Jinlong Li *,Hong Zhu,Changjun Peng, *,Honglai Liu

1 Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology and School of Petrochemical Engineering,Changzhou University,Changzhou 213164,China

2 State Key Laboratory of Chemical Engineering and Department of Chemistry,East China University of Science and Technology,Shanghai 200237,China

Keywords:Ionic liquid mixture Density Viscosity Excess property

ABSTRACT The density and viscosity of 1-butyl-3-methylimidazolium tetrafluoroborate[BMIM][BF4]and 1-butyl-3-methylimidazolium chloride[BMIM][Cl]and their binary mixtures within the temperatures from 303.15 K to 323.15 K and at ambient pressure were determined in this work.The temperature dependences of density and viscosity were satisfactorily described with the linear model and the Vogel-Tammann-Fulcher type equation,respectively.The molar volume and viscosity of binary IL mixtures were predicted through ideal mixing rules showing that almost null deviations for IL mixtures were observed and their mixing was remarkably close to linear ideal behavior in the molar volumes,while comparatively large errors in viscosity occurred.Additionally,the molar volume of the investigated pure ILs and their mixtures could well be predicted by a predictive model presented by Valderrama et al.(Fluid Phase Equilib.,275(2009)145).

1.Introduction

Room temperature ionic liquids(ILs),known as salts with a melting temperature usually below a conventional temperature of 100 °C[1]and benign solvents[2],have drawn much attention in the past few years and reached to many different potential applications[3-7].The notable characteristic of ILs is that they can be tailed based on task specific requirements,where the specific properties like high ionic conductivity,nonflammability,high thermal and chemical stabilities,negligible vapor pressure and so on can be reached by simple rearrangements of chemical structures and functional groups in the cation and/or in the anion,based on which IL mixtures provides more opportunities to tailor their properties and characteristics[8].For the study of IL mixtures,a pioneering work on their excess molar volumes was firstly carried out by Canongia Lopes et al.[9]To conveniently character the mixture of ILs,Niedermeyer et al.[10]proposed a nomenclature for such mixtures in a critical review on the use of IL mixtures,in which the IL mixture could be expressed as[A][X]x[Y](1-x)or[A]x[B](1-x)[X]for bearing a common cation or a common anion respectively,where x is the mole fraction of one IL.

Recently,the thermophysical properties of IL mixtures have been extensively investigated[11-13],and they have also been used for some process design[14].In a previous work[15],the IL mixture of 1-butyl-3-methylimidazolium tetrafluoroborate[BMIM][BF4]and 1-butyl-3-methylimidazolium chloride[BMIM][Cl]was found to be effective as an entrainer in the separation of acetonitrile and water,in which the thermodynamic properties(like density,viscosity and so on)of the mixed entrainer are very important for understanding their transportation characteristics and transfer properties.However,it is unfortunate that no mixing properties for[BMIM][BF4]and[BMIM][Cl]mixtures are available in opening literatures so far though some data for their pure compounds were measured,where the density and viscosity of[BMIM][BF4][8,11,16-39]at various pressure and temperature were thorough investigated while ones of[BMIM][Cl][40-43]comparatively sparse.Ongoing our work,the density and viscosity of pure ILs[BMIM][BF4]and[BMIM][Cl]and their corresponding binary mixtures were investigated in this work,to provide the fundamental understanding for their mixing properties and the further potential application in industrial.

2.Experimental

2.1.Materials

The materials used in the experimental measurement contained ILs and Karl Fisher reagent.The ILs of both[BMIM][Cl]and[BMIM][BF4]with a purity≥99.0% mass fraction were purchased from Chengjie Chemicals Ltd.,Shanghai,China.The analytical Karl Fischer reagent was provided by Merck KGaA,Darmstadt,Germany.Before preparing the IL mixtures,the pure ILs were firstly carefully degassed in a rotary evaporator and dehydrated in a vacuum drying oven(pressure 0-5 kPa)for at least 24 h at 353.15 K.Afterward,seven samples of ILs at 0,0.1989,0.3321,0.5003,0.6670,0.8003 and 1.0 mol fraction of[BMIM][BF4]that were numbered from(1)to(7)successively were gravimetrically prepared with a digital balance(METTLER TOLEDO,XP8002S)with an uncertainty of 0.0001 g,and the magnetic stirring in closed gall vials was carried out at least 20 min to ensure a completely mixing.The prepared samples were finally moved and kept in a closed glass container filled 4A molecular sieve desiccants prior to use.Before measurement of densities and viscosities,the water content of each pure or mixtures of ILs was determined using coulometric Karl Fischer titrator with an accuracy of 0.1 μg and a measurement range from 1 x10-6to 100%(METTLER TOLEDO,C20 KF Titrator),and the contents of water in all were less than 500.

2.2.Apparatus and procedure

The densities and viscosities of[BMIM][BF4]and[BMIM][Cl]and their binary mixtures were measured in the temperature range from 303.15 to 323.15 K,at atmospheric pressure.Density measurements were carried out using a digital DMA4500M Anton Paar densimeter with the absolute uncertainties in density and temperature of 0.00005 g·cm-3and 0.03 K respectively.The densimeter was firstly calibrated with water and air before use.The prepared IL samples were then put into the U glass tube using a specific syringe,meanwhile avoiding any bubbles to be taken into it.Finally,the density was measured and recorded at each temperature.Viscosity determinations were performed using DV-II+programmable controlled viscometer(Brookfield)with a temperature uncertainty of 0.1 K.The relative uncertainty in viscosity is 1%according to the manufacturer.After each measurement of viscosity,the water content contained in the samples was immediately analyzed again with Karl Fischer titrator due to the samples opening to air and being moisture absorption.

3.Results and Discussion

3.1.Density

In Table 1,the measured densities of pure ILs[BMIM][BF4]and[BMIM][Cl]and their corresponding binary mixtures at various compositions and temperatures(from 303.15 to 323.15 K)are listed.In Fig.1,the dependence of density with temperature and the comparisons of pure ILs between our work and literature data[8,37,38,40,41]are illustrated.It can be seen that the densities of our measurements for pure ILs were in good agreement with literature data and the relative average absolute deviations(AADs)for[BMIM][BF4]and[BMIM][Cl]were respectively 0.17%and 0.04%,suggesting that the samples used in experiment and the experimental method were reliable.Meanwhile,one can see that all mixing IL systems at various compositions showed a good linear dependence of density with temperature.

Table 1 Experimental density(g·cm-3)of[BMIM][BF4](xA)+[BMIM][Cl](1-xA)at various temperatures①

Fig.1.Experimental density of mixing IL[BMIM][BF4](xA)+[BMIM][Cl](1-xA):◆,pure ILs of this work;□,[38,40];△,[37,41];○,[8];lines with diamonds,mixing ILs of this work.

The density values(ρ)for pure ILs and their mixtures were correlated as a function of temperature,T(K),by a least-squares method,using the linear equation expressed as where Aρa(bǔ)nd Bρa(bǔ)re adjustable parameters,which have been listed in Table S1 in Supporting Information(SI),together with the respective correlation coefficients.One can see that all correlation coefficients at various compositions were close to 1,indicating that the linear function was enough to capture the measured density data within the temperature range investigated although the use of a linear or a second-order polynomial equation to fit experimental density data might raise some controversy[44].Furthermore,to obtain a generalized expression of the density equation at various compositions and temperatures,the obtained parameters of Aρa(bǔ)nd Bρwere further fitted to composition using a second-order equation,as presented in Table 2,also together with the correlation coefficient.The calculated molar volumes according to the fitting parameters are depicted in Fig.2,in which the experimental results were included too.The molar volume(Vm)was calculated by

Table 2 A generalized parameters for density of pure ILs and binary mixtures①

Fig.2.Molar volume of pure IL and their mixtures:□,303.15;◇,308.15;△,313.15;○,318.15;×,323.15 K;dash lines,correlation with parameters in Table 2.

where xiis the mole fraction of component i in a mixture;M represents the molecular weight and ρ denotes the experimental or correlated density.One can see that the correlated and experimental results were in good agreement with each other,and showed a good linear relationship of molar volume with IL mole fraction.The detailed values for experimental and correlated molar volume are provided in Table S2 of SI.As we all know that the molar volume of a mixture was directly related to the chemical potentials of the pure compounds in the mixture,so that the good linear relationship between molar volume and mole fraction was observed indicating that ideal mixing might occur for the investigated systems.To clearly show the very small deviations of the experimental molar volumes from the ideality,the excess molar volumewas further calculated by

where ρiis the density of pure component i in a mixture.The experimental and correlatedand their comparisons are listed in Table S3 and made in Fig.S1 of SI respectively,showing that(tenths of the unit)were small compared to the real molar volume(in order of hundreds of the unit)within the investigated temperature range.It is noted that the small experimental error in density was directly translated in considerable error in the excess molar volume due to their small values.Thus,the accuracy was very important in density measurements when the excess molar volume was discussed.

For the density of pure ILs,a prediction model was ever proposed by Valderrama et al.[45]and expressed as

where the constants of α,β,θ,τ and δ were determined based on literature data and were 0.3411,2.0433,0.5386,0.0393 and 1.0476,respectively.The subscripts‘c’and‘b’severally represent critical and boiling properties.In order to extend this model to predict the density of IL mixture,the properties of critical temperature and volume,boiling temperature and molecular weight for the corresponding mixtures have to be firstly determined and could be calculated from ones of pure ILs combining with a simple mixing rule as

where F means the property of a mixture,and Firepresent one of pure component i.The boiling point and the critical temperature and volume for pure IL[BMIM][BF4]and[BMIM][Cl]were from literature[45]and listed in Table S4 of SI,based on which,together with Eq.(7),the required properties for mixtures at various compositions were obtained and provided in Table S5 of SI.The predicted density at each condition is listed in Table S6 of SI,and the absolute average relative deviation of density calculated in Eq.(8)was 1.74%,indicating that the predicted model of IL density could well be used for the investigated systems.In Fig.3,the predicted deviations are depicted and one can see that the deviations for pure[BMIM][Cl]was the largest and ones for pure[BMIM][BF4]the smallest.Meanwhile,the deviations of density for mixtures increased with the increasing of[BMIM][Cl]in mixtures.Generally,the predicted densities for pure and mixing ILs were reasonable.

Fig.3.Predicted deviations of density for mixing IL[BMIM][BF4](xA)+[BMIM][Cl](1-xA)with literature model:xA=0.0(◆);0.1989(■);0.3321(▲);0.5003(×);0.6670(Ж);0.8003(●);1.0(+).

where the superscripts of‘exp’and‘cal’respectively represent experimental and calculated value,and N denotes the number of data.

3.2.Viscosity

The experimental viscosity of IL mixture[BMIM][BF4](xA)+[BMIM][Cl](1-xA)within the temperatures from 303.15 to 323.15 K and at various compositions are provided in Table 3,where the water contents that were immediately determined after finishing the viscosity measurement are also included.One can see that the viscosity of[BMIM][Cl]was much larger than one of[BMIM][BF4],and the water content increased in IL mixture with the increasing of[BMIM][Cl].Actually,the viscosity of pure IL[BMIM][BF4]and[BMIM][Cl]has also been measured by other researchers[8,11,31,34-37,39,42],especially for[BMIM][BF4][8,31,34-37,39].The comparisons of experimental viscosity from the above different sources for both pure ILs are illustrated in Fig.4.The experimental viscosity of[BMIM][BF4]from our work was in good agreement with the literature data,while one of[BMIM][Cl]was smaller than one measured in literatures.The disagreement of viscosity for[BMIM][Cl]was mainly introduced by water(see Table 3),which were absorbed by[BMIM][Cl]from moisture in air during experimental measurement due to the samples opening to air and its strong ability of moisture absorption.To show the interactions between water and the investigated ILs,the geometry optimizations in Dmol3 with GGA/VWN-BP and the DNP+methods for molecular cluster of water-[BMIM][Cl]and water-[BMIM][BF4]were made,and the optimized results and detailed information are given in Table S7-S9 and Fig.S2.One can see that the interactions between water and[BMIM][Cl]are stronger than ones between water and[BMIM][BF4]leading that more moisture were absorbed by[BMIM][Cl]during experiment.Therefore,the water content increased with the increasing of[BMIM][Cl]concentration in mixed ILs,as shown in Table 3.The dependence of viscosity for mixing ILs with the IL mole fraction was drawn in Fig.5,and a nonlinear relationship between them was observed though the linear volumetric behavior of them occurred.

Table 3 Experimental viscosity in mPa·s of[BMIM][BF4](xA)+[BMIM][Cl](1-xA)at various temperatures①

Fig.4.Experimental viscosity of pure[BMIM][BF4]and[BMIM][Cl]at various temperatures from our work(●for[BMIM][BF4]and ◆for[BMIM][Cl])and literature data(◇and ○for[BMIM][Cl]and other symbols for[BMIM][BF4])[8,11,31,34-37,39,42].

Fig.5.The dependence of experimental viscosity with composition for mixing IL[BMIM][BF4](xA)+[BMIM][Cl](1-xA):◆,303.15;■,308.15;▲,313.15;×,318.15;●,323.15 K.

The experimental viscosity data of pure ILs and IL mixtures were fitted as a function of temperature using the Vogel-Tammann-Fulcher(VTF)type model[46-48]expressed as

where η is viscosity in mPa·s;T means temperature in K,and Aηand Bηare the adjustable parameters.The adjustable parameters were determined by fitting the experimental viscosity values and reported along with the absolute average relative deviation(AADη)in Table S10 of SI.The AADη calculation was the same as the AADρ method given in Eq.(8)where replacing density with viscosity,and the AADη is only 0.82%for all experimental viscosity data indicating that Eq.(9)can well be used for the replacement of experimental results within the investigated temperature range.In Fig.6,the comparisons between experimental and correlated results were depicted and good agreements between experiment and correlation were observed.Based on the dependence of viscosity with temperature,the energy barrier(Eη)that was required to overcome to move ions upon the other ions in the ILs could be determined by[8]

Fig.6.Experimental(symbols),correlated(lines)and predicted(dash lines)viscosity of[BMIM][BF4](xA)+[BMIM][Cl](1-xA):xA=0.0(◆);0.1989(■);0.3321(▲);0.5003(×);0.6670(Ж);0.8003(●);1.0(+).

where R is the universal gas constant 8.314 J·mol-1·K-1.The higher the Eηis,the more difficult it is for moving the ions past each other.The calculated energy barrier within the temperatures from 303.15 to 323.15 K is illustrated in Fig.7,and it can be seen that the pure IL[BMIM][Cl]has a higher Eη,while[BMIM][BF4]presents lower Eη,indicating that the ions in[BMIM][BF4]fluid are easier to move past each other than ones in[BMIM][Cl].However,the change of the energy barrier with the concentration did not capture a strictly linear relationship,especially in the high concentration of[BMIM][BF4],which might be introduced by experimental measurement or correlation deviations since the energy barrier is only related to the parameter Bη,which was determined from experiment in this work.In addition,the viscosity of mixtures can also be calculated from the pure IL viscosity using the ideal Grunberg and Nissan mixing rules[49]as

Fig.7.Energy barrier of[BMIM][BF4](xA)+[BMIM][Cl](1-xA)within the temperatures from 303.15 to 323.15 K.

The predicted results were also compared in Fig.6 to the experimental viscosity for all mixing systems.The overall AADη from the predicted viscosity was 8.22%and generally captured the experimental trends.

4.Conclusions

New experimental density and viscosity of both pure ILs of[BMIM][BF4]and[BMIM][Cl]and their mixtures within temperatures from 303.15 to 323.15 K and at atmospheric pressure were reported.Five mole fractions(0.1989,0.3321,0.5003,0.6670 and 0.8003)of[BMIM][BF4]in binary mixtures were prepared and characterized in terms of density and viscosity.The relationships between the measured densities of pure ILs and mixtures and temperatures were well correlated using a linear equation and the mixing property in molar volume was very small and almost followed an ideal behavior.The temperature dependence of viscosity was satisfactorily described with VTF type model and comparably larger deviations from ideality for viscosity were observed.In addition,a predictive model for the density of pure ILs was extended to predict the density of the investigated IL mixtures with AADρ 1.74%.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.04.016.