Densities,conductivities,and viscosities of aqueous solutions of N-hexyl,methylpyrrolidinium bromide and N-butyl,methylpyrrolidinium bromide at different temperatures☆

Yufei Chen,Yufeng Hu*,Jianguang Qi,Yu Sun,Zheyu Li

State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase Behavior&Property Research Laboratory,China University of Petroleum,Beijing 102249,China

Keywords:Density Conductivity Viscosity Prediction Ionic liquids

ABSTRACT The densities,conductivities,and viscosities were measured for ternary solutions of N-hexyl,methylpyrrolidinium bromide([PP1,6]Br)-N-butyl,methylpyrrolidinium bromide([PP1,4]Br)-H2O and its binary subsystems[PP1,6]Br-H2O and[PP1,4]Br-H2O at(298.15,303.15,308.15,and 313.15)K,respectively.The results were used to test the predictability of the simple equations established for the prediction of density,conductivity,and viscosity of the mixed electrolyte solutions.The results show that the examined simple equations can offer good predictions for density,conductivity,and viscosity of the mixed ionic liquid solutions in terms of the corresponding properties of its binary subsystems of equal ionic strength.

1.Introduction

Ionic liquids(ILs)are composed entirely of ions.ILs have many attractive properties,e.g.,essentially no vapor-pressure,outstanding catalytic property,high thermal stability,and exceedingly high ionic conductivity[1].Furthermore,such properties can be optimized either by tuning the structure of constituent cation and/or anion[1]or by mixing different ILs[2,3].These properties have endowed ILs with widely potential applications in various fields.One of such applications is the use of the binary and/or ternary aqueous solutions of ILs in synthesis of trioxane,in which ILs serve as the catalyst and water as the solvent[4-9].In addition,aqueous solutions of ILs have been used in the colloid and surfactant fields[10,11].

Investigation on the properties of aqueous solutions of ILs is essential not only to technical and industrial applications,but also to the test/development of electrolyte theories.Up to now a number of groups have studied the physical properties of aqueous solutions of ILs[12-15],but few data have been reported for mixed IL solutions.Therefore,appropriate theories/models must be developed to provide predictions for properties of mixed IL solutions using the corresponding information on their binary subsystems.In this study we measure the densities,conductivities,and viscosities of the ternary solutions of N-hexyl,methylpyrrolidinium bromide([PP1,6]Br),N-butyl,methylpyrrolidinium bromide([PP1,4]Br)and H2O and its binary subsystems[PP1,6]Br-H2O and[PP1,4]Br-H2O at different temperatures.The results are used to test the applicability of the well known equations established for mixed electrolyte solutions to mixed IL solutions,including Patwardhan and Kumar's equation[16,17],Young and Smith's rule[18],and the equation of Hu et al.[19-23].Our preliminary experimental results show that these binary and ternary aqueous IL solutions can be used in synthesis of trioxane.

2.Experimental Section

2.1.Chemicals and procedures

All chemicals used in this study are reagent grade and their purity is＞99%(by mass).N-butyl,methylpyrrolidinium bromide and N-hexyl,methylpyrrolidinium bromide were supplied by Lanzhou Greenchem ILS,LICP.CAS.China and were dried under vacuum for 2 days at 343.15 K before use.

The experimental procedures are the same as those used in our previous studies[24]and are described brie fly as follows.The binary aqueous solutions were prepared by using a Sartorius CT225D balance with the precision of±5× 10?5g.In addition,the molalities of these IL solutions were analyzed by potentiometric titration of Br?with silver nitrate using an automatic potentiometric titrator(ZDJ-5,LEICI,Shanghai,China)with a precision of±5× 10?3ml.The differences between molalities gained by weighing and those determined by titration were in the order of 10?4mol·kg?1.The ternary solutions were prepared by mixing the binary solutions.All solutions were prepared immediately before use and the uncertainty was ±5 × 10?5mol·kg?1.The solutions were placed into stoppered bottles and stirred for 2 h to ensure complete dissolution and full mixing.

2.2.Property measurements

The conductivity was measured using a METLER TOLEDO Seven Easy TM conductivity meter calibrated with the standard aqueous potassium chloride solutions[21].The temperature was measured with a calibrated calorimeter thermometer,with the uncertainty of±0.01 K.

The densities of solutions were measured with a KEM oscillating tube digital densimeter(DA-505).The temperature was monitored with a digital thermometer and the thermostated stability was±0.01 K[14,20].The densimeter was calibrated with doubledistilled water and dry air[25,26].The densities of water at different temperatures were available in literature[27].The densities of dry air at different temperatures were taken from Ref.[28].In addition,hexane[29]was used as a calibration substance.The densities of H2O-[MEA][BF4](2-aminoethanol tetra fluoroborate)at 313.15 K and H2O-[Emim][BF4](1-ethyl-3-methylimidazolium tetra fluoroborate)at 293.15 K have been well established[13,30].Therefore,in our preliminary measurements these two binary IL solutions were also used as calibration substances.The results are(1.11076 vs 1.11085[30])g·cm?3for[MEA][BF4]-H2O(x［MEA］［BF4］=0.2987)at 313.15 K,and(1.29993 vs 1.2998[13])g·cm?3for[Emim][BF4]-H2O(x［Emim］［BF4］=0.1066)at 293.15 K.The uncertainty in density measurements was±5×10?5g·cm?3.

A modified Cannon-Ubbelohde suspended level capillary viscometer was used to measure the viscosity in a glass sided water thermostat controlled to±0.01 K.The capillary viscometers are calibrated and credited by the company,and with a stated precision of±0.1%[31],leading to a relative standard uncertainty of less than 0.5%in the viscosity measurements.

The viscosity of the solution is given by[26,27]

where ηois the viscosity of water,ρ and ρoare the densities of experimental solution and water,respectively,τ and τoare the flow time of the solution and water,respectively.The accuracy of density measurements of±5×10?5g·cm?3resulted in a relative standard uncertainty of less than 0.09%in the viscosity measurement[31].The efflux time of solutions was recorded using a digital electronic watch at specific temperature and this uncertainty was estimated to be±0.01 s,leading to a relative standard uncertainty of less than 0.03%in the viscosity measurement[31].Triplicate measurements were performed for each sample.The estimated overall expanded uncertainty,including temperature,efflux time,the accuracy of the density measurement,and calibration uncertainties,was 1.0%.

The viscosities of[MEA][BF4]-H2O at 313.15 K and[Emim][BF4]H2O at 293.15 K have been well established[13,30].In our preliminary measurements,water and these two binary IL solutions were used as calibration substances.The results are(0.8905 vs 0.8903[32])mPa·s for water at 298.15 K,(0.8570 vs 0.8573[30])mPa·s for[MEA][BF4]-H2O(x［MEA］［BF4］=0.2987)at313.15 K,and(10.3703 vs 10.365[13])mPa·s for[Emim][BF4]-H2O(x［Emim］［BF4］=0.1066)at 293.15 K.The agreement is good.

3.Results and Discussion

3.1.Equations for prediction of density and conductivity of aqueous solutions of ionic liquids

In the following section,the variables with superscript(o,I)and subscript MiXidenote the quantities of component MiXiin the binary solution MiXi-H2O(i=1,2)having the same ionic strength as that of a mixed solution,and those without superscript(o,I)denote the corresponding quantities in the mixed solution.

The equation of Patwardhan and Kumar[16,17]can be expressed as

with YMiXi=yMiXi+mMiXiMMiXi,where y,m,ρ,and M denote ionic strength fraction,molality,density,and molar mass,respectively.

Young's rule[33]for the conductivity of a mixed IL solution M1X1-M2X2-H2O in terms of conductivities of its binary solutions MiXi-H2O(i=1,2)of equal ionic strength can be expressed as

On the basis of Eyring's absolute rate theory and the rule of Patwardhan and Kumar[16,17],Hu[22]derived an equation for prediction of the viscosity of a mixed IL solution:

3.2.Comparisons with experimental data

Experimental data of densities,conductivities,and viscosities are used to test Eqs.(2)-(4).The procedure is briefly summarized as follows.

(1)Fit the measured data(density,conductivity,and viscosity)of binary solutions(cf.Tables 1-3)by the following polynomial equations

(2)Determine the composition()of the binary solutions having the same ionic strength as that of the multicomponent solution of given molalitiesi=1 and 2）.

Table 1 Densities of binary systems[PP1,6]Br-H2O and[PP1,4]Br-H2O at different temperatures

(4)Compare the predicted and experimental data.

The average relative differences between predicted and measured densities(δρ),conductivities(δσ),and viscosities(δη)over the entire experimental composition range of the mixed solution are defined as[14]

Table 2 Conductivities of binary systems[PP1,6]Br-H2O and [PP1,4]Br-H2O at different temperatures

3.3.Verifications of equations

Tables 1-3 show the measured densities,conductivities,and viscosities of binary solutions[PP1,6]Br-H2O and[PP1,4]Br-H2O at different temperatures.Tables 4-6 show the fitted parameters for binary systems[PP1,6]Br-H2O and[PP1,4]Br-H2O at different temperatures.

Figs.1-3 compare the predicted and measured densities,conductivities,and viscosities for ternary system[PP1,6]Br-[PP1,4]Br-H2O at different temperatures.The agreements between measured and predicted results are quite good.

Table 3 Viscosities of binary systems[PP1,6]Br-H2O and[PP1,4]Br-H2O at different temperatures

Table 4 The fit parameters for the density of the binary solutions

Table 5 The fit parameters for the conductivity of the binary solutions

Table 6 The fit parameters for the viscosity of the binary solutions

Fig.1.Comparisons of measured and predicted densities for ternary system[PP1,6]Br-[PP1,4]Br-H2O at different temperatures.

Fig.2.Comparisons of measured and predicted conductivities for ternary system[PP1,6]Br-[PP1,4]Br-H2O at different temperatures.

Fig.3.Comparisons of measured and predicted viscosities for ternary system[PP1,6]Br-[PP1,4]Br-H2O at different temperatures.

4.Conclusions

The densities,conductivities,and viscosities ofternary system[PP1,6]Br-[PP1,4]Br-H2O and its binary subsystems[PP1,4]Br-H2O and[PP1,6]Br-H2O were measured at three temperatures.The simple equations for the prediction of these properties of mixed electrolyte solutions are extended to the corresponding properties of mixed IL solutions.Their predictabilities are tested by comparisons with the measured data of ternary solution[PP1,6]Br-[PP1,4]Br-H2O.The agreements are impressive.

Nomenclature

M molar mass,g·mol?1

m molality,mol·g?1

N number of experimental data

x mole fraction

y ionic strength fraction

ρ density,g·cm?3

σ conductivity,mS·cm?1

η viscosity,mPa·s

δ average relative difference between predicted and measured quantity

Subscripts

i component of ionic liquid

ρ density

σ conductivity

η viscosity

Calc. calculation

Expl. experiment

Superscript

o,I quantity of component MiXiin the binary solution having the same ionic strength as that of corresponding ternary solution