Synthesis, Characterization, and Thermophysical Properties of 1,8-Diazobicyclo[5.4.0]undec-7-ene Based Thiocyanate Ionic LiquidsKallidanthiyil Chellappan Lethesh,*,† Syed Nasir Shah,† and M. I. Abdul Mutalib‡

†PETRONAS Ionic Liquids Centre, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia‡Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia

*S Supporting Information

ABSTRACT: In this work, synthesis of 12 ionic liquids basedon 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) cation withanions such as chloride, bromide, and thiocyanate wereperformed. The new ionic liquids were characterized usingnuclear magnetic resonance spectroscopy and elementalanalysis. The effect of alkyl spacer length on thephysicochemical properties of DBU based ionic liquids wasstudied by attaching ethyl, butyl, hexyl, octyl, decyl, andtetradecyl groups to the DBU cation. The effects of temperature on the density and viscosity over a temperature range from293.15 K to 373.15 K were recorded at atmospheric pressure. From the experimental density values, the thermal expansioncoefficient values were calculated. The surface tension of the neat ionic liquids was measured at seven different temperatures(293.15 K, 303.15 K, 313.15 K, 323.15 K, 333.15 K, 343.15 K, and 353.15 K). The surface entropy and surface enthalpy werecalculated from the experimental surface tension value at 303.15 K. The thermal behavior of these ionic liquids was studied usingthermogravimetric analysis and differential scanning calorimetry.

■ INTRODUCTION

In the past 2 decades scientists from various disciplines wereattracted to ionic liquids (ILs) due to their special charactersticssuch as very low vapor pressure, large electrochemical window,large liqud range, high ionic conductivity, and high thermalstability. Ionic liquids can be termed as organic salts made upentirely of ions.1,2 Ionic liquids, which are liquids at roomtemperature, are known as room-temperature ionic liquids(RTILs). Usually ionic liquids are made up of ugly organiccations with anions of organic or inorganic nature. Theirproperties depend mostly on the nature of the cations andanions. For instance, ionic liquids containing bis-(trifluoromethylsulfonyl)imide (Tf2N) anion would be waterimmiscible.3 On the other hand, acetate anions will formhydrophilic (water-soluble) ionic liquids.4 Because of theirspecial properties, ionic liquids find applications in differentareas such as organic synthesis,2,5−8 separation processes,9−15

catalysis,16−18 electrochemistry,19−21 and biomass dissolu-tion,22−27 etc. 1,8-Diazobicyclo[5.4.0]undec-7-ene (DBU) is aclass of amidine compounds that has found applications inorganic synthesis as a catalyst and as a non-nucleophilic base.Because of its strong alkaline nature, the use of DBU in organicsynthesis has been investigated extensively.28−32 Although DBUwas successful in these reactions, it suffered from lessrecyclability, which is against the sustainable approach ofmodern science. Ionic liquids based on DBU were developed toaddress the issues associated with the use of DBU in organicsynthesis.33−35 Although some ionic liquids based on DBU hasbeen reported, useful data on the thermophysical properties ofDBU based ionic liquids is limited and that is a hindrance in

utilizing them for various applications in which the conven-tional solvents are not suitable. In this work attempt is made todescribe the synthesis, characterization, and thermophysicalproperties of DBU based cation with various alkyl chainlengths. An overview of ionic liquids presented in this work isshown in Figure 1.

■ EXPERIMENTAL SECTIONMaterials. All the chemical were purchased from Acros

Organics (Geel, Belgium), and no further purification step wasperformed.

Synthesis of Halide Salts. General Procedure. To asolution of 1,8-diazobicyclo[5.4.0]undec-7-ene (10 g, 65.68mmol) in acetonitrile (30 mL), 1-bromoethane (8.58 g, 78.82mmol) was added and stirred at 60 °C for 48 h. The reactionmixture was cooled using an ice bath, and acetonitrile was

Received: November 14, 2013Accepted: May 1, 2014Published: May 29, 2014

Figure 1. Overview of ionic liquids used in this study.

Article

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© 2014 American Chemical Society 1788 dx.doi.org/10.1021/je400991s | J. Chem. Eng. Data 2014, 59, 1788−1795

removed in a rotary evaporator. The white solid obtained waswashed with cyclohexane (3 × 25 mL) and dried in a vacuumoven at 70 °C for 24 h.Synthesis of Ionic Liquids. General Procedure. To a

solution of 1-ethyl-1,8-diazobicyclo[5.4.0]undec-7-ene bromide(8 g, 30.62 mmol) in dichloromethane (30 mL), sodiumthiocyanate (3.72 g, 45.94 mmol) was added and the reactionwas vigorously stirred using a mechanical stirrer at 25 °C for 24h. The precipitate formed was filtered off, and the dichloro-methane layer was washed with cold water (3 × 25 mL).Dichloromethane was evaporated under vacuum to give 1-ethyl-1,8-diazobicyclo[5.4.0]undec-7-ene thiocyanate ([DBU-Et][SCN]) as a pale yellow solid. The ionic liquid formed wasfurther dried in a vacuum oven at 60 °C for 24 h.Characterization. Carbon, hydrogen, nitrogen, and sulfur

content were analyzed using elemental analyzer (CE Instru-ments EA-1110). 1H and 13C NMR spectra were recorded on aBruker Avance 500 spectrometer. Water content was measuredin a coulometric Karl Fischer titrator (model DL39).Density and Viscosity Measurements. An Anton Paar

viscometer (model SVM3000) was used to measure theviscosity of ionic liquids. Density measurement was carriedout using an Anton Paar densitimeter (DMA 5000). Standarduncertainties are u(ρ) = ± 0.00001 g·cm−3, u(η) = ± 0.32 %mPa·s, and u(T) = ± 0.01 K.Measurement of Surface Tension. A pendant drop

method was used to measure the surface tension. A syringe wasused to generate the drop, and it was photographed using acamera (OCA 20). Software (SCA 22) was used to evaluate theshape of the drop. The measurements were recorded from293.15 K to 353.15 K.Thermal Decomposition. The thermal decomposition

temperature of the ionic liquids was measured using athermogravimetric analyzer (PerkinElmer, Pyris V-3.81).Samples were heated from 25 °C to 750 °C in a crucibleunder a nitrogen atmosphere. The heating rate was 10 °C·min−1.The accuracy of the measurement is better than ± 1 °C.Melting Point. The melting point was measured using DSC

(differential scanning calorimetry; PerkinElmer, model Pyris1).The samples were heated in sealed aluminum pans in nitrogenatmosphere from 25 °C to 150 °C and then cooled to −60.75°C and again heated to 150 °C. The heating and cooling rate is10 °C·min −1.Halide Content. The halide content was measured using an

ion chromatogram (Metrohm model 761 Compact IC). A 20μL aliquot of ionic liquid was used for one measurement. Anaqueous solution of Na2CO3 (3.2 mmol) and NaHCO3 (1.0

mmol) was used as eluent. One molar solution of sulfuric acidwas used as the solution for regeneration. The sample wasprepared by dissolving 100 mg of ionic liquid in a solution ofacetonitrile (20 mL) and water (30 mL). Software (MetrodataIC Net 2.3) was used to analyze the results.

■ RESULTS AND DISCUSSION

Synthesis. DBU based ionic liquids were prepared byfollowing Scheme 1.Synthesis of the DBU based ionic liquids involves two steps.

The halide salt was prepared by the quaternization of 1,8-diazobicyclo[5.4.0]undec-7-ene with corresponding alkyl hal-ides at 70 °C for 48 h in acetonitrile. The ionic liquids obtainedafter the removal of the solvent were washed with cyclohexaneand dried under vacuum. The halide salts were obtained inmore than 90 % yield. Anion metathesis with Na[SCN]resulted in the corresponding [SCN]− based hydrophilic DBUbased ionic liquids. All of the ionic liquids based on thiocyanateanions were initially obtained as pale yellow liquid; three ofthem (3a, 3b, and 3f) crystallized upon standing at roomtemperature for 1 week.

Viscosity. Viscosity measurement was performed in atemperature range from 293.15 K to 373.15 K using anAnton Paar viscometer. Viscosity decreased as the temperatureincreased. The corresponding data are summarized in Table 1,Figure 2, and Figure 3 as a function of temperature. As can beseen in Figure 2 and Figure 3, the alkyl spacer length has astrong impact on the viscosity of the DBU based ionic liquids.

Scheme 1. Synthesis of DBU Based Ionic Liquids

Table 1. Experimental Viscosity (η) of Ionic Liquids as aFunction of Temperaturea

η

T (mPa·s)

(K) [DBU-Hex][SCN] [DBU-Oct][SCN] [DBU-Dec][SCN]

293.15 4198.8 1956.0 2604.0303.15 1504.3 806.71 1059.3313.15 635.21 379.33 490.58323.15 306.06 198.69 253.63333.15 164.15 114.14 143.66343.15 96.185 70.680 87.778353.15 60.575 46.570 57.259363.15 40.353 32.245 39.312373.15 28.295 23.311 27.999

aStandard uncertainties are u(η) = ± 0.32 % mPa·s and u(T) = ± 0.01K.

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The ionic liquids containing the alkyl groups such as ethyl,butyl, and tetradecyl were solids at room temperature.However, imidazolium based thiocyanate salts containingethyl and butyl groups are room-temperature ionic liquidshaving viscosity of 23.8 mPa·s and 59.8 mPa·s, respectively, at298.2 K.36,37 The DBU ionic liquids with alkyl spacer lengths ofsix, eight, and ten carbon atoms are room-temperature ionicliquids. Their viscosity increases in the order [DBU-Oct][SCN]< [DBU-Dec][SCN] < [DBU-Hex][SCN]. Higher viscositiesof the ionic liquids with longer alkyl spacer length are due tothe increased van der Waals interaction between the alkylgroups. The viscosity of N-hexyl isoquinolinium thiocyanate(745.13 mPa·s at 298.15 K) is considerably lower than that of[DBU-Hex][SCN] (4198.8 mPa·s at 298.15 K), while theviscosity of N-octylisoquinolinium thiocyanate (2388.0 mPa·sat 298.15 K) ionic liquid is much higher than that of [DBU-Oct][SCN] (1956.0 mPa·s) .38,39 Viscosity of both imidazoliumand isoquinolinium based ionic liquids also showed a lineardependence with temperature.

Density. Table 2 and Figure 4 show the effect of alkyl spacerlength on the density of DBU based ionic liquids at a

temperature range from 293.15 K to 373.15 K. The densityvalues decreased as the temperature increased. When the alkylchain length of the cation is increased, the density of DBU ionicliquids decreased and it is in the order [DBU-Hex][SCN] >[DBU-Oct][SCN] > [DBU-Dec][SCN]. The decrease indensity with an increase in alkyl spacer distance is due to theinsufficient close packing of the cations. Similar behavior wasobserved for imidazolium based ionic liquids.40 Density valuesof DBU based protic ionic liquids with different anions such asbis(trifluoromethanesufonyl)imide, acetate, methanesulfonate,trifluoroacetate, and trifluoromethanesulfonate are higher thanthiocyanate ionic liquids with DBU cation of varying alkyl chainlength.41 The density of thiocyanate ionic liquids with cationiccores such as imidazolium,36 pyridinium,42 pyrrolidinium,43 andisoquinolinium38,39,43 was reported to be in the same range asthat of DBU based ionic liquids. The density of all of theseionic liquids showed a linear dependency with temperature.

Estimation of Volumetric Properties. The experimentalvalues of density, ρ, for the studied ionic liquids with (T −298.15 K) were fitted by applying the following equation:

Figure 2. Plot of the viscosity as a function of temperature for thefollowing: ■, [DBU-Hex][SCN]; ▲, [DBU-Oct][SCN]; ●, [DBU-Dec][SCN].

Figure 3. Plot of log η as a function of temperature: ■, [DBU-Hex][SCN]; ▲, [DBU-Oct][SCN]; ●, [DBU-Dec][SCN].

Table 2. Experimental Density (ρ) of Ionic Liquids as aFunction of Temperaturea

ρ

T (g cm−3)

(K) [DBU-Hex][SCN] [DBU-Oct][SCN] [DBU-Dec][SCN]

293.15 1.0618 1.0288 1.0136303.15 1.0564 1.0230 1.0082313.15 1.0509 1.0172 1.0026323.15 1.0451 1.0116 0.9973333.15 1.0395 1.0053 0.9921343.15 1.0338 1.0005 0.9869353.15 1.0284 0.9953 0.9818363.15 1.0229 0.9897 0.9765373.15 1.0174 0.9843 0.9711

aStandard uncertainties are u(ρ) = ± 0.00001 g·cm−3 and u(T) = ±0.01 K.

Figure 4. Densities as a function of temperature: ■, [DBU-Hex][SCN]; ●, [DBU-Oct][SCN]; ▲, [DBU-Dec][SCN].

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ρ·

= − −−

⎡⎣⎢

⎤⎦⎥ A A Tln

g cm(( /K) 298.15)3 0 1

(1)

where c is a constant and A1 = (α/K) = −[(∂ ln ρ)/∂(T −298.15)]p, where α is the thermal expansion coefficient. Thecalculated values of correlation coefficients and the standarddeviation (SD) are shown in Table 3.

The molecular volumes, Vm, the standard molar entropy(S°), and the lattice energy of the ionic liquids [DBU-Hex][SCN], [DBU-Oct][SCN], and [DBU-Dec][SCN] havebeen calculated using the eqs 2, 3, and 4, respectively, and thevalues are listed in Table 4.

ρ=V

MNm

A (2)

° · · = +− −S V(303.15)/(J K mol ) 1246.5( /nm ) 29.51 1m

3

(3)

ρ· = +−U /(kJ mol ) 1981.2( /M) 103.8POT1 1/3

(4)

where M is molecular weight and NA is Avogadro’s number.Table 4 indicates that the molecular volume of the ionic

liquids increases as the alkyl spacer length increases. The meanimpact of a methylene (-CH2-) group to the molar volume is

0.028 nm3, which is in agreement with n-alcohols (0.0280 nm3)and n-paraffins (0.0267 nm3).44 As can be seen in Table 4, thelattice energy values of [DBU-Hex][SCN], [DBU-Oct][SCN],and [DBU-Dec][SCN] are 313 kJ·mol −1, 301 kJ·mol −1, and292 kJ·mol −1, respectively, and adjacent to previously reportedionic liquids.45−48 The studied ionic liquids have significantlylower lattice energies than those of the alkali halides.49 Forinstance, among alkali halides, CsI has the lowest lattice energy(613 kJ·mol−1), which is substantially higher than ionic liquids.

Surface Tension. The surface tension values of the DBUbased thiocyanate ionic liquids is given in Table 5 and Figure 5

in a temperature range between 293 K and 353 K. Surfacetension also shows a linear relationship with temperature as inthe case of density and viscosity. The highest surface tension

Table 3. Fitting Parameter (A and B) Values with R2 andStandard Deviation (SD)a for Empirical Correlation ofDensity,b Viscosity,c and Surface Tensiond of the MeasuredIonic Liquids

[DBU-Hex][SCN] [DBU-Oct][SCN] [DBU-Dec][SCN]

DensitySD 1.37·10−5 2.28·10−5 5.88·10−6

R2 0.9829 0.9851 0.9999A0 6.9655 6.9325 6.9184A1 5.00·10−4 6.00·10−4 5.00·10−4

ViscositySD 1.56·10−2 1.04·10−2 1.03·10−2

R2 0.9892 0.9909 0.9914A4 - 6.5231 - 5.7042 - 5.7861A5 2936.1 2606.1 2666.7

Surface TensionSD 4.92·10−4 1.14·10−1 1.03·10−2

R2 0.9717 0.9726 0.9999A2 4.9630 4.6525 4.0883A3 4.00·10−3 5.07·10−3 2.71·10−3

aStandard deviation values were calculated using

=−Z Z

nSD

( )exp cal2

DAT

where Z exp and Zcal are experimental and calculated data values,respectively. nDAT is the number of experimental points.

bEquation fordensity temperature dependence: ln ρ/ (kg·m−3) = A0 − A1(T −298.15), where

α ρ= = − ∂∂ −

⎛⎝⎜

⎞⎠⎟A

TKln

( 298.15)P

1

cEquation for viscosity temperature dependence: log η /(mPa·s) = A4+ (A5/T).

dEquation for surface tension temperature dependence: σ/(mN·m) = A2 − A3T.

Table 4. Calculated Values of Volume Properties of IonicLiquids at Temperature 303.15 K

ρ Vm S° UPOT

ionic liquid (kg·m−3 ) (nm3) (J·K−1·mol−1) (kJ·mol−1)

[DBU-Hex][SCN] 1056.4 0.464 608 313[DBU-Oct][SCN] 1023.0 0.525 684 301[DBU-Dec][SCN] 1008.2 0.579 751 292

Table 5. Experimental Surface Tension (σ) of Ionic Liquidsas a Function of Temperaturea

σ

T (mN·m−1)

(K) [DBU-Hex][SCN] [DBU-Oct][SCN] [DBU-Dec][SCN]

293.15 37.98 31.85 32.94303.15 37.37 31.19 32.68313.15 37.20 30.64 32.41323.15 36.82 29.88 32.14333.15 36.03 29.44 31.87343.15 35.94 29.18 31.59353.15 35.59 28.86 31.32

aStandard uncertainties are u(σ) = ± 0.04 mN··m−1 and u(T) = ±0.01 K.

Figure 5. Surface tension as a function of temperature: ■, [DBU-Hex][SCN]; ▲, [BDU-Oct][SCN]; ●, [DBU-Dec][SCN].

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value is observed for [DBU-Hex][SCN], and [DBU-Oct]-[SCN] showed the lowest value. The surface tensions of all ofthe ionic liquids under study are higher than common volatilesolvents such as methanol (22.07 mN·m−1), acetone (23.5 mN·m−1), and n-alkanes50−52 but lesser than water (71. 98 mN·m−1). Surface tension values of thiocyanate ionic liquids withimidazolium, pyrrolidinium, and pyridinium cations with analkyl chain length of four carbon atoms are considerably higherthan the ionic liquids in the present study.51,53 Surface tensionof all of these ionic liquids decreases as the temperatureincreased.The surface tension and the temperature are correlated using

the following equation.

σ · = −− A A T/(mN m )12 3 (5)

where σ represents surface tension, A2 and A3 are fittingparameters, and T is the temperature. Table 3 shows theestimated values of fitting coefficients along with the standarddeviation (SD). The measured surface tension values wereapplied to calculate the entropy and enthalpy of surfaceformation. It was possible to determine the surface entropyfrom the slope, Sa, of eq 6.

σ= = − ∂∂

⎜ ⎟⎛⎝

⎞⎠S A

T Pa 3

(6)

σ σ= = − ∂∂

⎜ ⎟⎛⎝

⎞⎠E A

T Ps 2

(7)

The surface enthalpy (Es) was calculated using eq 7 from thesurface tension at 303.15 K, and the results are tabulated inTable 6. The ionic liquids under study showed lower surface

entropy than the common organic solvents. It may be due tothe enhanced degree of surface orientation in ionic liquids. Thelower value of surface entropy of ionic liquids in the presentstudy is in good agreement with previously reported ionicliquids,46,54,55 and it is an indication of an enhanced surfaceorientation in ionic liquids. NaNO3 has a surface enthalpy valueof 146 mJ·m−2, which is noticeably higher than ionic liquids.56

It is a sign of the lower degree of interaction among the ions inionic liquids. The surface enthalpy value of the ionic liquids is

very close to the common organic solvents such as octane (51.1mJ·m−2) and benzene (67 mJ·m−2).Because of the intrinsic nature of the ionic liquids, it is

difficult to get reliable data of their critical temperature (Tc),which is an important parameter in correlating equilibrium andtransport properties of liquids.57 Hence Guggenheim58 (eq 8)and Eotvos59 (eq 9) empirical equations were used to predictthe critical temperature of the ionic liquids, and the results areshown in Table 7. Enthalpy of vaporization of ionic liquids wasestimated using eq 10.

σ = −σ⎛⎝⎜

⎞⎠⎟E

TT

1cG

11/9

(8)

σρ

= −⎛⎝⎜

⎞⎠⎟

MK T T( )

2/3

cE

(9)

σΔ ° = +H V N0.01121( ) 2.41g

m2/3

A1/3

(10)

where Eσ is the total surface energy of ionic liquids, whichequals the surface enthalpy because of the tiny volumedifference due to thermal expansion at the temperatures thatare not similar to the critical temperature Tc

G, K is a constant, σis surface tension, M is molecular weight, ρ is density, T is themeasured surface tension temperature, and NA is Avogadro’snumber.It is also possible to calculate boiling temperatures, Tb, of

ionic liquids from the critical temperature (Tc) by making useof the assumptions from Rebelo et al.60 According to them, therelation between Tb and Tc is Tb ≈ 0.6Tc for an ionic liquid.The calculated Tb values of ionic liquids are given in Table 7.

Thermal Decomposition. The thermal decompositiontemperature (Td) of all 12 ionic liquids and melting points(Tm) of solid ionic liquids are shown in Table 8. All of the DBUbased ionic liquids showed a good thermal stability (> 290 °C)at a scan rate of 10 °C·min−1. Among the ionic liquids studied,chloride salts have lower thermal stability compared to thebromide and thiocyanate anions. It may be due to the highernucleophilicity of the chloride anions which in turn decreasesthe thermal stability by decomposing the cationic core viabimolecular substitution (SN2) reaction of the easily accessiblealkyl group.61 The higher thermal stability of the bromide andthiocyanaye ionic liquids can be attributed to their bigger sizeand the less nucleophilic nature. Diop et al. have reported thatthe thermal stability of the DBU based ionic liquids withchloride anion is below 300 °C, which is in good agreementwith our results.62 According to Nowicki and co-workers,33 thethermal decomposition temperature of DBU ionic liquids withhydroxide anion was 362.8 °C and 362.1 °C, respectively. Thedifference in the thermal decomposition temperature of thesame cation with different anions is due to the difference in the

Table 6. Surface Thermodynamic Functions of Pure IonicLiquids at Temperature 303.15 K: Surface Entropy (Ss) andSurface Enthalpy (Es)

103·Ss Es

ionic liquid (mJ·K−1·m−2) (mJ·m−2)

[DBU-Hex][SCN] 40 49.49[DBU-Oct][SCN] 51 46.65[DBU-Dec][SCN] 27 40.86

Table 7. Critical Temperature (Tc) Normal Boiling Temperature (Tb), and Enthalpy of Vaporization (ΔH) of Ionic Liquids atTemperature 303.15 K

Guggenheim Eotvos

Tc Tb Tc Tb Δ1gHm°

ionic liquid (K) (K) (K) (K) (kJ·mol−1)

[DBU-Hex][SCN] 1475.83 885.5 1064.26 638.55 153.71[DBU-Oct][SCN] 1080.22 648.1 992.59 595.55 139.47[DBU-Dec][SCN] 1813.89 1088.33 1074.15 644.49 155.68

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basicity or the nucleophilicity of the anions.63 The thermalstability of DBU based protic ionic liquids with anions such asacetate, methanesulfonate, and trifluoroacetate, etc., are in therange from 171 °C to 451 °C.41 Imidazolium and pyrrolidiniumbased thiocyanate ionic liquids showed thermal behavior similarto DBU derived thiocyanate ionic liquids.36,43 On the contrary,pyridinium thiocyanate ionic liquids have lower thermalstability than DBU ionic liquids with the same anion.42 Figure6 shows thermogravimetric analysis (TGA) profiles for ionic

liquids [DBU-Et][Br], [DBU-Oct][Cl], [DBU-Dec][Br],[DBU-Oct][SCN], and [DBU-Dec][SCN]. In the DSC traces

at a scanning rate of 10 °C·min −1 (figure not shown), only anendothermic peak corresponding to Tm was detected for all ofthe ionic liquids which are solids at room temperature.

Interstitial Model for Ionic Liquids. A new theoreticalmodel called the interstitial model14,15 was developed for ionicliquids by abstracting the essence of the hole model for moltensalts.16 The model is based on four assumptions which can beseen elsewhere.8,17 In this model the interstitial volume, v, forthe ionic liquids was calculated using an equation from classicalstatistical mechanics.

σ=V k0.6791( T/ )b3/2

(11)

The value of the average volume of the interstices of ionicliquids [DBU-Hex][SCN], [DBU-Oct][SCN], and [DBU-Dec][SCN] are given in Table 9.The volume fractions of interstice, ∑v/V, for all the

measured ionic liquids are also given in Table 9. The valueswere between 10.76 % and 12.72 % and are in agreement withthe substances which show a volume expansion of approx-imately less than 15 % during the transformation from solids toliquids. The molar volume, V, is the summation of the inherentvolume, Vi, and the sum of the volumes of all interstices, ∑v =2NAv; i.e.,

= +V V N v2i A (12)

The coefficient of thermal expansion, α, was calculated byassuming that the ionic liquids expansion results only by theexpansion of the interstice during the temperature change.Hence the equation of α derived from the interstitial model isgiven below.

α = ∂∂

=⎜ ⎟⎜ ⎟⎛⎝

⎞⎠⎛⎝

⎞⎠V

VT

N vVT

1 3

P

A

(13)

The calculated and the experimental values of α for all of theionic liquids under study are similar (see Table 9). This resultindicates that this model is useful in the case of ionic liquids.

■ CONCLUSIONIn this work, synthesis of 12 new ionic liquids based on DBUcation was reported. The effects of alkyl chain length andtemperature on the physical properties of DBU based ionicliquids were studied. The viscosity, density, and surface tensionof the ionic liquids were determined in a wide temperaturerange, and they showed a linear relationship with temperature.From the experimental density values, molar volume, standardentropy, thermal expansion coefficient, and lattice energy weredetermined. The surface tension data were used to calculatesurface entropy and surface enthalpy. With the help ofGuggenheim and Eotvos equations, critical temperature wascalculated for the ionic liquids. The molar enthalpy ofvaporization was determined using the Zaitsau et al. method.64

The lower thermal decomposition temperature of the chloridesalts was due to the higher nucleophilicity of the chloride anion.

Table 8. Thermal Decomposition Temperature (Td) andMelting Points (Tm) of Ionic Liquidsa

Tm Td

ionic liquid (°C)b (°C)

[DBU-Et] [Br] 105 379[DBU-Bu] [Br] 82 371[DBU-Hex] [Cl] 44 303[DBU-Oct] [Cl] 30 294[DBU-Dec] [Br] nd 343[DBU-TetDec] [Cl] nd 294[DBU-Et] [SCN] 40 351[DBU-Bu] [SCN] 50 354[DBU-Hex] [SCN] nd 337[DBU-Oct] [SCN] nd 343[DBU-Dec] [SCN] nd 338[DBU-TetDec] [SCN] 48 334

aStandard uncertainties are u(T) = ± 1 K. bnd, not determined due toexperimental difficulty.

Figure 6. TGA profile for ionic liquids [DBU-Et][Br], [DBU-Oct][Cl], [DBU-Dec][Br], [DBU-Oct][SCN], and [DBU-Dec]-[SCN].

Table 9. Values of Parameters of Ionic Liquids for the Interstitial Model at 303.15 K

10−24·v ∑v 104·α(cal) 104·α(exp)

ionic liquid (cm3) (cm3) ∑v/ V (K−1) (K−1)

[DBU-Hex][SCN] 25.46 30.67 10.97 5.43 5.00[DBU-Oct][SCN] 33.40 40.23 12.72 6.29 6.00[DBU-Dec][SCN] 31.138 37.51 10.76 5.321 5.00

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■ ASSOCIATED CONTENT*S Supporting InformationText describing syntheses of the halide salts and the ionicliquids, and figures showing the 1H and 13C NMR spectra ofthe halide salts and ionic liquids. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: letheshkc@gmail.com. Phone: +60195084533. Fax:+6053687598.FundingThis work was supported by PETRONAS Ionic Liquids Centre(PILC). K.C.L. acknowledges the postdoctoral fellowship fromUniversity Teknologi PETRONAS (UTP), and S.N.S. acknowl-edges the Ph.D. scholarship from PETRONAS Ionic LiquidsCentre.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge all of the research officers in PILC for helpingwith the analysis of the ionic liquids.

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