Electrical conductivities for four ternary electrolyte aqueous solutions with one or two ionic liquid components at ambient temperatures and pressure☆

Qianqing Liang *,Yufeng Hu ,Wenjia Yue

1 School of Chemistry Engineering,Inner Mongolia University of Science&Technology,Baotou 014010,China

2 State Key Laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

3 China ShenHua Coal to Liquid and Chemical Co.,Ltd.,Beijing 100011,China

Keywords:Conductivity Young's rule Semi-ideal solution theory Binary system Ternary system

ABSTRACT This work provides a method to explore the transportproperty ofthe electrolyte aqueous solutions with one or two ionic liquids,especially focus on their electrical conductivity.The conductivities were measured for the ternary systems NaCl–[C6mim][Cl](1-hexyl-3-methylimidazolium chloride)–H2O,[C6mim][BF4]–[C6mim][Cl]–H2O,NaNO3–[C6mim][BF4](1-hexyl-3-methylimidazolium tetra fluoroborate)–H2O,and[C4mim][BF4](1-butyl-3-methylimidazolium tetra fluoroborate)–[C6mim][BF4]–H2O,and their binary subsystems NaNO3–H2O,NaCl–H2O,[C6mim][BF4]–H2O,[C6mim][Cl]–H2O,and[C4mim][BF4]–H2O,respectively.The conductivities of the ternary systems were also determined using generalized Young's rule and semi-ideal solution theory in terms of the data of their binary solutions.The comparison showed that the two simple equations provide good predictions for conductivity of mixed electrolyte solutions and the mixed ionic liquid solutions based on the conductivity of their binary subsystems.

1.Introduction

The transport properties of mixed aqueous solutions have always attracted considerable interestdue to extensive applications in many fields such as chemistry and chemical engineering,separation process,wastewater treatment,pollution control,and oil recovery.Electrical conductivity is admittedly one of the principal transport properties of aqueous electrolyte systems not only for its intrinsic interest but also for technical and industrial applications such as batteries and plating[1].

A number of groups have reported the physical properties of binary electrolyte solutions[2–6].However,it is still a considerable measurement work for the properties of the multicomponent solutions using the conventional testing method.Therefore,it is practically important to develop an efficient method to obtain more useful data of the multicomponent solutions according to the available information on the binary solutions[7–10].Several attempts have been made using the Young's rule[11,12]and the semi-ideal solution theory[13,14]to obtain good predictions for the thermodynamic properties of the mixed electrolytes solutions from their binary subsystems[15,16].Hu et al.have successfully verified and extended the applicability of the generalized Young's rule and the semi-ideal solution theory to the conductivity of mixed electrolytes solutions[17,18].For example,the conductivities of the ternary systems NaCl–LaCl3–H2O can be predicted very well with the conductivities of its binary subsystems Na Cl–H2O and LaCl3–H2O at 298.15 K[17,18].The measurementofthe conductivities[1]were also made forthe mixed electrolyte solution ternary systems Y(NO3)3–Ce(NO3)3–H2O,Y(NO3)3–Nd(NO3)3–H2O,and Ce(NO3)3–Nd(NO3)3–H2O,Y(NO3)3–La(NO3)3–H2O,La(NO3)3–Ce(NO3)3–H2O,and La(NO3)–Nd(NO3)3–H2O,and their binary subsystems Y(NO3)3–H2O,Ce(NO3)3–H2O,and Nd(NO3)3–H2O and La(NO3)3–H2O,at(293.15,298.15 and 308.15)K and up to Imax≤ 24.4 mol·kg-1.With the equations ofthe generalized Young's rule and the semi-idealsolution theory,the predictions of conductivities for ternary systems based on the data of their binary subsystems are in good agreement with the measured values.

Ionic liquids(ILs)have been recognized as noveldesignable solvents which can be tuned/controlled by tailoring their cationic and anionic structures to optimize their physicochemical properties[1].Thus,they are environmentally benign and non- flammable,and pose high thermal stability and a high solvation capacity[19,20].These unique features suggest their potential application in a wide variety of industrial and chemical processes,such as absorption media in gas absorption,heat transfer fluids and working fluids in electrochemical processes[21–23].

In all these applications,the utilization of aqueous solutions of ILs,especially cognition of the basic properties for these systems,will be inevitable in a practical approach or in the design of industry process.One of the basic properties of interest for electrochemical applications is electrical conductivity.Moreover,the presence of water in ILs can dramatically affect their physicochemical properties[24–26].Therefore,several groups[27–34]have studied the physical properties of aqueous solutions of ILs for the binary systems(water–IL).Up to now,the basic properties of aqueous solutions of IL mixtures are important not only for industrial applications,but also for the verification of the electrolyte theories although few measurements have been made on the electrical conductivities for ternary systems such as the aqueous solutions of IL mixtures and the mixed electrolyte solutions with IL.

Therefore,the experimental measurements of the conductivities of ternary systems NaCl–[C6mim][Cl]–H2O,[C6mim][BF4]–[C6mim][Cl]–H2O,NaNO3–[C6mim][BF4]–H2O,and[C4mim][BF4]–[C6mim][BF4]–H2O,and their binary subsystems NaNO3–H2O,NaCl–H2O,[C6mim][BF4]–H2O,[C6mim][Cl]–H2O,and[C4mim][BF4]–H2O were presented at room temperature,respectively.The above-mentioned predictive equations including generalized Young's rule and semi-ideal solution theory developed for the properties of mixed electrolyte solutions were extended to conductivities of mixed IL solutions and the mixed electrolyte solutions containing IL with lower concentration.On the otherhand,the measured conductivities ofthe above ternary systems and their binary subsystems were also used to test the generalized Young's rule and the semi-idealsolution theory for electricalconductivity of multicomponent solutions containing ILs and electrolytes.

2.Experimental

Deionized water was distilled in a quartz still,and its conductivity was 0.8–1.2 × 10-6S·cm-1.All chemicals used in this study were of reagent grade with the claimed purity＞99%.N-methylimidazole,n-C4H9Cl,n-C6H13Cl,and NaBF4were supplied by Shanghai Jiacheng ChemicalCo.,Ltd.These chemicals were re fined by fractionaldistillation[35].NaCl and NaNO3were dried under vacuum over CaCl2for 7 days at 423 K prior to their use[16].All the ionic liquids present in this work were prepared using well-established procedures[36–38].The chloride salts were prepared by reacting N-methylimidazole with RCl,where,R was n-C4H9or n-C6H13The products were purified by repeated extractions of the remaining starting materials with ethyl acetate.After the last extraction,the remaining ethylacetate was removed at 343.15 K under vacuum[39].The resulting[1-alkyl-mim][Cl]([C4mim][Cl]and[C6mim][Cl])were dried at 343.15 K under vacuum for 6 days.[C4mim][Cl]and NaBF4were dissolved in acetone separately with equimolar amounts.Then,the two solutions gradually formed a mixture with stirring.The precipitated sodium chloride was removed from the liquid by filtration.The excess acetone was evaporated away and crude product[C4mim][BF4]was dried in vacuum for 48 h.The obtained[C4mim][Cl]was white crystals at room temperature,and[C6mim][Cl]and[C6mim][BF4]were liquid at room temperature.After purification,the ILs were dried under vacuum over CaCl2for several days at 70°C and were then further dried with 0.3 nm molecular sieves for several days immediately before use.The water content after drying,measured by Karl Fisher titration,was within 0.012%(by mass).

All the samples were prepared by syringing weighed amounts of the pure liquids into stoppered bottles in a glove box.The binary aqueous solutionsofNaNO3,NaCl,[C6mim][BF4],[C6mim][Cl]and[C4mim][BF4]were prepared by mass from double-distilled deionized waterand the ILs using a Sartorius CT225D balance with a precision of±5×10-5g.The ternary systems were prepared by mixing the binary solutions with known mass concentration with uncertainty of±5 × 10-5mol·kg-1.All solutions prepared in a glass flask were placed into stoppered bottles and stirred for 2 h.The measurements were made one week after preparation to assure complete dissolution and aggregation.Due to the low solubility of the ionic liquids,all the prepared samples was in a narrow concentration range of 0.0004–0.3000 mol·kg-1.

The conductivities of prepared samples were measured with a METLER TOLEDO SevenEasyTM conductivity meter(cell constant=0.57 cm-1)calibrated with standard aqueous potassium chloride solutions[18].The temperature of the cell was kept constant to within±0.005 K by circulating thermostated liquid and the temperature was measured with a calibrated calorimeter thermometer(±0.006 K).

3.Predictive Equations for Conductivity

3.1.Generalized Young's rule for conductivity

The generalized Young's rule for the conductivity of the ternary electrolyte solutions[40]can be expressed as

with yi=Ii/(I1+I2),where I is ionic strength.σ,σ1and σ2are the conductivities of the ternary solution M1X1-M2X2-H2O and its binary subsystems MiXi-H2O(i=1 or 2)of equal ionic strength.

3.2.Semi-ideal solution theory for conductivity

The semi-ideal solution theory for the conductivity of the ternary electrolyte solutions can be expressed as[18,41,42]

where σiis the conductivity of the binary solution of salt i and water,MiXi-H2O(i=1 or 2),having the same water activity as that of the ternary solution M1X1-M2X2-H2O.ziis the ratio of the mole fraction of i?H2O(Li)(i=1 or 2)in the ternary ideal solution 1 ?H2O(L1)-2 ?H2O(L2)-H2O to the mole fraction of i?H2O(Li)in the binary ideal solution i?H2O(Li)-H2O i.e.,

with

and

3.3.Comparisons with the experimental data

The measured conductivities of ternary system are used to test Eqs.(1)and(2),and the test procedure[43]is briefly illustrated in the following schematic diagram in Fig.1.

The measured conductivities of the binary solutions are substituted by the following polynomial equations:

Fig.1.Test procedure diagram of comparisons with the experimental data.

Table 1 The parameters for the binary systems MiXi-H2O at different temperatures

The values of ciare determined using the water activities of the binary solutions.The values ofare inserted from Eq.(6)into Eq.(2)to yield the predictions for the ternary solutions of given mi(i=1 and 2),which are then compared with the corresponding experimental values.

The differences between predicted and measured conductivities are defined by

The average relative differences between the predicted and measured conductivities over the entire experimental composition range of the ternary solution are defined by

with i=1 and 2.

Table 2 Measured conductivities for the binary systems MiXi-H2O

Table 3 Measured conductivities for the binary systems MiXi-H2O

Table 4 Comparisons of measured and predicted conductivities for aqueous solution ofelectrolyte mixture NaNO3(B)–[C6mim][BF4](C)–H2O at 303.15 K.

4.Results and Discussions

4.1.Comparisons ofthe measured conductivitieswith the values reported in the literature

Table 2 compares the measured conductivities of the binary solutions NaCl–H2O at 298.15 K with the reported values[2].It could be seen that the agreements are good.The measured conductivities of the binary solutions[C6mim][BF4]–H2O at 303.15 K,and[C6mim][Cl]–H2O,[C4mim][BF4]–H2O,[C6mim][BF4]–H2O at 298.15 K are shown in Table 3,respectively.

4.2.Verifications of equations

Table 4 shows the comparisons of predicted and measured conductivities for aqueous solutions of(1:1+1:1),electrolyte mixtures of NaNO3–[C6mim][BF4]–H2O.It could be seen that the agreements are good for all the examined cases where mixing occurs at constant ionic strength and at constant water activity.The average relative differences between the predicted and the measured conductivities are δσ,eq1=2.2 × 10-2and δσ,eq2=2.0 × 10-2for NaNO3–[C6mim][BF4]–H2O at 303.15 K.

Tables 5,6 and 7 compare predicted and measured conductivities for the ternary solutions[C6mim][Cl]–NaCl–H2O,[C4mim][BF4]–[C6mim][BF4]–H2O and[C6mim][Cl]–[C6mim][BF4]–H2O at298.15 K.The average relative differences between the predicted and measured conductivities,δσ,eq1,are 2.55 × 10-2for[C6mim][Cl]–NaCl–H2O,2.54 × 10-2for[C4mim][BF4]–[C6mim][BF4]–H2O,and 1.50 × 10-2for[C6mim][Cl]–[C6mim][BF4]–H2O,respectively,which indicate that the predictions are almost in agreement with the measured values.Meanwhile,the average relative differences between the predicted and measured conductivities,δσ,eq2,are 2.65 × 10-2for[C6mim][Cl]–NaCl–H2O,2.70 × 10-2for[C4mim][BF4]–[C6mim][BF4]–H2O and 1.40 × 10-2for[C6mim][Cl]–[C6mim][BF4]–H2O,respectively,indicating that the predictions are almost in agreement with the measured values,which are also consistent with the predication results of Eq.(1).It is noted that the prediction of Eq.(1)is simpler since it only requires the conductivities of the binary subsystems MiXi-H2O(i=1 or 2)of equal ionic strength.The prediction of Eq.(2)requires both the conductivity and water activity of the binary subsystems.For all the examined cases,the above results indicated that,as for predicted results of[C6mim][Cl]–NaCl–H2O and[C4mim][BF4]–[C6mim][BF4]–H2O,Eqs.(1)and(2)are especially in good agreementwith the measured values for the ternary system of[C6mim][Cl]–[C6mim][BF4]–H2O at 298.15 K.

Table 5 Comparisons of measured and predicted conductivities for aqueous solution of electrolyte mixture[C6mim][Cl](B)–NaCl(C)–H2O at 298.15 K.

5.Conclusions

The conductivities were measured for the ternary systems NaNO3–[C6mim][BF4]–H2O at 303.15 K,and[C6mim][Cl]–Na Cl–H2O,[C4mim][BF4]–[C6mim][BF4]–H2O and[C6mim][Cl]–[C6mim][BF4]at 298.15 K,and their binary subsystems NaNO3–H2O,[C6mim][BF4]–H2O at 303.15 K,and NaCl–H2O,[C6mim][BF4]–H2O,[C6mim][Cl]–H2O,and[C4mim][BF4]–H2O at 298.15 K.The measured conductivities for the ternary solutions were used to verify the applicability of the generalized Young's rule and the semi-ideal solution theory forthe conductivity of the special ternary system in this work.The predicted results were almost in agreement with the measured values,indicating that the generalized Young's rule and the semiideal solution theory were suitable to predict the conductivities of the ternary electrolyte solutions from the data of their binary subsystems,especially for the ternary system of[C6mim][Cl]–[C6mim][BF4]–H2O.

Table 6 Comparisons of measured and predicted conductivities for aqueous solution ofelectrolyte mixture[C4mim][BF4](B)–[C6mim][BF4](C)–H2O at 298.15 K.

Table 7 Comparisons of measured and predicted conductivities for aqueous solution of electrolyte mixture[C6mim][Cl](B)–[C6mim][BF4](C)–H2O at 298.15 K.