of free chargesreaches a maximum with a maximum in the conductivity. As more and more [BMIM][BF4]is added, more and more charges in the solution are linked to the aggregates. Because of this,the equivalent conductance of [BMIM][BF4] decreases as the salt concentration increases,

0 20 40 60 80 100

0

10

20

30

40

50

Spe

cific

cond

ucta

nce/

mS

*cm

-1

[Bmim]BF mole fraction (%)4

0 1 2 3 4 5 6

0

10

20

30

40

50

Concentration of [BMIM][BF ]/mol*L4-1

Spe

cific

cond

ucta

nce/

mS

*cm

-1

(a) (b)

Fig. 5 Concentration dependence {mole fraction (a), molar concentration (b)} of specific conductivity foraqueous [BMIM][BF4] solutions

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0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.002

0.004

0.006

0.008

mΛΛ ΛΛ

/S*m

2 *mol

-1

C1/2

/mol1/2

*L-1/2

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.002

0.004

0.006

0.008

0.010

C1/2

/mol1/2

*L-1/2

m/S

m2 *m

ol-1

o=0.01044 S*m /mol

2

(a) (b)

ΛΛ

Fig. 6 Equivalent conductance, �m , versus the square root of concentration of aqueous [BMIM][BF4]solutions

from a high value due to the high mobility of the charges, down to the low value for thepure salt. In the salt-rich region, there is a mode of facilitated displacement of ions by thepresence of a network of interconnected micellar aggregates, labile and in permanent ex-change equilibrium with their surroundings, so the equivalent conductance is still not verylow (Table 3).

Fig. 7 Proposed model for an aqueous solution of [BMIM][BF4] in two different concentration regions: (a)the water-rich region, and (b) the salt-rich region (©− ) anion of low mobility (©+ ) cation of low mobility (©− )anion of high mobility (©+ ) cation of high mobility

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Table 3 The specific conductance and molar conductivity at 30 ◦C ofaqueous solution of [BMIM][BF4] of different concentrations

Concentration Specific conductance Molar conductivityEntry (mol·mL–1) (S·m–1) (S·m2·mol–1)

1 0.000206 12.13 588262 0.000397 17.92 451243 0.000574 23.00 400414 0.000739 27.50 371885 0.000893 30.90 345826 0.00104 33.80 325737 0.00117 36.10 307828 0.00130 38.00 292389 0.00142 39.60 27904

10 0.00153 41.10 2683211 0.00164 42.30 2582212 0.00174 43.70 2513313 0.00201 44.70 2223414 0.00221 45.60 2065615 0.00238 46.20 1938916 0.00254 46.60 1835017 0.00268 46.80 1745918 0.00282 47.10 1669219 0.00295 47.00 1595520 0.00306 46.90 1530921 0.00325 46.60 1434222 0.00346 45.90 1327023 0.00370 44.90 1214424 0.00397 42.80 1077725 0.00412 41.30 1001426 0.00429 39.20 914027 0.00447 36.60 819228 0.00466 32.80 703629 0.00487 27.20 558130 0.00496 23.90 481631 0.00506 19.90 393532 0.00516 15.11 293133 0.00526 8.64 164434 Pure water 0.00028935 Pure ILs 5.77

4. Conclusions

In conclusion, the effect of concentration on the physical properties of aqueous solutionsof the room-temperature ionic liquid, [BMIM][BF4], was studied so that they can be usedfor engineering design. The physical properties changed sharply in different concentrationregions. The mixture behaves as an ideal solution for the density and refractive indicies.The surface tension varied a great deal with concentration in dilute solutions whereas theviscosity changed much more in concentrated solutions. The conductivity increases sharplyin the water-rich region and decreases linearly in the salt-rich region. [BMIM][BF4] mainlyacts as a surfactant and salt in the surface tension and conductivity measurements. Interesting

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water-ionic liquid interactions have been identified and discussed. The model of two differentregions over the whole concentration range indicates a structural change in the mixtures.

It is very important to understand the interactions between the traditional organic solventsjust as for water and the RTILs, that can be studied by measurements of their physicalproperties. It is helpful for the bottom-up design of the RTILs and their further applications.

Acknowledgments The authors thank the Ministry of Education of the People’s Republic of China (105078)and Shanghai Science and Technology Committee (04JC14011) for their financial support.

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