J. Chem. Thermodynamics 45 (2012) 43–47

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J. Chem. Thermodynamics

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Properties of n-butylpyridinium nitrate ionic liquid and its binary mixtureswith water

Jian-ying Wang ⇑, Xiang-jing Zhang, Yong-qi Hu, Guo-di Qi, Li-ya LiangCollege of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, 70 Yuhua East Road, Shijiazhuang 050018, PR China

a r t i c l e i n f o

Article history:Received 5 May 2011Received in revised form 4 August 2011Accepted 8 September 2011Available online 16 September 2011

Keywords:Ionic liquidThermophysical propertiesSurface tensionDensity

0021-9614/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jct.2011.09.003

⇑ Corresponding author. Tel./fax: +86 311 8863229E-mail address: jenneywjy@126.com (J.-y. Wang).

a b s t r a c t

The density and surface tension of the pure ionic liquid n-butylpyridinium nitrate ([BuPy]NO3) weredetermined at temperature range from T = (293.15 to 338.15) K. The coefficient of thermal expansion,molecular volume and lattice energy of [BuPy]NO3 were calculated from the experimental values of den-sity. The surface entropy and enthalpy of [BuPy]NO3 were investigated. The IL studied show much lowersurface enthalpy and lattice energy in comparison with fused salts. The densities and surface tensions ofbinary mixtures of [BuPy]NO3 with water have been measured within the whole composition range atT = 298.15 K and atmospheric pressure. Excess molar volumes VE and surface tension deviations dc werethen deduced from the experimental results as well as partial molar volumes and excess partial molarvolumes. Excess molar volumes have a negative deviation from ideal behavior and the surface tensiondeviations are negative over the whole compositions range. VE and dc were correlated with suitable equa-tion respectively.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The rapid growth of literature in ionic liquids (ILs) shows thatILs attract much attention as negligible vapor pressure alternativesto traditional organic solvents [1]. At present, more efforts havebeen concentrated on the investigation of their potential applica-tions, such as new media for organic synthesis, catalytic reactions,biocatalysis, multiphase separations, and nanomaterial technolo-gies [2]. The complete design of industrial processes based on ILsis only achieved when their thermophysical properties, such asdensity and interfacial tension, are adequately characterized [3].

At present, the experimental data for density and surfacetension of ionic liquids are limited to the most commonly studiedimidazolium-based ionic liquids. The pyridinium-based ILs consti-tutes a very interesting family of compounds, since they have aspecially high thermal stability and constitute a lower cost alterna-tive to imidazolium-based ILs [4,5]. Moreover, pyridinium-basedILs exhibit interesting properties in some areas, for example, then-butylpyridinium nitrate ([BuPy]NO3) IL was a good microwaveabsorbent for the Knoevenagel condensation under microwaveirradiation, and the condensation proceed in the presence of IL toafford the desired product in high purity with high yield [6]. Theknowledge of physical properties of pyridinium-based ILs is very

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8.

important for their industrial application. However, the thermo-physical characterization of pyridinium-based ILs is very scarce.

This paper is a continuation of our systematic investigation onthe thermodynamic study of pure ILs and their mixtures withmolecular solvents [7–9]. In this work, the thermophysical proper-ties of pure n-butylpyridinium nitrate ([BuPy]NO3) ILs and its bin-ary mixture with water have been measured from (293.15 to338.15) K at atmospheric pressure. From the experimental valuesof density, we have obtained the coefficient of thermal expansion,molecular volume and lattice energy of [BuPy]NO3. The surfaceproperties of IL have been investigated. The densities and surfacetensions of binary mixtures of [BuPy]NO3 with water were mea-sured within the whole composition range at T = 298.15 K andatmospheric pressure. Excess molar volumes VE and surface ten-sion deviations dc were then obtained from the experimental re-sults as well as partial molar volumes and excess partial molarvolumes.

2. Experimental

2.1. Chemicals

N-butylpyridine nitrate, ([BuPy]NO3) (>99%), was acquired fromShanghai Chengjie Chemical Co. (in China). In order to reduce thewater content, the IL samples were dehydrated under vacuum ata temperature of 80 �C for at least 48 h prior to the measurements.

44 J.-y. Wang et al. / J. Chem. Thermodynamics 45 (2012) 43–47

Water content in the IL, determined by Karl Fisher titration, wasbelow 0.03%.

2.2. Density and surface tension measurements

Mixtures were prepared by weighing on a FC-204 analytical bal-ance (shanghai xinnuo instrument Co., China) covering the wholecomposition range of the mixture; the error in mole fraction beingestimated is less than 5 � 10�4. Densities of the pure liquids andtheir mixtures were measured using an Anton-Paar DMA4500 dig-ital vibrating-tube densimeter, which is based on mechanical oscil-lations of a U-shaped glass or metal tube filled with a fluid sample[10]. The repeatability and uncertainty in experimental measure-ment have been found to be lower than ±1 � 10�5 and ±5 � 10�5,respectively. The density measuring cell was thermostated with atemperature stability of ±0.01 K. The apparatus calibration wasperformed periodically, and doubly distilled, degassed water wereused for calibration. In order to avoid the adsorption of water, alldensities measurements were performed in nitrogen atmosphere.The measurements were done in three replicate runs and the aver-age value is considered for further study.

Measurements of surface tension were performed with a plati-num plate with a DCAT21 (Dataphysics, Germany) digital tensiom-eter. To avoid surface contamination and the absorption of water,all surface tension measurements were performed in nitrogenatmosphere. Samples were measured in a closed measuring cellwith a volume of about 40 cm3, and the temperature was con-trolled within ±0.02 K. Before a series of experiments, the plateand vessel were thoroughly cleaned by immersion in a concen-trated solution of nitric acid for several hours. Then the platewas rinsed with distilled water, carefully flamed in a Bunsen bur-ner, washed again with distilled water and dried. Each reported va-lue was an average of five measurements, where the maximumdeviation from the average value was always less than 0.5%. Theuncertainty of the measurements is ±0.15 mN/m.

3. Results and discussion

3.1. The properties for the pure [BuPy]NO3 ionic liquid

The density values of [BuPy]NO3 were measured at atmosphericpressure and over the temperature range from T = (293.15 to338.15) K. The experimental results of density and surface tensionof [BuPy]NO3 are listed in table 1.

The variation of the density of IL studied in this work with tem-perature was linear. Density results for [BuPy]NO3 were used to de-rive the coefficient of thermal expansion. The experimental valuesof lnq against T are shown in Supplementary figure S1. The valueswere fitted by the method of the least-square. An empirical equa-tion was obtained as follows:

TABLE 1Experimental density (q), and surface tension values (c) of[BuPy]NO3 at T = (293.15 to 338.15) K.

T/K q/(g � cm�3) c/(mN �m�1)

293.15 1.19022 52.3298.15 1.18605 51.5303.15 1.18183 51.2308.15 1.17812 51.0313.15 1.17404 50.5318.15 1.17014 50.0323.15 1.16651 49.5328.15 1.16276 49.3333.15 1.15932 49.0338.15 1.15603 48.6

In½q=ðg cm�3Þ� ¼ 0:3643� 6:500� 10�4T=K: ð1Þ

The correlation coefficient is 0.9996. The coefficient of thermalexpansion of [BuPy]NO3 studied is defined by the followingequation:

a ¼ 1V

@V@T

� �P¼ @ ln q

@T

� �P; ð2Þ

where a is the coefficient of thermal expansion, V is the volume ofthe IL and q is the density of the [BuPy]NO3. The value of a obtainedfrom the slope of the linear fit is 6.500 � 10�4 K�1 for [BuPy]NO3. Thea value is comparable to coefficients of imidazolium-based IL[C1mim](CH3O)2PO2 (6.21 � 10�4 K�1), [C2mim](CH3CH2O)2PO2

(6.82 � 10�4 K�1) [7], and the pyridinium-based IL n-butyl-3-meth-ylpyridinium tetrafluoroborate ([b4mpy][BF4], 5.72 � 10�4 K�1) [11].

From the experimental density, the molecular volume of [Bu-Py]NO3, Vm, was calculated at T = 298.15 K using the followingequation:

Vm ¼ M=ðN � qÞ; ð3Þ

where M is the molar mass of [BuPy]NO3 = 198.2 g �mol�1, N is theAvogadro’s constant, q is the density, and Vm is the molecular vol-ume of the IL. The calculated value of Vm for [BuPy]NO3 is0.2776 nm3.

Density is also critically needed for the estimation of lattice en-ergy (UPOT) of an ionic species which then can be utilized to calcu-late the heat of formation [12]. By Glasser’s theory [13], UPOT, maybe estimated using the following equation:

UPOT=kJ �mol�1 ¼ 1981:2ðq=MÞ1=3 þ 103:8: ð4Þ

The lattice energy of [BuPy]NO3 calculated form equation (4)was found to be 463.5 kJ �mol�1. This value is close to the latticeenergy of series of imidazolium-based ILs [Cnmim][BF4] (n = 2, 4,6) [13], and the lattice energy of [Cnmim][BF4] (n = 2, 4, 6) are inthe range of 432 to 470 kJ �mol�1. The lattice energy of the studiedpyridinium-based IL is much less than that of fused salts. Forexample, UPOT is 613 kJ �mol�1 for fused CsI, which has the lowestlattice energy among all the alkali halides. The low lattice energymay explain the low liquid-state temperature of room temperatureILs [14].

The experimental surface tension values for the [BuPy]NO3 arelisted in table 1. A linear variation of surface tension with temper-ature for pure [BuPy]NO3 was found as shown in figure 2. The tem-perature dependence of the surface tension is presented as

c=ðmN �m�1Þ ¼ a� bT=K; ð5Þ

where a = 75.37 and b = 0.0794 were obtained by the fitting, thecorrelation coefficient is 0.9941. The surface entropy and surfaceenthalpy can be calculated from the experimental surface tensionvalues [3,15]. From the equation (5), we can obtain the surfaceentropy (Ss = b = �(oc/oT)P) and surface enthalpy (Es = a = c � T(oc/oT)P) values. The obtained surface entropy and the surface enthalpyvalue for the [BuPy]NO3 are 0.0794 mJ � K�1 �m�2 and 75.37mJ �m�2, respectively. The value of Es is close to imidazolium-basedIL, for example, 85.46 mJ �m�2 for BMIZnCl3 [16]. In comparisonwith fused salts (for example, surface enthalpy of fused NaNO3 is146 mJ �m�2), the IL studied in this work shows much lower valueof surface enthalpy. This fact indicates that interaction energy be-tween ions in [BuPy]NO3 is much less than that in inorganic fusedsalts because the surface enthalpy is dependent on interactionenergy (lattice energy) between ions [17].

Critical temperature (Tc) of ILs is one of the most relevant ther-mophysical properties since it can be used in many correspondingstates correlations for equilibrium and transport properties of flu-ids [3]. It is necessary to achieve values for critical temperature Tc.However, reliable data are relatively hard to acquire due to the

TABLE 3Surface tension values and their deviations for the binary mixtures of ([BuPy]NO3 + water) at different mole fraction of [BuPy]NO3 (xIL) at T = 298.15 K andatmospheric pressure.

xIL c/(mN/m) dc/(mN/m)

1.0000 51.50.9023 50.8 �2.80.8226 49.6 �5.60.7604 49.3 �7.20.6330 48.0 �11.00.4470 46.1 �16.80.3247 44.0 �21.40.2067 41.7 �26.10.1136 40.0 �29.70.0816 38.0 �32.30.0593 37.6 �33.20.0498 37.5 �33.50.0359 37.6 �33.70.0150 40.1 �31.60.0097 43.7 �28.10.0000 72.0

0.0 0.2 0.4 0.6 0.8 1.0

-0.8

-0.6

-0.4

-0.2

0.0

VE/cm

3 ·mol

-1

xIL

FIGURE 1. Excess molar volume (VE) plotted against mole fraction of ([BuPy]-NO3 + water) at T = 298.15 K. j, Calculated values from equation (7); line: fittedcurves using Redlich–Kister equation.

TABLE 4Fitting parameters and root mean square deviations (r) for ([BuPy]NO3 + water) at298.15 K.

B0 B1 B2 B3 B4 r

J.-y. Wang et al. / J. Chem. Thermodynamics 45 (2012) 43–47 45

intrinsic nature of ILs. In this work, prediction of the critical tem-perature was carried out by using the Guggenheim [18] empiricalequations, which were described by the following equation

c ¼ K 1� TTc

� �11=9

; ð6Þ

where c is the surface tension, Tc is critical temperature, K is anempirical constant. The equation reflect the fact that c becomes nullat the critical point. The estimated critical temperature value of[BuPy]NO3 is 1089 K, which is comparable with the value of imi-dazolium-based IL [hmim][PF6] (1039 K), [bmmim][PF6] (1091 K),and [bmim][Tf2N] (1032 K) [3].

3.2. The properties for the binary mixture of [BuPy]NO3 and water

The experimental density and surface tension values of the([BuPy]NO3 + water) binary mixture were determined over thewhole composition range at 298.15 K and are shown in tables 2and 3, respectively.

As shown in table 2, the density value shows a trend of increasewith the increase of molar fraction of IL (xIL). With the addition ofwater, the density of the mixtures increases sharply at the molfraction of IL xIL < 0.45 and then q grows slightly at xIL > 0.45.Therefore the physical properties of ILs can be adjusted by addinga certain amount of water to meet the needs of different applica-tions for hydrophilic ionic liquids.

Excess thermodynamic properties, which depend on the com-position, are of great importance in understanding the nature ofmolecular aggregation that exists in the binary mixtures [19]. Fromthe measured density values of ([BuPy]NO3 + H2O) mixtures, theexcess molar volumes VE are obtained by the following equation:

VE ¼ xILM1 þ ð1� xILÞM2

q� xILM1

q1� ð1� xILÞM2

q2; ð7Þ

where xIL is mole fraction of [BuPy]NO3 in the mixtures; q, q1 andq2 are densities of binary mixture, pure [BuPy]NO3 and pure water,respectively; M1 and M2 are molar mass of [BuPy]NO3 and water,respectively.

The calculated VE are listed in table 2 and figure 1. The excessquantities of the binary mixtures have been fitted to a Redlich–Kister [20,21] type equation

Z ¼ xIL 1� xILð ÞXM

p¼0

Bpð2xIL � 1Þp; ð8Þ

where Z is the excess property; xIL is the mole fraction of [BuPy]-NO3; Bp are the fitting parameters; and M is the degree of the poly-

TABLE 2Densities, and excess molar volumes for binary mixtures of ([BuPy]NO3 + water) atdifferent mole fraction of [BuPy]NO3 (xIL) at T = 298.15 K and atmospheric pressure.

xIL q/(g � cm�3) VE/(cm3 �mol�1)

1.0000 1.186050.9018 1.18536 �0.19420.8166 1.18427 �0.31880.7600 1.18342 �0.40070.6300 1.18086 �0.57800.5002 1.17721 �0.75440.4023 1.17202 �0.80760.3000 1.16019 �0.66110.2008 1.14106 �0.49950.1098 1.10701 �0.28770.0793 1.08921 �0.22940.0498 1.06281 �0.09730.0259 1.03626 �0.04040.0161 1.02322 �0.02660.0091 1.01202 �0.00340.0000 0.9970

VE/(cm3 �mol�1) �3.0141 1.5322 1.5487 �1.8885 �1.2024 0.018C n C0

dc/(mN �m�1) �44.22 0.0774 22.48 1.06

nomial expansion, which was optimized using the Marquardt’salgorithm. The fitting parameters Bp are listed in table 4 togetherwith the standard deviations (r). The deviation was calculated byapplying the following expression:

r ¼Xndat

1

ðzexp � zcalÞ2=ndat

" #1=2

; ð9Þ

where zexp, zcal, and ndat present the experimental value, calculatedvalue, and the number of experimental data, respectively.

It is evident that the values of VE are negative over the entirerange of compositions. The excess molar volume VE decreases withthe molar fraction of [BuPy]NO3 when xIL < 0.45 or so, showing aminimum at xIL � 0.45 with a value VE � �0.7600 cm3 �mol�1 and

0

46 J.-y. Wang et al. / J. Chem. Thermodynamics 45 (2012) 43–47

then VE increases with the increase of molar fraction of [BuPy]NO3.The negative values of the VE indicate that relatively small watermolecules fit into the free volume between the relatively large ionsof [BuPy]NO3 upon mixing. The published data for the binary mix-ture of ([Pyrr][C7CO2] (pyrrolidinium octanoate) + H2O) [22] aresimilar to ours, but they present more negative VE values (morenegative than those for [BuPy]NO3 mixtures). A different phenom-enon has been reported for ([C4mim][BF4] + H2O) [15,23],([C2mim][BF4] + H2O) [16] and ([emim][L-lactate] + H2O) [8] mix-tures, where the values of VE were positive over the entire rangeof compositions. In comparison with ([BuPy]NO3 + H2O) system inour work, the ([Bmpy]BF4 + H2O) [24] and ([BuPy]BF4 + H2O) [25]binary mixtures show a positive values within the whole composi-tion range. This indicated that pyridinium-based nitrate IL have astronger interactions with water than that of pyridinium-based tet-rafluoroborate IL with water. The negative value of VE for [BuPy]-NO3 + H2O system indicates a negative deviation from idealbehavior and suggests that the interaction between unlike mole-cules seems to be stronger than the intramolecular interactions.

The partial and excess partial molar volumes have frequentlybeen used to give an insight on (solute + solvent) interactions.The partial molar volumes of IL and water in the studied binarymixtures over the whole composition range have been calculatedusing the following equations (10) and (11) [26,27]:

V IL ¼ Vm;IL þ VE þ ð1� xILÞ@VE

@xIL

!P:T

; ð10Þ

VO ¼ Vm;O þ VE þ xIL@VE

@xIL

!P:T

; ð11Þ

where V IL and VO are the partial molar volumes of the IL and waterin solution, Vm;IL and Vm;O are molar volume of the pure ionic liquidand water. Where ð@VE=@xILÞP;T is calculated from equation (8) usingthe parameters in table 4. The excess partial molar volumes in abinary mixture can be determined from the equations:

VEIL ¼ V IL � Vm;IL; ð12Þ

VEO ¼ VO � Vm;O: ð13Þ

The partial molar volumes and the excess partial molar volumesare given in table 5. The excess partial molar volumes of [BuPy]NO3

for the binary system are negative over the whole compositionsrange. The VE

IL values are more negative at low bulk IL mole frac-tion. This confirms the existence of strong interactions of ionic li-quid and water at low IL mole fraction in the bulk phase.

TABLE 5Partial molar volumes, excess partial molar volumes for the binary system of([BuPy]NO3 + water) at 298.15 K and ambient pressure.

xIL V IL/(cm3 �mol�1)

VO

(cm3 �mol�1)VE

IL

/(cm3 �mol�1)VO /(cm3 �mol�1)

1.0000 167.11 21.07 0.00 3.020.9018 167.09 19.42 �0.02 1.370.8166 167.06 18.93 �0.05 0.880.7600 167.05 18.74 �0.06 0.690.6300 166.97 18.22 �0.14 0.170.5002 166.55 17.49 �0.56 �0.560.4023 166.00 17.03 �1.12 �1.020.3000 165.39 16.94 �1.72 �1.110.2008 164.83 17.10 �2.28 �0.950.1098 164.58 17.49 �2.53 �0.560.0793 164.57 17.62 �2.54 �0.430.0498 164.66 17.83 �2.45 �0.220.0259 164.72 17.95 �2.39 �0.100.0161 164.75 17.99 �2.36 �0.060.0091 164.78 18.03 �2.33 �0.020.0000 164.80 18.05 �2.31 0.00

Table 3 show the experimental surface tension of ([BuPy]-NO3 + water) systems studied, in function of composition at298.15 K. The surface tension of the mixture decreases sharply atlow mole fraction of IL, showing a minimum at xIL � 0.05 and thenc increases slightly with the increase of molar fraction of [BuPy]-NO3. The results show that [BuPy]NO3 mainly acts as a surfactantin aqueous solution [28]. A similar phenomenon (the occurrenceof the minimum value of surface tension) was observed previouslywith imdazolium-based IL [C4mim]Br, [C6mim]Br, [C8mim]Br and[C10mim]Br [29] in water. The phenomenon that the distinctbreakpoint point is observed at xIL � 0.05 are consistent with the([emim][L-lactate] + water) system [8].

The CMC (critical micelle concentration) is taken as the point atwhich the surface tension reaches minimum. As shown in table 3,the CMC of [BuPy]NO3 in water was 1.8 ± 0.5 mol � dm�3, which iscomparable to the CMC value of [C2mim]Br in water reported inreference [29]. When the concentrations of [BuPy]NO3 are higherthan CMC, the surface tension of [BuPy]NO3 shows a slight in-crease, this phenomenon may be attributable to a change in natureof the micelles.

The surface tension deviations (dc) were calculated by

dc ¼ c�X2

i¼1

xici; ð14Þ

where c and ci is the surface tensions of the mixture and the com-ponent i, and xi is the mole fraction of the component i.

As shown in table 3, the surface tension deviations (dc) for ([Bu-Py]NO3 + water) system were negative over the whole compositionrange and approach the minimum at xIL � 0.05. Similar behaviorswere observed for ([EMIM]BF4 + water) in reference [30], whilethe dc vs. xIL curve reveals that the minimum of dc was in the molefraction of 0.04 for ([BMIM]BF4 + water) system.

The data shown in table 3 and figure 2 are unusual surface ten-sion deviations, which cannot be fitted by the Redlich–Kister equa-tion. The data could be correlated with quality by the followingnovel equation

dc ¼ CxnILð1� xILÞ þ C0xILð1� xILÞ; ð15Þ

where C, n, C0 are fitting parameters. The corresponding fittingparameters with the standard deviations (obtained from equation(9)) are given in table 4. We plot the resulting curves as line infigure 2.

0.0 0.2 0.4 0.6 0.8 1.0

-35

-30

-25

-20

-15

-10

-5

xIL

δγ/(m

N/m

)

FIGURE 2. Deviations of surface tensions (dc) from equation (14) (—) plottedagainst mole versus molar fraction (xIL) of IL for the ([BuPy]NO3 + water) binarymixture at T = 298.15 K. j, Calculated values from equation (14); line: fitted curvesusing equation (15).

J.-y. Wang et al. / J. Chem. Thermodynamics 45 (2012) 43–47 47

4. Conclusions

This investigation measured the thermophysical properties forthe system of n-butylpyridinium nitrate and water, focusing onpure ionic liquid and the mixtures. Density and surface tensionof pure n-butylpyridinium nitrate IL have been experimentallymeasured in a broad range of temperatures. From the experimentalresults, the thermal expansion coefficient, molecular volume, lat-tice energy, surface entropy, surface enthalpy value and the criticaltemperature have been obtained. The present study contributesnot only to a better understanding of the structure-property rela-tionship of this type of pyridinium-based ILs, but also to a properchoice of RTILs for a specific application.

On the other hand, density and surface tension were measuredover the whole composition range for binary system of [BuPy]NO3

with water at 298.15 K. The density value decreases with increas-ing the mole fraction of IL. The excess molar volumes VE for themixture of IL and water were a slightly negative, indicating a neg-ative deviation from ideal behavior. The excess molar volumeshave been well fitted by Redlich–Kister polynomial. The surfacetension of the mixture decreases rapidly in the lower IL mole frac-tion and becomes almost constant in the higher IL mole fraction.From the surface tension results, we can affirm that the [BuPy]NO3

mainly acts as a surfactant in aqueous solution. The critical micelleconcentration of [BuPy]NO3 in water was 1.8 ± 0.5 mol � dm�3. Fi-nally, we have extracted the surface tension deviations on mix-tures and the resulting data were fitted to a novel fittingequation with good accuracy.

The present results considerably expand the available thermo-dynamic database for the pure [BuPy]NO3 and ([BuPy]NO3 + water)mixtures. Moreover, the current investigation is helpful for the de-sign of the ILs and their further applications.

Acknowledgements

The authors are grateful for the financial support of NationalNatural Science Foundation of China (No. 20776037) and CollegeFoundation of Hebei University of Science and Technology(XL200716).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jct.2011.09.003.

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JCT 11-168