Fluid Phase Equilibria 383 (2014) 32–42

Apparent molar volumes and isentropic compressions ofbenzylalkylammonium ionic liquids in dimethylsulfoxide from293.15 K to 328.15 K

Dasthaiah Keshapolla, Ramesh L. Gardas *Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

A R T I C L E I N F O

Article history:Received 19 June 2014Received in revised form 19 September 2014Accepted 23 September 2014Available online 28 September 2014

Keywords:Ionic liquidApparent molar volumeRedlich–Mayer equationIon–solvent interaction

A B S T R A C T

Densities, r and speeds of sound, u of four benzylalkylammonium ionic liquids (ILs), namely,benzylmethylammonium propionate (BMAP), benzylmethylammonium hexanoate (BMAH), benzyldi-methylammonium propionate (BDMAP) and benzyldimethylammonium hexanoate (BDMAH) withdimethylsulfoxide (DMSO) have been determined as a function of ILs concentration, ranging from (0.1 to0.8) mol kg�1, at temperatures from 293.15 K to 328.15 K with 5 K interval and at atmospheric pressure.The apparent molar volumes, Vm, apparent molar isentropic compression, Ks,m and limiting apparentmolar expansion, Em

1 of the ILs have been calculated from the experimental density and speed of sounddata. The Redlich–Mayer type of equation was fitted to the apparent molar volume and apparent molarisentropic compression data, to evaluate apparent molar volume at infinite dilution Vm

1 and apparentmolar isentropic compression, Km

1 at infinite dilution.ã 2014 Elsevier B.V. All rights reserved.

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journal homepage: www.else vie r .com/ locat e/fluid

1. Introduction

Ionic liquids (ILs) are a family of substances that areconstituted almost entirely with ions and melt below the boilingpoint of water. Their properties such as negligible vaporpressures, broad liquidus range, high thermal stability, wideelectrochemical window and high specific solvation abilitiesmake these potentially useful as “designer solvents” [1]. Thus, ILsfind extensive applications in chemical reactions, electrochemis-try, bio-catalysis, flue gases absorption, biomass dissolution,separation science and many more areas of varying interests andutility [2–7]. To exploit the tremendous potential of ionic liquidsand also design newer ones for specific purposes, it is essential tohave a thorough grasp on their physicochemical and thermody-namic properties, especially on the nature of the interactionsbetween constituent cations and anions with an added solvent, soour research group is engaged in systematic study of thermophysical properties of novel neat ionic liquids and their mixtureswith solvents [8–10].

Various reports are available in literature [11–16] on thedensity, r and speed of sound, u for the binary mixtures of ILs,however the studies on the thermodynamic and thermo physicalbehavior of ammonium based ILs solutions are limited. The

* Corresponding author. Tel.: +91 44 2257 4248; fax: +91 44 2257 4202.E-mail address: gardas@iitm.ac.in (R.L. Gardas).

0378-3812/$ – see front matter ã 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.fluid.2014.09.022

thermodynamic data of binary mixtures of ILs with organicsolvents or water are required not only to understand themolecular interactions existing in mixtures but also essential forthe efficient design of equipment's, engineering applicationsconcerning heat transfer, mass transfer and fluid flow etc. Asurvey of the literature reveals that merely a few studies such ashave been reported in comparison to those with pure ILs [17–19].Molecular structure and intermolecular forces play an importantrole in determining the thermodynamic properties of liquidswhich in turn depend upon the manner in which components areassociated with each other. Volumetric properties of binarymixtures have been extensively studied to understand the natureand extent of various intermolecular interactions existing amongvarious species present [20].

The purpose of the present study was to investigate theinteraction of benzylmethylammonium propionate (BMAP), ben-zylmethylammonium hexanoate (BMAH), benzyldimethylammo-nium propionate (BDMAP), and benzyldimethylammoniumhexanoate (BDMAH), with DMSO and solvation behavior throughthermo physical property measurements of these solutions sincevolumetric and acoustic properties have been used as potentialtools to understand and analyze the solvation behavior of solutes inaqueous and non-aqueous solutions [13–15].

The chosen solvent DMSO is an important polar solvent anduseful in the wide range of applications in applied chemistry [21].Due to its excellent solvating power and high dielectric constantvalue (e = 46.45 at T = 298.15 K) [22], DMSO is used as a solvent for

List of abbreviations

ILs Ionic LiquidsBMAP Benzylmethylammonium propionateBMAH Benzylmethylammonium hexanoateBDMAP Benzyldimethylammonium propionateBDMAH Benzyldimethylammonium hexanoateDMSO Dimethylsulfoxide

List of symbolsm Molalityr Densityu Speed of soundVm Apparent molar volumeVm

1 Apparent molar volume at infinite dilutionEm

1 Apparent molar expansion at infinite dilutionk Isentropic compressionKs,m Apparent molar isentropic compressionKm

1 Apparent molar isentropic compression at infinitedilution

M Molar mass

D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42 33

chemical reactions, polymerization reaction and displacementreactions.

2. Experimental

2.1. Chemicals

All the chemicals were used as such without any furtherpurification. Source, purity and CAS number of chemicals used inthis study are presented in Table 1. The purity of the dimethylsuf-oxide was further assessed by comparison of the experimentaldensity and speed of sound data with literature values presented inTable 2 [23–44].

2.2. Synthesis of benzylalkylammonium ionic liquids

The benzylalkylammonium based ILs were synthesizedaccording to the method followed by Anouti et al. [45].Benzylalkylamine was taken in a two-necked round-bottomedflask, immersed in an ice-bath and equipped with a refluxcondenser, a dropping funnel (to add the acid), and a thermometer.

Table 1Provenance and purity of chemicals used in this work.

Chemical name CAS number

Benzyl methylamine 103-67-3

Benzyldimethylamine 103-83-3

Propionic acid 79-09-4

Hexanoic acid 142-62-1

Dimethylsulfoxide 67-68-5

Benzylmethylammonium propionate (BMAP) –

Benzylmethylammonium hexanoate (BMAH) –

Benzyldimethylammonium propionate (BDMAP) –

Benzyldimethylammonium hexanoate (BDMAH) –

a Synthesized in our research laboratory.b From Gas Chromatography analysis.c From 1H NMR and 13C NMR spectroscopic techniques.d Checked by Karl Fischer Titrator from Analab (Micro Aqua Cal 100). Concentration calc

ionic liquids.

Under vigorous stirring, acid was added drop-wise to the amine.Since acid–base neutralization reaction is highly exothermic, thetemperature was maintained below 278.15 K. Reaction mixturewas stirred continuously for 24 h at ambient temperature and aviscous ionic liquid was obtained. Since equimolar acid–base usedin the reaction and ionic liquid formed without any byproduct; nofurther purification of ionic liquid required. However, the residualamine or acid, if any, was evaporated under vacuum, and theremaining liquid was further dried in vacuum at 50 �C for two daysto remove water content and stored under nitrogen atmosphere.Dried ILs were characterized by 1H NMR and 13C NMRspectroscopic techniques and also confirmed that no residualamine or acid is present in the IL. So, probable impurity in dried ILcould be water content only. Water content of studied ionic liquidsdetermined by Karl Fischer titration and presented in Table 1. The1H and 13C NMR of synthesized ILs were recorded on BrukarAvance 500 MHz spectrometer using CDCl3 as a solvent and TMS asthe internal standard. IR spectra were recorded on JASCO FT/IR-4100 spectrometer. NMR and IR spectra analysis for studied ionicliquids are presented in Supplementary material S1.

2.3. Apparatus and procedure

Binary solutions were made by weight in air-tight glass vials byusing an analytical balance (Sartorius, CPA225D) having a precisionof �0.0001 g. The chemicals were kept in tightly sealed bottles tominimize the absorption of atmospheric moisture. Binary sol-utions of the ILs were prepared by mass, and then diluted by DMSOto get the test samples. Concentrations of the dilute solutionsranged from (0.1 to 0.8) mol kg�1. The uncertainty in the molality ofbinary solutions, u (m) = 7.41 �10�6mol kg�1. The density, r andspeed of sound, u of the solutions were measured simultaneouslyby using vibrating-tube digital density and speed of sound analyzer(Anton Paar, DSA 5000 M) at T = (293.15–328.15) K with 5 K intervaland at atmospheric pressure, p = (�0.1 MPa). The two-in-oneinstrument is equipped with a density cell and a speed of soundcell, both the cells are temperature-controlled by a built-in Peltierthermostat (PT-100) having an accuracy of �0.01 K. The instrumentwas calibrated by double distilled, degassed and deionized waterand with dry air at atmospheric pressure according to theprocedure mentioned in the instrument manual. The speed ofsound can be regarded as thermodynamic property, as theultrasonic absorption is negligible due to the use of low frequency(3 MHz) and low amplitude of the acoustic waves [46,47]. Theuncertainties in the measurement of density and speed of soundwere �5.0 � 103 kg m3 and �0.5 m s1, respectively.

Source Purity in mass fraction

Initial Final Water contentd

Sigma–Aldrich >0.970 >0.980b 0.05%Sigma–Aldrich >0.990 >0.995b 0.05%Qualizens >0.990 >0.995b 0.02%Sigma–Aldrich >0.995 >0.997b 0.02%Merck >0.999 >0.999b 0.05%Present worka – >0.99c 0.25%Present worka – >0.99c 0.26%Present worka – >0.99c 0.22%Present worka – >0.99c 0.20%

ulations for binary mixtures have been done including this water content present in

Table 3Densities, r and apparent molar volumes, Vmof benzylalkylammonium based ionic liq

m r 10�3 Vm� 106 r � 10�3 Vm� 1mol kg�1 kg m�3 m3mol�1 kg m�3 m3mo

BMAP + DMSOT/K = 293.15 T/K = 298.15

0.00000 1.100407 1.095387

0.15880 1.097192 194.73 1.092131 195.930.21001 1.096384 193.97 1.091315 195.140.33994 1.094445 192.97 1.089346 194.140.42849 1.093247 192.49 1.088132 193.650.52787 1.092018 192.03 1.086891 193.170.61235 1.091036 191.71 1.0859 192.840.72355 1.089842 191.34 1.084699 192.440.77682 1.089273 191.21 1.084118 192.32

T/K = 313.15 T/K = 318.15

0.00000 1.080327 1.075307

0.1588 1.076987 199.38 1.071942 200.540.21001 1.07611 198.72 1.071048 199.910.33994 1.074039 197.74 1.068942 198.950.42849 1.072769 197.23 1.067652 198.450.52787 1.07148 196.71 1.066345 197.920.61235 1.070456 196.35 1.065309 197.540.72355 1.069225 195.9 1.064064 197.080.77682 1.068642 195.75 1.063476 196.92

BMAH + DMSOT/K = 293.15 T/K = 298.15

0.08525 1.097215 247.32 1.092191 248.640.17086 1.094427 245.92 1.089391 247.270.26383 1.091568 245.32 1.086518 246.680.32531 1.089684 245.29 1.08462 246.680.42572 1.08696 244.76 1.081898 246.110.50888 1.084817 244.45 1.079739 245.810.58316 1.083005 244.19 1.077939 245.520.66053 1.081196 243.96 1.07613 245.28

T/K = 313.15 T/K = 318.15

0.08525 1.077143 252.44 1.072127 253.730.17086 1.074286 251.39 1.069257 252.760.26383 1.071368 250.87 1.066325 252.270.32531 1.06945 250.87 1.064398 252.280.42572 1.06669 250.3 1.061627 251.71

Table 2The comparison of experimentally measured density, r and speed of sound, u ofpure DMSO at T = (293.15 to 313.15) K and p = � 0.1 MPa with the correspondingliterature values.

T/K r 10�3/(kg m�3) u /m s�1

Expt. Lit. Expt. Lit.

293.15 1.100407 1.10040a 1502.01 1502.6o

1.10041b 1506.7p

1.10050c

298.15 1.095387 1.09537d 1485.10 1484q

1.09562e 1485.12p

1.0957f 1485.5r

1.0954g 1487s

1.09537h 1485.12t

1.09527i 1489.2u

303.15 1.090367 1.09038j 1468.15 1470v

1.090467k 1469.2w

1.0904i

1.0904l

308.15 1.085348 1.08550m 1451.28 1452.66w

313.15 1.080327 1.0803n 1434.50 –

1.0804k

1.08032c

Standard uncertainties u are u (r) = 5.0 � 10�3kg m�3,u (u) = 0.02 m s�1, u (p) = 1 kPa,and u(T) = 0.01 K.aRef. [23], bRef. [24], cRef. [25], dRef. [26], eRef. [27], fRef. [28], gRef. [29], hRef. [30],iRef. [31], jRef. [32], kRef. [33], lRef. [34], mRef. [35], nRef. [36], 0Ref. [37], pRef. [38],qRef. [39], rRef. [40], sRef. [41], tRef. [42], uRef. [43], vRef. [44], and wRef. [45].

34 D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42

3. Results and discussion

3.1. Apparent molar volumes

The experimental densities, r for the studied solutions atvarious temperatures are reported in Table 3 as a function ofmolality of the ILs. The density values for the binary mixtures ofbenzylalkylammonium ionic liquids with DMSO decreases withthe increase in ILs concentration and temperature, and also at aparticular temperature density values for the binary mixturesdecrease with increasing the number of alkyl groups in the cationicpart of the ILs. The plot of density versus concentration of ILs atdifferent temperatures for (BDMAP + DMSO) binary solution isshown in Fig. 1. The trend is found to be quite consistent with thefinding that density generally decreases with the increase in alkylgroup of cation [48].

The apparent molar volumes, Vm of the ILs in DMSO werecalculated from the solution densities using the followingequation:

Vm ¼ Mr� r � ro

m�r�ro

� �(1)

In the above equation, m is the molality of the ILs in DMSO, r and ro

stand for the densities of the binary solutions and pure solvent,respectively, and M is the molar mass of the ILs. The standarduncertainty in the determined, apparent molar volume, Vm rangesfrom (0.01 to 0.05) � 106m3mol�1 for low and high concentration

uids in DMSO at T = (293.15 to 328.15) K and p = � 0.1 MPa.

06 r � 10�3 Vm� 106 r � 10�3 Vm� 106

l�1 kg m�3 m3mol�1 kg m�3 m3mol�1

T/K = 303.15 T/K = 308.151.090367 1.085348

1.087085 197.05 1.082035 198.22 1.086245 196.33 1.081176 197.53 1.084242 195.33 1.079139 196.53

1.083009 194.83 1.077888 196.03 1.081754 194.33 1.076617 195.52

1.080753 193.99 1.075607 195.16 1.079545 193.58 1.074387 194.73 1.078969 193.44 1.073808 194.58

T/K = 323.15 T/K = 328.151.070285 1.065262

1.066897 201.7 1.061856 202.85 1.065989 201.1 1.060936 202.27 1.06385 200.16 1.058762 201.37 1.062539 199.66 1.057432 200.88

1.061215 199.12 1.056085 200.34 1.060162 198.75 1.055019 199.96 1.058904 198.28 1.053746 199.48

1.05831 198.11 1.053143 199.31

T/K = 303.15 T/K = 308.15 1.087178 249.86 1.082159 251.17

1.084353 248.65 1.079319 250.02 1.081467 248.07 1.076415 249.47 1.079566 248.05 1.074507 249.46

1.076826 247.5 1.071757 248.89 1.074675 247.16 1.069603 248.55 1.072866 246.87 1.067792 248.25 1.071055 246.63 1.065979 247.99

T/K = 323.15 T/K = 328.15 1.067111 255.01 1.062097 256.28

1.064231 254.12 1.059209 255.47 1.061286 253.66 1.056252 255.05 1.059351 253.69 1.054307 255.1

1.05657 253.12 1.051515 254.54

Table 3 (Continued)

m r 10�3 Vm� 106 r � 10�3 Vm� 106 r � 10�3 Vm� 106 r � 10�3 Vm� 106

mol kg�1 kg m�3 m3mol�1 kg m�3 m3mol�1 kg m�3 m3mol�1 kg m�3 m3mol�1

0.50888 1.064531 249.94 1.059461 251.35 1.054396 252.76 1.049334 254.180.58316 1.062719 249.63 1.057646 251.03 1.052575 252.44 1.047507 253.860.66053 1.060902 249.37 1.055826 250.77 1.050751 252.17 1.045682 253.58

BDMAP + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.10215 1.096797 220.1 1.091802 221.03 1.086802 222.02 1.081798 223.060.18727 1.093972 219.85 1.088983 220.85 1.083987 221.89 1.078993 222.940.28834 1.090856 219.45 1.085859 220.52 1.080865 221.59 1.075885 222.630.35024 1.089105 219.09 1.084109 220.16 1.079126 221.22 1.074137 222.30.45766 1.086239 218.57 1.081237 219.67 1.07625 220.74 1.07126 221.840.55297 1.083804 218.28 1.078796 219.39 1.073794 220.5 1.068799 221.610.64516 1.081811 217.67 1.076795 218.79 1.071787 219.91 1.066789 221.030.72159 1.080317 217.15 1.075293 218.27 1.07028 219.4 1.065275 220.52

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.10215 1.076795 224.08 1.071792 225.12 1.066788 226.17 1.061783 227.220.18727 1.073999 223.99 1.069004 225.06 1.064009 226.12 1.059015 227.190.28834 1.070896 223.7 1.06591 224.78 1.060923 225.86 1.055938 226.950.35024 1.06915 223.38 1.064164 224.47 1.05918 225.56 1.054198 226.650.45766 1.066275 222.93 1.061292 224.03 1.056296 225.17 1.051317 226.280.55297 1.063812 222.72 1.058827 223.83 1.053845 224.95 1.048867 226.070.64516 1.061796 222.14 1.056809 223.27 1.051828 224.38 1.046849 225.510.72159 1.060278 221.64 1.055287 222.77 1.050303 223.9 1.045323 225.03

BDMAH + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.08608 1.095927 272.52 1.090937 273.67 1.085943 274.88 1.08095 276.090.17468 1.091657 271.96 1.086676 273.21 1.081699 274.45 1.076722 275.710.23738 1.088564 272.56 1.083598 273.81 1.078637 275.05 1.073672 276.330.30986 1.085271 272.52 1.08031 273.8 1.075362 275.05 1.07041 276.330.37187 1.08273 272.06 1.077777 273.34 1.072835 274.6 1.067893 275.880.46497 1.078893 271.96 1.073947 273.25 1.069015 274.53 1.064084 275.820.53368 1.07624 271.79 1.071299 273.1 1.066376 274.38 1.06145 275.680.61853 1.073157 271.54 1.068223 272.84 1.063305 274.14 1.058386 275.45

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.08608 1.075953 277.34 1.070958 278.58 1.065961 279.84 1.060966 281.080.17468 1.071744 276.98 1.066767 278.25 1.06179 279.53 1.056813 280.820.23738 1.068706 277.61 1.063744 278.89 1.058781 280.18 1.053817 281.480.30986 1.065458 277.61 1.060508 278.9 1.055558 280.2 1.050609 281.510.37187 1.062949 277.17 1.058007 278.47 1.053069 279.77 1.048129 281.090.46497 1.059151 277.13 1.054222 278.44 1.049296 279.75 1.04437 281.070.53368 1.056526 276.99 1.051606 278.3 1.046689 279.62 1.041774 280.940.61853 1.053471 276.76 1.048559 278.08 1.043652 279.4 1.038747 280.73

Standard uncertainties u are u(m) = 7.41 x 10�6mol kg�1, u(r) = 5.0 x 10�3 kg m�3,u(p) = 1kPa, and u(T) = 0.01 K, u (Vm) = (0.01 to 0.05) x 106 m3mol�1for low and highconcentrationrange of ILs, respectively.

1.04

1.05

1.06

1.07

1.08

1.09

1.10

0.0 0.2 0.4 0.6 0.8

ρ.10

-3/ k

g.m

-3

m / mol.kg-1

Fig. 1. Variation of density, r versus molality of ILs, m for BDMAP + DMSO binarysolution: (^) 293.15 K, (&) 298.15 K, (~) 303.15 K, (*) 308.15 K, (*) 313.15 K, (O)318.15 K, (&) 323.15 K, and (D) 328.15 K.

170

190

210

230

250

270

290

0.0 0.2 0.4 0.6 0.8

Vm

. 106

/ m3 .

mol

- 1

m / mol.kg-1

Fig. 2. Comparison of apparent molar volume, Vm at 298.15 K as a function of molalityof ILs, m for (^) BMAP, (~) BDMAP, (&) BMAH, (*) BDMAH in DMSO solutions.

D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42 35

Table 4The apparent molar volume at infinite dilution,Vm1, standard deviations s(Vm1), Svand Bvparameters calculated using Eq. (2) for the binary solutions of(benzylalkylammonium based ionic liquids + DMSO) at temperatures, T = (293.15 to 328.15) K and p = � 0.1 MPa.

T/K Vm1� 106m3mol�1 Sv� 106mol kg�1 Bv� 106 kg m�3 s(Vm

1) � 106m3mol�1

BMAP + DMSO293.15 199.73 �15.28 6.39 0.06298.15 200.95 �15.34 6.30 0.06303.15 201.75 �14.05 5.24 0.05308.15 202.71 �13.22 4.54 0.04313.15 203.62 �12.30 3.81 0.04318.15 204.51 �11.31 3.04 0.03323.15 205.43 �10.43 2.40 0.03328.15 206.28 �09.40 1.68 0.03

BMAH + DMSO293.15 250.38 �12.70 6.04 0.17298.15 251.51 �11.84 5.26 0.17303.15 252.35 �10.03 3.78 0.15308.15 253.39 �08.81 2.75 0.15313.15 254.32 �07.28 1.53 0.14318.15 255.33 �06.02 0.55 0.14323.15 256.35 �04.85 �0.30 0.14328.15 257.31 �03.50 �1.27 0.14

BDMAP + DMSO293.15 219.74 3.52 �7.65 0.08298.15 220.38 4.66 �8.32 0.08303.15 221.27 5.05 �8.45 0.09308.15 222.29 5.01 �8.26 0.09313.15 223.23 5.30 �8.35 0.09318.15 224.21 5.52 �8.40 0.09323.15 225.13 5.96 �8.64 0.08328.15 226.13 6.15 �8.66 0.09

BDMAH + DMSO293.15 271.63 4.08 �5.29 0.26298.15 272.53 5.18 �6.04 0.25303.15 273.71 5.26 �5.97 0.24308.15 274.80 5.79 �6.29 0.24313.15 276.01 5.96 �6.33 0.24318.15 277.16 6.29 �6.49 0.24323.15 278.37 6.50 �6.57 0.24328.15 279.47 7.12 �7.00 0.24

Standard uncertainties u are u (p) = 1 kPa, and u (T) = 0.01 K.

36 D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42

range of IL (level of confidence = 0.68). The apparent molar

Fig. 3. Plot of apparent molar volume, Vm versus concentration, m of (BDMAP + DMSO) binary solutions at different temperatures.

Table 5Coefficients of Eq. (3) and standard deviations of Fit (SD).

ILs A B C SDa

BMAP 100.41 0.477 �0.00047 0.07BMAH 195.08 0.181 0.00002 0.05BDMAP 223.66 �0.192 0.00060 0.08BDMAH 249.23 �0.059 0.00046 0.06

a Standard deviation.

D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42 37

volumes (Table 3) increase with an increase in temperature of eachbinary system. The apparent molar volume decreases with anincrease in concentration for each binary system, decreasing Vm

with increasing concentration of ionic liquid have also beenobserved [49] in the system ([MOA]NTf2 + ethanol), Fig. 2 showsthe dependence of Vm on molality for different ILs in DMSOsolutions studied at 298.15 K. Also a representative plot for(BDMAP + DMSO) binary solution as a function of ILs concentrationand temperature is shown in Fig. 3. The plot shows various regionsof different colors such that the variation of Vm color region with ILconcentration and temperature on moving from dark blue to greenand then to orange regions.

The apparent molar volumes at infinite dilution, Vm1 of ILs have

been evaluated by Redlich–Mayer type equation [50] and it wasfitted to the apparent molar volume data as:

Vm ¼ V1m þ Sv � m1=2 þ Bv � m (2)

where Sv and Bv are the empirical parameter, Vm1 is the apparent

molar volume at infinite dilution (equals to the standard apparentmolar volume), which is the limiting value of the apparent molarvolume when their concentration is close to zero. At infinitedilution, the interactions of cations with anions are usually absent.For the binary system, each IL ion is surrounded by only DMSOmolecules and is an infinitely large distance from the other ions,therefore, Vm

1 is unaffected by IL (ion–ion) interaction [51] andVm

1 measures only the ion–solvent interaction.Results presented in Table 4 shows that Vm

1 values all are largeand positive; this is due to the large intrinsic volumes of the cationand anion, strong ion–solvent interactions at infinite dilution (asion–ion interactions vanish at infinite dilution). The smallest valueof Vm

1 corresponding to the BMAP + DMSO system and largestvalues of Vm

1 was observed for BDMAH in DMSO solution, it wasshown in Fig. 4. On the other hand, the slope, Sv shows that the ion–ion interactions. In generally, Sv is positive, but it can be negativefor some electrolytes; particularly for tetraalkylammonium salts[52–54], and also have been observed in this work, values of Vm

1,Sv and Bv for the ILs + DMSO systems were obtained at varioustemperatures by a linear regression analysis, and the results arelisted in Table 4 along with their the standard deviations. It wasfound that the apparent molar volumes at infinite dilution, Vm

1 forthe ILs increase with the increase in the methyl group in the cation/

170

190

210

230

250

270

290

288 298 308 318 328

Vm∞

. 106

/ m3 .

mol

-1

T / K

Fig. 4. Comparison of apparent molar volume at infinite solution, Vm1 as a function

of temperature, T for (^) BMAP, (~) BDMAP, (~) BMAH, (*) BDMAH in DMSOsolutions.

anion part of ILs at a particular temperature. In addition, Vm1 for

the ILs also increases with increasing temperature due to therelease of DMSO molecules from the loose solvation layer of the ILs.This suggests that solvation of the ILs in DMSO was weakened athigher temperatures. A similar trend was also observed for 1-ethyl-3- methylimidazolium methyl sulfate, 1-hexyl-3-methylimidazo-lium methyl sulfate, and 1-hexyl-3-methylimidazolium ethylsulfate in aqueous solutions [55]. It is interesting to find thatthe Sv values for the binary solutions of BMAP + DMSO andBMAH + DMSO are negative where as for the binary solutions ofBDMAP + DMSO and BDMAH + DMSO the values are positive.

Generally, the Sv values are frequently used as a qualitativemeasure for the ion–ion interactions, there are many factorsaffecting the Sv values of electrolyte, such as the nature ofelectrolyte and solvent, electrolyte concentration. It was alsoobserved that the absolute values of Sv became larger withincreasing temperature, suggesting an increased electrostaticinteraction between cation and anion of the ILs and a decreasedsolvation interaction between the ILs and the solvent at highertemperatures. The magnitudes of Sv parameter for BMAP/BMAH inDMSO at all the temperatures are negative, which suggest that theion–solvent interactions are stronger than the ion–ion interac-tions, whereas values of Sv positive for (BDMAP/BDMAH in DMSO)binary systems. The value of Bv decreases with an increase intemperature for each binary system. The negative Bv valuesindicate an increase in ion–ion interactions for the systems(BDMAP/BDMAH + DMSO) and the simultaneous release of DMSOmolecules into the bulk solvent. The positive Bv values indicate thatdecreases ion–ion interaction for the binary solutions (BMAP/BMAH + DMSO).

The temperature dependence of apparent molar volume atinfinite dilution, Vm

1 can be expressed as the second-orderpolynomial of the absolute temperature:

V1m ¼ A þ BT þ CT2 (3)

where A, B, and C are empirical parameters, and their valuesobtained by regression analysis are listed in Table 5. The limitingapparent molar expansion Em

1 can be obtained by differentiatingEq. (3) with respect to temperature to be obtained.

E1m ¼ dV1m

dT

� �p¼ B þ 2CT (4)

It can be observed that at each temperature Em1 values

(Table 6) for the ILs in DMSO are positive. This suggests that, onheating some DMSO molecules may be released from the loosesolvation layer of the ions. In addition, the Em

1 values of decreaseswith increasing temperature, in the binary solution of BMAP +DMSO, as at higher temperatures there are less bound solventmolecules on the ions, so the expansibility of the ILs in solutionsbecomes weaker [56].

The apparent molar expansion at infinite dilution, Em1 are also

evaluated by considering the mid-point temperature, Tm [57] of thetemperature range studied through the following relation:

Table 6The infinite dilution apparent molar expansion, Em

1 of benzylalkylammonium based ILs in DMSO at different temperatures, T = (293.15 to 328.15) K and p = � 0.1 MPa.

ILs Em1� 10�6 /m3mol�1 K�1

T/K 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15

DMSOBMAP 0.2008 0.1961 0.1914 0.1867 0.1820 0.1773 0.1725 0.1678BMAH 0.1957 0.1960 0.1962 0.1965 0.1967 0.1969 0.1972 0.1974BDMAP 0.1649 0.1710 0.1771 0.1832 0.1893 0.1954 0.2015 0.2076BDMAH 0.2115 0.2161 0.2207 0.2254 0.2300 0.2346 0.2392 0.2438

The standard deviations in Em1 ranges from (0.02 to 0.04) � 10�6m3mol�1 K�1, Standard uncertainties u are u (p) = 1 kPa, and u(T) = 0.01 K.

38 D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42

V1m ¼ b1 þ b2 T � Tmð Þ þ b3 T � Tmð Þ2 (5)

The temperature derivative of Eq. (5) at constant pressure,Em

1 = (@Vm1/@T)p = b2 + 2b3� (T � Tm), where Tm= 310.65 K for the

studied temperature range resulting into (b2� 5b3) being equal toEm

1 at 308.15 K. The polynomial coefficients b1, b2 and b3; Em1 at

308.15 K are given in Table 7.

3.2. Apparent molar isentropic compression

From the speed of sound, u and density, r data the isentropiccompression, ks was calculated for the binary mixtures by usingthe Laplace–Newton equation:

ks ¼ 1r � u2 (6)

where r is the density and u is the speed of sound for studiedsolutions. The speed of sound (Table 8) decreases with an increasein temperature and concentration of the solute for the binarysolutions (BMAH/BDMAP/BDMAH + DMSO), this behavior was alsoobserved in the system ([MOA] NTf2 + methyl acetate/methanol)[58]. The plot of speed of sound versus concentration of ILs atdifferent temperatures for (BDMAP + DMSO) binary solution isshown in Fig. 5. In the binary system (BMAP + DMSO) we haveobserved that there is no change in speed of sound at lowertemperature range, but slightly decreases at higher temperature,this results suggested that the interactions between ion–solventare very less in this binary solution.

The isentropic compression is the sum of two contributions, ks(solvent intrinsic) and ks (solute intrinsic). The ks (solventintrinsic) is the isentropic compression due to the compressionof solvent molecules (DMSO), while ks (solute intrinsic) is theisentropic compression due to the compression of the hydrationshell of ions or penetration of solvent molecules into the ILs [52].The ks values increases with an increase in temperature for eachbinary system. Fig. 6 represents plot for BDMAP + DMSO at a fixedcomposition due to an increase in thermal agitation giving rise tosolvent molecules being released from the solute, resulting in anincrease in the solution volume thus making the solution morecompressible. The plot of ks versus molality of ILs at particulartemperature (at 298.15 K) for different ILs mixture investigated inthe studied solutions BDMAH/BDMAP + DMSO solutions are more

Table 7The polynomial coefficients of Eq. (5) and infinite dilution apparent molar expansion,

ILs b1�10�6m3mol�1 b2� 10�6m3mol�1 K�1

DMSOBMAP 203.18 0.184

BMAH 253.86 0.196

BDMAP 222.71 0.186

BDMAH 275.39 0.227

The standard deviations in Em1 ranges from (0.02 to 0.04) � 10�6m3mol�1 K�1, Standa

compressible than the BMAP/BMAH + DMSO solutions shown inFig. 7. These results suggested that increasing of methyl group inthe cation part of ionic liquids, increases the isentropic compres-sion. For each system the ks value increase with an increase inconcentration of solute because the ks (solvent intrinsic) effect isdominant over the ks (solute intrinsic) effect. The isentropiccompression of different ILs solutions follows the order DMSO in,BDMAH > BDMAP > BMAH > BMAP.

The apparent molar isentropic compression of solutions, Ks,m inthe studied solutions was computed from the density and speed ofsound experimental data according to the following equation:

Ks;m ¼ ks � mr

� k0s � r � ksr0

m � r � r0 (7)

where ks and ks0 are the isentropic compressions of the solution

and solvent, respectively. The apparent molar isentropic com-pressions of the solutions, Ks,m values are also given in theSupplementary material S2. The standard uncertainty in thedetermined apparent molar isentropic compression, Ks,m rangesfrom (0.06 to 0.10) 10�15m3mol�1 Pa�1 at low and high concen-tration range of IL (level of confidence = 0.68). In this work theapparent molar isentropic compression of each binary systemincreases with an increase in temperature, similar behavior hasbeen obtained for other systems studied for the [C3mim][Br] + H2Osystem [59]. A representative plot for (BDMAP + DMSO) binarysolution as a function of IL concentration and temperature isshown in Fig. 8. The plot depicts regions of different colors suchthat the Ks,m values increase with ILs concentration and tempera-ture on moving from dark blue to green and then to orange coloredregions. At a particular temperature the apparent molar isentropiccompression, Ks,m values for the BDMAH + DMSO mixture are morecompressible than the corresponding values for the BMAP + DMSOmixture, and slightly more compressible than the binary solutions(BMAH/BDMAP + DMSO). The strong ion association in BMAPcauses a loss of compression for the BMAP + DMSO mixturecompared to the other mixture.

An equation of the form was used for correlating theexperimental apparent molar isentropic compression data,

Ks;m ¼ K1m þ Sk � m1=2 þ Bk � m (8)

where Km1 is the limiting apparent molar isentropic compression,

and Sk and Bk are empirical parameters. The values of Km1, Sk, and

Em1 at 308.15 K and p = � 0.1 MPa for benzylalkylammonium based ILs in DMSO.

b3� 10�3m3mol�1 K�2 Em1� 10�6 /m3mol�1 K�1

�0.471 0.1860.023 0.1960.609 0.1830.461 0.225

rd uncertainties u are u (p) = 1 kPa, and u (T) = 0.01 K.

Table 8Speed of sound, u isentropic compression, ks and apparent molar isentropic compression, Ks,m of ionic liquids in DMSO at T = (293.15 to 328.15) K and p = � 0.1 MPa.

m mol kg�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1

BMAP + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.00000 1502.01 1485.10 1468.15 1451.280.15880 1501.25 404.40 1484.19 415.66 1467.44 427.18 1450.46 439.280.21001 1501.16 404.74 1484.32 415.90 1467.25 427.62 1450.17 439.810.33994 1501.25 405.41 1484.26 416.69 1467.02 428.55 1449.80 440.860.42849 1501.15 405.91 1484.07 417.26 1466.73 429.20 1449.44 441.590.52787 1501.42 406.22 1484.26 417.63 1466.84 429.64 1449.45 442.110.61235 1501.93 406.31 1484.80 417.70 1467.36 429.73 1449.92 442.240.72355 1502.11 406.66 1484.85 418.14 1467.32 430.23 1449.79 442.820.77682 1502.42 406.70 1484.81 418.39 1467.51 430.35 1449.96 442.95

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.00000 1434.50 1417.78 1401.17 1384.600.15880 1433.56 451.81 1416.71 464.80 1399.98 478.22 1383.32 492.140.21001 1433.21 452.40 1416.32 465.44 1399.52 478.94 1382.81 492.930.33994 1432.69 453.60 1415.65 466.80 1398.72 480.46 1381.87 494.610.42849 1432.23 454.43 1415.10 467.73 1398.07 481.50 1381.14 495.760.52787 1432.15 455.02 1414.93 468.41 1397.82 482.27 1380.79 496.640.61235 1432.56 455.20 1415.26 468.65 1398.07 482.58 1380.97 497.010.72355 1432.34 455.86 1414.98 469.38 1397.72 483.39 1380.56 497.910.77682 1432.48 456.02 1415.07 469.58 1397.75 483.64 1380.53 498.21

BMAH + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.08525 1501.17 404.4348 1484.21 415.6336 1467.56 427.07 1450.67 439.100.17086 1499.10 406.5856 1482.35 417.7483 1465.36 429.47 1448.37 441.660.26383 1498.20 408.1404 1481.34 419.4243 1464.25 431.27 1447.18 443.580.32531 1497.18 409.4033 1479.99 420.9251 1463.10 432.71 1446.00 445.090.42572 1496.50 410.8024 1479.62 422.1952 1462.43 434.21 1445.25 446.700.50888 1495.70 412.0544 1478.36 423.7608 1461.35 435.72 1444.12 448.300.58316 1495.09 413.0807 1478.04 424.6523 1460.74 436.82 1443.43 449.490.66053 1494.55 414.0709 1477.45 425.7059 1460.11 437.94 1442.76 450.67

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.08525 1433.87 451.55 1417.14 464.43 1400.50 477.77 1383.91 491.600.17086 1431.50 454.25 1414.69 467.29 1397.97 480.80 1381.34 494.780.26383 1430.21 456.31 1413.31 469.49 1396.51 483.14 1379.80 497.270.32531 1428.98 457.91 1412.04 471.19 1395.18 484.95 1378.42 499.190.42572 1428.17 459.62 1411.15 473.02 1394.23 486.89 1377.39 501.260.50888 1426.96 461.33 1409.88 474.84 1392.89 488.83 1376.01 503.310.58316 1426.23 462.59 1409.13 476.16 1392.10 490.23 1375.18 504.800.66053 1425.52 463.85 1408.35 477.51 1391.27 491.67 1374.30 506.33

BDMAP + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.10215 1498.54 406.01 1481.90 417.07 1465.02 428.70 1448.15 440.780.18727 1495.40 408.77 1478.72 419.95 1461.81 431.71 1444.91 443.910.28834 1491.91 411.85 1474.88 423.36 1457.85 435.31 1441.22 447.480.35024 1489.94 413.61 1472.95 425.15 1456.25 436.97 1439.30 449.400.45766 1486.61 416.56 1469.46 428.31 1452.66 440.31 1435.65 452.900.55297 1483.60 419.19 1466.37 431.09 1449.16 443.45 1432.05 456.230.64516 1481.29 421.27 1463.95 433.32 1446.67 445.81 1429.50 458.720.72159 1479.36 422.96 1461.95 435.11 1444.62 447.70 1427.40 460.73

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.10215 1431.37 453.27 1414.65 466.22 1398.03 479.61 1381.47 493.490.18727 1428.11 456.53 1411.38 469.60 1394.72 483.14 1378.16 497.160.28834 1424.41 460.23 1407.66 473.46 1390.97 487.17 1374.39 501.350.35024 1422.45 462.26 1405.67 475.58 1388.95 489.39 1372.36 503.660.45766 1418.73 465.94 1401.92 479.42 1384.78 493.68 1368.11 508.180.55297 1415.05 469.45 1398.18 483.11 1381.40 497.26 1364.72 511.900.64516 1412.45 472.07 1395.52 485.88 1378.72 500.15 1362.01 514.930.72159 1410.31 474.18 1393.34 488.10 1376.52 502.48 1359.79 517.37

BDMAH + DMSOT/K = 293.15 T/K = 298.15 T/K = 303.15 T/K = 308.150.08608 1498.02 406.61 1481.42 417.68 1464.58 429.30 1447.73 441.380.17468 1493.89 410.46 1476.97 421.84 1460.02 433.68 1443.15 445.930.23738 1490.86 413.30 1474.22 424.62 1457.37 436.50 1440.49 448.850.30986 1487.91 416.20 1471.10 427.72 1454.54 439.53 1437.74 451.940.37187 1485.35 418.62 1468.34 430.34 1451.71 442.29 1434.83 454.850.46497 1481.76 422.14 1464.67 434.04 1447.98 446.16 1431.05 458.890.53368 1479.26 424.62 1462.32 436.52 1445.70 448.67 1428.80 461.480.61853 1476.12 427.65 1459.01 439.76 1442.25 452.12 1425.28 465.10

D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42 39

Table 8 (Continued)

m mol kg�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1 u m s�1 ks� 1012 Pa�1

T/K = 313.15 T/K = 318.15 T/K = 323.15 T/K = 328.150.08608 1430.97 453.88 1414.29 466.82 1397.69 480.21 1381.16 494.090.17468 1426.71 458.39 1410.38 471.25 1393.84 484.76 1377.30 498.820.23738 1423.70 461.64 1407.04 474.84 1390.44 488.52 1373.91 502.710.30986 1420.98 464.82 1404.40 478.08 1387.83 491.86 1371.29 506.170.37187 1418.01 467.87 1401.29 481.34 1384.64 495.30 1368.08 509.750.46497 1414.20 472.08 1397.45 485.73 1380.78 499.86 1364.20 514.500.53368 1411.94 474.77 1395.18 488.52 1378.52 502.75 1361.91 517.520.61853 1408.39 478.55 1391.59 492.47 1374.92 506.86 1358.33 521.77

Standard uncertainties u are u (m) = 7.41 �10�6mol.kg�1, u (u) = 0.5 m.s�1, u (p) = 1 kPa, u (T) = 0.01 K, and u (ks) = 0.000181 �10�10 kg�1m s�2 for low and high concentrationrange of ILs, respectively.

1320

1360

1400

1440

1480

1520

0.0 0.2 0.4 0.6 0.8

u / m

.s-1

m / mol.kg-1

Fig. 5. Variation of speed of sound, u versus molality of ILs, m for BDMAP + DMSObinary solution: (^) 293.1 K, (&) 298.15 K, (~) 303.15 K, (*) 308.15 K, (*)313.15 K,(�) 318.15 K, (&) 323.15 K, and (D) 328.15 K.

40 D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42

Bk obtained for each mixture at the experimental temperatures arelisted in Table 9. There are two factors that contribute to Km

1: (a)large organic ions can have some solvent intrinsic compressibility(positive effect) due to the intermolecular free space that makesthe solution more compressible [60] and (b) penetration (negative

390

420

450

480

510

540

0.0 0.2 0.4 0.6 0.8

κ s /

T. P

a-1

m / mol.kg-1

Fig. 6. Variation of isentropic compression, ks versus molality of ILs, m, forBDMAP + DMSO binary solution: (^) 293.15 K, (&) 298.15 K, (~) 303.15 K, (*)308.15 K, (*)313.15 K, (�) 318.15 K, (&) 323.15 K, and (D) 328.15 K.

effect) of the solvent molecules into the intra-ionic free space dueto the interaction of ions with the neighboring solvent molecule.The solute intrinsic effect is essentially an electrostriction effectthat causes constriction in the solution volume, resulting in a morecompact and less compressible medium. Positive limiting apparentmolar isentropic compressions, for (ILs + DMSO), are due to thehigher solvent intrinsic compressibility than the penetrationeffect. This behavior also has been observed for [[MOA] NTf2 + ethylacetate/ethanol] binary solutions [49].

4. Conclusions

From the measured density and speed of sound data attemperatures from (293.15 to 328.15) K for mixtures of thebenzylalkylammonium ionic liquids with organic solvent (dime-thylsulfoxide). The apparent molar volumes and compressionswere calculated and fitted satisfactorily to the Redlich–Mayerequations. The obtained values for the limiting apparent molarvolume and compressions values were used to acquire someinformation with respect to ion–solvent interactions in theILs + DMSO mixtures. It was found that the strength of interactionbetween the ionic liquids with the studied organic solvent have theorder: BDMAH > BMAH > BDMAP > BMAP. The limiting apparentmolar expansion Em

1 values were obtained at different temper-atures. We note that at each temperature Em

1 values for ionicliquid in DMSO have positive values. The positive Km

1values of (ILs

410

420

430

440

0.0 0.2 0.4 0.6 0.8

κ s /

T. P

a-1

m / mol.kg-1

Fig. 7. Comparison of isentropic compression, ks at 298.15 K as a function ofmolality of ILs, m for (^) BMAP, (~) BDMAP, (&) BMAH, (*) BDMAH in DMSOsolutions.

Fig. 8. Plot of apparent molar isentropic compression, Ks,m versus concentration, m of (BDMAP + DMSO) binary solutions at different temperatures. (For interpretation of thereferences to color in the text, the reader is referred to the web version of this article.)

Table 9The apparent molar isentropic compression at infinite dilution, Km

1, Sk and Bk parameters calculated using Eq. (8) for binary mixtures of benzylalkylammonium based ionicliquids with DMSO at temperatures, T = (293.15 to 328.15) K and p = � 0.1 MPa.

T/K Km1� 1014m3mol�1 Pa�1 Sk� 1014m3mol�3/2 kg1/2 Pa�1 Bk� 1014m3mol�2 kg Pa�1 s(Km

1) � 1012a

BMAP + DMSO293.15 09.43 �1.90 0.60 0.01298.15 09.94 �2.50 1.07 0.03303.15 09.76 �0.87 �0.12 0.01308.15 10.05 �0.54 �0.39 0.01313.15 10.36 �0.22 �0.65 0.02318.15 10.70 0.04 �0.87 0.02323.15 11.05 0.35 �1.11 0.02328.15 11.37 0.76 �1.41 0.02

BMAH + DMSO293.15 11.27 2.54 �2.62 0.07298.15 11.68 2.54 �2.58 0.06303.15 11.43 4.73 �4.25 0.09308.15 11.74 5.35 �4.74 0.10313.15 12.04 6.04 �5.29 0.10318.15 12.34 6.81 �5.88 0.11323.15 12.68 7.51 �6.43 0.12328.15 13.04 8.21 �6.95 0.12

BDMAP + DMSO293.15 11.35 1.92 �1.75 0.01298.15 11.20 3.54 �2.82 0.02303.15 11.56 3.58 �2.71 0.05308.15 12.02 3.41 �2.46 0.04313.15 12.41 3.59 �2.52 0.04318.15 12.83 3.76 �2.58 0.04323.15 13.15 4.42 �3.05 0.04328.15 13.57 4.70 �3.19 0.04

BDMAH + DMSO293.15 14.67 1.80 �1.36 0.03298.15 14.59 3.63 �2.70 0.02303.15 15.01 3.85 �2.83 0.05308.15 15.50 4.07 �2.94 0.06313.15 15.98 4.19 �2.86 0.04318.15 16.50 4.17 �2.61 0.07323.15 17.05 4.33 �2.65 0.08328.15 17.54 4.78 �2.91 0.08

Standard uncertainties u are u (p) = 1 kPa, and u (T) = 0.01 K.a Standard deviation in apparent molar isentropic compression at infinite dilution.

D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42 41

42 D. Keshapolla, R.L. Gardas / Fluid Phase Equilibria 383 (2014) 32–42

in DMSO) binary solutions suggest the dominance of the intrinsiccompressibility over the penetration effect.

Acknowledgements

Authors are thankful to Council of Scientific and IndustrialResearch (CSIR), Department of Science and Technology (DST) andIIT Madras for their financial support. Authors acknowledgeVickramjeet Singh and Akash Gupta for the help in synthesis ofILs and drawing few figures.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fluid.2014.09.022.

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