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Thermochimica Acta 639 (2016) 130–147

Contents lists available at ScienceDirect

Thermochimica Acta

journa l homepage: www.e lsev ier .com/ locate / tca

xperimental densities and viscosities of binary mixture of-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide orlycerol with sulfolane and their molecular interaction by COSMO-RS

ohd Azlan Kassim a,d, Nor Asrina Sairi a,d,∗, Rozita Yusoff b,e, Anantharaj Ramalingam c,atimah Alias a,d, Mohamed Kheireddine Aroua b,e

Chemistry Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, MalaysiaChemical Engineering Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiaChemical Engineering Department, SSN College of Engineering, 603110 Tamil Nadu, IndiaUniversiti Malaya Center for Ionic Liquids, Chemistry Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, MalaysiaCentre for Separation Science & Technology, Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur,alaysia

r t i c l e i n f o

rticle history:eceived 12 November 2015eceived in revised form 4 July 2016ccepted 8 July 2016vailable online 12 July 2016

eywords:edlich-Kisterxcess molar volume

a b s t r a c t

Thermophysical properties of solvent are set of data that are essential for designing processes inindustry. But in the absence of experimental data, an accurate predictive method is required. In thiscontext, density (�) and viscosity (�) of sulfolane with glycerol and 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide have been measured over the entire range of composition with tem-perature ranging from 298.15 K to 363.15 K at atmospheric pressure. From these experimental values,thermal expansion (�), excess molar volume (VE), viscosity deviation (��) and Gibbs free energy (�G)were calculated. The predicted values were close to the corresponding experimental data with all thestandard deviation lower than 1 × 10−3. Quantum chemical based COSMO-RS was used to predict the

ulfolanelycerol

onic liquidOSMO-RS model

molecular interaction and non-ideal liquid phase activity coefficient for all mixtures. It has been inter-preted that strong interaction for the sulfolane + [BMIM][NTf2] system, meanwhile weak interaction wasdeduced for the sulfolane + glycerol system. The molar enthalpy (�H), entropy (�S) and Gibbs free energyof activation (�G) of viscosity were calculated. Simultaneous effects of composition and temperature forthe binary mixtures were also reported.

© 2016 Published by Elsevier B.V.

. Introduction

The understanding on variation of thermophysical propertiesnd thermodynamic with temperature and mole fraction involvinginary, ternary and multicomponent system is of substantial sig-ificant to both academic and industry due to the intermolecular

nteraction with in the mixture. In these systems, the moleculesssociated with one another and display a significant effect on thentermolecular interaction leading to variation of thermophysicalnd thermodynamic properties. The degree and nature of the varia-

ion is depends on the polarity and molecular size of the individualomponent. Furthermore, the thermophysical data is essential inhe chemical industry for designing and process optimization of

∗ Corresponding author at: Chemistry Department, Faculty of Science, Universityf Malaya, 50603 Kuala Lumpur, Malaysia.

E-mail address: asrina@um.edu.my (N.A. Sairi).

ttp://dx.doi.org/10.1016/j.tca.2016.07.005040-6031/© 2016 Published by Elsevier B.V.

a chemical process. Such data, density and viscosity are requiredto for determining the mass transfer, fluid flow, heat transfer andenergy consumption of the chemical process [1–3]. Additionally,designing and optimization of a chemical plants process requiresaccurate data of the considered properties and reliable correlatingmodels or, in the absence of experimental data, accurate predictivemethods are necessary.

For over 25 years, sulfolane, which is one of the commercialorganic solvent often used in the industries such as petrochem-ical, polymer and photographic chemical, textile, hydrocarbonextraction and plasticizer [4,5]. Even though sulfolane has ther-mal and hydrolytic stability with high density and boiling point[6], the application of sulfolane has some unfavorable impacton the environment [4]. Therefore, the combination with other

solvents would reduce the consumption of sulfolane, yet sus-tain the good advantage. The unique attributes of ionic liquids(ILs) such as non-flammable, tailorability and negligible vapor

himica Acta 639 (2016) 130–147 131

pvbttgtits

t[sn3a(pdbaaAs(3vfpa

bswVtrt(va(pm

(uoaCTca

2

2

atbdcg

N

N+

N-S S

O

O

O

O

C C

FF

F

F

F

F

S

OO

(a)

(b)

HO

OH

OH

H2C

CH

H2C

(c)

M.A. Kassim et al. / Thermoc

ressure [7–9] enable them to be desirable replacement forolatile organic compounds (VOC). 1-butyl-3-methylimidazoliumis(trifluoromethylsulfonyl)imide, [BMIM][NTf2] was selected forhis study due to the fact that it is commercially available in rela-ively high quantity with significantly lower cost [10]. Meanwhile,lycerol is a commonly used solvent in the industrial processes dueo its non-toxicity and is considered a good solvent for organic andnorganic compounds. Glycerol is used largely in chemical separa-ions such as the distillation of ethanol/water solution, wine andpirits industries [11].

A number of literatures have been reported on characteriza-ion of this type of mixture (sulfolane + solvent). Patrawi et al.,12] brought up the measurements of densities, viscosities andpeed of sound for binary mixture of sulfolane with ethyl acetate,-propyl acetate and n-butyl acetate at temperature of 303.15 K,08.15 K and 313.15 K. Morti et al. [13], showed the densitynd excess molar volume of binary mixtures sulfolane + alcoholsmethanol, n-propanol, n-butanol, and n-propanol) at the tem-erature of 298.15 K to 323.15 K. Moreover, Mesquita et al. [14],etermined the density, viscosity and its excess properties forinary mixtures of sulfolane + alcohols (2-butanol and 2-propanol)nd sulfolane + glycols (diethylene glycol and trietylene glycol)t the temperature ranging from 303.15 K to 343.15 K. Besides,wwad et al. [15] brought up that the relative permittivity, den-ity and refractive index of binary mixtures of sulfolane + glycolsethylene glycol, diethylene glycol, and poly(ethylene glycol)) at03.15 K. Additionally, Yang et al. [16], measured the density,iscosity and its excess molar volume of binary mixtures sul-olane + aromatic hydrocarbons (benzene, toluene, ethylbenzene,-xylene, o-xylene and m-xylene) at the temperature of 303.15 Knd 323.15 K.

Therefore, in this work, the density and viscosity of sulfolaneased binary system with glycerol and [BMIM][NTf2] are mea-ured at temperature ranging from 298.15 K to 363.15 K over thehole composition at atmospheric pressure. A linear equation andogel-Fulcher-Tammann (VFT) were used to correlate experimen-

al density and viscosity data of pure and their binary mixture,espectively. Using the experimental density data of these mix-ures, thermal expansion coefficient (�) and excess molar volumeVE) are calculated. Using experimental viscosity data, molar acti-ation entropy (�S), molar activation enthalpy (�H) and molarctivation Gibbs free energy (�G) along with viscosity deviation��) are obtained. Furthermore, simultaneous effect of the tem-erature and concentration on the density and viscosity of binaryixtures were also studied.

Lastly the quantum chemical-based COSMO-RS modelCOnductor-like Screening MOdel for Real Solvents) has beensed to predict the activity coefficient for known compositionf mixtures. This model is independent of experimental datand uses the molecular structure to determine the Screeningharge Densities (SCDs) or the sigma profile of the pure molecule.his is only indicator in computing the chemical potential of aomponent in solution. Computational details of COSMO-RS modelre described in the later section.

. Experimental section

.1. Material

The chemical used in this work were supplied with high puritynd summarized in Table 1 with source purity and CAS number ofhe chemicals. The water content for 1-butyl-3-methylimidazolium

is(trifluoromethylsulfonyl)imide, [BMIM][NTf2] was 0.005 wt%, asetermined by Karl Fisher titration using an 831 Karl Fischeroulometer. Fig. 1 shows the chemical structures of [BMIM][NTf2],lycerol and sulfolane. All chemicals were used as received.

Fig. 1. Chemical structures of (a) 1-butyl-3-methylimidazolium bis(trifluoro-sulfonyl)imide ([BMIM][NTf2]), (b) glycerol and (c) sulfolane.

2.2. Apparatus and procedure

Density measurement of the binary mixtures was carried outusing oscillating U-tube digital densitometer DDM 2910 (RudolphResearch, USA) from the temperature of 293.15 K to 363.15 K at 10 Kinterval with the temperature accuracy of ±0.05 K at 1 atm. Theapparatus is precise within 0.0001 g cm−3. The calibration of thedensitometer was performed using dry air and ultra-pure water(supplied) at given temperature and atmosphere pressure. Vis-cosity measurement of the binary mixtures was conducted usingRheometer MCR 301 (Anton Paar, Austria) from the temperatureof 293.15 K to 363.15 K with 10 K increment at 1 atm using doublegap cylinder (DG 26.7) measuring geometry. The temperature ofthe solutions was maintained within ±0.05 K with the accuracy ofmeasurement less than 5%. Each value reported was an average oftriplicate.

2.3. Computational details

The structure of the cation, anion, sulfolane and glycerol weredrawn and geometry optimized using TmoleX software package.Geometry optimization was performed at Hartree-Fock theory with6-31G* basic set. Geometry optimization calculation using Hartree-Fock level provides more meaningful accurate values while the *accounts for polarization effect of the species [17,18]. Using theoptimized geometry for each individual compounds, a single pointcalculation was conducted with activation of the .cosmo file gen-eration using density functional theory (DFT) with 6-31G* basicset. Then, the .cosmo file was imported into the COSMOthermXsoftware package (version C30 1401) with parameterization fileBP TZVP C30 1301.ctd. [18] to obtain the �-profile and �-potentialof the individual components and to calculate activity coefficientof the [BMIM][NTf2] + sulfolane and glycerol + sulfolane binary mix-tures. Within COSMOthermX software, a pseudo binary approachwas adopted for the calculation of the mixture compose of ionicliquid-sulfolane whereby the cation and anion of the ionic liquidwas input as separate compounds with equal mole fraction.

3. Result and discussion

3.1. Pure compounds and their binary mixtures

To verify the reliability of the equipment and procedures, thedensity and viscosity of pure [BMIM][NTf2], glycerol and sulfolanewere measured at different temperatures and compared with the

132 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Table 1The sample provenance table for the compounds system.a

Chemical [BMIM][NTf2] Glycerol SulfolaneMolecular formula C10H15F6N3O4S2 C3H8O3 C4H8O2SIUPAC name 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide propane-1,2,3-triol Tetrahydrothiophene 1,1-dioxideCASRN 174899−83-3 56-81-5 126−33-0Source Iolitech Merck MerckPurity grade AR AR ARMass fraction purity 0.99 0.99 0.99Purification Method None None NoneAnalysis Method HPLC GC GCWater (wt%) 0.005 0.392 0.461

a All materials are used without further purifications.

Table 2Comparison of measured density (�) and viscosity (�) with literature values for [BMIM][NTf2], glycerol and sulfolane at different temperature and at atmosphere pressureP = 101.3 kPa.a

Component T/K � (g cm−1) � (Pa.s)

Exp. Lit. Lit. Lit. Exp. Lit. Lit. Lit.

[BMIM][NTf2] 293.15 1.4412 1.4414[19] 1.4389[20] 1.4393[21] 0.0640 0.0620[21] 0.0638[22]298.15 1.4364 1.4366[19] 1.4341[20] 1.4343[21] 0.0495 0.0500[21] 0.0510[22]303.15 1.4315 1.4319[19] 1.4293[20] 1.4294[21] 0.0390 0.0412[21] 0.0414[23]313.15 1.4219 1.4223[19] 1.4198[20] 1.4196[21] 0.0267 0.0282[21] 0.0285[23] 0.0283[22]323.15 1.4124 1.4129[19] 1.4103[20] 1.4097[21] 0.0201 0.0206[21] 0.0206[23]333.15 1.4029 1.4035[19] 1.4009[20] 0.0155 0.0155[23] 0.0154[22]343.15 1.3935 1.3942[19] 0.0124 0.0120[23]

Glycerol 313.15 1.249 1.2495[24] 1.2493[25] 0.3030 0.284[26]323.15 1.2424 1.2429[24] 1.2428[25] 0.1393 0.1333[27] 0.142[26]333.15 1.2357 1.2360[24] 0.0855 0.0815[27] 0.081[26]343.15 1.2289 0.0505 0.0538[27] 0.051[26]353.15 1.2219 0.0312 0.0367[27] 0.032[26]363.15 1.2148 0.0199 0.0195[27] 0.021[26]

Sulfolane303.15 1.2594 1.2604[28] 1.2619[29] 0.0105 0.0104[30] 0.0102[29]313.15 1.2517 1.2505[28] 1.2534[29] 0.0081 0.0080[30] 0.0080[29]323.15 1.2440 1.2412[28] 1.2447[29] 0.0064 0.0064[30] 0.0063[29]333.15 1.2354 1.2326[28] 0.0052 0.0052[30]343.15 1.2250 1.2224[28] 0.0043 0.0043[30]

a Standard uncertainties u are: u(�) = 0.001 g cm−3, u(�) = 5%, u(T) = 0.05 K and u(P) = 0.5 kPa.

T / K

290 300 310 320 330 340 35 0

Rel

ativ

e de

viat

ion

of d

ensi

ty (%

)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Fig. 2. Relative deviation between density measured in this work and literature values for pure [BMIM][NTf2]: (�) [19]; (©) [20]; (�) [21]; pure glycerol: (�) [24], (�) [25],and pure sulfolane: (�) [28], (�) [29].

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 133

T / K

280 30 0 32 0 340 36 0 380

Rel

ativ

e de

viat

ion

of v

isco

sity

(%)

-10

-5

0

5

10

15

20

F alues fa

e(ttc

R

w“r

uiatm3

[tmtpdTiimrqtbm(HiThF

ig. 3. Relative deviation between viscosity measured in this work and literature vnd pure sulfolane: (�) [30]. (�) [29].

xperimental values that are given by other authors in literaturesTable 2). The measurement of density and viscosity obtained inhis study were in good agreement with the literature data at allhe temperatures. The relative deviations of these results were cal-ulated using Eq. (1) and are shown in Figs. 2 and 3.

elativeDeviation = Yexp − YlitYexp

, Y = �, � (1)

here � and � are the density and viscosity. The subscript “exp” andlit” denote experimental data of this work and literature values,espectively.

It is necessary to mention that the viscosity of pure sulfolane wasnable to be measured at T = 293.15 K due to its melting point which

s at T = 300.65 K. Moreover, the binary mixture glycerol + sulfolanere immiscible at the temperatures less than 313.15 K throughouthe entire mole fraction range. Therefore, all the analysis of binary

ixture glycerol + sulfolane were conducted from T = 313.15 K to63.15 K.

All the experimental density and viscosity values ofBMIM][NTf2] + sulfolane and glycerol + sulfolane mixtures areabulated in Tables 3 and 4, respectively. Fig. 4 presents the

easured density of binary mixture [BMIM][NTf2] + sulfolanehroughout the entire mole fraction composition with the tem-erature ranging from 293.15 K to 343.15 K. It is observed that theensity increases with the increased composition of [BMIM][NTf2].he density curves show a quasi-linear decrease in values with the

ncrement of temperature throughout the whole composition. Sim-larly, Fig. 5 presents the measured density of glycerol + sulfolane

ixtures throughout the whole composition and the temperatureanging from 313.15 K to 363.15 K. The density value decreaseduasi-linear manner with the increase of temperature throughouthe whole composition. Due to the comparable value of densityetween pure glycerol and sulfolane at room temperature, ainor increase of density was observed at lower temperature

313.15–323.15 K) with the increase of glycerol composition.owever, at higher temperature (333.15–363.15 K), significant

ncrease in density was observed in the glycerol rich composition.his is due to glycerol having a higher density than sulfolane atigher temperature hence the intersect of isotherm observed inig. 5.

or pure [BMIM][NTf2]: (�) [21], (�) [23], (�) [22], pure glycerol: (©) [27], (�) [26]

To acquire empirical correlation for the pure compounds withtemperature, the following functional relationship for density wasused. The influence of temperature on the density of pure andbinary mixtures of sulfolane and [BMIM][NTf2], glycerol was foundto be linear and correlated using following Eq. (2):

� = A + BT (2)

where � is the density, T is the temperature, and A, B are theadjustable parameters (Table 5). Standard deviation, s, is calculatedusing Equation below (3) [31]:

s =[

m∑1

(Zexp − Zcal

)2

(m − n)

] 12

(3)

where m denotes the number of experimental point.

3.2. Thermal expansion

The coefficients of thermal expansion (�) for the systems stud-ied in this work are calculated from the experimental density datausing the following Eq. (4) [32]:

= − 1�

(ı�

ıT

)= −

(ıln�

ıT

)(4)

where � and T refer to density and temperature.It can be observed from Tables 3 and 4 that the change in thermal

expansion coefficient values is not significant and the variation ofvolume expansion of the systems studied in the present work couldbe considered as independent of temperature.

3.3. Excess molar volume

The values of excess molar volume (VE), were calculated fromexperimental density of the mixture, �, density of the pure compo-nent, � , the corresponding mole fraction, x , and molar masses, M ,

i i i

using the following Eq. (5) [33].

VE = x1M1

(1�

− 1�1

)+ x2M2

(1�

− 1�2

)(5)

134 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Table 3Experimental and calculated densities (�), viscosity (�), excess molar volume (VE), thermal expansion coefficient (�) and viscosity deviation (��) of ([BMIM][NTf2](1) + sulfolane (2)) at different temperature and composition (x1 = mol fraction)a at atmospheric pressure P = 101.3 kPa.

[BMIM][NTf2] (1) + sulfolane (2)

Experimental Prediction

x1b �(g cm−3) � (Pa s) VE(cm3 mol−1) � x 10−3(K−1) �� (Pa s) �(g cm−3) � COSMO-RS �(Pa s) VE(cm3 mol−1) �� (Pa s)

T = 293.15 K0 – – – – – 1.2697 1.0000 0.0132 – –0.1000 1.3151 0.0167 – 0.6872 – 1.3152 0.9871 0.0170 – –0.1999 1.3459 0.0198 – 0.6852 – 1.3460 0.9494 0.0199 – –0.3001 1.3687 0.0238 – 0.6896 – 1.3687 0.8936 0.0238 – –0.4000 1.3861 0.0282 – 0.6780 – 1.3860 0.8260 0.0282 – –0.5001 1.4001 0.0329 – 0.6753 – 1.4000 0.7521 0.0330 – –0.5999 1.4113 0.0380 – 0.6714 – 1.4112 0.6763 0.0380 – –0.7000 1.4208 0.0432 – 0.6701 – 1.4207 0.6019 0.0433 – –0.7995 1.4286 0.0496 – 0.6668 – 1.4285 0.5312 0.0474 – –0.8985 1.4353 0.0551 – 0.6646 – 1.4353 0.4656 0.0554 – –1.0000 1.4412 0.0640 – 0.6623 – 1.4411 0.3928 0.0602 – –

T = 298.15 K0 1.2664 0.0119 0.0000 0.6939 0.0000 1.2653 1.0000 0.0115 0.0000 0.00000.1000 1.3106 0.0148 −0.1496 0.6896 −0.0008 1.3106 0.9876 0.0146 −0.1362 −0.00090.1999 1.3413 0.0170 −0.1671 0.6875 −0.0024 1.3414 0.9514 0.0169 −0.1349 −0.00230.3001 1.3646 0.0199 −0.1584 0.6917 −0.0033 1.3640 0.8978 0.0199 −0.1052 −0.00330.4000 1.3813 0.0232 −0.1309 0.6803 −0.0038 1.3813 0.8327 0.0229 −0.0844 −0.00390.5001 1.3953 0.0267 −0.1285 0.6776 −0.0041 1.3953 0.7613 0.0266 −0.0736 −0.00400.5999 1.4065 0.0308 −0.0928 0.6737 −0.0037 1.4065 0.6879 0.0305 −0.0625 −0.00380.7000 1.4160 0.0347 −0.0895 0.6723 −0.0036 1.4160 0.6155 0.0342 −0.0452 −0.00330.7995 1.4238 0.0398 −0.0557 0.6690 −0.0022 1.4238 0.5464 0.0401 −0.0252 −0.00250.8985 1.4305 0.0444 −0.0239 0.6668 −0.0014 1.4305 0.4820 0.0440 −0.0108 −0.00121.0000 1.4364 0.0495 0.0000 0.6645 0.0000 1.4363 0.4098 0.0508 0.0000 0.0000

T = 303.15 K0 1.2594 0.0105 0.0000 0.6977 0.0000 1.2609 1.0000 0.0101 0.0000 0.00000.1000 1.3061 0.0128 −0.1484 0.6920 −0.0005 1.3061 0.9881 0.0127 −0.1390 −0.00050.1999 1.3367 0.0143 −0.1634 0.6899 −0.0019 1.3367 0.9533 0.0145 −0.1427 −0.00180.3001 1.3591 0.0165 −0.1475 0.6945 −0.0025 1.3593 0.9017 0.0168 −0.1159 −0.00260.4000 1.3766 0.0188 −0.1261 0.6826 −0.0031 1.3766 0.8391 0.0190 −0.0957 −0.00300.5001 1.3905 0.0215 −0.1232 0.6800 −0.0033 1.3906 0.7702 0.0218 −0.0842 −0.00320.5999 1.4017 0.0247 −0.0889 0.6760 −0.0029 1.4017 0.6991 0.0251 −0.0724 −0.00320.7000 1.4111 0.0273 −0.0849 0.6747 −0.0031 1.4112 0.6287 0.0276 −0.0541 −0.00290.7995 1.4190 0.0311 −0.0529 0.6713 −0.0021 1.4190 0.5612 0.0337 −0.0322 −0.00220.8985 1.4257 0.0351 −0.0222 0.6691 −0.0010 1.4257 0.4980 0.0357 −0.0142 −0.00101.0000 1.4315 0.0390 0.0000 0.6668 0.0000 1.4316 0.4266 0.0427 0.0000 0.0000

T = 313.15 K0 1.2517 0.0081 0.0000 0.7020 0.0000 1.2521 1.0000 0.0078 0.0000 0.00000.1000 1.2970 0.0097 −0.1459 0.6968 −0.0003 1.2971 0.9890 0.0096 −0.1418 −0.00020.1999 1.3275 0.0107 −0.1560 0.6947 −0.0011 1.3275 0.9569 0.0107 −0.1504 −0.00110.3001 1.3494 0.0123 −0.1254 0.6995 −0.0014 1.3498 0.9092 0.0123 −0.1264 −0.00170.4000 1.3672 0.0134 −0.1161 0.6873 −0.0022 1.3672 0.8511 0.0134 −0.1067 −0.00190.5001 1.3811 0.0154 −0.1125 0.6846 −0.0020 1.3811 0.7870 0.0154 −0.0946 −0.00200.5999 1.3922 0.0176 −0.0809 0.6806 −0.0017 1.3923 0.7204 0.0176 −0.0820 −0.00220.7000 1.4016 0.0187 −0.0755 0.6792 −0.0024 1.4017 0.6540 0.0189 −0.0628 −0.00220.7995 1.4094 0.0212 −0.0473 0.6759 −0.0018 1.4095 0.5898 0.0232 −0.0390 −0.00170.8985 1.4161 0.0244 −0.0188 0.6736 −0.0005 1.4162 0.5291 0.0245 −0.0175 −0.00061.0000 1.4219 0.0267 0.0000 0.6713 0.0000 1.4220 0.4595 0.0293 0.0000 0.0000

T = 323.15 K0 1.2440 0.0064 0.0000 0.7064 0.0000 1.2433 1.0000 0.0062 0.0000 0.00000.1000 1.288 0.0077 −0.1434 0.7017 −0.0001 1.2880 0.9899 0.0075 −0.1444 0.00000.1999 1.3183 0.0084 −0.1484 0.6995 −0.0007 1.3183 0.9602 0.0082 −0.1578 −0.00070.3001 1.3402 0.0097 −0.1026 0.7043 −0.0008 1.3404 0.9161 0.0094 −0.1366 −0.00110.4000 1.3578 0.0102 −0.1059 0.6921 −0.0017 1.3578 0.8623 0.0099 −0.1174 −0.00120.5001 1.3716 0.0119 −0.1016 0.6893 −0.0013 1.3717 0.8026 0.0116 −0.1047 −0.00130.5999 1.3828 0.0136 −0.0728 0.6853 −0.0010 1.3828 0.7403 0.0132 −0.0913 −0.00150.7000 1.3921 0.0141 −0.0658 0.6839 −0.0018 1.3922 0.6777 0.0139 −0.0713 −0.00160.7995 1.3999 0.0160 −0.0416 0.6805 −0.0014 1.4000 0.6168 0.0158 −0.0456 −0.00130.8985 1.4066 0.0184 −0.0152 0.6782 −0.0003 1.4066 0.5587 0.0180 −0.0207 −0.00051.0000 1.4124 0.0201 0.0000 0.6758 0.0000 1.4125 0.4951 0.0198 0.0000 0.0000

T = 333.15 K0 1.2354 0.0052 0.0000 0.7113 0.0000 1.2345 1.0000 0.0050 0.0000 0.00000.1000 1.2790 0.0057 −0.1408 0.7066 0.0000 1.2790 0.9906 0.0060 −0.1470 0.00010.1999 1.3090 0.0063 −0.1405 0.7045 −0.0004 1.3091 0.9632 0.0065 −0.1650 −0.00050.3001 1.3310 0.0073 −0.0792 0.7092 −0.0005 1.3309 0.9224 0.0074 −0.1465 −0.00090.4000 1.3484 0.0075 −0.0954 0.6969 −0.0014 1.3484 0.8725 0.0077 −0.1277 −0.00090.5001 1.3622 0.0092 −0.0903 0.6941 −0.0008 1.3622 0.8170 0.0092 −0.1145 −0.00090.5999 1.3733 0.0105 −0.0643 0.6900 −0.0007 1.3733 0.7587 0.0104 −0.1005 −0.00100.7000 1.3826 0.0108 −0.0559 0.6886 −0.0014 1.3826 0.6999 0.0108 −0.0795 −0.0011

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 135

Table 3 (Continued)

[BMIM][NTf2] (1) + sulfolane (2)

Experimental Prediction

x1b �(g cm−3) � (Pa s) VE(cm3 mol−1) � x 10−3(K−1) �� (Pa s) �(g cm−3) � COSMO-RS �(Pa s) VE(cm3 mol−1) �� (Pa s)

0.7995 1.3904 0.0122 −0.0357 0.6851 −0.0011 1.3904 0.6423 0.0107 −0.0520 −0.00100.8985 1.3971 0.0141 −0.0116 0.6828 −0.0003 1.3971 0.5869 0.0140 −0.0239 −0.00041.0000 1.4029 0.0155 0.0000 0.6804 0.0000 1.4029 0.5219 0.0133 0.0000 0.0000

T = 343.15 K0 1.2250 0.0043 0.0000 0.7173 0.0000 1.2258 1.0000 0.0041 0.0000 0.00000.1000 1.2699 0.0046 −0.1381 0.7117 0.0000 1.2700 0.9913 0.0048 −0.1483 0.00000.1999 1.2998 0.0051 −0.1324 0.7095 −0.0004 1.2999 0.9660 0.0052 −0.1685 −0.00040.3001 1.3218 0.0059 −0.0551 0.7141 −0.0005 1.3215 0.9283 0.0060 −0.1513 −0.00070.4000 1.3391 0.0062 −0.0846 0.7017 −0.0010 1.3390 0.8820 0.0061 −0.1328 −0.00080.5001 1.3528 0.0073 −0.0786 0.6989 −0.0008 1.3528 0.8303 0.0075 −0.1193 −0.00080.5999 1.3639 0.0082 −0.0558 0.6948 −0.0007 1.3638 0.7759 0.0084 −0.1049 −0.00090.7000 1.3732 0.0086 −0.0456 0.6933 −0.0012 1.3731 0.7207 0.0086 −0.0836 −0.00110.7995 1.3810 0.0095 −0.0296 0.6898 −0.0011 1.3809 0.6662 0.0069 −0.0551 −0.00110.8985 1.3876 0.0107 −0.0078 0.6875 −0.0008 1.3876 0.6136 0.0111 −0.0254 −0.00081.0000 1.3935 0.0124 0.0000 0.6849 0.0000 1.3934 0.5511 0.0085 0.0000 0.0000

cAt T = 293.15 K, pure sulfolane is solid [28].aStandard uncertainty u are u(x) = 0.0005, u(T) = 0.05 K, u(�) = 0.001 g cm−3, u(�) = 5%, u(P) = 0.5 kPa and combined expanded uncertainties (confidence level, 95%)U(VE) = 0.0001 cm3 mol−1.

b x1 = mol fraction of [BMIM][NTf2].

F variou(

wx

TK

Y

wos

eaxdp

ig. 4. Density of [BMIM][NTf2] (1) + sulfolane (2) mixtures against temperature at�) 0.6 × 1; (♦) 0.7 × 1; (�) 0.8 × 1; (�) 0.9 × 1; ( ) 1 × 1.

here x1, �1 and M1 relate to [BMIM][NTf2] and glycerol, whereas2, �2 and M2 relate to sulfolane.

The values of excess molar volume are summarized inables 3 and 4 and represented by Figs. 6 and 7 using a Redlich-ister Equation (6) [21] where YE represent the excess properties:

E = x1x2

n∑i=0

Ai(2x1 − 1)i (6)

here Ai refers to the adjustable parameter and n is the numberf coefficient in the equation. The adjustable parameters, Ai, areummarized in Table 6.

In Fig. 6, it is shown that the VE values are negative over thentire mole fraction range of [BMIM][NTf2] at various temper-

tures with the minimum of the asymmetric curve which is at1 = 0.2 for all the studied systems. The excess molar volume, VE

epends primarily on the intermolecular forces between two com-onents of the mixture. The magnitude and sign of VE can be

s concentration; (�) 0 × 1; (©) 1 × 1; (�) 0.2 × 1; (�) 0.3 × 1; (�) 0.4 × 1; (�) 0.5 × 1;

qualitatively examined by accounting the physical, structural andchemical contributions [34]. The physical contribution comprisesthe dispersion of forces or weak dipole–dipole interaction thatleads to positive contribution of VE . The structural contributioninvolves the geometric effect enabling the fitting of two differentsizes molecules into each other’s structure leading to negative con-tribution to VE [35]. The chemical contribution comprises specificinteractions; formation of hydrogen bonding, formation of chargetransfer complexes, other complex forming interactions, and strongdipole–dipole interaction between the components, while leads tonegative VE values. Therefore, the negative VE of binary mixture[BMIM][NTf2] + sulfolane could be attributed to strong interactionbetween different molecules [36], i.e. [BMIM][NTf2] and sulfolane.Moreover, the relatively small sulfolane molecule easily fits into

the free volume between the comparatively large ions of the[BMIM][NTf2], resulting in a negative VE values [37]. The trend ofVE values for the binary mixture [BMIM][NTf2] + sulfolane becomesmore negative which can be due to the increases of kinetic energy

136 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Table 4Experimental and calculated densities (�), viscosity (�), excess molar volume (VE), thermal expansion coefficient (�) viscosity deviation (��) of binary mixture (glycerol(1) + sulfolane (2)) at different temperature and composition (x1 = mol fraction) at atmospheric pressure P = 101.3 kPa.a

Glycerol (1) + sulfolane (2)

Experimental Prediction

x1b �(g cm−3) � (Pa s) VE (cm3 mol−1) � x 10−3(K−1) �� (Pa.s) �(g cm−3) � COSMO-RS �(Pa.s) VE (cm3mol−1) �� (Pa.s)

T = 313.15 K0.0000 1.2517 0.0081 0.0000 0.7439 0.0000 1.2528 1.0000 0.0085 0.0000 0.00000.1002 1.2496 0.0085 0.1416 0.7093 −0.0292 1.2498 1.0278 0.0087 0.2048 −0.02870.2002 1.2488 0.0105 0.1807 0.7047 −0.0568 1.2488 1.0926 0.0106 0.2592 −0.05730.3002 1.2484 0.0144 0.1882 0.6980 −0.0823 1.2484 1.1845 0.0146 0.2353 −0.08320.4001 1.2478 0.0192 0.2073 0.6868 −0.1069 1.2489 1.3028 0.0194 0.1965 −0.10440.5001 1.2475 0.0304 0.2042 0.6717 −0.1194 1.2486 1.4499 0.0364 0.1769 −0.12090.5999 1.2475 0.0462 0.1854 0.6672 −0.1336 1.2484 1.6299 0.0516 0.1751 −0.13380.6999 1.2475 0.0707 0.1590 0.6326 −0.1432 1.2476 1.8483 0.0715 0.1634 −0.14290.7999 1.2485 0.1010 0.0729 0.6055 −0.1418 1.2487 2.1119 0.1023 0.1115 −0.14130.8993 1.2489 0.1640 0.0270 0.5797 −0.1078 1.2491 2.4289 0.1657 0.0252 −0.10831.0000 1.2490 0.3030 0.0000 0.5474 0.0000 1.2492 2.8084 0.3030 0.0000 0.0000

T = 323.15 K0.0000 1.2440 0.0064 0.0000 0.7485 0.0000 1.2435 1.0000 0.0067 0.0000 0.00000.1002 1.2409 0.0072 0.2261 0.7142 −0.0144 1.2409 1.0238 0.0070 0.1905 −0.01380.2002 1.2401 0.0085 0.2705 0.7096 −0.0278 1.2400 1.0810 0.0083 0.2521 −0.02840.3002 1.2397 0.0112 0.2829 0.7029 −0.0399 1.2397 1.1632 0.0108 0.2391 −0.04050.4001 1.2392 0.0144 0.3002 0.6915 −0.0514 1.2403 1.2693 0.0140 0.2085 −0.04990.5001 1.2391 0.0220 0.2931 0.6761 −0.0569 1.2402 1.4011 0.0233 0.1934 −0.05740.5999 1.2392 0.0322 0.2637 0.6717 −0.0623 1.2401 1.5621 0.0326 0.1928 −0.06300.6999 1.2397 0.0459 0.2135 0.6366 −0.0652 1.2397 1.7566 0.0444 0.1794 −0.06500.7999 1.2412 0.0665 0.1009 0.6091 −0.0605 1.2411 1.9906 0.0637 0.1225 −0.05980.8993 1.2419 0.1030 0.0433 0.5830 −0.0404 1.2418 2.2709 0.0986 0.0289 −0.04101.0000 1.2424 0.1393 0.0000 0.5503 0.0000 1.2424 2.6056 0.1537 0.0000 0.0000

T = 333.15 K0.0000 1.2354 0.0052 0.0000 0.7537 0.0000 1.2342 1.0000 0.0054 0.0000 0.00000.1002 1.2321 0.0058 0.2562 0.7193 −0.0076 1.2320 1.0204 0.0057 0.1757 −0.00640.2002 1.2313 0.0066 0.3124 0.7147 −0.0147 1.2312 1.0709 0.0066 0.2446 −0.01450.3002 1.2310 0.0083 0.3287 0.7079 −0.0210 1.2310 1.1442 0.0082 0.2429 −0.02100.4001 1.2307 0.0104 0.3466 0.6963 −0.0268 1.2318 1.2392 0.0104 0.2207 −0.02590.5001 1.2307 0.0146 0.3389 0.6807 −0.0296 1.2318 1.3572 0.0155 0.2101 −0.02980.5999 1.2311 0.0203 0.3055 0.6761 −0.0318 1.2317 1.5009 0.0213 0.2108 −0.03230.6999 1.2320 0.0288 0.2372 0.6405 −0.0323 1.2318 1.6743 0.0288 0.1956 −0.03240.7999 1.2337 0.0418 0.1235 0.6128 −0.0277 1.2335 1.8818 0.0413 0.1336 −0.02780.8993 1.2348 0.0617 0.0536 0.5863 −0.0156 1.2346 2.1297 0.0614 0.0326 −0.01681.0000 1.2357 0.0855 0.0000 0.5533 0.0000 1.2355 2.4246 0.0849 0.0000 0.0000

T = 343.15 K0.0000 1.2250 0.0043 0.0000 0.7601 0.0000 1.2249 1.0000 0.0044 0.0000 0.00000.1002 1.2232 0.0047 0.1649 0.7246 −0.0043 1.2232 1.0174 0.0047 0.1582 −0.00410.2002 1.2225 0.0053 0.2390 0.7198 −0.0083 1.2224 1.0619 0.0053 0.2347 −0.00860.3002 1.2223 0.0063 0.2731 0.7129 −0.0118 1.2223 1.1272 0.0064 0.2453 −0.01200.4001 1.2223 0.0077 0.2887 0.7012 −0.0150 1.2232 1.2122 0.0078 0.2327 −0.01460.5001 1.2227 0.0102 0.2858 0.6854 −0.0167 1.2234 1.3178 0.0107 0.2270 −0.01670.5999 1.2233 0.0136 0.2601 0.6807 −0.0175 1.2234 1.4462 0.0145 0.2287 −0.01790.6999 1.2241 0.0185 0.2273 0.6447 −0.0172 1.2239 1.6005 0.0194 0.2109 −0.01710.7999 1.2262 0.0266 0.1128 0.6165 −0.0134 1.2260 1.7848 0.0278 0.1427 −0.01320.8993 1.2275 0.0380 0.0567 0.5898 −0.0059 1.2274 2.0039 0.0398 0.0341 −0.00621.0000 1.2289 0.0505 0.0000 0.5564 0.0000 1.2287 2.2637 0.0504 0.0000 0.0000

T = 353.15 K0.0000 1.2150 0.0037 0.0000 0.7664 0.0000 1.2156 1.0000 0.0037 0.0000 0.00000.1002 1.2143 0.0039 0.0986 0.7299 −0.0026 1.2143 1.0149 0.0039 0.1446 0.00090.2002 1.2137 0.0043 0.1867 0.7251 −0.0050 1.2136 1.0540 0.0043 0.2287 −0.00260.3002 1.2136 0.0049 0.2346 0.7181 −0.0071 1.2136 1.1121 0.0050 0.2502 −0.00620.4001 1.2138 0.0060 0.2563 0.7062 −0.0088 1.2146 1.1881 0.0060 0.2455 −0.00860.5001 1.2144 0.0074 0.2571 0.6901 −0.0101 1.2151 1.2826 0.0076 0.2443 −0.01000.5999 1.2153 0.0098 0.2367 0.6854 −0.0104 1.2151 1.3973 0.0101 0.2477 −0.01080.6999 1.2161 0.0129 0.2247 0.6489 −0.0098 1.2160 1.5347 0.0135 0.2289 −0.01070.7999 1.2185 0.0180 0.1114 0.6204 −0.0069 1.2184 1.6984 0.0192 0.1563 −0.00920.8993 1.2202 0.0249 0.0520 0.5933 −0.0021 1.2201 1.8923 0.0268 0.0400 −0.00561.0000 1.2219 0.0312 0.0000 0.5595 0.0000 1.2218 2.1213 0.0317 0.0000 0.0000

T = 363.15 K0.0000 1.2060 0.0032 0.0000 0.7721 0.0000 1.2063 1.0000 0.0031 0.0000 0.00000.1002 1.2053 0.0033 0.1119 0.7353 −0.0016 1.2054 1.0128 0.0033 0.1282 −0.00160.2002 1.2048 0.0035 0.2053 0.7304 −0.0031 1.2048 1.0472 0.0036 0.2202 −0.00320.3002 1.2048 0.0040 0.2583 0.7233 −0.0045 1.2048 1.0989 0.0040 0.2538 −0.00450.4001 1.2051 0.0047 0.2860 0.7113 −0.0055 1.2060 1.1667 0.0047 0.2581 −0.00550.5001 1.2059 0.0058 0.2875 0.6950 −0.0065 1.2067 1.2512 0.0056 0.2618 −0.0064

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 137

Table 4 (Continued)

Glycerol (1) + sulfolane (2)

Experimental Prediction

x1b �(g cm−3) � (Pa s) VE (cm3 mol−1) � x 10−3(K−1) �� (Pa.s) �(g cm−3) � COSMO-RS �(Pa.s) VE (cm3mol−1) �� (Pa.s)

0.5999 1.2069 0.0074 0.2655 0.6902 −0.0066 1.2067 1.3537 0.0073 0.2666 −0.00670.6999 1.2080 0.0094 0.2492 0.6533 −0.0060 1.2081 1.4763 0.0096 0.2459 −0.00590.7999 1.2107 0.0127 0.1279 0.6244 −0.0037 1.2109 1.6216 0.0137 0.1679 −0.00360.8993 1.2127 0.0171 0.0626 0.5970 −0.0006 1.2129 1.7935 0.0186 0.0437 −0.00061.0000 1.2148 0.0199 0.0000 0.5628 0.0000 1.2150 1.9956 0.0209 0.0000 0.0000

a Standard uncertainty u are u(x) = 0.0005, u(T) = 0.05 K, u(�) = 0.001 g cm−3, u(�) = 5%, u(P) = 0.5 kPa and combined expanded uncertainties (confidence level, 95%)U(VE) = 0.0001 cm3 mol−1.

b x1 = mol fraction of glycerol.

Fig. 5. Density of glycerol (1) + sulfolane (2) mixtures against temperature at various concentrations; (�) 0 × 1; (©) 0.1 × 1; (�) 0.2 × 1; (�) 0.3 × 1; (�) 0.4 × 1; (�) 0.5 × 1; (�)0.6 × 1; (♦) 0.7 × 1; (�) 0.8 × 1; (�) 0.9 × 1; ( ) 1 × 1.

Table 5Fitting parameters of Eq. (2) together with correlation coefficient squared, R2, and standard relative deviations, �, for the density of binary mixtures [BMIM][NTf2] + sulfolaneand glycerol + sulfolane.

x1 A B (x10−4) R2 sa (x10−5)

[BMIM][NTf2] (1) + sulfolane (2)0.0000 1.5273 −8.7874 0.9954 89.30850.1000 1.5801 −9.0379 1.0000 4.82350.1999 1.6163 −9.2218 1.0000 4.98340.3001 1.6454 −9.4389 0.9996 3.18910.4000 1.6615 −9.3973 1.0000 4.51470.5001 1.6772 −9.4549 1.0000 4.96580.5999 1.6890 −9.4758 1.0000 4.99310.7000 1.6998 −9.5201 1.0000 7.14280.7995 1.7078 −9.5259 1.0000 6.12570.8985 1.7149 −9.5392 1.0000 4.08841.0000 1.7209 −9.5447 1.0000 8.5251

glycerol (1) + sulfolane (2)0.0000 1.5444 −9.3114 0.9978 74.75600.1002 1.5273 −8.8629 1.0000 9.24470.2002 1.5244 −8.8000 1.0000 6.09700.3002 1.5213 −8.7143 1.0000 2.91240.4001 1.5175 −8.5771 1.0000 7.11170.5001 1.5112 −8.3857 1.0000 7.34370.5999 1.5092 −8.3286 1.0000 8.92040.6999 1.4947 −7.8914 0.9999 13.37700.7999 1.4854 −7.5600 0.9999 15.39900.8993 1.4758 −7.2400 0.9999 14.97501.0000 1.4633 −6.8371 0.9998 16.5950

a � = standard deviation, Eq. (3).

138 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

X1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

VE /

cm3 m

ol-1

-0.18

-0.16

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

Fig. 6. Excess molar volume, VE of [BMIM][NTf2] (1) + sulfolane (2) mixtures against temperature as function of [BMIM][NTf2] mole fraction; (�) 298.15 K; (©) 303.15 K; (�)313.15 K; (�) 323.15 K; (�) 333.15 K; (�) 343.15 K.

X1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

VE /

cm3 m

ol-1

0.0

0.1

0.2

0.3

0.4

Fig. 7. Excess molar volume, VE of glycerol (1) + sulfolane (2) mixtures against temperature as function of glycerol mole fraction; (�) T = 313.15 K, (©) T = 323.15 K, (�)T = 333.15 K, (�) T = 343.15 K, (�) T = 353.15 K, (�) T = 363.15 K.

Table 6Redlich-Kister fitting coefficients Ai of the VE of [BMIM][NTf2] + sulfolane and glycerol + sulfolane binary mixtures as a function of various temperature along with their fittingdeviations, �.

T/K A0 A1 A2 A3 A4 R2 sa

[BMIM][NTf2] + sulfolane298.15 −0.2942 0.1858 −0.2596 1.0700 −0.8699 0.9977 0.0347303.15 −0.3369 0.2016 −0.2990 1.0392 −0.7887 0.9974 0.0227313.15 −0.3784 0.2169 −0.3373 1.0091 −0.7096 0.9971 0.0079323.15 −0.4188 0.2318 −0.3744 0.9798 −0.6326 0.9966 0.0128333.15 −0.4581 0.2462 −0.4106 0.9512 −0.5577 0.9962 0.0287343.15 −0.4773 0.2533 −0.4283 0.9372 −0.5210 0.9959 0.0421

glycerol + sulfolane313.15 0.8275 −0.0747 −0.2273 −1.1820 0.5667 0.9951 0.0125323.15 1.1915 −0.2220 −0.4812 −1.6863 1.4143 0.9973 0.0124333.15 1.3734 −0.3141 −0.5482 −1.7492 1.6454 0.9991 0.0131343.15 1.1554 −0.2155 −0.0875 −0.9485 0.2178 0.9943 0.0145353.15 1.0354 −0.1062 0.1939 −0.4663 −0.9001 0.9902 0.0164363.15 1.1594 −0.1026 0.1039 −0.5019 −0.7415 0.9915 0.0187

a � = standard deviation, Eq. (3).

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 139

Fig. 8. Activity coefficient for sulfolane as function of [BMIM][NTf2] at T = 298.15 K:.

unctio

woc

gotTooduacaas

Fig. 9. Activity coefficient for sulfolane as f

ith the increase of temperature. This leads to a lower interactionf similar molecules resulting in increased shrinkage of volume andonsequently decreasing the excess molar volume [38].

Fig. 7 shows that the excess molar volume, VE of binary mixturelycerol + sulfolane mixtures is positive at various temperaturesver the whole range of mole fraction of glycerol. This indicateshat the interaction between glycerol and sulfolane is not strong.he factors that contribute to the positive value of VE are: (i) lossf dipolar association; rupturing of H-bonding of component byther or breaking up of associates held by weaker force, namelyipole–dipole or by Van der Waals force, (ii) the geometry of molec-lar structure which restrict fitting of the one component intonother, (iii) the stearic hindrance which opposes proximity of theonstituent molecules. This could be due to an unfavorable inter-

ction between polar substituent and apolar groups resulting in

steric hindrance of molecule component which overcomes eachpecific interaction between different species [39]. Therefore, the

n of mole fraction of glycerol T = 313.15 K:.

positive VE of glycerol + sulfolane mixtures can be described by con-sidering the steric hindrance effect of the alkyl chain of glyceroland sulfolane alkyl ring that imparts the hydrophobic character ofsulfolane. Furthermore, when glycerol molecules are solvated bysolvents which are unable to form H-bond, all the three OH groupsof glycerol form intramolecular hydrogen bonds among them [40].

In order to verify the findings, we have predicted the activitycoefficients, �i of solutes in mixture using COSMO-RS model andsummarized in Tables 3 and 4. Fig. 8 shows the predicted activitycoefficient for [BMIM][NTf2] + sulfolane mixture at 298.15 K. Thepredicted activity coefficient decreases with increasing mole frac-tion of [BMIM][NTf2] in the mixture and has an activity coefficientvalue lower than 1 over the whole composition. This indicates

a stronger unlike interactions compare to like interaction whichleads to an attraction between unlike molecules. Similarly, Fig. 9shows the predicted activity coefficient for glycerol+ sulfolane mix-ture. The predicted activity coefficient increases with increasing

140 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Fig. 10. �-profile of [BMIM]+ cation, [NTf2]− anion, glycerol and sulfolane predicted by COSMO-RS model.

n, glyc

mfiar

oae(r

Fig. 11. �-potential of [BMIM]+ cation, [NTf2]−anio

ole fraction of glycerol in the mixture. The value of activity coef-cient is higher than 1 over the whole composition. This indicates

stronger like interactions than unlike interaction which lead toepulsion between unlike molecules in the binary mixture [41].

To further understand the molecular interaction in termf molecular polarity, electrostatic interaction, hydrogen bondffinity and hydrophobicity in [BMIM][NTf2] + sulfolane and glyc-

rol + sulfolane binary system, a 3D polarized charged distribution�, sigma) on the molecular surface of the individual componentsesulted from the quantum chemical calculation were generated

erol and sulfolane predicted by COSMO-RS model.

using COSMO-RS model. The 3D screening charge distribution onthe molecular surface are visualized using a histogram �-profile,which used to qualitatively describe the molecule and predict thepossible interaction of the components in liquid mixture [42]. The�-profile histogram is divided into 3 main regions; hydrogen bonddonor region (� < −0.0082 e/Å2), hydrogen bond acceptor region(� > 0.0082 e/Å2) and nonpolar region (−0.0082 < � < 0.0082 e/Å2).

Fig. 10 shows the �-profile of [BMIM]+ cation, [NTf2]− anion, glyc-erol and sulfolane molecules.

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 141

Fig. 12. Viscosity of [BMIM][NTf2] (1) + sulfolane (2) mixtures against temperature at various concentrations; (�) 0 × 1; (©) 0.1 × 1; (�) 0.2 × 1; (�) 0.3 × 1; (�) 0.4 × 1; (�)0.5 × 1; (�) 0.6 × 1; (♦) 0.7 × 1; (�) 0.8 × 1; (�) 0.9 × 1; ( ) 1 × 1.

F ious c(

bca−boocfcw

aca

ig. 13. Viscosity of glycerol (1) + sulfolane (2) mixtures against temperature at var�) 0.6 × 1; (♦) 0.7 × 1; (�) 0.8 × 1; (�) 0.9 × 1; ( ) 1 × 1.

In Fig. 10, sulfolane presents a highly polarized charged distri-ution at +0.012 e/Å2 which is assigned to the sulfonyl group (redolored sigma surface) showing its ability to act as hydrogen bondcceptor (basic character). Moreover, sulfolane presents a peak at0.006 e/Å2 within the nonpolar region due to the four carbon ringonded to the sulfur atom (green colored sigma surface). From theverall analysis of the �-profile of sulfolane, a distribution chargen the � polarity scale around hydrogen bond acceptor region indi-ates the ability of sulfolane to act as a base. Hence it is possible toorecast that component with hydrogen bond donor groups (acidicharacter) able to develop a favorable intermolecular interactionith sulfolane.

2

In the other hand, glycerol shows a broad peak at +0.015 e/Åssigned to the oxygen atom in the terminal hydroxyl group (redolored sigma surface), while a small peak at −0.017 e/Å2 due to thecidic hydrogen atom of the hydroxyl group. Additionally, glycerol

oncentrations; (�) 0 × 1; (©) 0.1 × 1; (�) 0.2 × 1; (�) 0.3 × 1; (�) 0.4 × 1; (�) 0.5 × 1;

presents a peak at −0.006 e/Å2 in the nonpolar region is corre-sponding to the aliphatic glycerol backbone of the molecule. Overallanalysis of the �-profile of glycerol, a distribution charge on the �polarity scale around hydrogen bond acceptor and hydrogen bondacceptor region indicates the ability of glycerol to act as a base andacid.

In regards to ions that constitute [BMIM][NTf2], Fig. 10 shows �-profile for [BMIM]+ cation is dominated by a main peak at −0.004e/Å2 with a large distribution in the nonpolar region of the his-togram attributed to the aliphatic alkyl chain and the aromaticheadgroup (green color sigma surface). A peak, −0.009 e/Å2 in thehydrogen bond donor region is due to the acidic hydrogen atom

of the aromatic ring (blue color sigma surface). On the other hand,Fig. 10 also shows �-profile for [NTf2]− anion with a prominentpeak at +0.003 e/Å2in the nonpolar region attributed to the non-polar fluorinated alkyl group (green color sigma surface). A peak,

142 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Table 7Fitting parameters of VFT equation together with correlation coefficient, R2, and standard relative deviations, �, for the viscosity of binary mixtures [BMIM][NTf2] + sulfolaneand glycerol + sulfolane.

x1 A (x10−6) B C R2 �a

[BMIM][NTf2] (1) + sulfolane (2)0.0000 2.30 2746 −24.15 0.9888 0.00020.1000 6.89 2024 33.97 0.9980 0.00030.2000 17.4 1495 80.89 0.9993 0.00040.3000 115 766 149.50 0.9993 0.00040.4000 151 637 171.30 0.9995 0.00060.5000 469 397 199.70 0.9995 0.00060.6000 494 411 198.60 0.9994 0.00070.7000 453 418 201.50 0.9995 0.00090.8000 7.89 × 103 4366 656.30 0.9814 0.00100.9000 435 487 192.50 0.9995 0.00121.0000 5.808 × 103 3951 637.90 0.9746 0.0013

glycerol (1) + sulfolane (2)0.0000 84.360 841 130.50 0.9999 0.00000.1000 7.782 2205 −1.15 0.9971 0.00000.2000 6.138 2170 22.11 0.9979 0.00010.3000 4.438 2127 50.46 0.9976 0.00010.4000 4.553 2063 66.30 0.9985 0.00020.5000 2.585 1953 108.70 0.9983 0.00040.6000 1.341 2315 93.88 0.9930 0.00130.7000 2.780 2064 109.90 0.9986 0.00080.8000 2.170 2340 95.70 0.9977 0.00150.9000 2.543 2253 109.90 0.9980 0.00231.0000 1.322 1384 175.30 1.0000 0.0005

a � = standard deviation, Eq. (3).

Table 8The molar enthalpy (�H), entropy (�S) and Gibbs free energy of activation (�G) of viscosity correlation coefficient (R2) and standard deviation (�) for the binary mixtures.

x1 �H(kJ mol−1) T�S(kJ mol−1) �G(kJ mol−1) R2

[BMIM][NTf2] (1) + sulfolane (2)0.0000 18.30 35.00 −16.70 0.99710.1000 19.63 34.79 −15.16 0.99790.2000 20.83 34.41 −13.59 0.99790.3000 21.61 34.41 −12.80 0.99810.4000 24.66 32.10 −7.44 0.99790.5000 23.58 33.79 −10.21 0.99810.6000 24.06 33.91 −9.86 0.99810.7000 25.61 32.88 −7.27 0.99710.8000 26.20 32.84 −6.64 0.99760.9000 25.90 33.59 −7.69 0.99791.0000 26.19 33.75 −7.56 0.9979

glycerol (1) + sulfolane (2)0.0000 17.99 35.37 −17.39 0.99840.1000 17.92 35.49 −17.57 0.99820.2000 20.39 33.61 −13.22 0.99890.3000 24.22 30.73 −6.50 0.99850.4000 26.41 29.31 −2.91 0.99930.5000 34.94 22.74 12.20 0.99620.6000 36.97 21.66 15.31 0.99400.7000 38.15 21.28 16.87 0.9986

+p

gvbntatsi

0.8000 39.28 21.080.9000 42.85 18.831.0000 50.75 12.70

0.011 e/Å2 in the hydrogen bond acceptor region is due to theolar sulfonyl group (red color sigma surface).

Fig. 11 displays �-potential of [BMIM]+ cation, [NTf2]− anion,lycerol and sulfolane molecules. Sulfolane presents a negativealue at � < −0.0082 e/Å2 which indicate an affinity to a hydrogenond donor and a positive value at � > 0.0082 e/Å2 correspondso affinity to hydrogen bond acceptor. Glycerol displays a nega-ive at both � < −0.0082 e/Å2 and � > 0.0082 e/Å2 which indicateffinity to both hydrogen bond donor and hydrogen bond accep-or. For the ions which constitute the [BMIM][NTf2], Fig. 11 also

hows a negative value � > 0.0082 e/Å2 for [BMIM]+ cation whichndicate an affinity to a hydrogen bond acceptor while a negative

18.20 0.999024.02 0.999338.04 0.9988

value at � < −0.0082 e/Å2 for [NTf2]− anion indicating an affinitytoward hydrogen bond donor.

Based on the �-profile and �-potential of each of the compo-nent, it is believed that [BMIM][NTf2] + sulfolane binary mixturehas a stronger molecular interaction in comparison to the glyc-erol + sulfolane binary mixture. This is due to a more prominentpeak in the hydrogen bond donor region of [BMIM]+ cation thanglycerol suggesting higher acidity of [BMIM][NTf2] to favorablyinteract with the sulfolane molecule through hydrogen bondinginvolving the basic sulfonyl group of sulfolane. Furthermore, the �-

potential of nonpolar region for [BMIM][NTf2] is much lower thansulfolane and glycerol, which could contribute to nonpolar inter-

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 143

Fig. 14. Viscosity deviation from ideality of [BMIM][NTf2] (1) + sulfolane (2) mixtures against temperature as function of [BMIM][NTf2] mole fraction; (�) 298.15 K; (©)303.15 K; (�) 313.15 K; (�) 323.15 K; (�) 333.15 K; (�) 343.15 K.

Table 9Redlich-Kister fitting coefficients Ai of the �� of [BMIM][NTf2] + sulfolane and glycerol + sulfolane binary mixtures as a function of various temperature along with theirfitting deviations, �.

T/K A0 A1 A2 A3 A4 R2 �a

[BMIM][NTf2] + sulfolane298.15 −0.0160 0.0006 −0.0005 −0.0042 0.0105 0.9962 0.000126303.15 −0.0126 −0.0012 −0.0078 −0.0028 0.0223 0.9949 0.000122313.15 −0.0079 −0.0032 −0.0133 0.0005 0.0285 0.9694 0.000210323.15 −0.0052 −0.0024 −0.0124 −0.0018 0.0258 0.9278 0.000240333.15 −0.0037 −0.0011 −0.0099 −0.0041 0.0206 0.8840 0.000235343.15 −0.0033 −0.0013 −0.0058 −0.0068 0.0062 0.8620 0.000117

glycerol + sulfolane313.15 −0.4835 −0.2896 −0.3135 −0.4113 −0.1874 0.9998 0.000974323.15 −0.2296 −0.1321 −0.1413 −0.0879 0.0380 0.9996 0.000660333.15 −0.1190 −0.0662 −0.0635 0.0092 0.0750 0.9994 0.000616343.15 −0.0667 −0.0362 −0.0269 0.0342 0.0656 0.9994 0.000226353.15 −0.0400 −0.0215 −0.0092 0.0362 0.0485 0.9995 0.001834363.15 −0.0255 −0.0137 −0.0007 0.0316 0.0336 0.9997 0.000064

a � = standard deviation, Eq. (3).

Fig. 15. Viscosity deviation from ideality of glycerol (1) + sulfolane (2) mixtures against temperature as function of glycerol mole fraction; (�) 313.15 K; (©) 323.15 K; (�)333.15 K; (�) 343.15 K; (�) 353.15 K; (�) 363.15 K.

144 M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147

Fig. 16. Effect of temperature (T) and mole fraction (x1) on density of binary mixtures of [BMIM][NTf2] + sulfolane.

(x1) o

ai

3

ufe[lo

Fig. 17. Effect of temperature (T) and mole fraction

ction between [BMIM][NTf2] and sulfolane, leading to a strongernteraction.

.4. Viscosity

The present measured viscosities against temperature are tab-lated at different composition as summarized in Tables 3 and 4

or the binary mixtures of [BMIM][NTf2] + sulfolane and glyc-

rol + sulfolane, respectively. The results show that pureBMIM][NTf2] had greater viscosity than sulfolane and simi-arly glycerol had greater viscosity than sulfolane. The viscosityf [BMIM][NTf2] + sulfolane mixtures increased exponentially

n density of binary mixtures of glycerol + sulfolane.

as the mole fraction of [BMIM][NTf2] increased, likewise theviscosity of glycerol + sulfolane mixtures also increased drasticallywith the increment of glycerol mole fraction. Figs. 12 and 13show viscosity against temperature for [BMIM][NTf2] + sulfolaneand glycerol + sulfolane mixtures, respectively. The result showsthat the viscosity of all the pure and binary mixtures decreaseddrastically with the increase of temperature. Moreover, as shownin Fig. 12, the viscosity of pure [BMIM][NTf2] is higher than the

viscosity of sulfolane. Therefore, the viscosity of binary mixturewas enhanced as the concentration of sulfolane in the mixtureincreased. Similarly, as shown in Fig. 13, the viscosity of pure glyc-

M.A. Kassim et al. / Thermochimica Acta 639 (2016) 130–147 145

Fig. 18. Effect of temperature (T) and mole fraction (x1) on viscosity of binary mixtures of [BMIM][NTf2] + sulfolane.

(x1) on

eg

T

�

w

p

Fig. 19. Effect of temperature (T) and mole fraction

rol is higher than sulfolane and increasing as the concentration oflycerol increased.

The viscosity values, �, were fitted using Vogel-Fulcher-ammann (VFT) Eq. (7) [43].

= Aexp(

B

T − T0

)(7)

here A, B and T0 are adjustable parameter.Table 7 summarizes the fitting parameters of VFT equation of

ure components and binary mixtures.

viscosity of binary mixtures of glycerol + sulfolane.

Furthermore, using Eyring equation, the activation enthalpy andentropy for the viscosity of pure components and binary mixturesare described and correlated through the following Eq. (8) [28]:

� =(hNAVm

)exp

(−�SR

)exp

(�H

R

)(8)

where � is the viscosity, Vm is the molar volume, h and NA are Plankand Avogadro’s constants, respectively.

Correspondingly, �S, �H, R and T are the molar activationentropy, molar activation enthalpy, gas constant and temperature,

1 himic

rbuf

R

dAc

�

a

3

ta

�

wa

v[TiaTesME

3

t[FhtmetcTarwotito

4

sgtv

[

[

[

[

[

[

46 M.A. Kassim et al. / Thermoc

espectively. The values of �S and �H for pure components andinary mixtures were obtained by fitting the experimental datasing the rearranged linear Eyring equation against 1/T (10) as

ollow:

ln(�VmhNA

)= �H

T− �S (9)

From the intercept, �S as a function of concentration wereetermined and from the slope, the values of �H were obtained.dditionally, the molar Gibbs free energy of activation, �G can bealculated at mean temperature using the given Eq. (10):

G = �H − T�S (10)

Table 8 summarizes the experimental derived data for the molarctivation �S and �H of pure components and binary mixtures.

.5. Viscosity deviation

Using the experimental viscosity data of pure components andheir binary mixtures, the viscosity deviations of the mixtures, ��re calculated using the following Eq. (11) [44].

� = � − (x1�1 + x2�2) (11)

here, the subscript ‘1′ and ‘2′ denote [BMIM][NTf2] or glycerol,nd sulfolane, respectively.

Figs. 14 and 15 show the composition dependence oniscosity deviation for mole fraction of binary mixturesBMIM][NTf2] + sulfolane and glycerol + sulfolane, respectively.he viscosity deviations, ��, for both binary mixtures at var-

ous temperatures are negative over the entire mole fractionnd become less negative with increasing the temperature.he negative �� values for [BMIM][NTf2] + sulfolane and glyc-rol + sulfolane mixtures can be explained by the fact that thetrong dipole interaction is dominant in these mixtures [44].oreover, these deviations are regressed using the Redlich-Kister

q. (6). The adjustable parameters, Ai, are summarized in Table 9.

.6. Simultaneous effect of the temperature and composition

The simultaneous effect of the temperature and concen-ration on the density and viscosity of binary mixtures ofBMIM][NTf2] + sulfolane and glycerol + sulfolane are presented inigs. 16–19. Fig. 16 shows that the concentration of [BMIM][NTf2]as more prominent effect on density in comparison to tempera-ure. However, the statement is vice versa for glycerol + sulfolane

ixtures. Apparently, temperature and concentration have similarffects on viscosity for both binary mixtures (Fig. 17). Fig. 18 showshat the influence of concentration and temperature are signifi-ant due to higher viscosity of [BMIM][NTf2] relative to sulfolane.herefore, the viscosity of the mixtures increases significantly withddition of small amount of [BMIM][NTf2]. In Fig. 19, an intenseeduction of viscosity for glycerol + sulfolane mixtures is observedith increasing temperature, predominantly at high concentration

f glycerol. Due to a higher viscosity of glycerol relative to sulfolane,he addition of small amount of glycerol contributes to significantncrement in viscosity for the mixtures. However, at high tempera-ure, the concentration of glycerol does not exert significant impactn viscosity.

. Conclusion

In this research, we conduct a density and viscosity mea-

urements for binary mixtures of [BMIM][NTf2] + sulfolane andlycerol + sulfolane at whole range of composition and tempera-ure ranging from 293.15 to 363.15 K. Based on the densities andiscosities, we computed the excess properties, thermal expansion

[

a Acta 639 (2016) 130–147

coefficient, and Gibbs free energy. Both binary mixtures display atemperature independent volume expansion. Binary mixtures of[BMIM][NTf2] + sulfolane exhibit a negative value in excess molarvolume and negative viscosity deviation values indicating for astrong interaction between components. Binary mixtures of glyc-erol + sulfolane display a positive value in excess molar volume andnegative viscosity deviation values indicating for a weak interactionbetween components. Behaviors of binary solutions are justifiedusing COSMO-RS based on the �-profile and �-potential and activ-ity coefficient of each of the component. The molar enthalpy (�H),entropy (�S) and Gibbs free energy of activation (�G) of viscos-ity were also calculated. Simultaneous effect of composition andtemperature for the binary mixtures were also illustrated.

Acknowledgements

The author would like to express acknowledgment to Uni-versiti Malaya for granting this project under Universiti MalayaResearch Grant UMRG (RP006F-13SUS) and Postgraduate ResearchFund (PG102-2013A). Our sincere appreciations are also extendedto University of Malaya Centre of Ionic Liquids (UMCiL).

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