Surface tension of binary mixtures of 1-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with alcohols

Article abstract of DOI:10.1007/s10953-014-0128-9

New experimental surface tension data have been provided at 283.15, 298.15, 313.15 K and atmospheric pressure for binary mixtures of 1-butyl-3-methyl- imidazolium bis(trifluoromethylsulfonyl)imide and 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with the alcohols: methanol, ethanol, 1-propanol, 2-propanol, l-butanol and 1-pentanol. The experimental results show that the surface tensions of these mixtures depend systematically on the alkyl chain length of the ionic liquid and alcohol, composition and temperature. Surface tension changes on mixing have been calculated and adequately fitted by the Redlich-Kister polynomial equation. The adjustable parameters and the standard deviations between experimental and calculated values are reported.

Full text of DOI:10.1007/s10953-014-0128-9

J Solution Chem (2014) 43:404–420
DOI 10.1007/s10953-014-0128-9
Surface Tension of Binary Mixtures of 1-Alkyl-3-Methyl-
Imidazolium Bis(trifluoromethylsulfonyl)imide Ionic
Liquids with Alcohols
Alfonsina E. Andreatta Eva Rodil Alberto Arce Ana Soto
·
·
·
Received: 12 March 2013 / Accepted: 31 October 2013 / Published online: 25 January 2014
© Springer Science+Business Media New York 2014
Abstract New experimental surface tension data have been provided at 283.15, 298.15,
313.15 K and atmospheric pressure for binary mixtures of 1-butyl-3-methyl-imidazolium
bis(trifluoromethylsulfonyl)imide and 1-octyl-3-methyl-imidazolium bis(trifluoromethyl-
sulfonyl)imide ionic liquids with the alcohols: methanol, ethanol, 1-propanol, 2-propanol,
l-butanol and 1-pentanol. The experimental results show that the surface tensions of these
mixtures depend systematically on the alkyl chain length of the ionic liquid and alcohol,
composition and temperature. Surface tension changes on mixing have been calculated and
adequately fitted by the Redlich–Kister polynomial equation. The adjustable parameters
and the standard deviations between experimental and calculated values are reported.
Keywords Surface tension · [Cnmim][NTf2] · Alcohols
1 Introduction
In recent years, ionic liquids (ILs) have attracted much attention from the scientific com-
munity due to their large variety of applications in industry and applied chemistry [1]. The
ionic nature of these compounds results in a unique combination of properties [2, 3]. Of these,
negligible vapour pressure may be the most attractive from the environmental point of view
and it also implies a high potential for recycling. Most ILs exhibit other interesting properties
such as wide liquid range, chemical and physical stability, non-flammability, high thermal
conductivity, and the ability to dissolve a broad range of compounds. Moreover, tunable
A. E. Andreatta
IDTQ- Grupo Vinculado PLAPIQUI – CONICET, Universidad Tecnologica Nacional, Fac. Reg. San
´
Fco. Av de la Universidad 501, San Francisco, Cordoba, Argentina
´
E. Rodil · A. Arce · A. Soto (&)
Department of Chemical Engineering, University of Santiago de Compostela, 15782 Santiago, Spain
e-mail: ana.soto@usc.es
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J Solution Chem (2014) 43:404–420
405
physical, chemical and biological properties obtained by combination of the cation and anion
of an ionic liquid generates a huge number of applications for these salts.
To design any process involving ILs on an industrial scale, it is necessary to know a
range of physical properties, including density, viscosity, surface tension, etc. The surface
tension of a liquid mixture plays an important role in affecting the mass and heat transfer at
the interface. It is needed in many industrial applications such as liquid–liquid extraction,
gas absorption, distillation, condensation and so on. These data are needed not only for
process and product design but also for development of correlation and prediction models.
A key factor in evaluating simulation and design programs is the reliability of predicted
thermophysical properties and phase equilibria. Unfortunately, these data are limited and in
several cases contradictory.
Moreover, the surface tension reveals information about the structure and energetics of
the surface region between two phases. Variation in surface energy depends on variation in
molecular forces and of the density of packing or molecular size. For this reason, the
determination of this property for liquid mixtures lets us study interactions among the
components. In the literature, there are few works collecting surface tension data for
mixtures of ILs and alcohols. Furthermore, interactions follow very different patterns.
Aggregation appears in several cases [4, 5] and strong intermolecular interactions which
lead to continuous variations occur in several other systems [6, 7].
This work reports experimental measurements of surface tension for 1-butyl-3-methyl-
imidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]) or 1-octyl-3-methyl-imi-
dazolium bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]) IL and alcohol (methanol,
ethanol, 1-propanol, 2-propanol, 1-butanol, or 1-pentanol) binary mixtures at 283.15,
298.15, 313.15 K and atmospheric pressure. Surface tension changes of mixing are cal-
culated and fitted by Redlich–Kister polynomial equation.
2 Experimental
2.1 Materials
The source and purity of the methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 1-
pentanol are shown in Table 1. No further purification of these products was carried out.
The 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2])
and 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]),
were synthesized in our laboratory by reaction of 1-methylimidazole (Aldrich, [99 mass%,
GC) with an excess of 1-bromobutane (Merck, [98 mass%) and with an excess of
1-chlorooctane (Aldrich, 99 mass%) to produce [C4mim][Br] and [C8mim][Cl], respectively.
These bromide and chloride salts were mixed with lithium bis(trifluoromethanesulfonyl)
imide, Li[NTf2] (Solvionic, [99 mass%) in deionized water, to obtain [C4min][NTf2] and
[C8min][NTf2], respectively, by ion metathesis. After washing, the purification of the ILs was
completed by heating them under high vacuum for at least 48 h (1 mbar, 353.15 K). More
details about the experimental procedure can be found in earlier publications [8]. No bromide
concentration was detected by means of capillary electrophoresis in the [C4min][NTf2] while
the chloride concentration detected in the [C8min][NTf2] by the same technique was
14.2 ppm. The ILs were analyzed by 1H and 13C NMR to confirm the absence of any major
impurities. Estimated purity is [99.5 mass% for both ILs.
123

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J Solution Chem (2014) 43:404–420
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407
123

408
J Solution Chem (2014) 43:404–420
Fig. 1 Comparison of
(a)
48
46
44
42
40
38
36
34
32
30
28
experimental surface tension
available in the literature of a
[C4mim][NTf2] and b [C8mim]
[NTf2]: filled circle this work;
open circle [7]; filled triangle
[24]; times [16]; asterisk [17];
open triangle [18]; plus [19];
open square [25]; minus [26];
filled diamond [27]; open
σ
diamond [28]; filled square [29];
hyphen [22, 31]; filled grey circle
[30]; filled grey diamond [23]
280
285
290
295
300
305
310
315
T / K
(b)
36
35
34
33
32
31
30
29
28
σ
280
285
290
295
300
305
310
315
T / K
Table 1 shows a comparison between experimental and literature [6, 7, 9–31] density
data at 298.15 K and surface tension data at 283.15, 298.15, 313.15 K and atmospheric
pressure. The water content of pure components, also reported in this table, was determined
by means of a Karl Fisher titration method carried out in a Metrohm 737 KF.
Figure 1 shows a comparison of the surface tension determined here for the ILs [C4min]
[NTf2] and [C8min][NTf2] at 283.15, 298.15, 313.15 K with data available in the literature
[7, 16–19, 22–31]. In spite of the fact that trace amounts of impurities have dramatic
effects on the physical properties of ILs, it can be seen that our data are in good agreement
with previously measured values. In the case of the measurements at 298.15 K, many data
have been published and some of them are clearly dispersed.
Figure 2 illustrates the influence of the anion on the surface tension at 298.15 K of ILs
with [C4mim] or [C8mim] cations [6, 7, 17, 18, 25–27, 32–40]. It can be seen that the
influence of the anion is greater for the shorter alkyl substituent on the imidazolium cation.
A detailed analysis of the influence of each specific anion can be found in the interesting
review of Tariq et al. [39].
2.2 Experimental Apparatus and Procedure
Densities ofthepure components were measured using anAnton PaarDMA 5000densimeter.
This instrument, calibrated by air and water, automatically corrects for the effect of viscosity
onthe density measurement overthe whole viscosity range. The uncertainty in the densityand
the temperature measurements are 3 9 10−5 g·cm−3 and 0.01 K, respectively.
123

J Solution Chem (2014) 43:404–420
409
70
60
50
40
30
20
[C4mim][anion]
70
60
50
40
30
20
[C8mim][anion]
Fig. 2 Experimental surface tension at 298.15 K for different anions in [C4mim] and [C8mim] cations
available in the literature. a [C4mim]: filled circle this work; open circle [25]; filled triangle [17]; open triangle
[32]; asterisk [33]; filled square [34]; plus [35]; open square [36] at 293.15 K; minus [7]; filled diamond [18];
open diamond [6]; times [37]; filled grey circle [38] at 293 K; filled grey diamond [39] at 293K. b [C8mim]: filled
circle this work; open circle [26]; filled triangle [17]; filled square [34]; open square [27] at 293 K; dash [40]
Mixtures were prepared by mass with a Mettler Toledo AT 261 balance (repeatabil-
ity 1 9 10−4 g), and the deviation in the mole fraction was estimated to be 0.0002.
The Wilhelmy plate method was used for the measurement of the surface tensions. All
measurements were carried out in a Kru¨ss K11 tensiometer, equipped with an specially
adapted platinum plate of cylindrical shape (Kru¨ss accessory reference PL 22; dimensions:
10mmheight9 20mm baseperimeter9 0.1mmwidth), suitablefortheproductionofreliable
measurements with lower amounts of sample than a conventional plate. The temperature
inside the sample vessels (cylindrical open-top glass vessels with a diameter of 30 mm) was
kept constant by means of an oil bath controlled by a circulating water bath connected to a
Julabo F12 cryogenic thermostat. The uncertainty in the temperature measurement is 0.1 K.
Each valueofsurface tensionreportedinthis work istheaverageoftenconsecutive immersion
measurements, after two initial immersion measurements that were systematically disre-
garded. The method described has an estimated uncertainty of 0.1 mN·m−1.
3 Results and Discussion
Experimental results of surface tension, σ, and surface tension changes of mixing, Δσ, were
measured for [C4mim][NTf2] or [C8mim][NTf2] and alcohols (methanol, ethanol,
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J Solution Chem (2014) 43:404–420
Table 2 Experimental surface tension (σ) and surface tension changes of mixing (Δσ) of [C4mim][NTf2]
with methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol at different temperatures
283.15 K
298.15 K
313.15 K
x
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
{x [C4mim][NTf2] + (1 − x) Methanol}
0.1049
0.2076
0.3025
0.4087
0.5037
0.5994
0.7168
0.7523
0.8215
0.8649
0.9164
0.9577
26.41
28.14
29.40
30.61
31.21
31.79
32.62
32.87
33.14
33.35
33.42
33.44
2.095
2.761
3.039
3.150
2.770
2.356
1.972
1.853
1.408
1.168
0.706
0.298
25.52
27.43
28.61
29.79
30.52
31.09
31.78
32.03
32.18
32.40
32.50
32.52
2.281
3.114
3.300
3.367
3.102
2.668
2.128
2.006
1.431
1.196
0.757
0.344
{x [C4mim][NTf2] + (1 − x) Ethanol}
0.0999
0.1949
0.2962
0.4001
0.5069
0.6376
0.7183
0.7764
0.8126
0.8615
0.9245
0.9601
24.63
26.05
27.40
28.69
29.83
31.15
31.93
32.46
32.76
32.96
33.33
33.42
0.456
0.883
1.174
1.379
1.403
1.357
1.294
1.217
1.139
0.827
0.539
0.257
23.77
25.25
26.50
27.79
29.18
30.45
31.22
31.68
31.92
32.06
32.43
32.51
0.673
1.148
1.326
1.517
1.777
1.664
1.580
1.426
1.283
0.905
0.604
0.312
22.66
24.18
25.60
26.89
28.24
29.63
30.45
30.87
31.09
31.31
31.57
31.66
0.728
1.210
1.522
1.677
1.859
1.816
1.759
1.544
1.369
1.053
0.625
0.326
{x [C4mim][NTf2] + (1 − x) 1-Propanol}
0.1041
0.2052
0.2969
0.3736
0.4946
0.6132
0.7079
0.8031
0.9018
0.9333
25.12
25.95
26.83
27.66
29.08
30.45
31.57
32.39
33.04
33.23
−0.155
−0.263
−0.233
−0.113
0.185
0.456
0.698
0.636
0.370
0.268
24.35
25.14
26.01
26.87
28.41
29.87
30.93
31.59
32.30
32.45
−0.055
−0.192
−0.163
−0.006
0.425
0.797
0.989
0.776
0.580
0.442
23.11
23.99
24.93
25.92
27.48
29.02
30.03
30.78
31.50
31.65
−0.024
−0.119
−0.063
0.188
0.582
0.979
1.076
0.908
0.676
0.523
{x [C4mim][NTf2] + (1 − x) 2-Propanol}
0.1006
0.1975
0.2878
22.68
23.57
24.72
−0.467
−0.702
−0.598
21.95
22.82
24.06
−0.435
−0.668
−0.455
20.77
21.63
22.88
−0.298
−0.591
−0.415
123

J Solution Chem (2014) 43:404–420
411
Table 2 continued
283.15 K
298.15 K
313.15 K
x
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
0.3871
0.4810
0.5517
0.6311
0.7059
0.8232
0.8961
0.9772
26.11
27.63
28.78
29.86
30.80
31.94
32.62
33.34
−0.361
0.070
0.400
0.560
0.631
0.411
0.245
0.024
25.58
26.94
28.02
29.11
30.08
31.28
31.83
32.43
−0.066
0.226
0.501
0.688
0.806
0.672
0.392
0.069
24.45
26.05
27.21
28.30
29.22
30.53
31.13
31.65
−0.027
0.456
0.774
0.920
0.949
0.864
0.596
0.151
{x [C4mim][NTf2] + (1 − x) 1-Butanol}
0.1048
0.1989
0.3045
0.3940
24.68
25.07
25.65
26.43
27.82
29.27
30.12
31.43
32.34
32.54
−0.518
−0.909
−1.204
−1.167
−0.737
−0.162
0.008
23.52
24.09
24.67
25.58
26.99
28.45
29.45
30.68
31.60
31.71
−0.461
−0.710
−1.049
−0.918
−0.516
0.026
0.5099
0.6154
0.6974
0.8081
0.9065
0.9563
28.47
29.88
30.86
32.15
33.00
33.36
−1.012
−0.485
−0.190
0.174
0.313
0.401
0.579
0.202
0.495
0.644
0.145
0.282
0.320
1-propanol, 2-propanol, 1-butanol, 1-pentanol) binary systems at 283.15, 298.15, and
313.15 K and are summarized in Tables 2 and 3, respectively. Measurements for mixtures
of ILs and methanol were not carried out at 313.15 K since the proximity to the methanol
boiling point caused problems of evaporation, preventing accurate measurements. The
[C4mim][NTf2] system with 1-butanol is partially miscible at 283.15 K and with 1-pent-
anol is immiscible at all the temperatures.
The surface tension changes of mixing, Δσ, were calculated as:
X
Dr=ðmNꢀmꢁ1Þ ¼ rM
ꢁ
xiri
ð1Þ
i
where σM and σi are the surface tension of the mixture and pure component, respectively.
The surface tension deviations, Δσ, were fitted with composition data by using the
Redlich–Kister polynomial equation [41], which for binary mixtures is:
X
ꢀ
ꢁ
k
Drij ¼ xixj
Ak xi ꢁ xj
ð2Þ
k
where, xi is the mole fraction of component i, Ak is the polynomial coefficient, and k is the
number of the polynomial coefficient.
The Redlich–Kister coefficients for the binary systems and the standard deviations
obtained in the correlations are listed in Table 4 for all temperatures studied. In each case,
the coefficients were obtained by fitting Eq. 2 by least-squares regression. Fisher’s F-test
was used to define the polynomial degree.
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J Solution Chem (2014) 43:404–420
Table 3 Experimental surface tension (σ) and surface tension changes of mixing (Δσ) of [C8mim][NTf2]
with methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 1-pentanol at different temperatures
283.15 K
298.15 K
313.15 K
x
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
{x [C8mim][NTf2] + (1 − x) Methanol}
0.0978
0.1821
0.2826
0.3885
0.5129
0.6131
0.7002
0.8036
0.8449
0.9368
25.93
27.46
28.51
29.38
30.10
30.59
31.02
31.09
31.21
31.62
1.835
2.620
2.782
2.716
2.336
1.941
1.601
0.757
0.511
0.108
25.05
26.81
27.87
28.56
29.31
29.73
30.04
30.06
30.22
30.62
2.053
3.075
3.254
3.017
2.677
2.220
1.767
0.881
0.679
0.273
{x [C8mim][NTf2] + (1 − x) Ethanol}
0.1042
0.1872
0.2691
0.3961
0.4723
0.5549
0.6223
0.7248
0.7632
0.8252
0.8560
0.9136
0.9784
24.81
26.18
27.27
28.64
29.40
30.00
30.31
30.83
31.00
31.26
31.36
31.60
31.96
0.748
1.377
1.734
1.969
2.047
1.909
1.616
1.220
1.047
0.753
0.577
0.302
0.083
24.04
25.31
26.35
27.75
28.54
29.25
29.55
30.01
30.12
30.23
30.24
30.49
30.81
1.076
1.612
1.926
2.201
2.315
2.294
1.996
1.548
1.318
0.879
0.616
0.356
0.101
22.93
24.23
25.32
26.93
27.76
28.38
28.78
29.22
29.30
29.35
29.35
29.42
29.64
1.162
1.719
2.085
2.565
2.716
2.601
2.401
1.929
1.667
1.161
0.891
0.449
0.092
{x [C8mim][NTf2] + (1 − x) 1-Propanol}
0.0974
0.1935
0.2894
0.3793
0.4688
0.5552
0.5967
0.6916
0.7790
0.8556
0.9461
25.32
26.24
27.11
27.84
28.53
29.17
29.46
30.08
30.62
31.14
31.72
0.254
0.429
0.554
0.587
0.582
0.552
0.519
0.403
0.265
0.190
0.068
24.49
25.47
26.30
27.03
27.70
28.25
28.47
29.07
29.61
30.07
30.58
0.315
0.579
0.694
0.755
0.757
0.664
0.574
0.468
0.356
0.246
0.082
23.28
24.28
25.23
26.07
26.79
27.42
27.65
28.23
28.66
28.94
29.46
0.409
0.678
0.898
1.054
1.092
1.065
0.979
0.837
0.602
0.299
0.125
{x [C8mim][NTf2] + (1 − x) 2-Propanol}
0.1006
0.1910
0.3141
23.08
24.21
25.87
0.085
0.302
0.720
22.39
23.53
25.13
0.178
0.444
0.855
21.25
22.56
24.27
0.387
0.804
1.299
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J Solution Chem (2014) 43:404–420
413
Table 3 continued
283.15 K
298.15 K
313.15 K
x
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
σ/mN·m−1
Δσ/mN·m−1
0.4209
0.5131
0.5878
0.6857
0.7940
0.8899
0.9343
27.47
28.65
29.54
30.28
30.93
31.29
31.52
1.244
1.492
1.629
1.382
0.938
0.330
0.113
26.66
27.81
28.63
29.43
30.00
30.28
30.46
1.355
1.613
1.712
1.566
1.090
0.443
0.195
25.74
26.89
27.85
28.54
28.84
29.14
29.24
1.716
1.955
2.178
1.902
1.133
0.486
0.149
{x [C8mim][NTf2] + (1 − x) 1-Butanol}
0.1161
0.2051
0.3127
0.4123
0.5216
0.6175
0.7401
0.8254
0.8794
25.61
26.09
26.81
27.65
28.55
29.37
30.24
30.83
31.23
−0.406
−0.535
−0.552
−0.394
−0.243
−0.080
−0.050
−0.044
−0.019
24.86
25.38
26.06
27.00
27.86
28.66
29.40
29.90
30.23
−0.233
−0.297
−0.324
−0.039
0.103
23.70
24.28
25.07
26.03
27.07
27.71
28.44
28.83
29.12
−0.145
−0.158
−0.085
0.210
0.521
0.521
0.433
0.249
0.184
0.273
0.207
0.147
0.122
{x [C8mim][NTf2] + (1 − x) 1-Pentanol}
0.1008
0.2042
0.2977
0.4024
0.4977
0.5917
0.7053
0.7968
0.9040
26.42
26.72
27.11
27.61
28.08
28.74
29.52
30.22
31.01
−0.327
−0.639
−0.802
−0.922
−1.016
−0.913
−0.805
−0.647
−0.492
25.52
25.88
26.34
26.85
27.31
27.91
28.65
29.28
30.06
−0.227
−0.460
−0.536
−0.625
−0.712
−0.650
−0.561
−0.456
−0.290
24.28
24.76
25.21
25.82
26.37
27.00
27.78
28.42
29.12
−0.155
−0.285
−0.386
−0.394
−0.407
−0.331
−0.221
−0.121
−0.054
Figure 3 shows, as function of the IL mole fraction, the surface tensions for the binary
mixtures of [C4min][NTf2] + alcohol and [C8min][NTf2] + alcohol at 283.15, 298.15 and
313.15 K.
In the case of [C4min][NTf2], Fig. 3a shows that, at any temperature, when methanol or
ethanol is added to the IL, the surface tension increases from that of the alcohol to that of
the IL. The graph becomes flat at high concentrations of the IL (xIL [ 0.8). In the case of
the other alcohols, there is a change in the curvature of the plots. The initial slopes of
curves are low (tensions near the alcohol value). The property increases and, similar to
alcohols of lower molecular weight, plots are flat near the pure IL. The same qualitative
behavior is found for [C8min][NTf2] (Fig. 3b). In this case, the curves corresponding to
mixtures with 1-pentanol are flat at low concentrations of IL, but increase practically
linearly up to the alcohol surface tension.
Figure 4 shows, as function of the IL mole fraction, the surface tension changes of
mixing for the binary mixtures of [C4min][NTf2] + alcohol and [C8min][NTf2] + alcohol
at 283.15, 298.15 and 313.15 K.
123

414
J Solution Chem (2014) 43:404–420
Table 4 Redlich-Kister coefficients and standard deviations (S) of Eq. 1 for Δσ for the investigated systems
at 283.15, 298.15 and 313.15 K
T/K
A0
A1
A2
A3
S
{x [C4mim][NTf2] + (1 − x) Methanol}
283.15
298.15
11.179
12.332
−6.6776
−7.5623
6.5836
–
–
0.081
0.078
6.6046
{x [C4mim][NTf2] + (1 − x) Ethanol}
283.15
298.15
313.15
5.9671
7.1291
7.7774
1.2495
1.2128
1.3085
–
–
–
–
–
–
0.056
0.074
0.071
{x [C4mim][NTf2] + (1 − x) 1-Propanol}
283.15
298.15
313.15
0.9457
1.8874
2.5392
4.7008
5.7129
5.9220
–
–
–
–
–
–
0.053
0.080
0.069
{x [C4mim][NTf2] + (1 − x) 2-Propanol}
283.15
298.15
313.15
0.6161
1.3216
1.9808
8.2434
6.8189
7.5823
−3.3124
−2.8800
−2.0320
−5.5719
0.024
0.045
0.071
–
–
{x [C4mim][NTf2] + (1 − x) 1-Butanol}
298.15
313.15
−3.2114
−2.2817
7.2382
7.5247
4.4363
4.9535
–
–
0.062
0.057
{x [C8mim][NTf2] + (1 − x) Methanol}
283.15
298.15
9.6736
10.912
−6.4380
−8.1547
3.3724
5.0394
−8.2417
−6.9313
0.046
0.070
{x [C8mim][NTf2] + (1 − x) Ethanol}
283.15
298.15
313.15
7.5581
8.8837
10.742
−3.0287
−3.2260
−2.1819
–
–
–
0.077
0.097
0.073
–
−2.6541
{x [C8mim][NTf2] + (1 − x) 1-Propanol}
283.15
298.15
313.15
2.2659
2.8163
4.1591
−0.9694
−1.3920
−0.9781
–
–
–
–
0.013
0.019
0.051
{x [C8mim][NTf2] + (1 − x) 2-Propanol}
283.15
298.15
313.15
5.9650
6.4225
8.0338
2.9338
3.0242
4.8406
−6.4679
−5.2579
−5.4774
–
0.081
0.081
0.047
–
−8.2424
{x [C8mim][NTf2] + (1 − x) 1-Butanol}
283.15
298.15
313.15
−1.3717
0.0051
1.6165
2.6216
2.6347
2.7252
–
–
–
–
0.062
0.075
0.058
–
−3.1433
{x [C8mim][NTf2] + (1 − x) 1-Pentanol}
283.15
298.15
313.15
−3.9305
−2.6850
−1.5612
−0.2912
−0.1758
0.8320
–
–
–
–
0.046
0.021
0.011
−0.2540
0.7396
123

J Solution Chem (2014) 43:404–420
415
(a)
(b)
34.0
34.0
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
TT=2=83.2158ºK3.15 K
T=283.15 ºK
T=283.15 K
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
34.0
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
34.0
T=298.15 ºK
T= 298.15 ºK
T=298.15 K
T=298.15 K
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
34.0
34.0
T=313.15 ºK
T=313.15 K
T= 313.15 ºK
T=313.15 K
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
32.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x 1
Fig. 3 Plots of σ against mole fraction of [C4mim][NTf2] (a) and [C8mim][NTf2] (b) at 283.15, 298.15 and
313.15 K for IL + methanol (filled diamond), ethanol (open square),1-propanol (open triangle), 2-propanol
(filled circle), 1-butanol (filled square), and 1-pentanol (x) binary systems; the points are experimental points
and the lines are calculated from Eq. 2 using the parameters listed in Table 4
In terms of surface tension changes of mixing, Fig. 4a shows that mixtures of
[C4min][NTf2] with methanol or ethanol have positive surface tensions of mixing at all IL
compositions and temperatures studied in this work. The maximum is displaced to low
concentrations of IL in the case of methanol. Systems with other alcohols show an s-shaped
123

416
J Solution Chem (2014) 43:404–420
(a)
(b)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
T=283.15 K
TT==22883.315.1ºK5 K
T=283.15 ºK
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
T=298.15 K
T=298.15 K
T= 298.15 ºK
T=298.15 ºK
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
TT==331133.1.51ºK5 K
T=313.15 K
T=313.15 ºK
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
Fig. 4 Plot of Δσ against mole fraction of [C4mim][NTf2] (a) and [C8mim][NTf2] (b) at 283.15, 298.15 and
313.15 K for IL + methanol (filled diamond), ethanol (open square),1-propanol (open triangle), 2-propanol
(filled circle), 1-butanol (filled sqaure), and 1-pentanol (x) binary systems; the points are experimental points
and the lines are calculated from Eq. 2 using the parameters listed in Table 4
dependence on composition, whereas Δσ is negative at high alcohol mole fractions;
positive values are found at high IL concentrations.
Figure 5 shows a comparison between surface tension data measured in this work for
[C4min][NTf2] + 1-propanol and [C4min][NTf2] + 1-butanol with data reported in the
123

J Solution Chem (2014) 43:404–420
417
3.0
2.0
Fig. 5 Comparison between
surface tension changes of
mixing data from this work (open
triangle {[C4mim][NTf2] + 1-
propanol}; open circle {[C4mim]
[NTf2] + 1-butanol}) and from
the literature Ref. [7] (filled
triangle, filled circle,
1.0
respectively) at 298.15 K; the
lines are Redlich–Kister
correlations
0.0
-1.0
-2.0
-3.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
(a) 34.0
(b) 34.0
32.0
32.0
30.0
28.0
26.0
24.0
22.0
20.0
30.0
28.0
26.0
24.0
22.0
20.0
18.0
18.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
Fig. 6 Effect of temperature on surface tension, σ, for [C4mim][NTf2] + 2-propanol (a) and [C8mim]
[NTf2] + 2-propanol (b) binary systems at (open circle) 283.15, (filled triangle) 298.15, (open square)
313.15 K; the symbols are experimental points and the lines are calculated from Eq. 2 using the parameters
listed in Table 4
literature at 298.15 K [7]. Data series are in relatively good agreement for the
[C4min][NTf2] + 1-propanol binary system. However, surface tension changes of mixing
determined in this work for [C4min][NTf2] + 1-butanol binary system are higher than
those published by Wandschneider et al. [7] This can be justified since, as discussed
previously, trace amounts of impurities can have dramatic effects on physical properties of
ILs and their mixtures. No other comparable data were found in the literature.
As it can be seen from Fig. 4b, in the case of [C8min][NTf2], surface tension changes of
mixing are positive for systems with methanol, ethanol, 1-propanol, and 2-propanol for all
compositions and temperatures. [C8min][NTf2] + 1-butanol shifts from negative values of
the property at 283.15 K to an s-shaped dependence on composition at higher temperatures.
Δσ for systems with 1-pentanol are negative for all compositions and temperatures. No
comparable data were found in the surveyed literature for [C8min][NTf2] + alcohol
systems.
123

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J Solution Chem (2014) 43:404–420
(a) 2.5
(b) 2.5
2.0
1.5
2.0
1.5
1.0
1.0
0.5
0.5
0.0
0.0
-0.5
-1.0
-0.5
-1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x1
x1
Fig. 7 Effect of temperature on surface tension changes of mixing, Δσ, for [C4mim][NTf2] + 2-propanol
(a) and [C8mim][NTf2] + 2-propanol (b) binary systems at (open circle) 283.15, (filled triangle) 298.15,
(open square) 313.15 K; the symbols are experimental points and the lines are calculated from Eq. 2 using
the parameters listed in Table 4
Figure 6 shows the effect of temperature on the surface tensions of [C4min][NTf2] + 2-
propanol and [C8min][NTf2] + 2-propanol. Figure 7 shows the effect of temperature on the
surface tension changes of mixing for these systems. The surface tension decreases with
temperature as is typical for organic solvents. Surface tension changes of mixing increase
with temperature. This effect is found for the two binary systems studied.
For the pure ILs, the surface tension decreases with increasing alkyl chain length, as was
showed previously [22, 42]. Consequently, the same behavior is found for the mixtures
with an alcohol, with lower values of this property in the case of mixtures with [C8min]
[NTf2]. For a system with an alcohol at one temperature, values of surface tension changes
of mixing become more positive in going from [C4min][NTf2] to [C8min][NTf2] except in
the case of methanol where the values of Δσ are slightly lower.
4 Conclusions
In this study new experimental data for the [C4min][NTf2] or [C8min][NTf2] + alcohol
systems at 283.15, 298.15 and 313.15 K have been measured. The results show that the
surface tension of these mixtures depends systematically on alkyl chain length of the ionic
liquid and alcohol, composition and temperature. It was observed that alcohols with small
hydrocarbon backbones, such as methanol and ethanol, show positive surface tension
changes of mixing. With increasing number of carbon atoms in their hydrocarbon chain,
this becomes negative at high alcohol concentrations and positive at higher IL concen-
trations; it becomes completely negative for systems with [C4min][NTf2] and 1-pentanol.
The calculated surface tension changes of mixing were adequately correlated by means of
the Redlich–Kister equation.
A large amount of experimental work and computational simulations are needed to
establish a clear correlation among the surface tension and the molecular structure of the
ionic liquids.
123

J Solution Chem (2014) 43:404–420
419
Acknowledgments The authors are grateful to the Ministry of Economy and Competitiveness of Spain for
financial support through project CTQ2012-33359, including European Regional Development Fund
(ERDF) advanced funding, and to the Directorate General for R+D+i of the Xunta de Galicia through
Galician Network on Ionic Liquids, REGALIs (CN 2012/120). A. E. Andreatta also wants to thank
CONICET from Argentine.
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