Surface tension of four binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid + water or + ethanol)

J. Chem. Thermodynamics 49 (2012) 165–171

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Surface tension of four binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid + water or + ethanol) E. Rilo a, M. Domínguez-Pérez a, J. Vila a, L.M. Varela b, O. Cabeza a,⇑ a b

Mixtures Group, Dept. of Physics, Science Faculty, University of A Coruña, Campus da Zapateira s/n, 15008 A Coruña, Spain Nanomaterials and Soft Matter Group, Department of Condensed Matter Physics, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain

a r t i c l e

i n f o

Article history: Received 21 November 2011 Received in revised form 27 January 2012 Accepted 28 January 2012 Available online 6 February 2012 Keywords: Surface tension Binary mixtures Alkyl sulphate Imidazolium ionic liquid Water Ethanol Pseudolattice model

a b s t r a c t In this work, we present surface tension experimental measurements for eight binary systems containing water or ethanol and an ionic liquid (IL) of the 1-ethyl-3-methyl imidazolium alkyl sulphate family, being the alkyl chain of the anion: ethyl, butyl, hexyl and octyl. Measurements were performed at the temperature of 25.0 °C and atmospheric pressure. All four ILs are completely miscible with water and ethanol, but for a concentration range of the octyl sulphate IL aqueous system the mixture jellifies, and so it is not possible to measure its surface tension. These measurements allow us to study the influence of the anion size on the surface tension for the pure IL compounds, and the role of the two different solvents in the surface tension behaviour. Thus, we observe that it is completely different when mixed with water or with ethanol, as also happens in other mixtures with different ionic liquids. From the experimental data, we extract surface tension deviations using the most popular definition. The calculated deviations for the ethanol based system are fitted using the Redlich–Kister equation and a novel one previously reported by us. Furthermore, we have also calculated the reduced surface pressure for the aqueous mixtures, which is fitted with good agreement using a theoretical equation obtained from the Bahe–Varela pseudo-lattice model. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Ionic liquids (ILs) are salts with melting points below 100 °C consisting of an anion and a large organic cation. The first, ethyl ammonium nitrate, was discovered in 1914, but their study started in the last decade of twentieth century when new ionic liquids were discovered, and applications for them were proposed [1]. In general one or both ions are large and the organic cation has usually a low degree of symmetry. Because of this fact inter-ionic interactions are weak and hence their melting points are usually below room temperature (in fact they are also known as room temperature ionic liquids, RTIL). Physical and chemical properties of ILs are strongly dependent on the ions presented in their formulation. Furthermore, they are known as design solvents due to their ability to be tailor made for a specific purpose. So, for these reasons, we could obtain ILs with previously chosen physico-chemical properties by changing the anion-cation combination. Due to their ionic nature, all ILs have negligible vapour pressure (or it is very small as recently measured [2]), which means that ILs do not evaporate. Also many of them present interesting properties such as: good solvents for organic and inorganic compounds

⇑ Corresponding author. E-mail address: [email protected] (O. Cabeza). 0021-9614/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2012.01.023

including metal salts, electrical conductivity and high thermal and electrochemical stability. These characteristics make ILs promising compounds to be used for batteries, organic synthesis, extractions and alloy electro-deposition as well as potential ‘‘green solvent’’ replacements for volatile organic compounds (VOCs) [1]. Potential uses and applications of ILs require knowledge of their physical properties such as density, viscosity, surface tension, conductivity, not only for the pure substances, but also for their possible mixtures with different solvents. Due to the lack of theoretical models to predict behaviour, the only way to determine their physical properties is by experimentation. However, reported measurements of physical properties of ILs mixed with different substances was scarce until 2005, but from that year many papers have been published that perform these measurements on mixtures of ILs with different kinds of solvents such as water, ethanol, aromatic compounds or other ionic liquids [3,4], and the number of published papers is continually growing. Surface tension (r) is an important quantity since many chemical and physical processes take place at the liquid surface. An example of their importance is catalysis processes between ILs and overlying organic phases [5]. Moreover, surface tension provides information about the nature of the surface energy and about surfactants such as the self assembly phenomena. So measurements of this physical property are of great interest from both the experimental and theoretical points of view. Despite this, there

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E. Rilo et al. / J. Chem. Thermodynamics 49 (2012) 165–171

are few experimental data on surface tension available in literature [6–13]. Concerning theoretical models for ILs surface microstructure, some advances have been performed using the Bahe–Varela pseudolattice theory [14]. This model assumes that, for an aqueous mixture of IL, the anions in the mixture form a continuum structureless neutralizing background, and the organic cations and water molecules are placed in the nodes of a pseudolattice. The surface pressure of IL-water mixtures is calculated using a localized model for the adsorption of particles in the surface of the mixture and a mean-field Bragg–Williams approximation for the chemical potential of the adsorbed particles in the pseudolattice [15]. In this work, we selected four ionic liquids that belong to the 1ethyl-3-methyl imidazolium alkyl sulphate family (EMIM-CnS), being the alkyl chain ethyl (ES), butyl (BS), hexyl (HS) and octyl (OS). Then, we prepared binary mixtures of these compounds with water and with ethanol covering the whole composition range and measured their surface tensions at 25.0 °C and atmospheric pressure. The aim of this work is to study the influence of the alkyl chain length of the anion on surface tension for the mixtures described. We extracted the surface tension deviations, dr, and the reduced surface pressure, p. This last quantity was fitted by the theoretical equation deduced from Bahe–Varela theory for (IL + water) mixtures [15].

2. Experimental The ionic liquids were purchased from Solvent Innovation (actually absorbed by Merck) and Iolitec. Their purity, expressed as mass fracture are: EMIM-ES (Iolitec, 0.99), EMIM-BS (Solvent Innovation, 0.98), EMIM-HS (Solvent Innovation, 0.98) and EMIM-OS (Solvent Innovation, 0.98). Ethanol was from Panreak with purity better than 0.995 and the water employed was Milli-Q grade. The original bottles of ILs were opened within a sealed chamber under an inert atmosphere and relative humidity below 10% to prevent water adsorption, since these ILs are highly hygroscopic, mainly those with shorter alkyl chain [16]. Note that EMIM-methyl sulphate hydrolyzes with water and so it is not interesting to measure this aqueous system [12]. All binary mixtures were prepared by weighing with the use of a Mettler AT201 balance, whose sensitivity is 105 g, giving an uncertainty in the mole fraction value of the ±104. The mixture was bottled and sealed before taking them out of the dry environment of the chamber. Surface tension (r) measurements were performed using a drop volume tensiometer Lauda TVT1, with sensitivity of 105 N  m1. Temperature was maintained at 25.0 °C with an external bath, with accuracy better than 0.1 °C. All measurements were performed two times minimum to ensure reproducibility, which is better than 5  105 N  m1. In table 1, we include important data for the pure chemical compounds used (including water and ethanol) as mole mass, water content (measured with a Karl Fisher 701 KF Titrino Coulo-

meter), measured density (necessary to calculate the surface tension), measured surface tension and values published in literature for density [17–22] and surface tension [21–25] (majority compiled in the IL-thermo database [3]). We have not found any published values of surface tension for pure EMIM-BS and EMIMHS, while data published for EMIM-OS [21] are very similar to ours, and for EMIM-ES our value is in the range of those published (which present differences among them higher than 6%) [23,24]. Density (q), was measured using an Anton Paar SVM 3000 Stabinger viscodensimeter, which was thermostatted with an inner Peltier cell with an uncertainty of ±0.02 °C. The resolution in the measurement of the density is 1  105 g  cm3, and its uncertainty is 5  105 g  cm3. These measurements have been carried out taking into account the viscosity of the sample which is measured by the same apparatus at the same time. In the literature, we found density data for the four ILs studied, being the values measured here very similar to some of those already published. As observed, the surface tension decreases with ionic liquid alkyl chain length of the anion, as happens for other IL families but with the alkyl chain length of the cation [11]. It is interesting to note that EMIM-OS is miscible with water for the entire range of concentration, but the mixture jellifies for mixtures with the IL mole fraction (xIL) between 0.14 and 0.28. Obviously, we could not measure the surface tension for those concentrations with our apparatus. In figure 1, we show the resulting jelly for a given concentration of xIL  0.2, where it is possible to appreciate the marks made with a cutter. Figure 2 gives the concentration region (in mole fraction of the EMIM-OS) where this phenomenon happens at room temperature (note that for certain regions it gets jellified only after some time at absolute rest). For completeness, we include in figure 3 the X-ray diffractogram of a (water + EMIM-OS) mixture that jellifies, where we observe that it is mainly glassy (amorphous) but some order exists as observed by the presence of two broad, but well defined, peaks at 4° and 21° angles in the 2h scale. In any case, a more profound study of this jelly transition by adding water to EMIM-OS is needed. To our knowledge, it is the first time that a transition from liquid to jelly is described just by adding water.

3. Results and discussion 3.1. Surface tension In table 2 and figure 4(a), we show the experimental results for the four aqueous systems measured at 25.0 °C and ambient pressure. As observed, the surface tension does not vary when water is added to the ionic liquid except at very high water content (xIL < 0.25), except for EMIM-ES system where the r increase is a bit more progressive. In the system containing EMIM-BS, we have observed a strange hump minimum not measured in the other systems but already published for other (IL + water) ones [6,7]. We

TABLE 1 Molar mass, M; water weight fraction measured with Karl Fisher titrator, w; density, q, and surface tensions, r, of the chemicals used at 25.0 °C. For density and surface tension, data published in literature are also given. M/g  mol1

106 w

q/kg  m3

qlit/kg  m3

r/mN  m1

rlit/mN  m1 46.96[22] 45.43[23] 47.00[24]

EMIM-ES

236.29

200

1238.30

1237.63[17] 1238.80[18]

47.13

EMIM-BS EMIM-HS EMIN-OS

264.35 292.40 320.46

1200 600 <1000

1175.65 1129.60 1093.75

39.64 34.77 31.29

30.90[21]

18.02 46.07

2000

997.00 785.85

1180[19] 1130[19] 1094.15[21] 1100[19] 997.02[20] 785.46[22]

71.35 22.01

71.81[25] 21.97[25]

Water Ethanol

Experimental uncertainties are ±0.05 kg  m3 for r and ±0.05 mN  m1 for r.

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E. Rilo et al. / J. Chem. Thermodynamics 49 (2012) 165–171 TABLE 2 Surface tension of the binary mixtures (IL + water) at 25.0 °C.

FIGURE 1. Jelly obtained by simply mixing water with EMIM-OS for a given concentration (xIL  0.2, which means about 18% by weight of water).

Solid Solid if at rest Liquid

xIL

r/mN  m1

1.0000 0.9041 0.7939 0.6957 0.6066

47.16 47.39 47.72 47.94 48.30

1.0000 0.9034 0.7997 0.7030 0.5868 0.4992 0.4764 0.4017 0.3957

39.62 39.67 39.77 39.82 39.89 40.01 39.93 40.31 40.04

1.0000 0.8997 0.7893 0.7050 0.5943 0.4957

34.77 34.77 34.82 34.87 34.95 35.05

1.0000 0.8904 0.8287 0.7660 0.7373 0.7018 0.6934

30.91 30.79 30.71 30.81 30.70 30.77 30.74

xIL

r/mN  m1

EMIM-ES + water 0.5026 48.79 0.4046 49.42 0.3017 50.45 0.2020 52.23 0.1006 55.64 EMIM-BS + water 0.2973 40.81 0.2585 40.83 0.2004 41.48 0.1947 41.59 0.1716 41.83 0.1464 40.85 0.1330 40.29 0.1200 39.74 0.1010 39.84 EMIM-HS + water 0.4034 35.25 0.3009 35.59 0.1997 36.16 0.0997 37.79 0.0797 38.25 0.0602 38.88 EMIM-OS + water 0.6347 30.71 0.5677 30.67 0.5127 30.72 0.4000 30.48 0.2846 31.01 0.0937 33.62 0.0774 33.87

xIL

r/mN  m1

0.0600 0.0402 0.0199 0.0050 0.0000

57.64 59.68 62.42 67.23 71.35

0.1004 0.0987 0.0934 0.0805 0.0758 0.0602 0.0579 0.0403 0.0389

39.42 39.17 39.08 39.96 39.85 41.70 40.83 45.93 44.48

0.0396 0.0196 0.0050 0.0020 0.0008 0.0005

39.55 40.27 42.13 49.54 58.45 62.58

0.0506 0.0349 0.0198 0.0110 0.0050 0.0007 0.0002

34.96 35.30 36.69 37.19 37.84 42.45 56.92

Experimental uncertainties are ±0.05 mN  m1 for r and ±0.0004 for xIL.

0

0.2

0.4

xIL

0.6

0.8

1

FIGURE 2. State of the mixture (jelly, jelly if at rest and liquid) depending of the mole fraction of ionic liquid EMIM-OS in aqueous solution.

FIGURE 3. X-ray diffractogram of the jelly phase obtained for the aqueous mixture of EMIM-OS shown in figure 1.

associate that minimum to the presence of certain contaminants in the IL, as was recently published [24]. Note also that for the aqueous system with EMIM-OS there are no data for the concentration interval 0.14 < xIL < 0.28 because the mixture jellifies, as explained in the previous section. The surface tension behaviour of all aqueous systems suggests that the IL ions occupy all the surface of the sample, probably forming a pseudo-lattice with water within its interior thereby forming water clusters [13,26], as was observed very recently through numerical simulations [27]. This model agrees with the observed hygroscopic behaviour of the ILs, where adsorbed

water from moisture is desorbed by dry air without absorption from the bulk of the IL, even for hydrophilic ILs as those studied here [16]. That behaviour was recently explained theoretically using a modified Brunauer–Emmet–Teller isotherm approach, where it is suggested that a pseudolattice is formed in the surface of the ILs [28]. In table 3 and figure 4(b), we show the experimental values for the four systems with ethanol measured here at 25.0 °C and ambient pressure. We include measurements for the system EMIMmethyl sulphate (-MS) recently published [12]. As observed, the surface tension behaviour is completely different from that for aqueous systems. The values for all five alkyl sulphate (IL + ethanol) systems decrease nearly linearly from its value when pure to a given common value, r(xIL = 0.3)  28 mN  m1. Then they decrease linearly again to the value of pure ethanol (see table 1), following roughly the straight line: r/mN  m1  22 + 20xIL, which is plotted in figure 4(b). These results suggest that the liquid surface is composed of ions and ethanol molecules in the same proportion, and that below xIL = 0.3 the alkyl tails of the anions are not present in the surface, probably oriented to the bulk of the mixture. We have not found any published data of surface tension for aqueous or ethanol mixtures of the ILs studied here, except one set published for (EMIM-ES + water) [24]. Data presented in this reference show great uncertainties and they do not cover the diluted range of concentrations. Those measurements are included in a recent review where all data published are compiled for binary mixtures of ILs with water and ethanol for many physical properties [4]. 3.2. Surface tension deviations From the surface tension values, tabulated in tables 2 and 3, we have calculated the surface tension deviations (dr) using the usual relationship, which reads:

dr ¼ r  xIL rIL  xS rS

ð1Þ

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(a)

(b) 55

75

50

65

-1

45 40

σ / mN·m

σ /mN·m

-1

55

45

35 30

35

25

25 20

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

x IL

x IL

FIGURE 4. Plot of surface tension (r) against the mole fraction of IL (xIL) for all binary mixtures. Symbols represent mixtures with EMIM-ES, rhombus; EMIM-BS, square; EMIM-HS, triangle, and EMIM-OS, circle. (a) Aqueous mixtures. (b) Mixtures with ethanol. Triangle symbols represent mixtures with EMIM-MS from Ref. [12].The straight line plotted from xIL = 0.3 to 0 represents the common linear behaviour found.

where r is the surface tension of the mixture and S corresponds to the solvent (water or ethanol). The results are shown in figure 5(a) and (b) and, as expected, the behaviour is completely different for both solvents. In spite of the fact that dr magnitude has not any physical meaning, it can be useful to compare with results published in other papers for the same magnitude [8,11]. Thus, in figure 5(a) we observe that dr for aqueous mixtures presents a minimum at about the concentration where the surface tension value equals that of the pure corresponding IL. Also, it is observed that the position of that minimum moves at more diluted mixtures when the alkyl chain on the anion increases. Also, the absolute value of dr increases with the alkyl chain length, due to the fact that r value for the pure ILs decreases with it. Surface tension deviations for the four ILs increase linearly with the IL mole fraction from xIL = 0.25 up to 0 for the pure IL. Obviously, dr for the EMIM-BS aqueous system presents the anomaly obtained for r at the minimum, which we attribute to impurities according Ref. [26]. In figure 5(b), we present the dr data for the systems with ethanol. Now the behaviour is completely different from that presented by the aqueous mixtures, but also very different among the four systems. Thus, while for the system with ES, the curve is negative with a minimum in xIL = 0.6, for that with BS the curve is a sigmoid, negative at rich IL mixtures and positive in the other

TABLE 3 Surface tensions of the binary mixtures (IL + ethanol) at 25.0 °C. xIL

r/mN  m1

1.0000 0.9048 0.8252 0.7030

47.16 43.81 41.48 38.12

1.0000 0.8794 0.7916 0.6866

39.62 37.35 35.75 33.99

1.0000 0.9409 0.7951 0.7189 0.5946

34.77 34.31 32.84 32.08 30.86

1.0000 0.9530 0.9432 0.9164 0.8317 0.7387

30.91 30.82 30.79 30.80 30.50 29.96

r/mN  m1

xIL

EMIM-ES + ethanol 0.5984 35.40 0.4999 32.99 0.3989 30.68 0.3025 28.52 EMIM-BS + ethanol 0.4993 31.05 0.4691 30.63 0.2904 27.76 0.1536 25.36 EMIM-HS + ethanol 0.4691 29.48 0.3860 28.60 0.3138 27.69 0.2267 26.53 0.1253 25.00 EMIM-OS + ethanol 0.6269 29.60 0.5059 29.14 0.4582 28.92 0.4002 28.61 0.3506 28.18 0.2985 27.64

xIL

r/mN  m1

0.2056 0.0988 0.0000

26.37 24.18 22.01

0.1347 0.0660

25.01 23.60

0.0977 0.0679 0.0300

24.48 23.87 22.98

0.2445 0.2150 0.1595 0.0980 0.0712 0.0399

27.01 26.62 25.82 24.48 23.71 22.89

Experimental uncertainties are ±0.05 mN  m1 for r and ±0.0002 for xIL.

0

4

(a)

(b) 3

δσ / mN·m

-1

δσ / mN·m

-1

-9

-18

2 1 0

-27 -1 -2

-36 0

0.2

0.4

0.6 x IL

0.8

1

0

0.2

0.4

x IL

0.6

0.8

1

FIGURE 5. Plot of surface tension deviations dr against the mole fraction of the IL (xIL) for all binary mixtures. Symbols represent mixtures with water of EMIM-ES, rhombus; EMIM-BS, square; EMIM-HS, triangle, and EMIM-OS, circle. (a) Aqueous mixtures. (b) Mixtures with ethanol. Continuous and dotted lines are the best fit of, respectively, equations (2) and (3) to the data.

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E. Rilo et al. / J. Chem. Thermodynamics 49 (2012) 165–171 TABLE 4 Fitting parameters K, K0 , n y n0 from equation (2) and fitting parameters Ak/mN  m1 from equation (3), and the corresponding standard deviations, s. + Ethanol

K/mN  m1

n

K0 /mN  m1

n0

s/mN  m1

EMIM-ES EMIM-BS EMIM-HS EMIM-OS

10.88 2.34 3.93 6.72

2.13 3.58 0.60 1.35

4.33 7.70 5.50 18.42

2.75 3.45 4.23 2.78

0.06 0.02 0.04 0.10

+ Ethanol

A0

A1

A2

A3

A4

s/mN  m1

EMIM-ES EMIM-BS EMIM-HS EMIM-OS

6.23 1.09 5.90 10.54

2.38 4.48 3.77 9.26

1.30 0.06 0.83 4.43

2.87 0.32 3.17 5.11

3.07 7.21 2.36

0.03 0.01 0.03 0.07

(a)

(b)

1

1

0.8

0.6

0.6

π

π

0.8

0.4

0.4

0.2

0.2

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

0.6

0.8

1

xIL

xIL

(c)

(d)

1

1

0.8

0.6

0.6

π

π

0.8

0.4

0.4

0.2

0.2

0

0

0.2

0.4

0.6

0.8

1

0 0

0.2

0.4

xIL

xIL

FIGURE 6. Plot of reduced surface pressure, p, against the mole fraction of IL (xIL) for the aqueous binary mixtures. Line is the best fit of the theoretical equation (5) from Bahe–Varela model. (a) EMIM-ES, (b) EMIM-BS, (c) EMIM-HS and (d) EMIM-OS.

region. Finally, both systems with HS and OS present positive values for the entire concentration range, with a maximum in xIL = 0.3. We have fitted these curves using a novel phenomenological equation which reads [29]

dr ¼

KxnIL xS

þK

0

0

0 xIL xnS

ð2Þ

0

where K, K , n and n are fitting parameters, being the exponents always positive. Also we fitted the surface tension deviations using the well known Redlich–Kister equation, [30]

dr ¼ xIL xS

m X

Ak ðxIL  xS Þk

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 i¼1 ðdrcal  drexp Þ s¼ N1

ð4Þ

where, drcal is the value obtained from equation (2), (3), drexp is that calculated from the original value and N is the number of data points. The curves obtained are plotted in figure 5(b), continuous lines for the novel equation (2) and dotted lines for Redlich–Kister equation (3). As observed, both equations fit properly the data for the ethanol mixtures, while for dr for aqueous systems neither of them fits the calculated data.

ð3Þ

k¼0

where Ak are fitting parameters and m the polynomial grade. Table 4 shows the best fitting parameters for each system and both equations (2) and (3) and the corresponding standard deviations s, defined as

4. Application of Bahe–Varela theory As mentioned in the introduction, up to now the most accuracy theory to explain IL properties is the Bahe–Varela pseudolattice

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E. Rilo et al. / J. Chem. Thermodynamics 49 (2012) 165–171

model [14]. As it was published by some of us recently, the surface tension of the aqueous ionic liquid systems can be easily explained (and evaluated) using a derivation of that pseudolattice theory [15], but unfortunately it is not appropriate for describing ethanol adsorption on the free surface of (IL + ethanol) mixtures. This could be related to the well-known differences of water and ethanol in what clustering in ILs is concerned: while water tends to form relatively large clusters in (IL + water) mixtures, ethanol tends to be more finely dispersed in the bulk mixtures, what could induce non localized adsorption of the alcohol in the free surface of the mixture [31]. The B–V model fits the surface tension data well for the aqueous mixtures for different ILs as demonstrated for some imidazolium and ammonium systems [15]. This theoretical model leads to a simple expression to fit the surface tension, which reads 0

p¼

xIL CeC ð1xIL Þ 0

2

ð5Þ

2

1 þ xIL CeC ð1xIL Þ

where C and C0 are fitting parameters with physical meaning and p is the reduced surface pressure (which indicates the IL mole fraction at the surface of the mixture). This last is defined as,

p¼

r  rS : rIL  rS

ð6Þ

As expected, we have about p = 1 for mole fractions above xIL = 0.3. For a binary mixture of solvent molecules (B) and an adsorbate (A) formed by the IL, the parameters C and C0 can be expressed as

 bcxAA  bcx C ¼ exp e 2 and C 0 ¼ : 2

ð7Þ

As reported elsewhere [15], b is the thermal energy (=1/kBT), c stands for the number of nearest neighbours of an ion in the pseudolattice, x = xAA + xBB  2xAB represents the energy of interaction valance at the surface between identical entities (xAA for the IL and xBB for the solvent) or with the different ones (xAB). If x > 0 then the segregation of species is favoured while if x < 0 the mixing process takes place easily. In figure 6(a) to (d), we plot the reduced surface pressure obtained, including the best fit of equation (5) for all samples studied and the fitting parameters obtained are compiled in table 5. This expression fits quite well the data for the four systems analyzed, except for a range of concentrations where the minimum in the dr curves appears, i.e. some water molecules begin to populate the surface. It is possible that discrepancies were due to some simplifications used to formulate the theory. The observation of the C’ values obtained and given in table 5 confirms that segregation of IL ions to the surface increases with alkyl chain length, which had been already suggested [6,11,13]. In contrast, parameter C of equation (5), related with the inverse of the interactions between IL ions, presents its higher value for EMIM-HS (i.e. six carbon atoms in the alkyl chain). This result indicates that the force between IL ions is lower for that alkyl chain, which agrees with many other papers published previously, where it is reported that a change of tendency of many physical magnitudes happens for six carbon atoms in the alkyl chain [2], including melting temperature [32].

TABLE 5 Best fitting parameters C and C0 from equation (5) for the aqueous mixtures of the four ILs studied. + Water

EMIM-ES

EMIM-BS

EMIM-HS

EMIM-OS

C C’

24.74 0.0598

121.18 0.236

63.45 2.348

202.77 2.715

5. Conclusions We present experimental measurements of surface tension for binary mixtures of four members of the 1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid family with water and ethanol. The alkyl chains of the anion studied are ethyl, butyl, hexyl and octyl, and the majority of data presented here had not being published previously. The surface tension behaviour with concentration is completely different for the two solvents used. Thus, for the aqueous systems, the ionic liquid (IL) acts as a surfactant, being the surface composed only by IL ions for the majority of water concentrations, so the surface tension value corresponds to that of pure IL down to a high water concentration mixtures (with water mole fraction above 0.8). Then the surface tension value increases abruptly to the pure water value for most of the diluted mixtures. That surfactant-like behaviour of the IL increases with the alkyl chain length of its anion. In contrast, the value of the surface tension for the four systems with ethanol decreases about linearly from the pure IL value to a common 28 mN  m1 value at the IL mole fraction of 0.3. Then its value for all systems decreases following the same linear trend to the pure ethanol value, independently of the alkyl chain of the IL anion. From the experimental data, we calculate the surface tension deviations and the reduced surface pressure. This last quantity is fitted for the aqueous mixtures with an equation derived from Bahe–Varela pseudolattice model, obtaining a very good fitting quality. Acknowledgments Thanks to the staff of the Structural Analysis of SAI in the UDC for the X-Ray diffractograms. This work was supported by the Spanish Ministry of Science and Innovation (Grants N° FIS2007-66823-C0201 and N° FIS2007-66823-C02-02), and by the Directorate General for R+D+i of the Xunta de Galicia (Grants N° 10-PXIB-103-294 PR and 10-PXIB-206-294 PR). All these research projects have been co financed with the European Regional Development Fund funds. References [1] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, Wiley-Verlag, Weinheim, 2003; N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123–150. [2] M.A.A. Rocha et al., Phys. Chem. B 115 (2011) 10919–10926. [3] Q. Dong, C.D. Muzny, A. Kazakov, V. Diky, J.W. Magee, J.A. Widegren, R.D. Chirico, K.N. Marsh, M. Frenkel, J. Chem. Eng. Data 52 (2007) 1151–1159. IL thermo database, http://www.ilthermo.boulder.nist.gov/ILThermo/mainmenu. uix. [4] O. Cabeza, S. García-Garabal, L. Segade, M. Domínguez-Pérez, E. Rilo, L.M. Varela, in: A. Kokorin (Ed.), Ionic Liquids: Theory, Properties, New Approaches, INTECH, Zagreb, 2011, pp. 111–136. [5] Bin. Dong, Na. Li, Liqiang. Zheng, T. Li Yu, Inoue, Langmuir 23 (2007) 4178– 4182. [6] I. Bou Malham, P. Letellier, M. Turmine, J. Phys. Chem. B 110 (2006) 14212– 14214. [7] J. Sung, Y. Jeon, D. Kim, T. Iwahashi, T. Iimori, K. Seki, Y. Ouchi, Chem. Phys. Lett. 406 (2005) 495–500. [8] W. Liu, L. Cheng, Y. Zhang, H. Wang, M. Yu, J. Mol. Liquids 140 (2008) 68–72. [9] W. Liu, T. Zhao, Y. Zhang, H. Wang, M. Yu, J. Sol. Chem. 35 (2006) 1337–1346. [10] L.A.S. Ries, F.A. do Amaral, K. Matos, E.M.A. Martini, M.O. de Souza, R.F. de Souza, Polyhedron 27 (2008) 3287–3293. [11] E. Rilo, J. Pico, S. García-Garabal, L.M. Varela, O. Cabeza, Fluid Phase Equilibr. 285 (2009) 83–89. [12] Jian.-Ying. Wang, Feng.-Yun. Zhao, Yu.-Min. Liu, Xiao.-Ling. Wang, Hu. YongQi, Fluid Phase Equilibr. 305 (2011) 114–120. [13] M. Blesic, M.H. Marques, N.V. Plechkova, K.R. Seddon, L.P.N. Rebelo, A. Lopes, Green Chem. 9 (2007) 481–490. [14] L.M. Varela, J. Carrete, M. García, J.R. Rodríguez, L.J. Gallego, M. Turmine, O. Cabeza, in: A. Kokorin (Ed.), Ionic Liquids: Theory, Properties, New Approaches, INTECH, Zagreb, 2011, pp. 347–366. [15] L.M. Varela, J. Carrete, M. Turmine, E. Rilo, O. Cabeza, J. Phys. Chem. B 113 (2009) 12500–12505. [16] S. Cuadrado-Prado, M. Domínguez-Pérez, E. Rilo, S. García-Garabal, L. Segade, C. Franjo, O. Cabeza, Fluid Phase Equilibr. 278 (2009) 36–40.

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JCT-11-535