Temperature dependence of physical properties of ionic liquid 1,3-dimethylimidazolium methyl sulfate

Article abstract of DOI:10.1021/je050434f

This paper reports on the synthesis of the ionic liquid 1,3-dimethylimidazolium methyl sulfate [MMIM][MeSO₄]. Experimental densities, speed of sounds, and refractive indices were determined from (283.15 to 343.15) K. Dynamic viscosities were measured from (293.15 to 343.15) K. Surface tensions were measured from (288.15 to 313.15) K. The coefficient of thermal expansion and molecular volume of [MMIM][MeSO₄] were calculated from experimental values of density.

Full text of DOI:10.1021/je050434f

952
J. Chem. Eng. Data 2006, 51, 952-954
Temperature Dependence of Physical Properties of Ionic Liquid
1,3-Dimethylimidazolium Methyl Sulfate
Ana B. Pereiro,† Francisco Santamarta,‡ Emilia Tojo,‡ Ana Rodr´ıguez,*,† and Jose´ Tojo†,§
Chemical Engineering Department and Organic Chemistry Department, Vigo University, 36310 Vigo, Spain
This paper reports on the synthesis of the ionic liquid 1,3-dimethylimidazolium methyl sulfate [MMIM][MeSO4].
Experimental densities, speed of sounds, and refractive indices were determined from (283.15 to 343.15) K.
Dynamic viscosities were measured from (293.15 to 343.15) K. Surface tensions were measured from (288.15 to
313.15) K. The coefficient of thermal expansion and molecular volume of [MMIM][MeSO4] were calculated
from experimental values of density.
ultrasonically, dried over freshly activated molecular sieves
(types 3 Å and 4 Å, supplied by Aldrich) for several weeks,
and kept in an inert argon atmosphere as soon as the bottles
were opened. Their mass fraction purities supplied by the
company were more than 99.0 % for 1-methylimidazole and
dimethyl sulfate (supplied by Fluka), more than 99.9 % for
toluene (supplied by Merck), and more than 99.5 % for ethyl
acetate (supplied by Aldrich). Chromatographic (GLC) tests of
the solvents showed purities that fulfilled purchaser specifica-
tions.
Introduction
Room-temperature ionic liquids (RTILs) are organic salts that
melt below 373.15 K and have an appreciable liquid range.1
The most commonly studied ionic liquids (ILs) contain am-
monium, phosphonium, pyridinium, or imidazolium cations,
with varying heteroatom functionality. In this paper, we have
considered the use of imidazolium cation and methyl sulfate
[CH3SO4]- as anion.
Recently, RTILs have received more attention because of their
unusual properties. Thus, they have great potential as “green”
solvents for industrial processes,2 possibly replacing currently
used organic solvents due to their unique properties such as
negligible vapor pressures, broad liquid temperature range, and
high specific solvent abilities. Despite the importance of RTILs
and their interest, accurate values for many of their fundamental
physical-chemical properties are either scarce or even absent,
but a few authors3-5 have studied the structural organization of
ILs and the dependence of their physical-chemical properties
with different parameters of some ILs.
Densities, refractive indices, dynamic viscosities, and other
physical properties are very useful industrially. An exhaustive
literature survey reveals no published work on the physical
properties of the ionic liquid 1,3-dimethylimidazolium methyl
sulfate [MMIM][MeSO4]. With the aim of characterizing the
pure component, experimental densities, speed of sounds,
refractive indices, dynamic viscosities, and surface tensions at
a temperature range of (283.15 to 343.15) K of the [MMIM]-
[MeSO4] have been determined. From the experimental density,
the coefficient of thermal expansion and the molecular volume
have been calculated.
Synthesis of 1,3-Dimethylimidazolium Methyl Sulfate. 1,3-
Dimethylimidazolium methyl sulfate was prepared according
to a slightly modified literature procedure.6 Figure 1 shows the
[MMIM][MeSO4] structure.
Dimethyl sulfate was added dropwise to a solution of equal
molar amounts of 1-methylimidazol in toluene (150 mL/0.42
mol of starting 1-methylimidazol) and cooled in an ice bath
under nitrogen at a rate to maintain the reaction temperature
below 313.15 K, due to the reaction being highly exothermic.
The reaction mixture was stirred at room temperature for (1 to
4) h depending on the amount of starting materials (the progress
of the reaction was monitored by thin-layer chromatography
using aluminum sheets silica gel 60 GF-254, dichloromethane
+ 10 % methanol as eluent). The upper organic phase of the
resulting mixture was decanted, and the lower IL phase was
washed with ethyl acetate (4 × 70 mL per 0.4 mol of starting
1-methylimidazol). After the last washing, the remaining ethyl
acetate was removed by heating under reduced pressure. The
ionic liquid obtained was dried by heating to (343.15 to 353.15)
1
K and stirring under high vacuum (2 × 10-1 Pa) for 48 h. H
NMR (400 MHz, D2O, ppm): δ 8.69 [s, 1 H, H-2], 7.48 [d, J
) 1.4 Hz, 2 H, H-4,5], 3.95 [6 H, NCH3], 3.79 [s, 3 H, OCH3].
The ionic liquid was kept in bottles with inert gas. To reduce
the water content to negligible values (mass fraction lower than
0.03 %, determined using a 756 Karl Fisher coulometer),
vacuum (2 × 10-1 Pa) and moderate temperature (343.15 K)
were applied to the IL for several days, always immediately
prior to their use.
In this paper, the IL is characterized as liquid in this interval
due to the solid-liquid transition6 being broad, between -18
°C (super-cooling temperature for crystallization in the cooling
cycle) and 43 °C (melting point in heating cycle).
Experimental Section
Chemicals. The reagents used for the synthesis of the IL were
of Lichrosolv quality. Before use, the reagents were degassed
Experimental Procedure. The density and speed of sound
of the pure liquid were measured with an Anton Paar DSA-
5000 digital vibrating-tube densimeter. The repeatability and
the uncertainty in experimental measurements have been found
to be lower than (( 2 × 10-6 and ( 10-5) g‚cm-3 for the
* To whom correspondence should be addressed. Tel.: +34 986812312.
Fax: +34 986812382. E-mail: aroguez@uvigo.es.
† Chemical Engineering Department.
‡ Organic Chemistry Department.
§ Deceased.
10.1021/je050434f CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/09/2006

Journal of Chemical and Engineering Data, Vol. 51, No. 3, 2006 953
Table 1. Density G, Speed of Sound u, Refractive Index nD,
Dynamic Viscosity η, and Surface Tension σ of [MMIM][MeSO4] at
Several Temperatures
T/K
F/g‚cm-3
u/m‚s-1
nD
η/mPa‚s
σ/mN‚m-1
Figure 1. Schematic structure of [MMIM][MeSO4].
283.15
288.15
293.15
298.15
303.15
308.15
313.15
318.15
323.15
328.15
333.15
338.15
343.15
1.33824
1.33454
1.33088
1.32725
1.32365
1.32009
1.31657
1.31305
1.30955
1.30606
1.30259
1.29914
1.29570
1851
1838
1826
1813
1801
1789
1777
1765
1753
1742
1730
1719
1708
1.48659
1.48525
1.48392
1.48270
1.48129
1.47999
1.47867
1.47726
1.47593
1.47421
1.47294
1.47154
1.47027
60.9
60.3
59.8
59.5
59.1
58.9
density and (( 0.01 and ( 0.1) m‚s-1 for the speed of sound.
The apparatus was calibrated by measuring the density of
Millipore quality water and ambient air according to manual
instruction. The calibration was checked with pure liquids with
known density and speed sound.
92.76
72.91
58.35
39.21
27.78
21.01
16.01
The refractive indices were determined by the automatic
refractometer ABBEMAT-WR Dr. Kernchen with a resolution
of ( 10-6 and an uncertainty in the experimental measurements
of ( 4‚10-5. The apparatus was calibrated by measuring the
refractive index of Millipore quality water and tetrachloroeth-
ylene (supplied by the company) before each series of measure-
ments according to manual instruction. The calibration was
checked with pure liquids with known refractive index.
Kinematic viscosities were determined experimentally using
an automatic viscometer Lauda PVS1 with two Ubbelhode
capillary microviscometers with a diameter of (7‚10-4 and
1.26‚10-3) m. Gravity fall is the principle of measurement on
which this viscometer is based. The capillary is maintained in
a D20KP Lauda thermostat with a resolution of ( 0.01 K. The
capillaries were calibrated and were credited by the supplier
company. The calibration was checked with pure liquids with
known dynamic viscosity. The uncertainty of the capillary
diameter is ( 0.005 mm. The uncertainty in the experimental
measurements has been found to be ( 0.01 mPa‚s. The
equipment has a control unit PVS1 (processor viscosity system)
that is a PC-controlled instrument for the precise measurement
of liquid viscosity using standardized glass capillaries with an
uncertainty of ( 0.01 s.
Table 2. Fitting Parameters of Equations 2 and 3 and Standard
Deviations (eq 4) To Correlate the Physical Properties of
[MMIM][MeSO4]
physical properties
A0
A1
SD
F/g‚cm-3
1.538397
0.443489
2522.9
1.564301
1532.2
83.385
-0.000708
-0.000538
-2.3794
-0.000274
3.2799
0.00014
0.00010
0.8
0.00012
0.02
0.1
ln(F/g‚cm-3
u/m‚s-1
nD
)
log(η/mPa‚s)
σ/mN‚m-1
-0.0787
Density, Speed of Sound, RefractiWe Index, Dynamic
Viscosity, Surface Tension, and Miscibility. The density F,
speed of sound u, refractive index nD, dynamic viscosity η, and
surface tension σ values were fitted by the method of least
squares using the following equations:
z ) A0 + A1T
(2)
(3)
log η ) A0/T - A1
where z is F, u, nD, or σ; T is the absolute temperature; and A0
and A1 are adjustable parameters. The correlation coefficient
of the linear regression is 0.9999 for the density, 0.9997 for
the speed of sound, 0.9995 for the refractive index, 0.9968 for
the dynamic viscosity, and 0.9728 for the surface tension. The
correlation parameters are listed in Table 2 together with the
standard deviations (SD). These deviations were calculated by
applying the following expression:
The kinematic viscosity is determined from the following
relationship:
ν ) k(t - y)
(1)
where y is the Hagenbach correction, t is the flow time, and k
is the Ubbelhode capillary microviscometer constant, being y
and k supplied by the company.
The surface tension of pure liquid was measured with the
tensiometer Lauda TVT2 by the hanging drop tensiometer
method. The measured tank was thermostatized in a Polyscience
controller temperature with a temperature stability of ( 0.01
K, which is regulated in a D20KP Lauda thermostat with a
resolution of ( 0.01 K. The equipment has both a control unit
and a mechanical one that are connected to a PC-controlled
instrument for the precise measurement of liquid with an
uncertainty of ( 0.1 mN‚m-1. The radiuses of the needles were
calibrated and were credited by the supplier company. The
calibration was checked with pure liquids with known surface
tension.
nDAT
1/2
2
(zexp - zadjust
)
∑
i
SD )
(4)
(
)
nDAT
where property values and the number of experimental and
adjustable data are represented by z and nDAT, respectively.
Figures 2 to 5 show the physical properties against T.
Thermodynamic Properties. The values of density were fitted
by the method of the least-squares using the following empirical
equation:
ln F ) A0 + A1T
(5)
Results and Discussion
The physical properties of [MMIM][MeSO4] were measured
experimentally from (283.15 to 343.15) K for the density, speed
of sound, and refractive index from (293.15 to 343.15) K for
the dynamic viscosity and from (288.15 to 313.15) K for the
surface tension. The values are listed in Table 1.
The frequency of an Anton Paar densimeter is affected by
the viscosity of the sample,7 at high viscosity a correction has
to be applied to the density. Our samples have low viscosities,
and this correction must not be applied.
where T is the absolute temperature and A0 and A1 are fitting
parameters. Figure 2 shows the experimental and adjustable
values of ln F against T. The correlation coefficient of the linear
regression is 0.9999.
The coefficient of thermal expansion of [MMIM][MeSO4]
is defined by the following equation:
1 ∂V
V ∂T P
∂ ln F
∂T
R )
) -
(6)
(
)
(
)
P

954 Journal of Chemical and Engineering Data, Vol. 51, No. 3, 2006
Figure 2. Plot of experimental values of ln F (O) and F (0) against T and
fitted curves for [MMIM][MeSO4].
Figure 5. Plot of experimental values of surface tension σ (O) and
σ‚Vmolec (0) against T and fitted curves for [MMIM][MeSO4].
2/3
where M is the molar mass (208.27 g‚mol-1), N is Avogadro’s
number, and Vmolec is the molecular volume. The molecular
volume at 298.15 K for [MMIM][MeSO4] is 0.2606 nm3.
In general, σ of many liquids almost linearly decreases while
temperature increases, and the relationship is expressed in the
Eo¨tvo¨s equation:8
σ‚Vmolec2/3 ) k(Tc - T)
(8)
where Vmolec is the molecular volume of the liquid, Tc is the
critical temperature, and k is an empirical constant. The linear
regression of σ‚Vmolec2/3 obtained from this experimental values
against T was made, and the equation σ‚Vmolec2/3/10-18‚J )
0.0314-0.00002 (T/K) was obtained. The standard deviation
of the linear regression is 0.001. The value of k is 0.2‚10-24
J‚K-1. The fused salts have large polarity, and their values of
k are low, for example, k is 0.6‚10-24 J‚K-1 for fused NaCl.8
This implies that [MMIM][MeSO4] has a similar polarity than
the fused salts. Therefore, the magnitude of k can represent the
polarity of ionic liquids. Figure 5 shows the experimental and
Figure 3. Plot of experimental values of speed of sound u (O) and nD (0)
against T and fitted curves for [MMIM][MeSO4].
2/3
adjustable values of σ‚Vmolec against T.
Literature Cited
(1) Holbrey, J. D.; Rogers, R. D. Ionic Liquids in Synthesis; VCH-
Wiley: Weinheim, 2002.
(2) Rogers, R. D.; Seddon, K. S. Ionic Liquids Industrial Applications
for Green Chemistry; ACS Symposium Series 818; American Chemi-
cal Society: Washington, DC, 2002.
(3) Chiappe, C.; Pieraccini, D. Ionic liquids: solvent properties and organic
reactivity. J. Phys. Org. Chem. 2005, 18, 275-297.
(4) Wasserscheid, P.; Keim, W. Ionic liquidsnew solutions for transition
metal catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772-3789.
(5) Seddon, K. R.; Stark, A.; Torres, M. J. Influence of chloride, water,
and organic solvents on the physical properties of ionic liquids. Pure
Appl. Chem. 2000, 72, 2275-2287.
(6) Holbrey, J. D.; Reichert, W. M.; Swatloski, R. P.; Broker, G. A.; Pitner,
W. R.; Seddon, K. R.; Rogers, R. Efficient, halide free synthesis of
new, low cost ionic liquids: 1,3-dialkylimidazolium salts containing
methyl- and ethyl-sulfate anions. Green Chem. 2002, 4, 407-413.
(7) Cerdeirina, C. A.; Troncoso, J.; Romani, L.; Najdanovic-Visak, V.;
Esperanca, J. M. S. S.; Rebelo, L. P. N. Criticality in C4mimBF4 +
water mixture. Presented at the ACS National Meeting, Washington,
DC, 2003; IEC-012.
Figure 4. Plot of experimental values of log η (O) against 1/T and fitted
curve for [MMIM][MeSO4].
where R is the coefficient of thermal expansion, V is the volume
of the ionic liquid, and F is the density of the ionic liquid. A
value R ) 5‚10-4 K-1 was obtained from this equation. The
molecular volume of [MMIM][MeSO4] at 298.15 K was
calculated from the experimental density using the following
equation:
(8) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley: New
York, 1976 (Translation by T. R. Gu, Science Press: Beijing, 1986).
Received for review October 19, 2005. Accepted February 13, 2006.
JE050434F
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Vmolec
)
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