Densities and surface tensions of ionic liquids/sulfuric acid binary mixtures☆

Tao Zhang,Jing Hu,Shengwei Tang*

Multi-phase Mass Transfer and Reaction Engineering Lab,School of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:Ionic liquid Sulfuric acid Density Surface tension Excess property

ABSTRACT The densities and surface tensions of[Bmim][TFO]/H2SO4,[Hmim][TFO]/H2SO4and[Omim][TFO]/H2SO4binary mixtures were measured by pycnometer and Wilhelmy plate method respectively.The results show that densities and surface tensions of the mixtures decreased monotonously with increasing temperatures and increasing ionic liquid(IL)molar fraction.IL with longer alkyl side-chain length brings a lower density and a smaller surface tension to the ILs/H2SO4binary mixtures.The densities and surface tensions of the mixtures are fitted well by Jouyban–Acree(JAM)model and LWW model respectively.Redlich?Kister(R–K)equation and modified Redlich?Kister(R–K)equation describe the excess molar volumes and excess surface tensions of the mixtures well respectively.Adding a small amount of ILs(xIL＜0.1)into sulfuric acid brings an obvious decrease to the density and the surface tension.The results imply that the densities and surface tensions of ILs/H2SO4binary mixtures can be modulated by changing the IL dosage or tailoring the IL structure.

1.Introduction

Acid catalysts are widely used in chemical reactions to produce a lot of valuable intermediates or products.To date,mineral acids,such as H2SO4,HF and HCl,are still important liquid acid catalysts in the industrial processes.However,mineral acids are not considered as environmentally benign catalysts for its serious corrosiveness and costly regeneration.In the recent decades,a lot of research work focuses on the exploration of innovative catalysts to replace mineral acids.

Recently,ionic liquids(ILs)arouse significant attentions for their unique physicochemical properties[1–3]and the acidic ionic liquids are thought to be potential alternative catalysts of mineral acids.The catalytic properties of acidic ILs were studied in many reactions,such as esterification[4],Friedel–Crafts alkylation[5]and oligomerization[6],etc.However,the expensive price impedes the industrial application of ILs.

Fortunately,researchers have observed that adding a small amount of specific ionic liquid into traditional mineral acid brings a notable improvement to the catalytic reaction and catalyst life.Wasserscheid et al.[7]usedILs/H2SO4mixture to catalyze the Friedel–Crafts alkylation of benzene with 1-decene.When the molar fraction of ILs was only 2.2%,the yield of monoalkyl benzene was increased more than 90%compared with the pure sulfuric acid catalyst.Tang[8]found that the imidazolium-based acidic ILs could be used as additives of sulfuric acid(H2SO4)or trifluoromethanesulfonic acid(TFOH)to improve the is obutane/1-butene alkylation reaction.Zhang's group systematically investigated the catalytic properties of the binary mixtures of strong acids and ILs additives in isobutane alkylation[9–12].The results showed that the addition of ionic liquid in mineral acids led to an increased yield of desired C8alkylate,a suppression of side reactions and a prolonged catalyst life.Tang et al.[13,14]also successfully used acidic ILs to modulate the adsorption reactants ratio of alkane/ole fin on solid acid catalyst MCM-22 or MCM-36,so as to improve the C4alkylation reaction catalyzed by solid acid catalysts.

Although synergistic catalysis effect of acidic ionic liquid coupled with mineral acid has been observed,the synergistic mechanism has not been elucidated clearly.In the alkylation process of is obutane and ole fins,it is believed that the addition of ionic liquid brings to a tunable acidity of the catalyst,an improved reactant solubility and an enhanced mass transfer between the catalyst phase and hydrocarbon phase.In order to verify these hypotheses,we measured the Hammett acidity functions H0of the binary mixture of trifluoromethanesulfonic acid(TFOH)and ionic liquids[15].The ideal H0value for the C4alkylation process is about?8.1to?12.7[16].We found that H0values of the binary mixtures can be effectively controlled by tailoring the structure of IL or tuning the dosage of IL.We also measured the solubility of isobutanein differentionic liquids[17].Dissolution properties of isobutane in acid catalyst phase can be improved by the addition of ionic liquid.The increased isobutane solubility is beneficial to the objective reaction.

The mass transfer between the catalyst phase and the hydrocarbon phase plays a crucial role in the reaction[18].However,in the C4alkylation process with sulfuric acid as catalyst,the big density difference between the two phases hinders the interphase mixing.The surface tension of the catalyst phase also has an important Influence on the dispersibility of the catalyst phase.Both density and surface tension are important physicochemical properties for the application of the novel composite catalyst of mineral acid coupled with ionic liquids.However,little information about the density and surface tension of ILs/H2SO4can be found from the literatures.

Ongoing our work,we measured the densities and surface tensions of ILs/H2SO4binary mixtures to provide the necessary fundamental properties for the further industrial application of the composite catalyst.The results are also helpful to elucidate the effect of the addition of ionic liquid on the density and surface tension of the catalyst phase.And then the Influence of ionic liquid addition on the interphase mass transfer can be analyzed qualitatively.

2.Experimental Section

2.1.Materials

Ionic liquids and chemicals used in this study are listed in Table 1.The ILs were dried over 24 h at～353 K and ～10?2Pa.The mass fractions of water in 1-buty-3-methylimidazolium trifluoromethanesul fonate([Bmim][TFO]),1-hexyl-3-methylimidazolium trifluoromethanesulfonate([Hmim][TFO])and1-octyl-3-methylimidazolium trifluorome thanesulfonate([Omim][TFO])were measured as 0.38%,0.34%and 0.28%respectively by Karl Fischer titration method.

Pure sulfuric acid(w=1.0)was prepared by mixing sulfuric acid(w=0.95–0.98)with fuming sulfuric acid according to the procedures described elsewhere[15].Binary mixtures of ILs/H2SO4with different molar fractions of ILs were prepared by simply mixing the prepared pure sulfuric acid with desired amount of dried ILs.The mass of ILs and sulfuric acid was weighed by a Mettler balance with a precision of 0.0001 g.Based on the mass of each component in samples and the accuracy of the balance,the standard uncertainty of molar fractions u(x)were less than 0.0001 in this study.

2.2.Apparatus and procedures

The densities of[Bmim][TFO]/H2SO4,[Hmim][TFO]/H2SO4and[Omim][TFO]/H2SO4with different ILs fractions were measured at the temperature range from 293.2 K to 313.2 K by a pycnometer method.The pycnometer(10 ml)was dried to constant weight in an oven at 383.2 K.The mass of the dry pycnometer was weighed at room temperature by a Mettler balance.The dry pycnometer full of water or samples were kept at the desired temperature for at least 15 min by a thermostatic water bath with precision of±0.1 K.Then the mass of the dry pycnometer full of water or samples was weighed.Three identical samples were measured to obtain the average density.The densities of the binary mixture of ILs/H2SO4

were obtained by the following equation:

where

mPthe mass of the dry pycnometer,g,

mMthe mass of the dry pycnometer full of ILs/H2SO4binary mixtures,g,

mWthe mass of the dry pycnometer full of water,g,

ρMthe density of ILs/H2SO4binary mixtures,g·cm?3,

ρWthe density of water,g·cm?3.

The surface tensions of the mixtures described above were measured in the temperature range from 293.2 K to 313.2 K by a Wilhelmy plate method(JK99C surface tensiometer,Shanghai Powereach Digital Technology Equipment Co.,Ltd).The beaker in the surface tensiometer was filled with 20–30 ml binary mixture and was sealed timely to prevent absorbing water.When the sample temperature was kept at the desired temperature,the surface tensions of the mixtures were measured through the platinum plate.The temperature was controlled by a thermostatic water bath and monitored by a thermal resistor of Pt100 with precision of±0.1 K.For the strong hygroscopicity of sulfuric acid,the sample chamber was kept dry by desiccant and the measurement was operated carefully.At least three identical samples were measured to obtain the average surface tension.The precision of surface tensiometer is 0.05 mN·m?1in the measuring range of 0–500 mN·m?1.

2.3.Uncertainty and the reliability of measurement

The combined standard uncertainty u of density and surface tension measurement could be obtained from the propagation of the system uncertainty and the random uncertainty[19,20]:

where

uSysthe system uncertainty of the measurement apparatus,g·cm?3or mN·m?1,

uRanthe random uncertainty of the measurement,g·cm?3or mN·m?1.

The system uncertainty of surface tension measurement depends on the precision of the apparatus,so uSys(γ)=0.05 mN·m?1.For the system uncertainty and random uncertainty of the density measurement,it could be calculated as follows:

Table 1 Specifications of chemicals used

where

uSys(ρ) the system uncertainty of the density measurement apparatus,g·cm?3,

uRan(ρ) the random uncertainty of the density measurement,

g·cm?3,

uSys(mP)the system uncertainty of the mass of the dry pycnometer,g,

uSys(mM)the system uncertainty of the mass of dry pycnometer full of ILs/H2SO4binary mixtures,g,

uSys(mW)the system uncertainty of the mass of dry pycnometer full

of water,g,

uRan(mP)the randomuncertainty of the mass of the dry pycnometer,g,

uRan(mM)the random uncertainty of the mass of dry pycnometer full of ILs/H2SO4binary mixtures,g,

uRan(mW)the random uncertainty of the mass of dry pycnometer full of water,g.

The precision of Mettlerbalanceis±0.0001 g.Therefore,uSys(mP)=uSys(mM)=uSys(mW)=0.0001 g.

Besides,the random uncertainty of density and surface tension measurement could be calculated as follows.

s the standard measurement deviation of density or surface tension,g·cm?3or mN·m?1,

xithe experimental value of density or surface tension,g·cm?3or mN·m?1,

xaveragethe average experimental value of density or surface tension,g·cm?3or mN·m?1,

n the number of parallel measurements.

Taking the random and system uncertainty into account,the combined standard uncertainty of density u(ρ)of the ILs/H2SO4mixtures was estimated to be 0.0002 g·cm?3.The combined standard uncertainty of surface tension u(γ)was estimated to be 0.19 mN·m?1.

In order to verify the reliability of the density measurement method and procedures,the densities of[Bmim][TFO],[Hmim][TFO],[Omim][TFO]and pure H2SO4(w=1.0)were measured and compared with the data from different literatures.The results are listed in Table 2.The results show that the experimental data agree with the data from literatures very well.All relative standard deviations(RSD)are less than 0.53%.

Surface tensions of[Hmim][TFO],[Omim][TFO],pure H2SO4(w=1.0),ethanol and water were also measured and compared with the data from literatures to verify the reliability of surface tension measurement.The results are also listed in Table 2 and the relative deviations are less than 1%.Overall,the results show that the measurement methods of density and surface tension are reliable.

3.Results and Discussion

3.1.Densities of ILs/H2SO4binary mixtures

The densities of[Bmim][TFO]/H2SO4,[Hmim][TFO]/H2SO4and[Omim][TFO]/H2SO4were measured at atmospheric pressure.The temperature ranged from 293.2 K to 313.2 K and the IL molar fraction ranged from 0 to 1(the results are listed in Table S1 in the supporting information.).In the process of measurement,ILs and H2SO4were miscible.

At293.2K,the densities of[Bmim][TFO],[Hmim][TFO],[Omim][TFO]and H2SO4are 1.3025 g·cm?3,1.2385 g·cm?3,1.1912 g·cm?3and 1.8344 g·cm?3respectively.Densities of ILs are much lower than that of H2SO4.When the temperature increased to 313.2 K,their densities decreased to 1.2869 g·cm?3,1.2227 g·cm?3,1.1776 g·cm?3and 1.8144 g·cm?3respectively.The decrease percentages were all less than 2%.Meanwhile,densities of ILs/H2SO4binary mixtures slightly but monotonically decreased with increasing temperature.The results agree with the principle of“expand when heated and contract when cooled”.

Fig.1 illustrates the effects of ILs molar fraction(xIL)on the densities of ILs/H2SO4binary mixtures.The densities of the three ILs/H2SO4binary mixtures all decrease continuously with increasing ILs molar fraction at the measured temperatures.It is undoubtedly for that the densities of ILs are smaller than that of sulfuric acid.From Fig.1,the decreasing slopes at the range of 0＜xIL＜0.3 are obviously steeper than that at the range of xIL＞0.3.It implies that the addition of a small amount of IL in sulfuric acid brings an obvious decrease to the density.Forexample,adding ILs of xIL=0.1([Bmim][TFO],[Hmim][TFO]or[Omim][TFO])in sulfuric acid,the densities of acid phase at 293.2 K decreased 10.29%,12.16%,14.10%respectively.

The density of ILs depends on its structure.Based on the order of[Bmim][TFO],[Hmim][TFO]and[Omim][TFO],the densities of ILs decrease with the increasing length of the alkyl side chain sequentially,and the densities of ILs/H2SO4binary mixtures also decrease in the above-mentioned order at same ILs molar fraction.Thus,the density of ILs/H2SO4binary mixture can be easily tuned in the range from the density of pure ILs to the density of pure H2SO4by changing the ILs dosage,meanwhile,the tunable density range of the binary mixture can even be broadened by tailoring the structure of ILs.

Table 2 Experimental data and literature data of density and surface tension at different temperature

The decreased density of ILs/sulfuric acid mixture narrows the density difference between the catalyst phase and the reactant phase,it is helpful to intensify the interphase mass transfer of the organic reactions with sulfuric acid as catalyst,such as the process of C4alkylation.

In order to describe the effects of ILs molar fractions on the densities of ILs/H2SO4binary mixtures and to provide an estimation method of the mixtures densities,the Jouyban–Acree(JAM)model[34]was used to fit the experimental densities.The Jouyban–Acree(JAM)model is as follows:

where

xILthe molar fraction of ILs in ILs/H2SO4binary mixtures,

xSAthe molar fraction of H2SO4in ILs/H2SO4binary mixture,

ρILthe density of ILs,g·cm?3,

ρSAthe density of H2SO4,g·cm?3,

Aiadjustable parameters of Jouyban–Acree(JAM)model.

The fitted parameters Aiare listed in Table 3.The fitting curves are shown in Fig.1.For all the three ILs/H2SO4binary mixtures,the average relative deviations(ARD/%)between the calculated resultsand experimental results are less than 0.3%.The fitting curves in Fig.1 clearly show that the Jouyban–Acree(JAM)model describes the densities of ILs/H2SO4binary mixtures well.

Table 3 Fitted parameters Aiin Jouyban–Acree(JAM)model

The excess property reflects the non-ideal ity of the ILs/H2SO4binary mixture.To fur the runderst and the interaction between ILs and concentrated sulfuric acid,the excess molar volume(VE,cm3·mol?1)was calculated as follows:

where

MILthe molecular weight of ILs,g·mol?1,

MSAthe molecular weight of H2SO4,g·mol?1.

The calculated excess molar volumes(VE)are illustrated in Fig.2.From Fig.2,we found that all VEare≥0 in the range of IL molar fraction from 0–1,which demonstrates that mixing sulfuric acid with ILs leads to a volume expansion and an increase of the intermolecular free volume between H2SO4and ILs[35].Interaction between ILs and H2SO4,steric effect and intra molecular hydrogen bonds all have effects on the excess molar volume.For example,strong intra molecular hydrogen bonds exist in both pure H2SO4[36]and pure ILs[37–39].When ILs are mixed with H2SO4,the hydrogen bonds in pure H2SO4and pure ILs are partly replaced by the interactions between ILs and H2SO4.The cation group of ILs is large and almost hydrophobic.It has a strong steric effect on the interaction between ILs and sulfuric acid.The weakened hydrogen bonds bring an expansion of the volume and the steric effect leads to an increase of the intermolecular free volume between H2SO4and ILs.

Although densities of the binary mixtures decrease with the increasing temperature generally,we did not observe any obvious effect of temperature on the excess molar volume of these binary mixtures.Almost all the excess molar volumes achieve the maximum at the IL molar fraction of 0.3–0.4.At the same conditions,the excess molar volume follows the order of[Bmim][TFO]/H2SO4＞[Omim][TFO]/H2SO4＞[Hmim][TFO]/H2SO4.The excess molecular volume depends on the interactions bet ween ILs and H2SO4,intra mole cularhydrogen bonds,steric effects and hydrophobicity–hydrophilicity property,etc.Even the anion exchange is probably occurred in the system.The excess molar volume is determined by these complex factors in the ILs/H2SO4binary mixtures.In this study,we do not know the details of these complicated factors.

Fig.2.Plots of excess molar volume against molar fraction of ILs.(a)T=293.2 K;(b)T=298.2 K;(c)T=303.2 K;(d)T=308.2 K;(e)T=313.2 K;solid line:Fitted by Redlich?Kister model;scattered points:excess molar volumes:[Bmim][TFO]/H2SO4[Hmim][TFO]/H2SO4;[Omim][TFO]/H2SO4.

Table 4 Fitting parameters(Bi/cm3·mol?1)of Redlich–Kister equation and standard deviation

Redlich?Kister(R–K)equation was used to fit the excess molar volumes of the ILs/H2SO4binary mixtures.The Redlich?Kister(R–K)equation[40]is as follows:

where

Bithe parameter of the Redlich?Kister equation,cm3·mol?1,

k the degree of the polynomial expansion,k=2.

Fig.3.Effect of ILs molar fraction on the surface tensions of ILs/H2SO4binary mixtures.(a)Square,T=293.2 K;(b)Circle,T=298.2K;(c)Triangle,T=303.2K;(d)Inverted Triangle,T=308.2 K;(e)Diamond,T=313.2 K.Solid line:Fitted by LWW model.Scattered points:Experimental results:Dark():[Bmim][TFO]/H2SO4;Violet():[Hmim][TFO]/H2SO4;Green():[Omim][TFO]/H2SO4.

Table 5 Fitted parametersand Λ21in LWW model

Table 5 Fitted parametersand Λ21in LWW model

Model(LWW) 293.2 K 298.2 K 303.2 K 308.2 K 313.2 K[Bmim][TFO]/H2SO4（?A） 0.14 0.14 0.14 0.14 0.14 Λ21 18.49 18.96 19.20 19.91 19.62[Hmim][TFO]/H2SO4（?Λ21?A） 0.21 0.20 0.20 0.20 0.20 Λ21 26.42 26.35 26.80 26.43 26.70[Omim][TFO]/H2SO4（?Λ21?A） 0.27 0.26 0.26 0.26 0.25 Λ21 30.55 30.49 30.65 30.52 30.36?Λ21

The fitting results are shown in Fig.2.The fitting parameters and standard deviations(S)are listed in Table 4.From Fig.2,the Redlich?Kister equation describes the excess molar volume of ILs/H2SO4binary mixtures well.

3.2.Surface tensions of ILs/H2SO4binary mixtures

Surface tensions of[Bmim][TFO]/H2SO4,[Hmim][TFO]/H2SO4and[Omim][TFO]/H2SO4were measured under atmospheric conditions.The temperature ranged from 293.2 K to 313.2 K and the ILs molar fraction ranged from 0 to 1.The results are listed in Table S4 in supporting information.

At 293.2 K,the surface tensions of[Bmim][TFO],[Hmim][TFO]and[Omim][TFO]are 33.59 mN·m?1,32.60 mN·m?1,28.76 mN·m?1respectively,while the surface tension of H2SO4is 52.62 mN·m?1.The surface tensions of ILs are much lower than that of H2SO4.With increasing temperature from 293.2 K to 313.2 K,the surface tensions of[Bmim][TFO],[Hmim][TFO],[Omim][TFO]and H2SO4decreased to 32.36 mN·m?1,31.85 mN·m?1,27.94 mN·m?1and 51.87 mN·m?1,respectively.Weals of ound that the surfacetensions of ILs/H2SO4binary mixtures decreased slightly but monotonically with the increasing temperature.

Fig.3 clearly shows the effects of ILs molar fraction(xIL)on the surface tensions of the mixtures.Obviously,the surface tensions of the ILs/H2SO4binary mixtures decrease with increasing ILs molar fraction.Especially,at the ILs molar fraction range of 0–0.1,the surface tensions decrease steeply with increasing ILs content.When the ILs([Bmim][TFO],[Hmim][TFO]or[Omim][TFO])molar fraction is about 0.1,the decrease percentages of surface tensions are about 24.48%,27.31%and 31.47%respectively.The results imply that the surface tensions of sulfuric acid can be modulated by adding a small amount of ILs.

The structure of ILs also has effects on the surface tensions of ILs/H2SO4binary mixtures.As described above,the length of alkyl side chain increases sequentially with the order of[Bmim][TFO],[Hmim][TFO]and[Omim][TFO].Interestingly,the surface tensions of ILs decrease sequentially with the same order.So as,the surface tensions of ILs/H2SO4binary mixtures also decrease with the increasing order of alkyl chain length at the same ILs molar fraction.

A low surface tension is bene ficial to the interphase mass transfer of two immiscible phases.The results provide an effective way to modulate the surface tensions of sulfuric acid catalyst by tuning the amount of added ILs or tailoring the ILs structure.

In order to provide a method to estimate surface tensions of ILs/H2SO4binary mixtures,the LWW model proposed by Li et al.[34]was used to fit the surface tensions of ILs/H2SO4binary mixtures:

where

γILthe surface tension of ILs,mN·m?1,

γSAthe surface tension of H2SO4,mN·m?1,

γmthe surface tension of ILs/H2SO4binary mixtures,mN·m?1,

R universal gas constant,R=8.3145 Pa·m3·mol?1·K?1,

The fitting parameters are listed in Table 5 and the fitting curves are shown in Fig.3.The average relative deviation(ARD/%)between the measured surface tensions of the mixtures([Bmim][TFO]/H2SO4,[Hmim][TFO]/H2SO4,[Omim][TFO]/H2SO4)and the calculated values were no more than 0.7%.The results show that LWW model can be used to predicate the surface tension of the three ILs/H2SO4binary mixture well.

The excess surface tensions or surface tension deviations(γE/mN·m?1)were calculated as follows:

The calculated excess surface tensions(γE)were illustrated in Fig.4.

All the excess surface tensions are negative(Fig.4).It is similar with the binary mixture of pyridinium-based ionic liquid and water[41].The breaking of the hydrogen bond network in the ILs and sulfuric acid may be responsible for the decrease of the surface tension of the mixture and the negative excess surface tensions.

From Fig.4,we did not observe obvious Influences of temperature on the excess surface tensions of ILs/H2SO4binary mixtures.Butthestructure of ILs and ILs molar fraction have obvious effects on it.For all the three types of ILs/H2SO4binary mixtures,the excess surface tensions reach the minimum at ILs molar fraction range of 0.1–0.2.And we found that the absolute values of excess surface tensions increase sequentially with the order of[Bmim][TFO],[Hmim][TFO]and[Omim][TFO].It coincides with the increasing order of alkyl side length.The steric effect leads to an enhanced packing and a decrease of interactions.It is probably responsible for the change of excess surface tensions with the IL structure.

Excess surface tension reflects the non-uniform distribution degree of the species between the surface region and the bulk region of the binary solution[31,42].The component with lower surface tension would enrich in surface region due to its positive adsorption[43].As shown in Fig.4,all excess surface tensions are＜0.It indicates that the concentration of ILs in the surface region is higher than that in the bulk,because the ILs has a lower surface tension than that of sulfuric acid.With the order of[Bmim][TFO],[Hmim][TFO]and[Omim][TFO],the distribution non-uniform degree increases with increasing length of the alkyl side chain.

Modified Redlich?Kister(R–K)equation proposed by Myers and Scott[44]was used to fit the excess surface tension of ILs/H2SO4binary mixtures

where,Dn,C are fitting parameters.

The standard deviations and the fitting parameters are listed in Table 6.The fitting curves are displayed in Fig.4.The results show that modified Redlich?Kister(R–K)equation has a nice performance in fitting the excess surface tension of ILs/H2SO4binary mixtures.

Fig.4.Plots of excess surfacetension(γE)against ILs molar fraction(xIL).(a)T=293.2 K;(b)T=298.2K;(c)T=303.2K;(d)T=308.2K;(e)T=313.2 K;Solidline:Fitted by modified Redlich?Kister model.Scattered points:Experimental results.Symbol:[Bmim][TFO]/H2SO4;[Hmim][TFO]/H2SO4;[Omim][TFO]/H2SO4.

Table 6 Fitting parameters of modified Redlich?Kister equation and standard deviation

4.Conclusions

At the temperature of 293.2 K–313.2 K,the densities of the ILs([Bmim][TFO],[Hmim][TFO]and[Omim][TFO])are lower than that of sulfuric acid.The densities of ILs/H2SO4binary mixtures decreased monotonously with increasing temperatures and increasing molar fraction of ILs.A longer alkyl chain length of ILs brings a lower density to the ILs/H2SO4binary mixtures.Jouyban-Acree(JAM)model describes the densities of ILs/H2SO4binary mixtures well.The addition of a small amount of IL(xIL＜0.1)in sulfuric acid can dramatically decrease the density of the solution.The density of ILs/H2SO4binary mixture can be tuned by changing the ILs dosage or tailoring the structure of ILs.

The surface tensions of the ILs([Bmim][TFO],[Hmim][TFO]and[Omim][TFO])are much lower than that of sulfuric acid at the temperaturerange of 293.2K–313.2 K.The surface tensions ofILs/H2SO4binary mixtures decrease monotonically with increasing temperatures and increasing molar fraction of ILs.IL with a longer alkyl side chain brings a lower surface tensions to the ILs/H2SO4binary mixtures.LWW model can be used to fit the surface tensions of ILs/H2SO4binary mixtures well.Modified Redlich?Kister(R–K)equation fitted well with the excess surface tensions.In the range of IL fraction xIL＜0.1,the surface tensions of ILs/H2SO4binary mixtures decrease sharply with increasing IL fraction.The results imply that the surface tensions of ILs/H2SO4binary mixtures can be modulated easily by changing the ILs dosage or tailoring the ILs structure.

The addition of ILs,such as[Bmim][TFO],[Hmim][TFO]and[Omim][TFO],brings a decrease of density and surface tensions to the H2SO4phase.It is beneficial to the organic reactions with sulfuric acid as catalyst,such as the C4alkylation process.Because a small surface tension and a narrow density difference between the catalyst phase and the reactant phase are helpful to intensify the interphase mass transfer.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.02.001.