www.elsevier.com/locate/molliq

Journal of Molecular Liq

Effects of alkyl chain on transport properties in

1-alkyl-3-methylimidazolium hexafluorophosphates

Tatsuya Umecky, Mitsuhiro Kanakubo*, Yutaka Ikushima

Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology (AIST),

4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan

Available online 14 November 2004

Abstract

We studied the transport properties in a series of 1-alkyl-3-methylimidazolium hexafluorophosphates with the alkyl side chains being

butyl, hexyl, and octyl, which are expressed by [BMIM][PF6], [HMIM][PF6], and [OMIM][PF6], respectively. The self-diffusion coefficients

of the cation (Dcation) and anion (Danion) species in the ionic liquids were independently determined at a fixed temperature of 323.2F0.1 K by

observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique together with the electric conductivities. Based on these

experimental results, the effects of alkyl side chain on transport properties in 1-alkyl-3-methylimidazolium hexafluorophosphates were

discussed in terms of interionic interactions.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Ionic liquid; 1-alkyl-3-methylimidazolium hexafluorophosphate; NMR spectroscopy; Pulsed-field gradient spin-echo technique; Self-diffusion

coefficient; Electric conductivity

1. Introduction

In the early stage of 21st century, the solvent-free

technology to reduce the use of harmful volatile organic

compounds is one of the most crucial research subjects for

the achievement of environmentally benign green chemical

processes. There have been several approaches proposed so

far, for example, to exploit no-solvent systems, aqueous

solutions, supercritical fluids, and ionic liquids. The latter

two media of supercritical fluids and ionic liquids have

recently gained much attention, and also are expected to

have a promising wide application such as extraction,

separation, chemical reaction, and material processing

media [1,2]. Supercritical fluids, which consist of water

and carbon dioxide, have the merits of economical benefit,

easy solvent removal, and readily tunable solvent properties

by temperature and pressure, while ionic liquids, of which

the melting points are lower than room temperature, are

0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.molliq.2004.10.011

* Corresponding author. Tel.: +81 22 237 5211; fax: +81 22 237 6839.

E-mail address: m-kanakubo@aist.go.jp (M. Kanakubo).

nonvolatile, nonflammable, and the solvent properties can

be modified by a choice of cation and anion combinations.

Thus, such a new class of solvents has several advantages

over conventional liquid organic solvents; however, a lack

of fundamental properties sometimes prevents much pro-

gress of its utility in a variety of research fields.

Ionic liquids, of which the cations are based on a five-

membered imidazolium unit (Fig. 1), have relatively low

melting points, and high thermal and chemical stabilities

[2]. Moreover, the physicochemical properties can be

widely varied by the alkyl side chains (R1 and R2) attached

to the nitrogen atoms as well as the anion species. Seddon et

al. [3] measured the densities and viscosities of a series of 1-

alkyl-3-methylimidazolium (R1=CnH2n+1 and R2=CH3 with

n being mainly 2, 4, 6, and 8) ionic liquids of BF4�, PF6

�,

Cl�, CF3SO3�, and NO3

� anions, and found that the

densities decreased with increasing alkyl chain length

whereas the viscosities increased in any combinations of

cation and anion species. Suarez et al. [4] determined the

densities, viscosities, and electric conductivities of 1-butyl-

3-methylimidazolium ionic liquids of BF4� and PF6

�. Noda

et al. [5] measured the densities, viscosities, electric

uids 119 (2005) 77–81

Fig. 1. Molecular structure of five-membered imidazolium cation.

T. Umecky et al. / Journal of Molecular Liquids 119 (2005) 77–8178

conductivities, and self-diffusion coefficients of both cation

and anion species in 1-ethyl-3-methylimidazolium ionic

liquids of BF4� and N�(SO2 CF3)2. Only a few experimental

data concerning transport properties are available even for

the relatively popular 1-alkyl-3-methylimidazolium ionic

liquids, and the effects of the alkyl side chain on transport

properties have not been examined completely. In the

present work, hence, we focus on transport properties of a

series of 1-alkyl-3-methylimidazolium hexafluorophosphate

ionic liquids, where the abbreviations of [BMIM][PF6],

[HMIM][PF6], and [OMIM][PF6] are used throughout for

R1=CnH2n+1 and R2=CH3 with n being 4, 6, and 8,

respectively. The self-diffusion coefficients of cation and

anion species in the ionic liquids are independently

determined at a fixed temperature of 323.2F0.1 K by

observing 1H and 19F nuclei with the pulsed-field gradient

spin-echo NMR technique together with the electric

conductivities.

2. Experimental

The samples of [BMIM][PF6], [HMIM][PF6], and

[OMIM][PF6] were synthesized according to the literature

procedures [6,7]. First, 1-alkyl-3-methylimidazolium chlor-

ide was prepared by mixing equal molar amounts of 1-

methylimidazole and 1-chloroalkane at 343 K for more than

50 h. The resulting viscous liquid was washed several times

with ethyl acetate, and then the remaining ethyl acetate was

removed by heating to 343 K under vacuum. To exchange

the chloride anion for hexafluorophosphate, a stirred

mixture of 1-alkyl-3-methylimidazolium chloride and water

was cooled to 273 K, and hexafluorophosphoric acid was

added very slowly. After stirring the biphasic mixture for 4

h, the upper aqueous phase was decanted, and the lower

ionic liquid was washed more than five times with water and

with saturated aqueous NaHCO3 solutions to ensure

neutrality. The ionic liquid was heated under vacuum at

343 K for 50 h to remove any excess water, and then kept in

a glove box filled with inert Ar gas. It was confirmed that

the residual chloride was very minor in any ionic liquids by

the elemental titration analyses. Found (C, 33.97; H, 5.25;

N, 9.80) and calculated (C, 33.81; H, 5.32; N, 9.86) for

[BMIM][PF6], found (C, 39.79; H, 6.03; N, 8.95) and

calculated (C, 38.47; H, 6.13; N, 8.97) for [HMIM][PF6],

and found (C, 42.98; H, 6.74; N, 8.18) and calculated (C,

42.35; H, 6.81; N, 8.23) for [OMIM][PF6].

Since a small amount of impurities such as water and

chloride anion remarkably affected physicochemical proper-

ties of ionic liquids in some cases [8], we have paid much

attention to the sample preparation. The ionic liquids for

electric conductivity experiments were transferred into an

airtight container in a glove box, and degassed by Ar gas

before each measurement. The ionic liquids for NMR

experiments were dried and degassed by several freeze–

pump–thaw cycles under high vacuum less than 2 mPa, and

sealed into cylindrical Pyrex tubes. The sealed sample tube

was inserted in a standard 5-mm tube filled with deuterated

lock solvent.

The electric conductivities, j, were recorded with an

electric conductivity measurement system 369 presented by

Fuso Electro Chemical System. A pair of Pt electrodes were

fixed with a poly(etheretherketone) tube, which had several

holes allowing the sample liquids to come and go, and

washed with nitric acid before each measurement. The cell

constant of 2.99 cm�1 was determined with a known value

of n of 0.1 mol dm�3 KCl aqueous solution at 298.15 K [9],

and the temperature dependence of the cell constant was

negligibly small. The electric conductivities of ionic liquids

were measured at 323.1F0.1 K and at the fixed frequency of

1 kHz.

NMR measurements were performed on Varian Inova

300 and 500 spectrometers with 5-mm pulsed-field gradient

probes. Before each measurement, the sample was held for

12 h in the probe, which was controlled at 323.2F0.1 K

with a variable temperature control unit using heated dry air.

The sample temperature was calibrated with a thermistor

thermometer (TAKARA, D641). For the determination of

self-diffusion coefficients, 1H of the 3-methyl group in the

imidazolium cation and 19F in PF6� were used. The number

of accumulation times was fixed at 4 in all measurements.

The self-diffusion coefficients, D, were measured with the

pulsed-field gradient spin-echo technique. The natural

logarithm of the ratio of signal integrals, A and A0, in the

presence and absence of the pulsed-field gradient can be

proportional to the square, g2, of the gradient magnitude

[10]:

ln A=A0Þ ¼ � Dc2 D � d=3ð Þd2g2�

ð1Þ

where c is the magnetogyric ratio, D is the interval between

the two gradient pulses, and d is the gradient pulse width.

Constant values of 5 and 50 ms were used for d and D,

respectively. g was calibrated with a known value of D,

3.96�10�9 m2 s�1, for water [11], and varied up to ~0.5 T

m�1. In each measurement, a set of more than 20 different

values of g was inputted. Diffusion measurements were

repeatedly performed five times or more in both Varian

Inova 300 and 500 spectrometers, and the average values of

D were obtained.

3. Results and discussion

In Table 1, we summarize the fundamental physicochem-

ical quantities of molar mass (M), density (q), viscosity (g),

Table 1

Values of M, q, g, Vm, VvdW(cation), VvdW(anion), VvdW(total), and VvdW(total)/Vm in [BMIM][PF6], [HMIM][PF6], and [OMIM][PF6] at 323.2 K

M (g mol�1) q (g cm�3)a g (mPa s)a Vm (dm3 mol�1) VvdW(cation)

(dm3 mol�1)bVvdW(anion)

(dm3 mol�1)bVvdW(total)

(dm3 mol�1)

VvdW(total)/Vm

[BMIM][PF6] 284.2 1.3473 82 0.211 0.081 0.041 0.122 0.58

[HMIM][PF6] 312.2 1.2681 125 0.246 0.092 0.041 0.133 0.54

[OMIM][PF6] 340.3 1.2013 152 0.282 0.102 0.041 0.143 0.51

a Cited from Ref. [3].b Cited and calculated from Refs. [12,13].

T. Umecky et al. / Journal of Molecular Liquids 119 (2005) 77–81 79

molar volume (Vm=M/U), van der Waals molar volumes of

cation (VvdW(cation)) and anion (VvdW(anion)), and their

total volume (VvdW(total)=VvdW(cation)+VvdW(anion)) in

[BMIM][PF6], [HMIM][PF6], and [OMIM][PF6] at 323.2

K. As the alkyl chain length increases, q decreases slightly

whereas g increases remarkably. In this series of ionic

liquids, the van der Waals volumes of the plane imidazolium

cations are much larger than that of the almost spherical

hexafluorophosphate anion. Values of Vm and VvdW(total)

increase with increasing alkyl chain length; however, their

ratio of VvdW(total)/Vm decreases gradually. This suggests

that ionic species with shorter alkyl chain are packed more

closely.

The electric conductivities (j) and molar conductivities

(�=jM/U) in [BMIM][PF6], [HMIM][PF6], and [OMIM]

[PF6] were presented in Table 2. The present determined

value of 0.471 S m�1 for j of [BMIM][PF6] is in a good

agreement with the literature values of 0.458 S m�1 [4] and

0.470 S m�1 [14] at the same temperature. Table 2 clearly

shows that K decreases with the alkyl chain length. This is

considerably attributable to the viscosity increase with the

alkyl chain; however, the Walden products of Kg still

remain variable. This difference cannot be compensated by

considering the size effect of alkyl side chain as discussed

later.

The self-diffusion coefficients of the cation (Dcation) and

anion (Danion) species in [BMIM][PF6], [HMIM][PF6], and

[OMIM][PF6] were independently determined at 323.2F0.1

K by observing 1H and 19F nuclei with the pulsed-field

gradient spin-echo NMR technique. For example, logarith-

mic values of normalized spin-echo amplitudes (A/A0) were

plotted against the square of field gradient ( g2) in Fig. 2.

The self-diffusion coefficient can be obtained from the slope

divided by c2(D�d/3)d2. As shown in Fig. 2, the slope

Table 2

Values of j, K, Kg, Dcation, Danion, Dcation/(Dcation+Danion), KD, VD(cation), VD(a

323.2 K

j K Kg Dcationa Danion

a

S m�1 10�5 S m2

mol�1

10�6 S N s

mol�1

10�12

m2 s�1

10�12

m2 s�1

[BMIM][PF6] 0.471 9.93 8.14 25.9 (0.2) 20.6 (0.5)

[HMIM][PF6] 0.200 4.93 6.16 14.5 (0.9) 13.2 (0.6)

[OMIM][PF6] 0.114 3.22 4.90 8.8 (0.6) 9.1 (0.6)

a The 95% confidence limit is given in parentheses.

becomes larger in the magnitude as the alkyl chain is

shorter. Such a change in the slope is remarkable for 1H of

cations with a large magnetogyric ratio. The self-diffusion

coefficients collected by two different instruments Varian

Inova 300 and 500, remained virtually the same, and the

average values of Dcation and Danion are presented with the

experimental errors in Table 2. Since the decreases in the

spin-echo amplitudes for less diffusive species are very

small up to at most ~20% in this range of g2, the self-

diffusion coefficients obtained for such species involve

relatively large experimental errors.

As is expected from the conductivity data, Dcation and

Danion remarkably decrease with increasing alkyl chain

length due to the viscosity increase. The Nernst–Einstein

equation can give the relationship between K and D [15]:

K ¼ jzcationjDcation þ jzanionjDanionÞF2=RT�

ð2Þ

where F is the Faraday constant, R is the gas constant, and

zcation and zanion are charge numbers of cation and anion,

respectively. The charge numbers are equal to 1 in this

case. Using Eq. (2), we calculate the apparent molar

conductivity, KD, from Dcation and Danion in each ionic

liquid (Table 2). If each ion has the activity of unity without

ionic association, the experimentally determined conductiv-

ity, K, should be identical with KD within the theoretical

framework. The ratios of K/KD are 0.62, 0.51, and 0.52 in

[BMIM][PF6], [HMIM][PF6], and [OMIM][PF6], respec-

tively. Much smaller values of K/KD than unity could

indicate the presence of ionic association in any ionic

liquids.

Although VvdW(cation) is, as mentioned above, much

larger than VvdW(anion) in three ionic liquids,Dcation is larger

nion), and VD(total) in [BMIM][PF6], [HMIM][PF6], and [OMIM][PF6] at

Dcation KD VD(cation) VD(anion) VD(total)

Dcation+Danion 10�5 S m2

mol�1

dm3

mol�1

dm3 mol�1 dm3 mol�1

0.56 16.1 0.0035 0.0070 0.0105

0.52 9.6 0.0056 0.0074 0.0130

0.49 6.2 0.0138 0.0128 0.0266

Fig. 2. Plots of natural logarithms of A/A0 against g2 in the pulsed-field

gradient spin-echo technique at 323.2 K. These example data were collected

with a Varian Inova 300 spectrometer.

T. Umecky et al. / Journal of Molecular Liquids 119 (2005) 77–8180

than Danion in [BMIM][PF6] and [HMIM][PF6], and Dcation

is comparable with Danion in [OMIM][PF6]. The apparent

cationic transference number, Dcation/(Dcation+Danion),

decreases with increasing alkyl chain length. This is mainly

attributable to the fact that the friction for the translational

motions can work more effectively for the bulky spherical

PF6� anion than for the plane imidazolium cations. Thus, in

view of their molecular sizes, the imidazolium cations are

considered to be good electron carriers.

The hydrodynamic radii of cation (rcation) and anion

(ranion) species in the ionic liquids can be estimated from

Dcation and Danion, respectively, using the well-known

Stokes–Einstein equation [16]:

ri ¼ kBT=6pDig; ð3Þ

where kB is Boltzmann constant and the subscript biQrepresents cation or anion. In order to compare the hydro-

dynamic size with the van der Waals molar volume, we

calculate the average hydrodynamic molar volume (VD(i)=

NA4pri3/3) on the assumption that each ion is regarded as a

sphere. As shown in Table 2, the present data provide three

important features; (i) VD is much smaller than the

corresponding value of VvdW in both the cation and anion

species, (ii) VD(cation) increases with increasing alkyl chain

length; however, the ratio of VD(cation)/VvdW(cation) does

not remain constant and increases with the alkyl chain

length, and (iii) more interestingly, although three ionic

liquids have the common anion of PF6�, VD(anion) increases

with the alkyl chain length.

The magnitude of VD thus calculated may be signifi-

cantly influenced by the friction responsible for the

translational motions. The effective friction should be

sensitive to the geometrical shape and free space around

the ionic species as well as inter-ionic interactions between

the ions and their surroundings. Much smaller values of

VD than VvdW in both the cation and anion species indicate

the rather ineffective friction, although interactions in the

ionic liquids are expected to be very strong from the

macroscopic point of view of high viscosities. At the

present stage, this fact cannot be rationalized straight-

forwardly. However, there is relatively a lot of free space

in the ionic liquids as shown by comparison between

VvdW(total) and Vm in Table 1, and this could make a

negative contribution to the effective friction. It was also

found that VD/VvdW of the bulky spherical PF6� anion is

larger than those of the plane imidazolium cations in three

ionic liquids. This fact suggests that the presence of free

space will decrease the effective friction in the non-

spherical plane cations more remarkably.

In three ionic liquids, it was found that VD/VvdW values

of imidazolium cations increase with the alkyl chain

length. The elongation of the alkyl side chain brings about

changes in the geometrical shape and free space as well as

in the interionic interactions. The free space increases with

the alkyl chain length in view of VvdW/Vm, which reduces

the effective friction in contrast to the experimental

results. If one assumes that the geometrical shape will

not change drastically in the three imidazolium cations,

the increase in VD/VvdW indicates that inter-ionic inter-

actions become stronger as the alkyl side chain is

elongated. This is strongly supported by the experimental

results of VD(anion) of PF6�. Although the geometrical

shape of the common PF6� anion remains unchanged in

the three ionic liquids, VD(anion) increases with the alkyl

chain length. This is a clear evidence of stronger inter-

ionic interactions in the ionic liquids with longer alkyl

side chains. This conclusion is reasonably in harmony

with the results of K/KD.

In this study, three transport quantities of Dcation, Danion,

and � were exactly determined by independent experiments

in a series of 1-alkyl-methylimidazolium hexafluorophos-

phates. It has been revealed that interionic interactions are

significantly influenced by the alkyl side chain in the ionic

liquids. In the present results, there will be no contradiction

to a proposed type of interionic interaction, i.e., weak

hydrogen bonding between hydrogens (in particular, C2-H)

in the imidazolium ring and fluorines in PF6� [4,17,18].

However, in order to elucidate obviously what kind of inter-

ionic interactions can exist effectively, we are going to

determine the site-specific rotational dynamics in this series

of ionic liquids.

Acknowledgements

This work was partially supported by Industrial Tech-

nology Research Grant Program in 2003 from New Energy

and Industrial Technology Development Organization

(NEDO) of Japan.

T. Umecky et al. / Journal of Molecular Liquids 119 (2005) 77–81 81

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