Determination of physical properties for the mixtures of[BMIM]Cl with different organic solvents☆

Hina Saba ,Xinjun Zhu ,2,Ye Chen ,Yumei Zhang ,*

1 State Key Laboratory for Modi fi cation of Chemical Fibers and Polymer Materials,Donghua University,Shanghai 201620,China

2 Department of Material and Engineering,Luoyang Institute of Science and Technology,Luoyang 471023,China

Keywords:Ionic liquid Viscosity Conductivity Refractive index Density Binary mixtures Organic solvents

A B S T R A C T Physical properties including refractive index,density,viscosity and conductivity for binary mixtures of 1-butyl-3-methyl imidazolium chloride([BMIM]Cl)and different organic solvents at 298.15 Khave been investigated.Excess molar volumes have been calculated and obtained data has been fitted by the Redlich–Kister equation.The density and refractive index were found to increase with increasing concentration of[BMIM]Cl,however,exceptions do exist as in the case of dimethyl sulfoxide(DMSO)/[BMIM]Cl.For DMSO/[BMIM]Cl,the density decreases with increasing concentration.The addition of different organic solvents was able to disrupt the interactions within mixtures,leading to free mobility of ions.The free mobility of ions has been found to enhance conductivity and decrease viscosity to varying extents in all mixtures studied.It has been observed that solubility parameters,dielectric constants and composition of the solvents used play a vital role in determining the resultant properties.The data obtained will play an important role in understanding the effect of the addition of organic solvents in ILs to enhance their applicability.

1.Introduction

Now adays,many interesting air-stable room temperature ionic liquids(RTILs)have been developed that are being increasingly employed as replacements for organic solvents in basic research and several applications[1].These green solvents have not only led us towards sustainable chemistry but also provided a great range of properties to work with as the properties of ionic liquids(ILs)could be easily tailored by altering the choice of anion and cation.Various aspects of these novel solvents along with their utility within different technological processes have been widely reviewed by different scientists and researchers[2–9].Their diverse applicability in certain fields has been limited due to their high viscosity which hinders their performance within reaction media.The problem could be solved by heating the ILs which will lead to low viscosity and hence high mobility of ions.But this is not a suitable tactic to overcome the limitation as usually pure ILs have de composition temperature lower(not higher than 400 K)than their boiling point[10].A much more effective alternative for decreasing viscosity and enhancing its electrical conductivity is to dilute ionic liquids by a co-solvent.Properties of the binary mixtures of ILs with water[11–16]and certain organic solvents[17–25]have already been reported in literatures.Heintz et al.have studied the densities and viscosities of pyridinium based IL and methanol mixtures at different temperatures and suggested that methanol was able to decrease the viscosity of the mixture due to its ability to fit within empty spaces of the bulky IL and by disrupting strong coulombic interactions between ions[17].Later,Gonzáles et al.illustrated same type of mechanism while investigating mixtures of IL and different organic solvents at different temperatures[18].How ever,Wang et al.suggested an additional aspect of these studies and elucidated that the change in densities and viscosities is more dependent on the composition of the organic solvent used than its identity[19].This finding was further confirmed by Arce et al.[20].Certain scientists have considered the dielectric constant of the organic solvent as the driving force for the resultant properties.Comminges et al.[21]and some other researchers[22–25]confirmed that the addition of a cosolvent will decrease viscosity exponentially with the solvent mole fraction and as a result ion mobility increases more than ten times for a given IL/solvent concentration.The resultant properties of these binary mixtures will be dependent on the choice of co-solvent and IL being used.Alkyl chain length of cation,choice of anion and dielectric constant of the co-solvent play a vital role in determining the underlying interactions and thereafter,resultant properties of these mixtures.All these studies have helped us to further disclose various aspects of mixtures of IL with different organic solvents.

To date several review s related to the binary mixtures of different ILs have been reported.The scope of these reviews was mostly related to the properties of pure ionic liquids and their mixtures,including gas and liquid solubility in common organic solvents[26],recent applications of ionic liquids along with some results of measurements of liquid–liquid equilibrium and partition coefficients[27],and the ability of ionic liquids to affect the mechanistic aspects of some organic reactions[28].

1-Butyl-3-methyl imidazolium chloride([BMIM]Cl)has been discovered as a neoteric solvent for cellulose processing[29],how ever,the high viscosity of the cellulose solution has brought some difficulty to the engineering.Inspired by the results that the viscosity could be decreased by adding organic solvent to IL,the physical properties besides viscosities of the mixture of IL were studied in our lab.Since all the possible mixtures of IL/organic solvent systems could not be studied within same time period,it is necessary to make measurements on certain discriminating systems in order to provide information that can be used for further applications.Current studies will play avital role in the development of methods for designing new processes and improving energy conditions,and in the determination of certain environmental hazards.

2.Experimental

2.1.Materials

Butyl chloride(＞99%)and 1-methyl imidazole(99.7%)were purchased from Shanghai Chemical Reagents Company.Doubly distilled water was used in all experiments.Acetone,acetonitrile,DMSOand ethanol have been used as an organic solvent system.All organic solvents used were purchased from Shanghai Chemical Reagents Company and were of HPLC-grade.Experimental and theoretical data for refractive indices(n),density(ρ)and viscosity(η)of the pure components have been shown in Table 1.

2.1.1.Synthesis of[BMIM]Cl

Synthesis of[BMIM]Cl was based on the reported procedure[30].Equal volumes of chlorobutane and 1-methylimmidazole were added to flaks and reflux condensation was carried out for 24–47 h at 70 °C with constant stirring.Product was separated from the reaction media by means of ethyl acetate.The process of purification by ethyl acetate was repeated at least three times.The product was further heated under vacuum at 70°C in order to remove any organic solvent.The[BMIM]Cl was yellowish in color and was crystalline at room temperature.The purity of[BMIM]Cl was verified in terms of nuclear magnetic resonance(NMR)analysis(＞99%).The chlorine mass fraction was smaller than 1.4×10?4.The water mass fraction of IL(＜0.19%)was determined by the Karl Fisher titration(ZSD-2 KF Cany Precision Instruments Co,Ltd.).

2.2.Instrumentation

A 2W AJ Abbe refractometer was used for the measurements of the refractive index.Calibration of the instrument was done by means of deionized water.The uncertainty in the refractive index was found to be±1.0×10?4.The temperature was maintained at(298.15±0.01)K with an external temperature controller(HX-1008 Low constant temperature circulating water baths).

The conductivity measurements were performed with a DDSJ-308A conductometer(Shanghai Optical Instrument Factory,cell constant=1.0 cm?1)with an uncertainty of 0.5%.The temperature was maintained at(298.15±0.01)K with a DC-2006 low-temperature thermostat(Shanghai Hengping Instrument Factory).

The viscosity was determined by an Ubbelohde viscometer(inner diameter=1.5 mm).The temperature of the sample was maintained at(298.15±0.01)K with an external temperature controller(SYP Electro-thermostatic water cabinet).The measurements were carried out three times for each sample.The uncertainty was found to be 2.0 ± 10?3m Pa·s.As[BMIM]Cl was a highly viscous compound at higher concentrations,its viscosity was not determined due to the limitation of the apparatus used.The ionic liquid was dried in a vacuum oven at 70°C for 24 h before carrying out the studies in order to avoid the water interference with the mixtures.Moreover,as[BMIM]Cl should be a super cooled liquid at room temperature due to its melting point at 70°C,it was warmed for 10 min before every reading in order to obtain more precise results.

Before measuring the densities,a DA-130N Density Specific Gravity Meter was calibrated with pure water at(298.15±0.01)K.The uncertainty in density was estimated to be 5×10?3g·cm?3.All measurements were repeated three times and then average values were calculated.

3.Results and Discussion

3.1.Density and refractive index

The effect of increasing molar concentration(M)and mole fraction(x)of IL on density of the mixtures is illustrated in Figs.1 and 2,respectively.Both experimental and calculated values are shown in Table 2.The calculated values are derived from an ideal molar volume equation as follow s:

It can be transformed as:

w here x is the mole fraction,V is the volume used,d is the density,Vmis the molar volume,ρ is the density of mixtures,MW is the molecular w eight and φ is the volume fraction(subscripts 1,2 and m represent both components of mixture used and mixture,respectively).In the case of[BMIM]Cl,its density is recorded as 1.081 g·cm?3(it is quite in agreement with the value already being cited in literature[30]),which is less compared to other 1-butyl-3-imidazolium based ionic liquids due to the presence of a relatively less bulky anion i.e.Cl?[31,32].The density of the mixtures is found to follow the molar volume equation especially at lower concentrations.The density of the mixtures increases with increasing concentration with the exception of[BMIM]Cl/DMSO where the addition of[BMIM]Cl leads to a decrease in density.Except for the DMSO/[BMIM]Cl system,the density is found to increase more gradually with increasing mole fractions as compared to molar concentration.These results are quite in agreement with the results of our group published previously for aqueous mixtures of ILs[16]aswell as with several studies acquired for IL/organic solvent mixtures[32–34].

Table 1Purity grades with experimental and theoretical values for properties of the pure components and[BMIM]Cl

Fig.1.Relationship between density and mole fraction(x)of IL in different organic solvents at 298.15 K.*represents the calculated data from Eq.(4).

Fig.2.Relationship between density and molar concentration(M)of[BMIM]Cl in different organic solvents at 298.15 K.▲ DMSO;● ethanol;? acetone;■ acetonitrile.

In order to elucidate the extent of interactions within solute–solvent molecules,the excess molar volumes(VE)are calculated by Eq.(3):

where ρ depicts the density of mixtures,and x and M represent the mole fractions and molar masses,respectively.Subscripts 1 and 2 correspond to pure ionic liquid and organic solvents,respectively.All values of excess molar volume have been fitted by using the Redlich–Kister polynomial equation:

where Aiis the adjustable parameter.The data of excess molar volume and coefficients of the Redlich–Kister equation are given in Tables 2 and 3.The standard deviations(σ)have been calculated by using Eq.(5):

w here n is the number of experimental data and p is the number of coefficients of the Redlich–Kister equation.

For all the systems(except IL/DMSO),the VEis negative almost over all concentration ranges.These results are similar to several studies reported previously[35–37].The negative VEvalues clearly indicate the existence of interactions and certain associations among the molecules of IL and organic solvents.Fig.3 represents the relationship between VEand the mole fraction of acetonitrile.How ever,the IL/DMSO systemhas shown quite anomalous behavior by showing positive VEvalues.Positive values of molar excess volumes clearly indicate the existence of hydrogen bonding as a source of intermolecular interactions.For such systems,contraction in the volume on mixing is governed by the component which has smaller free volume making use of larger free volume of the other component molecules.Moreover,the dielectric constant for DMSO is larger than other solvents(see Table 4).It means that the electrostatic interaction between cation and anion is weaker due to the larger dipole moment of DMSO and hence leads to the ionization of IL and free mobility of ions in DMSO solvent,making the VEvalues positive(see Table 3).

Table 2Coefficients of the Redlich Kister equation for V E of[BMIM]Cl and different organic solvents at 298.15 K

Table 3Experimental and calculated densities(ρ,ρcal.),and excess molar volumes(V E)for mixtures of[BMIM]Cl with different organic solvents at 298.15 K

Fig.3.Plot of excess molar volumes against compositions for the mixture of[BMIM]Cl with acetonitrile at 298.15 K.Solid line represents the Redlich–Kister fit.

As VEis the resultant of contributions from some opposing effects.These effects could be chemical,physical or structural.The positive VErelates to effects rendered by physical or non-specific interactionsbetween the species present in the mixture.How ever,the negative values of VEare due to the chemical intermolecular interactions which lead to volume decrease,and include charge transfer type forces and complex forming interactions.Although the[BMIM]Cl/DMSO does not behave like other ionic liquid systems reported in literature how ever this behavior could be related to the various solvent mixtures[38,39].It could be suggested that due to higher density of DMSO as compared to IL,the mixture is unable to form certain specific interactions.The only interactions dwelling in these mixtures are the hydrogen bonding and weak dipole–dipole interactions.As the concentration of IL is increased within the mixture,it is easy for the IL molecules to reside within a denser network of organic molecules by simply forming hydrogen bonding.

Table 4Hildebrand,Hansen solubility parameters and dielectric constants for the organic solvents at 298.15 K

It has also been observed that the resultant properties are more characteristic of concentrations being used as compared to the selection of the organic solvents itself(as shown in Fig.2).With the only exception of IL+DMSO mixtures,the density data for all mixtures seem to overlap each other in distinctive of their nature and extent of polarity.However,for IL+DMSO mixtures the interactions are observed to be the driving factor more than its concentration.This finding is in agreement with the work presented by Iulian and Ciocirlan[34].

Fig.4.Relationship between refractive index and molefraction(x)of[BMIM]Clin different organic solvents at 298.15 K.▲ DMSO;● ethanol;? acetone;■ acetonitrile.

These interactions of DMSO could be attributed to its highest dielectric constant as shown in Table 4.Moreover,the solubility parameters for DMSO(Table 4)show high hydrogen bonding,polarity and dispersion content as compared to others.Only ethanol has more hydrogen bonding content,but it is masked by its polarity and dispersion values which are far less than DMSO.The solubility parameters and dielectric constant of DMSO clearly make it best suited for mixing IL,leading to hydrogen bonded interactions.

The refractive index data have been determined for the binary mixtures of[BMIM]Cl and organic solvents,i.e.,ethanol,acetone,acetonitrile and DMSO.Refractive index is mostly used to have an insight about the polarizability/dipolarity of the system[40].The relationship of the refractive index with mole fraction has shown a gradual trend with increasing mole fraction(Fig.4)for all the IL/organic solvent mixtures as suggested by many studies reported previously in literature[41,42].Relevant data for the all mixtures are illustrated in Table 5.

The linear increase of refractive index with increasing concentration shows the maximum interaction of light within a solution and hence less free space within the molecules.Therefore,the occurrence of intermolecular interactions and aggregate formation is quite evident.As a result,it could be evaluated that the diverse inter-and intra-molecular interactions between/within the molecules of IL and organic solvent will lead to aggregate formation which would increase the refractive index.For the IL/acetone system,the refractive index could not be determined at low concentrations of IL.At low concentrations of IL,the solution is not found clear due to its poor miscibility of IL.As dielectric constant is believed to be the measure of miscibility,the behavior of these solvents could be related to their extent of polarity and interaction within mixtures as follows:DMSO＞acetonitrile＞ethanol＞acetone.

Generally,polar and dipolar solvents of high dielectric constants show better miscibility with ILs(Table 4),indicating that polar solvents have stronger molecular interactions with ILs.However,certain concentrations of acetone have resulted in phase separation.But at higher concentrations of IL or acetone,no phase separation has been observed.The anomalous behavior of the acetone/IL system among all other systems could be suggested due to its low est dielectric constant and low est value of solubility(Table 4)among the other solvents used.The trendof the refractive index of these solvents with[BMIM]Cl is found to be:DMSO＞acetone～ethanol＞acetonitrile.Although,the dielectric constant of acetonitrile is more as compared to acetone and ethanol,its hydrogen bonding content is the least among all(as shown in Table 4),leading to the least interactions within mixtures and hence yielding low refractive indices as compared to other systems being investigated.

Table 5Conductivity(κ)and viscosity(η)data for mixtures of[BMIM]Cl with different organic solvents at 298.15 K

Fig.5.Relationship between conductivity and mole fraction(x)of[BMIM]Cl in different organic solvents at 298.15 K.▲ DMSO;● ethanol;? acetone;■ acetonitrile.

3.2.Conductivity and viscosity

Conductivity and viscosity for the binary mixtures of[BMIM]Cl and other organic solvents such as ethanol,acetone,acetonitrile and DMSO are determined.The conductivity of the mixture is found to be increased in a region with high organic solvent concentration and a steady decrease is observed in a high IL concentration region.This trend is similar to the studies conducted before in this regard[43,44].An almost similar trend is observed for conductivity with increasing mole fraction(Fig.5)for all the IL/organic solvent binary mixtures.The increase of conductivity in the organic-rich region is due to free mobility of the ions with less viscosity.But as the concentration of ionic liquid increases within the mixture,less free space will be available and hence ion mobility is restricted.This restricted ion mobility is due to the presence of more bulky ions of IL as well as the interactions between the IL and organic solvent molecules,leading to a gradual decrease in the conductivity of the mixture.Related data are shown in Table 6.

The mobility of ions could be related to over all resultant conductivity as follows[45]:

where nirepresents the number of charged moieties of species i,qidepicts the charge and μishows the ion mobility.Thus,it can be elucidated that increasing concentration of IL will lead to strong interactions and hinder the ion mobility.The association of ILs results mainly from the hydrogen-bond interactions between the hydrogen atoms of the cations([BMIM]+)and the anions(Cl?),the possible π–π interactions between the imidazolium cations,the aliphatic interaction and columbic interactions within the system.When the IL/organic mixtures are compared,the conductivity pattern is found to be:acetonitrile＞ethanol＞DMSO＞acetone.This finding could be attributed to the formation of the weakest hydrogen interactions by acetonitrile(as predicted by the Hansen solubility parameters in Table 4),resulting in free mobility of ions and less aggregate formation.

How ever,the viscosity increases gradually with increasing concentration of ionic liquid as shown in Fig.6.Similar behavior has been observed previously for different mixtures of ILs with organic solvents[46,47].At higher concentrations,the viscosity could not be determined due to a limitation of the instrument used.Due to the occurrence of diverse interactions and aggregation within pure ILs,their addition in a certain mixture will lead to enhance viscosities and therefore hinder mobilities of ions.

Table 6Refractive index(n)for mixtures of[BMIM]Cl with different organic solvents at 298.15 K

Fig.6.Relationship between viscosity and molefraction(x)of[BMIM]Cl in different organic solvents at 298.15 K.▲ DMSO;● ethanol;? acetone;■ acetonitrile.

Both transport properties could be related by implying Walden's rule as follow s[48]:

w here κmrepresents the molar conductivity,η is the viscosity and K is constant.As shown in Fig.7,the product is nearly constant for a certain range of concentrations for almost all mixtures.However,it increases for mixtures of[BMIM]Cl with ethanol,acetone and DMSO,but it tends to decrease for mixtures of IL+acetonitrile.These results are quite in agreement with previously reported results[49].

Fig.7.Product of molar conductivity by viscosity and molar concentration for binary mixtures of[BMIM]Cl+organic solvents.▲ DMSO;● ethanol;? acetone;■ acetonitrile.

4.Conclusions

Current studies take into account four binary mixtures of[BMIM]Cl with different organic solvents.The organic solvents have diverse extent of polarity or dielectric constants and solubility values,leading to varying miscibility with IL.DMSO is found to have maximum miscibility with IL and hence shown exceptional behavior in each case studied.This mixture shows anomalous behavior due to its ability to form more physical interactions.On the contrary,mixtures of acetone/IL indicate immiscibility with the IL at certain concentrations due to its low est dielectric constant among all the solvents used.Apart from these two exceptional cases,acetonitrile,ethanol and acetonitrile have shown moderate miscibility.Except for the DMSO/[BMIM]Cl mixture,the density increases with increasing concentration.Refractive index shows ideal behavior for all the mixtures but its refractive index could not be determined over the whole concentration range due to immiscibility of acetone at certain concentrations.The conductivity of the mixtures is found to increase in the systems with high concentration of organic solvent as compared to IL and vice versa.A gradual increase in viscosity is observed by the addition of IL within the mixture.Different organic solvents show a distinct extent of interactions which clearly affects the resultant properties.The anomalous behaviors could be attributed to either aggregate formation within ILs or difference of dielectric constants of organic solvents or the extent of intermolecular interactions between cations and anions.