Synthesis, Characterization, and Thermophysical Properties of 1,8

Article pubs.acs.org/jced

Synthesis, Characterization, and Thermophysical Properties of 1,8Diazobicyclo[5.4.0]undec-7-ene Based Thiocyanate Ionic Liquids Kallidanthiyil 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 based on 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) cation with anions such as chloride, bromide, and thiocyanate were performed. The new ionic liquids were characterized using nuclear magnetic resonance spectroscopy and elemental analysis. The effect of alkyl spacer length on the physicochemical properties of DBU based ionic liquids was studied by attaching ethyl, butyl, hexyl, octyl, decyl, and tetradecyl groups to the DBU cation. The effects of temperature on the density and viscosity over a temperature range from 293.15 K to 373.15 K were recorded at atmospheric pressure. From the experimental density values, the thermal expansion coefficient 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 were calculated from the experimental surface tension value at 303.15 K. The thermal behavior of these ionic liquids was studied using thermogravimetric analysis and differential scanning calorimetry.

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INTRODUCTION In the past 2 decades scientists from various disciplines were attracted to ionic liquids (ILs) due to their special characterstics such as very low vapor pressure, large electrochemical window, large liqud range, high ionic conductivity, and high thermal stability. Ionic liquids can be termed as organic salts made up entirely of ions.1,2 Ionic liquids, which are liquids at room temperature, are known as room-temperature ionic liquids (RTILs). Usually ionic liquids are made up of ugly organic cations with anions of organic or inorganic nature. Their properties depend mostly on the nature of the cations and anions. For instance, ionic liquids containing bis(trifluoromethylsulfonyl)imide (Tf2N) anion would be water immiscible.3 On the other hand, acetate anions will form hydrophilic (water-soluble) ionic liquids.4 Because of their special properties, ionic liquids find applications in different areas such as organic synthesis,2,5−8 separation processes,9−15 catalysis,16−18 electrochemistry,19−21 and biomass dissolution,22−27 etc. 1,8-Diazobicyclo[5.4.0]undec-7-ene (DBU) is a class of amidine compounds that has found applications in organic synthesis as a catalyst and as a non-nucleophilic base. Because of its strong alkaline nature, the use of DBU in organic synthesis has been investigated extensively.28−32 Although DBU was successful in these reactions, it suffered from less recyclability, which is against the sustainable approach of modern science. Ionic liquids based on DBU were developed to address the issues associated with the use of DBU in organic synthesis.33−35 Although some ionic liquids based on DBU has been reported, useful data on the thermophysical properties of DBU based ionic liquids is limited and that is a hindrance in © 2014 American Chemical Society

utilizing them for various applications in which the conventional solvents are not suitable. In this work attempt is made to describe the synthesis, characterization, and thermophysical properties of DBU based cation with various alkyl chain lengths. An overview of ionic liquids presented in this work is shown in Figure 1.

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

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EXPERIMENTAL SECTION Materials. All the chemical were purchased from Acros Organics (Geel, Belgium), and no further purification step was performed. Synthesis of Halide Salts. General Procedure. To a solution of 1,8-diazobicyclo[5.4.0]undec-7-ene (10 g, 65.68 mmol) in acetonitrile (30 mL), 1-bromoethane (8.58 g, 78.82 mmol) was added and stirred at 60 °C for 48 h. The reaction mixture was cooled using an ice bath, and acetonitrile was Received: November 14, 2013 Accepted: May 1, 2014 Published: May 29, 2014 1788

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

Journal of Chemical & Engineering Data

Article

Scheme 1. Synthesis of DBU Based Ionic Liquids

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

removed in a rotary evaporator. The white solid obtained was washed with cyclohexane (3 × 25 mL) and dried in a vacuum oven 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), sodium thiocyanate (3.72 g, 45.94 mmol) was added and the reaction was vigorously stirred using a mechanical stirrer at 25 °C for 24 h. The precipitate formed was filtered off, and the dichloromethane layer was washed with cold water (3 × 25 mL). Dichloromethane was evaporated under vacuum to give 1ethyl-1,8-diazobicyclo[5.4.0]undec-7-ene thiocyanate ([DBUEt][SCN]) as a pale yellow solid. The ionic liquid formed was further dried in a vacuum oven at 60 °C for 24 h. Characterization. Carbon, hydrogen, nitrogen, and sulfur content were analyzed using elemental analyzer (CE Instruments EA-1110). 1H and 13C NMR spectra were recorded on a Bruker Avance 500 spectrometer. Water content was measured in a coulometric Karl Fischer titrator (model DL39). Density and Viscosity Measurements. An Anton Paar viscometer (model SVM3000) was used to measure the viscosity of ionic liquids. Density measurement was carried out using an Anton Paar densitimeter (DMA 5000). Standard uncertainties 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 was used to generate the drop, and it was photographed using a camera (OCA 20). Software (SCA 22) was used to evaluate the shape of the drop. The measurements were recorded from 293.15 K to 353.15 K. Thermal Decomposition. The thermal decomposition temperature of the ionic liquids was measured using a thermogravimetric analyzer (PerkinElmer, Pyris V-3.81). Samples were heated from 25 °C to 750 °C in a crucible under 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 nitrogen atmosphere from 25 °C to 150 °C and then cooled to −60.75 °C and again heated to 150 °C. The heating and cooling rate is 10 °C·min −1. Halide Content. The halide content was measured using an ion chromatogram (Metrohm model 761 Compact IC). A 20 μL aliquot of ionic liquid was used for one measurement. An aqueous solution of Na2CO3 (3.2 mmol) and NaHCO3 (1.0

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RESULTS AND DISCUSSION Synthesis. DBU based ionic liquids were prepared by following Scheme 1. Synthesis of the DBU based ionic liquids involves two steps. The halide salt was prepared by the quaternization of 1,8diazobicyclo[5.4.0]undec-7-ene with corresponding alkyl halides at 70 °C for 48 h in acetonitrile. The ionic liquids obtained after the removal of the solvent were washed with cyclohexane and dried under vacuum. The halide salts were obtained in more than 90 % yield. Anion metathesis with Na[SCN] resulted in the corresponding [SCN]− based hydrophilic DBU based ionic liquids. All of the ionic liquids based on thiocyanate anions were initially obtained as pale yellow liquid; three of them (3a, 3b, and 3f) crystallized upon standing at room temperature for 1 week. Viscosity. Viscosity measurement was performed in a temperature range from 293.15 K to 373.15 K using an Anton Paar viscometer. Viscosity decreased as the temperature increased. The corresponding data are summarized in Table 1, Figure 2, and Figure 3 as a function of temperature. As can be seen in Figure 2 and Figure 3, the alkyl spacer length has a strong impact on the viscosity of the DBU based ionic liquids. Table 1. Experimental Viscosity (η) of Ionic Liquids as a Function of Temperaturea η T

(mPa·s)

(K)

[DBU-Hex][SCN]

[DBU-Oct][SCN]

[DBU-Dec][SCN]

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

4198.8 1504.3 635.21 306.06 164.15 96.185 60.575 40.353 28.295

1956.0 806.71 379.33 198.69 114.14 70.680 46.570 32.245 23.311

2604.0 1059.3 490.58 253.63 143.66 87.778 57.259 39.312 27.999

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

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Density. Table 2 and Figure 4 show the effect of alkyl spacer length on the density of DBU based ionic liquids at a Table 2. Experimental Density (ρ) of Ionic Liquids as a Function of Temperaturea ρ (g cm−3)

T (K)

[DBU-Hex][SCN]

[DBU-Oct][SCN]

[DBU-Dec][SCN]

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

1.0618 1.0564 1.0509 1.0451 1.0395 1.0338 1.0284 1.0229 1.0174

1.0288 1.0230 1.0172 1.0116 1.0053 1.0005 0.9953 0.9897 0.9843

1.0136 1.0082 1.0026 0.9973 0.9921 0.9869 0.9818 0.9765 0.9711

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

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

[DBU-

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

The ionic liquids containing the alkyl groups such as ethyl, butyl, and tetradecyl were solids at room temperature. However, imidazolium based thiocyanate salts containing ethyl and butyl groups are room-temperature ionic liquids having viscosity of 23.8 mPa·s and 59.8 mPa·s, respectively, at 298.2 K.36,37 The DBU ionic liquids with alkyl spacer lengths of six, eight, and ten carbon atoms are room-temperature ionic liquids. Their viscosity increases in the order [DBU-Oct][SCN] < [DBU-Dec][SCN] < [DBU-Hex][SCN]. Higher viscosities of the ionic liquids with longer alkyl spacer length are due to the increased van der Waals interaction between the alkyl groups. 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 the viscosity of N-octylisoquinolinium thiocyanate (2388.0 mPa·s at 298.15 K) ionic liquid is much higher than that of [DBUOct][SCN] (1956.0 mPa·s) .38,39 Viscosity of both imidazolium and isoquinolinium based ionic liquids also showed a linear dependence with temperature.

temperature range from 293.15 K to 373.15 K. The density values decreased as the temperature increased. When the alkyl chain length of the cation is increased, the density of DBU ionic liquids decreased and it is in the order [DBU-Hex][SCN] > [DBU-Oct][SCN] > [DBU-Dec][SCN]. The decrease in density with an increase in alkyl spacer distance is due to the insufficient close packing of the cations. Similar behavior was observed for imidazolium based ionic liquids.40 Density values of DBU based protic ionic liquids with different anions such as bis(trifluoromethanesufonyl)imide, acetate, methanesulfonate, trifluoroacetate, and trifluoromethanesulfonate are higher than thiocyanate ionic liquids with DBU cation of varying alkyl chain length.41 The density of thiocyanate ionic liquids with cationic cores such as imidazolium,36 pyridinium,42 pyrrolidinium,43 and isoquinolinium38,39,43 was reported to be in the same range as that of DBU based ionic liquids. The density of all of these ionic liquids showed a linear dependency with temperature. Estimation of Volumetric Properties. The experimental values of density, ρ, for the studied ionic liquids with (T − 298.15 K) were fitted by applying the following equation:

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

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⎡ ρ ⎤ ⎥ = A 0 − A1((T /K) − 298.15) ln⎢ ⎣ g·cm−3 ⎦

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

(1)

where c is a constant and A1 = (α/K) = −[(∂ ln ρ)/∂(T − 298.15)]p, where α is the thermal expansion coefficient. The calculated values of correlation coefficients and the standard deviation (SD) are shown in Table 3. Table 3. Fitting Parameter (A and B) Values with R2 and Standard Deviation (SD)a for Empirical Correlation of Density,b Viscosity,c and Surface Tensiond of the Measured Ionic Liquids [DBU-Hex][SCN]

a

SD R2 A0 A1

1.37·10−5 0.9829 6.9655 5.00·10−4

SD R2 A4 A5

1.56·10−2 0.9892 - 6.5231 2936.1

SD R2 A2 A3

4.92·10−4 0.9717 4.9630 4.00·10−3

[DBU-Oct][SCN]

5.88·10−6 0.9999 6.9184 5.00·10−4 1.03·10−2 0.9914 - 5.7861 2666.7

UPOT

(J·K−1·mol−1)

(kJ·mol−1)

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

1056.4 1023.0 1008.2

0.464 0.525 0.579

608 684 751

313 301 292

σ (mN·m−1)

T

1.03·10−2 0.9999 4.0883 2.71·10−3

(Zexp − Zcal)2 nDAT

(K)

[DBU-Hex][SCN]

[DBU-Oct][SCN]

[DBU-Dec][SCN]

293.15 303.15 313.15 323.15 333.15 343.15 353.15

37.98 37.37 37.20 36.82 36.03 35.94 35.59

31.85 31.19 30.64 29.88 29.44 29.18 28.86

32.94 32.68 32.41 32.14 31.87 31.59 31.32

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

where Z exp and Zcal are experimental and calculated data values, respectively. nDAT is the number of experimental points. bEquation for density temperature dependence: ln ρ/ (kg·m−3) = A0 − A1(T − 298.15), where

A1 =

S°

(nm3)

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

Standard deviation values were calculated using SD =

Vm

(kg·m−3 )

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, the lattice energy values of [DBU-Hex][SCN], [DBU-Oct][SCN], and [DBU-Dec][SCN] are 313 kJ·mol −1, 301 kJ·mol −1, and 292 kJ·mol −1, respectively, and adjacent to previously reported ionic liquids.45−48 The studied ionic liquids have significantly lower lattice energies than those of the alkali halides.49 For instance, 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 DBU based thiocyanate ionic liquids is given in Table 5 and Figure 5

[DBU-Dec][SCN]

Density 2.28·10−5 0.9851 6.9325 6.00·10−4 Viscosity 1.04·10−2 0.9909 - 5.7042 2606.1 Surface Tension 1.14·10−1 0.9726 4.6525 5.07·10−3

ρ ionic liquid

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

⎛ ⎞ ∂ ln ρ α = −⎜ ⎟ K ⎝ ∂(T − 298.15) ⎠ P

Equation for viscosity temperature dependence: log η /(mPa·s) = A4 + (A5/T). dEquation for surface tension temperature dependence: σ/ (mN·m) = A2 − A3T. c

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

Vm =

M NAρ

(2)

S°(303.15)/(J ·K−1·mol−1) = 1246.5(Vm/nm 3) + 29.5 (3) −1

1/3

UPOT/(kJ· mol ) = 1981.2(ρ /M)

+ 103.8

(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 mean impact of a methylene (-CH2-) group to the molar volume is

Figure 5. Surface tension as a function of temperature: ■, [DBUHex][SCN]; ▲, [BDU-Oct][SCN]; ●, [DBU-Dec][SCN]. 1791



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very close to the common organic solvents such as octane (51.1 mJ·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 and transport properties of liquids.57 Hence Guggenheim58 (eq 8) and Eötvos59 (eq 9) empirical equations were used to predict the critical temperature of the ionic liquids, and the results are shown in Table 7. Enthalpy of vaporization of ionic liquids was estimated using eq 10.

value is observed for [DBU-Hex][SCN], and [DBU-Oct][SCN] showed the lowest value. The surface tensions of all of the ionic liquids under study are higher than common volatile solvents 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 with imidazolium, pyrrolidinium, and pyridinium cations with an alkyl chain length of four carbon atoms are considerably higher than the ionic liquids in the present study.51,53 Surface tension of all of these ionic liquids decreases as the temperature increased. The surface tension and the temperature are correlated using the following equation. σ /(mN·m−1) = A 2 − A3T

(5)

where σ represents surface tension, A2 and A3 are fitting parameters, and T is the temperature. Table 3 shows the estimated values of fitting coefficients along with the standard deviation (SD). The measured surface tension values were applied to calculate the entropy and enthalpy of surface formation. It was possible to determine the surface entropy from the slope, Sa, of eq 6. ⎛ ∂σ ⎞ Sa = A3 = −⎜ ⎟ ⎝ ∂T ⎠ P Es = A 2 = σ −

Table 6. Surface Thermodynamic Functions of Pure Ionic Liquids at Temperature 303.15 K: Surface Entropy (Ss) and Surface Enthalpy (Es) Es −2

ionic liquid

(mJ·K ·m )

(mJ·m−2)


40 51 27

49.49 46.65 40.86

(9) (10)

where E is the total surface energy of ionic liquids, which equals the surface enthalpy because of the tiny volume difference due to thermal expansion at the temperatures that are not similar to the critical temperature TGc , K is a constant, σ is surface tension, M is molecular weight, ρ is density, T is the measured surface tension temperature, and NA is Avogadro’s number. It is also possible to calculate boiling temperatures, Tb, of ionic liquids from the critical temperature (Tc) by making use of the assumptions from Rebelo et al.60 According to them, the relation 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 decomposition temperature (Td) of all 12 ionic liquids and melting points (Tm) of solid ionic liquids are shown in Table 8. All of the DBU based 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 the bromide and thiocyanate anions. It may be due to the higher nucleophilicity of the chloride anions which in turn decreases the thermal stability by decomposing the cationic core via bimolecular substitution (SN2) reaction of the easily accessible alkyl group.61 The higher thermal stability of the bromide and thiocyanaye ionic liquids can be attributed to their bigger size and the less nucleophilic nature. Diop et al. have reported that the thermal stability of the DBU based ionic liquids with chloride anion is below 300 °C, which is in good agreement with our results.62 According to Nowicki and co-workers,33 the thermal decomposition temperature of DBU ionic liquids with hydroxide anion was 362.8 °C and 362.1 °C, respectively. The difference in the thermal decomposition temperature of the same cation with different anions is due to the difference in the

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

−1

⎛ M ⎞2/3 σ ⎜ ⎟ = K (TcE − T ) ⎝ρ⎠

σ

(7)

103·Ss

(8)

Δ1g Hm° = 0.01121(σV 2/3NA1/3) + 2.4

(6)

⎛ ∂σ ⎞ ⎜ ⎟ ⎝ ∂T ⎠ P

11/9 ⎛ T ⎞ ⎜ ⎟ σ=E 1− G Tc ⎠ ⎝ σ

entropy than the common organic solvents. It may be due to the enhanced degree of surface orientation in ionic liquids. The lower value of surface entropy of ionic liquids in the present study is in good agreement with previously reported ionic liquids,46,54,55 and it is an indication of an enhanced surface orientation in ionic liquids. NaNO3 has a surface enthalpy value of 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 in ionic liquids. The surface enthalpy value of the ionic liquids is

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

Eötvos

Tc

Tb

Tc

Tb

Δg1Hm °

ionic liquid

(K)

(K)

(K)

(K)

(kJ·mol−1)


1475.83 1080.22 1813.89

885.5 648.1 1088.33

1064.26 992.59 1074.15

638.55 595.55 644.49

153.71 139.47 155.68

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at a scanning rate of 10 °C·min −1 (figure not shown), only an endothermic peak corresponding to Tm was detected for all of the ionic liquids which are solids at room temperature. Interstitial Model for Ionic Liquids. A new theoretical model called the interstitial model14,15 was developed for ionic liquids by abstracting the essence of the hole model for molten salts.16 The model is based on four assumptions which can be seen elsewhere.8,17 In this model the interstitial volume, v, for the ionic liquids was calculated using an equation from classical statistical mechanics.

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

Td

ionic liquid

(°C)b

(°C)

[DBU-Et] [Br] [DBU-Bu] [Br] [DBU-Hex] [Cl] [DBU-Oct] [Cl] [DBU-Dec] [Br] [DBU-TetDec] [Cl] [DBU-Et] [SCN] [DBU-Bu] [SCN] [DBU-Hex] [SCN] [DBU-Oct] [SCN] [DBU-Dec] [SCN] [DBU-TetDec] [SCN]

105 82 44 30 nd nd 40 50 nd nd nd 48

379 371 303 294 343 294 351 354 337 343 338 334

V = 0.6791(k bT/σ )3/2

(11)

The value of the average volume of the interstices of ionic liquids [DBU-Hex][SCN], [DBU-Oct][SCN], and [DBUDec][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 values were between 10.76 % and 12.72 % and are in agreement with the substances which show a volume expansion of approximately less than 15 % during the transformation from solids to liquids. The molar volume, V, is the summation of the inherent volume, Vi, and the sum of the volumes of all interstices, ∑v = 2NAv; i.e.,

Standard uncertainties are u(T) = ± 1 K. bnd, not determined due to experimental difficulty.

a

basicity or the nucleophilicity of the anions.63 The thermal stability of DBU based protic ionic liquids with anions such as acetate, methanesulfonate, and trifluoroacetate, etc., are in the range from 171 °C to 451 °C.41 Imidazolium and pyrrolidinium based thiocyanate ionic liquids showed thermal behavior similar to DBU derived thiocyanate ionic liquids.36,43 On the contrary, pyridinium thiocyanate ionic liquids have lower thermal stability than DBU ionic liquids with the same anion.42 Figure 6 shows thermogravimetric analysis (TGA) profiles for ionic

V = Vi + 2NAv

(12)

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

3NAv ⎛ 1 ⎞⎛ ∂V ⎞ ⎜ ⎟⎜ ⎟ = ⎝ V ⎠⎝ ∂T ⎠ P VT

(13)

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

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CONCLUSION In this work, synthesis of 12 new ionic liquids based on DBU cation was reported. The effects of alkyl chain length and temperature on the physical properties of DBU based ionic liquids were studied. The viscosity, density, and surface tension of the ionic liquids were determined in a wide temperature range, and they showed a linear relationship with temperature. From the experimental density values, molar volume, standard entropy, thermal expansion coefficient, and lattice energy were determined. The surface tension data were used to calculate surface entropy and surface enthalpy. With the help of Guggenheim and Eötvos equations, critical temperature was calculated for the ionic liquids. The molar enthalpy of vaporization was determined using the Zaitsau et al. method.64 The lower thermal decomposition temperature of the chloride salts was due to the higher nucleophilicity of the chloride anion.

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

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

Table 9. Values of Parameters of Ionic Liquids for the Interstitial Model at 303.15 K 10−24·v

∑v

104·α(cal)

(cm )

(cm )

∑v/ V

(K )

(K−1)


25.46 33.40 31.138

30.67 40.23 37.51

10.97 12.72 10.76

5.43 6.29 5.321

5.00 6.00 5.00

3

1793

−1

104·α(exp)

ionic liquid

3



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ASSOCIATED CONTENT

S Supporting Information *

Text describing syntheses of the halide salts and the ionic liquids, and figures showing the 1H and 13C NMR spectra of the halide salts and ionic liquids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +60195084533. Fax: +6053687598. Funding

This work was supported by PETRONAS Ionic Liquids Centre (PILC). K.C.L. acknowledges the postdoctoral fellowship from University Teknologi PETRONAS (UTP), and S.N.S. acknowledges the Ph.D. scholarship from PETRONAS Ionic Liquids Centre. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge all of the research officers in PILC for helping with the analysis of the ionic liquids. REFERENCES

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