Chapter 2

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Synthesis and Physicochemical Characterization of Imidazolium

Based Ionic Liquids

—————————————————————————————-

PreludeThe present chapter is a detailed discussion about the synthesis, purification and physic-

ochemical characterization of N−alkylimidazolium based room temperature ionic liq-

uids (RTILs) viz. 1−butyl−3−methylimidazolium tetrafluoroborate ([BMIM][BF4]),

1−butyl−2, 3−dimethylimidazolium tetrafluoroborate ([BDMIM][BF4]), 1−butyl−3−

methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1− butyl− 2, 3−dimethyl−

imidazolium hexafluorophosphate ([BDMIM] [PF6]). In first part of this chapter we

present detailed description about synthesis, characterization and purification procedures

which we have optimized in our laboratory for synthesis of RTILs with electrochem-

ical grade purity. A two step synthetic procedure involving quaternization reaction of

1− methyl− imidazole or 1, 2 −methylimidazole by chlorobutane followed by anion

exchange of product from this step through its acid base neutralization reaction with

hexafluorophosphoric acid or tetrafluoroboric acid, was used for synthesis of above men-

tioned RTILs. The RTILs were characterized by 1H NMR, 13C NMR, mass spectrometry

and infrared (IR) spectroscopy.

In second part of this chapter we present a detailed account of our investigations related

to effect of temperature and acetonitrile (cosolvent) on the interfacial and bulk properties

of [BMIM][BF4], [BDMIM][BF4] and [BMIM][PF6] RTILs. The variations monitored

through surface tension, conductivity, photoluminescence, Infrared and UV-visible spec-

troscopic measurements are explained in terms of RTIL molecular structure. Temper-

ature was found to distort the interfacial order as [BDMIM][BF4] > [BMIM][BF4] >

[BMIM][PF6]. Excess surface energy and excess surface entropy was determined from

the temperature dependence of surface tension values. Addition of acetonitrile to RTILs

besides breaking the structural organization of RTIL was found to show positive devia-

tions from ideal behavior.

36

2.1 (A) Synthesis and chemical characterization of N-alkylimidazolium based ionic

liquids

2.1.1 Introduction

Tunable physicochemical characteristics, high thermal stability, ability to dissolve wide

range of organic and inorganic substrates and above all the eco-green features of room

temperature ionic liquids (RTILs) have established them as innovative green alternative

to conventional solvents in scientific research [1–3]. From electrochemical point of view,

their wide electrochemical window, appreciable conductivity and above all the stereose-

lective interactions of RTIL constituent ions with electrogenerated species have attracted

considerable attention [4–7]. It is in this context, that use of RTILs as solvents or sup-

porting electrolytes for electrochemical investigations has been the domain of many pub-

lications in recent years. Due to air and moisture stability, alkylimidazolium based RTILs

are preferred for electrochemical investigations [4, 6].

Main concern for researchers working with RTILs is the barely affordable price of com-

mercially available RTILs. Besides, many groups have demonstrated narrow electrohem-

ical window and not smooth base line features in cyclic voltammetry experiments with

commercially available RTILs, which they ascribe to some chemical changes due to inter-

action of RTILs with containers/atmosphere during their prolonged storage periods after

synthesis [3, 8]. It is in this context that most groups involved with electrochemical inves-

tigations in RTILs prefer the in house synthesis and purification of RTILs of their choice.

A detailed discussion about the synthesis, purification and chemical characterization of

RTILs viz. 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl-2,3-

dimethylimidazolium tetrafluoroborate ([BDMIM][BF4]), 1-butyl-3-methylimidazolium

hexafluorophosphate ([BMIM][PF6]) and 1−butyl −2, 3− dimethylimidazolium hex-

afluorophosphate ([BDMIM][PF6]) is presented in this section.

“Voltammetrically pure” ionic liquids, having low background currents were synthesized

through slightly modified reported procedures [9–11]. Gordon et al. recommendations

[12] were followed for the purification of starting materials, optimization of experimental

parameters for quaternization and anion exchange reactions and purification of synthe-

sized RTILs.

37

2.1.2 Chemicals

Chemicals used were of Analytical Reagent grade. 1-methylimidazole, 1-chlorobutane,

tetrafluoroboric acid (42% aqueous solution-HBF4) and hexafluorophosphoric acid (62%

aqueous solution-HPF6) were procurred from Spectrochem India. 1, 2− dimethylim-

idazole was purchased from Acros Organics. Acetonitrile (ACN), ethyl acetate(EA),

dichloromethane (DCM) of HPLC grade, sodium bicarbonate (NaHCO3) and sodium

sulfate (Na2SO4) were purchased from Merck, India. All solvents were dried and freshly

distilled prior to use.

2.1.3 Purification of Chemicals

For the synthesis of pure RTILs, purification of starting materials is recommended.

Hence care was taken to purify all the chemicals and solvents as per the literature re-

ports [13].

Purification of 1-methylimidazole

10 g of KOH pellets were added to 500 mL of 1-methylimidazole. Some porcelain

granules were charged into the mixture to assure homogeneous heating. The mixture

was allowed to reflux for 3 hours, followed by distillation of the distillate and collection

of the same over dried molecular sieves. Finally the distilled 1-methylimidazole was

stored in an inert atmosphere under a blanket of argon gas.

Purification of acetonitrile

10 g of phosphorous pentaoxide (P2O5) were added to 500 mL of ACN. The mixture was

allowed to reflux for 4 hours followed by distillation. The first fraction was thrown while

the remaining amount was collected over dried molecular sieves and stored under argon

blanket.

Purification of ethylacetate

10 g of phosphorous pentaoxide (P2O5) were added to 500 mL of EA. The mixture was

allowed to reflux for 4 hours followed by distillation. The distillate was collected over

molecular sieves and stored under inert atmosphere under argon blanket.

38

Purification of 1-chlorobutane

1-chlorobutane was dried over calcium chloride (CaCl2) for 4 hours, filtered, distilled

and collected over molecular sieves.

2.1.4 Experimental measurements

IR spectra were recorded on a Bruker Vector 22 instrument using chloroform (CHCl3).1H NMR was recorded on a Bruker DPX 200 instrument in CDCl3 using TMS as inter-

nal standard for protons. 1H NMR chemical shifts and coupling constants J are given

in ppm and Hz respectively. 13C NMR was recorded on a Bruker DPX 500 instrument.

Mass spectra were recorded on EIMS (Shimadzu) and ESI-esquire 3000 Bruker Dalton-

ics instrument. Mass-spectrometric (MS) data is reported in m/z. Elemental analysis was

carried out using Elemental Vario EL III elemental analyser. Elemental analysis data is

reported in % standard.

2.1.5 Synthesis and chemical characterization of N-alkylimidazolium based ionic

liquids

A two step procedure was used in the synthesis of N-alkylimidazolium based ionic liq-

uids. In the first step, quaternization of 1-methylimidazole or 1,2-dimethylimidazole by

its reaction with 1-chlorobutane was done to produce alkylated halide precursor 1-butyl-

3-methylimidazoilum chloride [BMIM][Cl] or 1− butyl−2, 3− dimethylimidazoilum

chloride [BDMIM][Cl]. The halide ion of these salts in next step was exchanged with

tetrafluoroborate (BF−4 ) or hexafluorophosphate (PF−6 ) anion on acid base netralization

with HBF4 or HPF6 respectively.

2.1.6 Synthesis of 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]

30 g of dry 1−methylimidazole and 42 g of dry 1−chlorobutane (1:1.2 molar ratio) were

placed in a three neck, 500 mL round bottomed flask under argon atmosphere. 20 mL

of dried ACN were charged into this mixture under inert conditions followed by deoxy-

genation of the mixture by flushing it with argon gas for ca. 15 minutes. The mixture

was allowed to reflux with continuous stirring under argon atmosphere at 75 oC for 96

hours. The prolonged heating periods for lower temperatures rather than shorter periods

39

with elevated temperatures were found to be more effective for clean synthesis in this

step. Slight excess of 1-chlorobutane was used to assure complete consumption of 1-

methylimidazole and ACN was added to reduce viscosity of reaction mixture and hence

eliminating long reaction periods. After reflux, the reaction mixture was washed three

times with dried EA. Volume of EA used in each washing was half the volume of the re-

action mixture. The washed mixture was concentrated and then dried under high vacuum

to remove remaining EA and ACN. The dried mixture left overnight led to crystalliza-

tion of pure white 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) crystals whose

spectral data is given below:1H NMR (200 MHz, CDCl3): δ 0.96 (t, 3H, J = 7.21), 1.38 (m, 2H), 1.92 (m, 2H), 4.12

(s, 3H), 4.33 (t, 2H, J = 7.35), 7.41 (d, 1H, J = 1.60), 7.54 (d, 1H, J = 1.50), 10.48 (s,

1H).13C NMR (500 MHz, CDCl3): δ 13.40, 19.44, 32.13, 36.57, 49.77, 121.86, 123.56,

137.94.

IR (CDCl3, cm−1): 667.53, 756.27, 1135.89, 1250.79, 1465.98, 1539.65, 1588.69,

1637.37, 2111.43, 2875.08, 2936.74, 2962.73, 3416.57.

Mass (ESI-MS): 139 (M+).

Anal Calcd. for C8H15N2Cl: C: 55.01, H: 8.66, N: 16.04; found C: 55.08, H: 8.62, N:

16.06.

2.1.7 Synthesis of 1−butyl−2, 3−dimethylimidazolium chloride ([BDMIM][Cl])

35 g of dry 1,2−dimethylimidazole and 42 g of dry 1−chlorobutane (1:1.2 molar ratio)

were placed in a three neck, 500 mL round bottomed flask under argon atmosphere.

20 mL of dried ACN were added to this mixture and the mixture deoxygenated by

flushing it with argon gas for ca. 15 minutes. The mixture was allowed to reflux with

continuous stirring under argon atmosphere at 75 oC for 96 hours. Slight excess of

1−chlorobutane was added to assure complete consumption of 1,2−dimethylimidazole

and ACN was added to reduce viscosity of reaction mixture and hence eliminating long

reaction periods.

After reflux, the reaction mixture was washed three times with dried EA. Volume of EA

used in each washing was half the volume of the reaction mixture. The washed mixture

40

was concentrated and then dried under high vacuum to remove remaining EA and ACN.

The dried mixture left overnight led to crystallization of pure white 1− butyl−2,3−

dimethylimidazolium chloride [BDMIM][Cl] crystals whose spectral data is given be-

low:1H NMR (200 MHz, CDCl3): δ 0.96 (t, 3H, J = 7.16), 1.39 (m, 2H), 1.81 (m, 2H), 2.84

(s, 3H), 4.07 (s, 3H), 4.26 (t, 2H, J = 7.39), 7.63 (d, 1H, J = 1.99), 7.89 (d, 1H, J = 1.95).13C NMR (500 MHz, CDCl3): δ 10.26, 13.28, 19.29, 31.60, 35.66, 48.40, 121.17,

123.02, 143.26.

IR (CDCl3, cm−1): 755.76, 1136.06, 1250.87, 1466.10, 1588.64, 1636.82, 2111.47,

2875.07, 2962.82, 3481.46.

Mass (ESI-MS): 152.9 (M+).

Anal Calcd. for C9H17N2Cl: C: 57.29, H: 9.08, N: 14.85; found C: 57.26, H: 9.03, N:

14.89.

2.1.8 Synthesis of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])

For the anion exchange reaction, 25 g of freshly prepared [BMIM][Cl] were dissolved

in 150 mL Millipore water. The solution was ice cooled and hexafluorophosphoric acid

(44 mL) was added drop wise with continuous stirring. Slow addition of hexafluorophos-

phoric acid minimizes the amount of heat generated during the reaction, which otherwise

can lead to hydrolysis of PF−6 . Two immiscible phases were formed which were stirred

continuously for 1 hour. Use of water as solvent in this step allows the anion metathesis

reaction to be rapid and quantitative, besides it helps in easy purification of the ionic

liquid. The aqueous phase was decanted and the organic phase was repeatedly washed

with ice cooled Millipore water till the washings were no more acidic. The product was

then dissolved in dichloromethane to which, 2 mg activated charcoal and 20 mg Na2SO4

were added. After an hour, the organic phase was filtered out and dried under vacuum

at 75 oC for 12 hours. A colorless liquid whose NMR spectrum matched well with that

reported for [BMIM][PF6] [9] was obtained. The liquid was stored in a polypropylene

bottle under vacuum dried conditions. Storage in glass bottles was particularly avoided

due to the appearance of noxious fumes followed by accumulation of white solid in the

41

storage bottle. This is attributed to HF etching and the generation of volatile silicon flu-

oride and acid, which is formed due to hydrolysis of PF−6 in a moist environment [14].

Spectral data of 1−butyl−3-methylimidazolium hexafluorophosphate is given below:1H NMR (200MHz, CDCl3): δ 0.96 (t, 3H, J = 7.21), 1.29 (m, 2H), 1.78 (m, 2H), 3.83

(s, 3H), 4.09 (t, 2H, J = 7.40), 7.25 (d, 1H, J = 3.51), 7.30 (d, 1H, J = 5.35), 8.35 (s, 1H).13C NMR (500 MHz, CDCl3): δ 13.33, 19.38, 31.52, 36.13, 49.86, 122.57, 123.83,

135.87.

IR (CDCl3, cm−1): 623.27, 755.04, 1030.54, 1053.53, 1572.27, 1636.71, 2875.27,

2933.93, 2961.27, 3155.33, 3422.71.

Mass (ESI-MS): 138.9 (M+).

Anal Calcd. for C8H15N2PF6: C: 33.81, H: 5.32, N: 9.86; found C: 33.84, H: 5.38, N:

9.85.

2.1.9 Synthesis of 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([BDMIM]

[PF6])

30 g of freshly prepared [BDMIM][Cl] were dissolved in 150 mL Millipore water. The

solution was ice cooled and hexafluorophosphoric acid (44 mL) was added drop wise

with continuous stirring. Two immiscible phases were formed, which were stirred con-

tinuously for 1 hour. For drying, purification and storage, procedure similar to one men-

tioned for [BMIM][PF6] was used. Spectral data of [BDMIM][PF6] is given below:1HNMR (200 MHz, CDCl3): δ 0.98 (t, 3H, J = 7.19), 1.41 (m, 2H), 1.79 (m, 2H), 2.66

(s, 3H), 3.83 (s, 3H), 4.08 (t, 2H, J = 7.42), 7.31 (d, 1H, J = 1.73), 7.34 (d, 1H, J = 1.72).13C NMR (500 MHz, CDCl3): δ 9.68, 13.72, 19.81, 31.82, 35.39, 48.69, 120.97,

122.89, 146.20.

IR (CDCl3, cm−1): 757.05, 1032.70, 1059.24, 1299.70, 1588.11, 1638.50, 2927.01,

2960.33, 3418.83.

Mass (ESI-MS): 152.9 (M+).

Anal Calcd. for C9H17N2PF6: C: 36.25, H: 5.75, N: 9.39; found C: 36.28, H: 5.72, N:

9.37.

42

2.1.10 Synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM] [BF4])

24 g of freshly prepared [BMIM][Cl] were dissolved in 10 mL Millipore water. The

solution was ice cooled and tetrafluoroboric acid (20.0 mL, 45% aqueous) was added

dropwise with continuous stirring. The solution was left to stir overnight. The mix-

ture was cooled and [BMIM][BF4] extracted through batch extraction using ice cooled

CH2Cl2 (DCM). Cooling of DCM and the reaction mixture ensures minimum solubility

of [BMIM][BF4] in DCM that in turn minimizes loss of RTIL during extraction process.

All fractions of DCM were collected and to this collection, activated charcoal was added.

After 4 hours, the organic phase was separated by filtration through Whatmann paper.

The DCM phase containing [BMIM][BF4] was dried over 10 g anhydrous Na2SO4. Af-

ter 1 hour, the organic phase was filtered out and dried under vacuum at 75 oC for about

12 hours. A colorless liquid as [BMIM][BF4] was obtained. The liquid was stored in a

polypropylene bottle under vacuum dried conditions. Spectral data of [BMIM][BF4] is

given below:1H NMR (200MHz, CDCl3): δ 0.92 (t, 3H, J = 7.18), 1.34 (m, 2H), 1.86 (m, 2H), 3.96

(s, 3H), 4.19 (t, 2H, J = 7.33), 7.28 (d, 1H, J = 3.42), 7.38 (d, 1H, J = 1.50), 8.85 (s, 1H).13C NMR (500 MHz, CDCl3): δ 13.54, 19.58, 32.13, 36.47, 50.03, 122.49, 123.93,

136.61.

IR (CDCl3, cm−1): 623.27, 755.04, 1030.54, 1053.53, 1572.27, 1636.71, 2875.27,

2933.93, 2961.27, 3155.33, 3422.71.

Mass (ESI-MS): 138.9 (M+).

Anal Calcd. for C8H15N2BF4: C: 42.51, H: 6.69, N: 12.39; found C: 42.52, H: 6.65, N:

12.37.

2.1.11 Synthesis of 1−butyl−2,3−dmethylimidazolium tetrafluoroborate ([BDMIM]

[BF4])

30 g of freshly prepared [BDMIM][Cl] were dissolved in 10 mL Millipore water. The

solution was ice cooled and tetrafluoroboric acid (20 mL, 45% aqueous) was added drop-

wise with continuous stirring. The solution was left to stir overnight. The mixture was

cooled and [BDMIM][BF4] extracted through batch extraction using ice cold CH2Cl2

(DCM). All fractions of DCM were collected and to it, activated charcoal was added.

43

After 4 hours, the organic phase was separated by filtration using Whatmann paper. The

DCM phase containing [BDMIM][BF4] was dried over 10 g of anhydrous Na2SO4. Af-

ter 1 hour, the organic phase was filtered and the filterate completely evaporated and

dried under vacuum at 75 oC for ca. 12 hours. A colorless liquid as [BDMIM][BF4]

was obtained. The liquid was stored in a polypropylene bottle under the vacuum dried

conditions. Spectral data of [BDMIM][BF4] is given below:1HNMR (200 MHz, CDCl3): δ 0.96 (t, 3H, J= 7.18), 1.37 (m, 2H), 1.75 (m, 2H), 2.61

(s, 3H), 3.81 (s, 3H), 4.06 (t, 2H, J=7.39), 7.27 (d, 1H, J=1.70), 7.30 (d, 1H, J=1.69).13C NMR (500 MHz, CDCl3): δ 9.50, 13.50, 19.60, 31.63, 35.24, 48.49, 120.88,

122.70, 145.

IR (CDCl3, cm−1): 757.01, 1032.70, 1054.24, 1299.70, 1588.11, 1635.50, 2927.01,

2959.73, 3418.63.

Mass (ESI-MS): 152.9 (M+).

Anal Calcd. for C9H17N2BF4: C: 45.03, H: 7.14, N: 11.67; found C: 45.09, H: 7.10, N:

11.63.

2.1.12 Conclusion

Performing quaternization reaction at lower temperatures for prolonged time periods

(more than 90 hours), rather than at elevated temperatures for a shorter period leads

to better purity of synthesized RTILs. Use of acids HBF4 and HPF6 should be preferred

instead of metal (especially silver) salts of BF−4 and PF−6 in the anion exchange reac-

tion, for synthesis of electrochemical grade imidazolium based RTILs. This is because

of appreciable solubility of metal halides in the RTILs, whose presence besides altering

the physicochemical properties interferes with the electrochemical investigations in these

RTILs. An additional advantage with the use of acid is that it ensures easy workup and

purification of RTILs. Prolonged storage of PF−6 anion containing RTILs in glass vials

leads to the development of pressure inside the containers, besides corroding the glass.

Hence these RTILs must be stored in polypropylene base like containers.

44

2.2 (B) Effect of Temperature and Cosolvent on Interfacial and Bulk Character-

istics of N-alkylimidazolium Ionic Liquids

2.2.1 Introduction

Room temperature ionic liquids (RTILs) are attracting significant attention as novel

‘green’ solvents for electrochemical investigations and applications [1, 3, 4]. Use of

RTILs as solvents or supporting electrolytes in electrochemical investigations requires a

comprehensive understanding about their transport and interfacial properties. Presently

their high viscosity which is responsible for slow mass transport in RTILs, is a main

hurdle standing in the way of their applications in electrochemical setups. Working at

high temperatures [15–17] and/or addition of a cosolvent to RTILs to reduce their vis-

cosity are advocated as two operationally simple options, that can be used to overcome

mass transport related drawbacks of RTILs [18–20]. RTIL mixture with cosolvents has

been proposed to posses altered and in some cases improved physicochemical proper-

ties [21, 22]. RTILs, especially those containing imidazolium cations are highly ordered

fluids [23] and most of the solvent specific effects of RTILs on heterogeneous electron

transfer and associated chemical reactions are ascribed to the structural organization that

prevails in RTILs. Since fluid structuring in RTILs is expected to be very sensitive to

rise in temperature and/or addition of cosolvents, a cautionary approach needs to be fol-

lowed in use of any one of these options to overcome viscosity related challenges in

RTILs. Hence a proper understanding about impact of temperature and cosolvent on

bulk and interfacial properties of RTILs is a perquisite for deciding the proper limits of

their dilution by cosolvent or elevation of temperature while using RTILs in combina-

tion with cosolvents or at elevated temperatures. Due to its electrochemical, solubility

and polarity features, acetonitrile (ACN) [24] seems to be a good choice as cosolvent

for imidazolium based RTILs. Surface tension, electrical conductivity and optical char-

acteristics are supposed to be a good measure of the structural ordering prevailing at

the interface and in the bulk of RTILs. Aim of work presented in this section of thesis

was to explore the effect of temperature and ACN as cosolvent on structural, transport

and interfacial properties of alkylimidazolium based RTILs. Surface tension, conductiv-

ity, emission characteristics, UV-visible and infrared spectroscopic measurements were

used for the said purpose. While surface tension is a good indicator of the cohesive

45

forces as well as microscopic ordering prevailing at surface, conductivity and optical

properties give sufficient insight about bulk characteristics in fluid phases. Present study,

besides giving new insight into the structural features of imidazolium based RTILs is

supposed to be of considerable importance for selection of optimum coslovent frac-

tion and/or temperature for the desired interfacial and bulk characteristics in 1-butyl-3-

methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl-2,3-dimethylimidazolium

tetrafluoroborate ([BDMIM][BF4]) and 1-butyl-3-methylimidazolium hexafluorophos-

phate ([BMIM][PF6]) for their use in electrochemical research and biphasic catalysis.

2.2.2 Experimental

Surface tension measurements were made with Kruss-9 tensiometer by the platinum ring

detachment method as reported earlier [25]. Temperature was maintained at the desired

value (within ± 0.1 oC) by the circulating water from a HAAKE GH thermostat. Con-

ductivity was recorded by a digital microprocessor based conductivity meter (CYBER-

SCAN CON 500) from Eutech instruments having sensitivity of 0.1 µScm−1, details are

reported elsewhere [26]. Steady state photoluminescence (PL) spectra were recorded at

room temperature with the help of Shimadzu RF-5301PC spectrofluorometer. Fourier

transform infrared spectroscopy (FTIR) of liquid samples was performed with a Thermo

SCIENTIFIC Nicolet - 6700 spectrometer. Numerical calculations and data fitting were

performed through codes written in, Origin 6.0 (Microcal Software Inc.). Abinitio calcu-

lations were performed to obtain gas phase optimized geometries and electronic energies

by using GAUSSIAN 03 set of codes [27], at the density functional theory level with

exchange functional B3LYP [28] and 6-31 G( d, p) basis set. Frequency analysis of the

resultant geometries was performed at the same level of theory and basis set, to check

whether the obtained structures are the stable minimum energy structures and no imagi-

nary frequencies were found.

2.2.3 Effect of Temperature

Surface tension

Figure 2.1 shows the variation of surface tension (γ) with temperature for [BMIM]

[BF4], [BDMIM][BF4], [BMIM][PF6]. Like most other liquids γ varies linearly with

46

Figure 2.1: Temperature dependence of γ for [BMIM][BF4], [BDMIM][BF4] and[BMIM][PF6]. The straight line shows linear fit of the data in accordance with equation 2.1.

temperature, obeying equation 2.1

γ = a− b.T (2.1)

with a regression coefficient≈ 0.99, and throughout the temperature range retains the or-

der, [BDMIM][BF4] > [BMIM][BF4] > [BMIM][PF6] in accordance with earlier report

by Freire et al. [29]. Magnitude of interfacial tension and relative order for the RTIL

molecular structure is an index of molecular organization at the surface [30]. While

surface tension is a measure of cohesive energy prevailing at surface, its temeprature

dependence gives a good idea about microscopic ordering of surface molecule. Elec-

troneutrality requirement warrants that the surface of RTILs can not be enriched by any

of the constituent ions, instead bulk stoichiometry should be preserved up to the surface.

This fact has been proven through Direct Recoil Spectrometry (DRS) experiments on

RTILs by Gannon et al. [31].

According to Langmuir [30] each part of a molecule possesses a local surface energy and

the surface tension is an indicator of the orientation of constituents at interface. Hence,

surface tension and its sensitivity to temperature in investigated RTILs can give much in-

sight about the relative orientations of their constituents at RTIL surface. Since the con-

stituent cations of RTILs are comparatively of larger size, the stoichiometric requirement

of 1 cation : 1 anion at surface warrants the dominance of cation on surface properties.

The extent of cationic dominance on surface characteristics will be affected by their in-

teractions with the counter anions and other cations at interface that will determine their

orientation at interface. The experimental values of γ for the investigated RTILs are close

47

to 40.3 mJ/m2 as reported for the imidazole [32], this is an evidence for cationic dom-

inance on surface properties in the investigated RTILs. The cation-anion interactions

in presently investigated RTILs seem to be purely coulombic, while the cation-cation

interactions are expected to have considerable contribution from van der Waals interac-

tions in addition to coulombic repulsion. Abinitio calculations in gas phase were used

to characterize the charge distribution, interaction energy and optimized geometry of

constituent ions and cation-anion combinations in RTILs. Taking into consideration the

conformers with the lowest energy for constituent ions and the cation-anion combina-

tions, the ion interaction energy was found to be -160.01, -88.47 and -94.99 kcalmol−1

for [BMIM][BF4], [BMIM][PF6] and [BDMIM][BF4] respectively, indicating that the

interaction energy is stronger in presence of [BF4]− than [PF6]−. This is well expected

as per the Lewis basicity of anions and is evident from Figure 2.2 that shows IR spectra

recorded for pure [BMIM][BF4] and [BMIM][PF6]. Lower frequencies for C-H vibra-

Figure 2.2: FTIR absorption spectra in the range 2500-3500 cm−1 for pure [BMIM][BF4] and[BMIM][PF6] ionic liquids. Frequency assignments were done as per reference 22.

tions of imidazoilum cation in case of [BMIM][BF4] in comparison to [BMIM][PF6] are

a direct evidence for weaker cation-anion interactions in the later [33]. This we suppose

is the reason for higher γ of [BMIM][BF4] in comparison to that of [BMIM][PF6] in

accordance with the hypothesis proposed by Deetlefs et al. [34]. According to this hy-

pothesis increase in ion size and increase of diffuse nature of charge on the RTIL ions

leads to decrease in the overall interaction among RTIL constituents, hence leading to a

decrease in surface tension of the RTIL. This feature of RTILs makes them to behave dif-

ferently than conventional organic solvents wherein increase in size of constiuents leads

to an increase in surface tension.

48

Since introduction of methyl group at the C-2 position of imidazolium cation has been

proved to decrease the cation-anion interaction and we also observed this in our quan-

tum mechanical calculations, higher value of γ for [BDMIM][BF4] as compared to

[BMIM][BF4] is quite unexpected. Similar trend has earlier been reported by Hunt et al.

[35] for the melting point and viscosity measurements in imidazolium based RTILs. We

attribute the higher surface tension of [BDMIM][BF4] than [BMIM][BF4] to increased

structural order of former on account of methyl substitution [35]. Besides inhibiting butyl

chain rotation due to steric repulsion, introduction of methyl group restricts the number

of otherwise thermodynamically allowed ion pair conformers for an imidazolium (cation)

plus anion combination. Both these factors are expected to enhance molecular order and

hence should lead to higher surface tension. In othewords, on substitution of C2-H of

imidazole with CH3, the loss in interactions due to steric reasons is lesser than the gain

that arises on account of loss in entropy. This argument is well supported by the higher

viscosity values of [BDMIM][BF4] in comparison to that of [BMIM][BF4] [38]. Simu-

lation studies, neutron and X-ray reflectivity measurements [39–42] have shown that at

RTIL/air interface, two types of molecular arrangements prevail - one with side chain

(butyl in present study) normal to surface and other with side chain parallel to the sur-

face. The former has imidazolium rings parallel to the surface and hence will have lower

surface tension than the later wherein the imidazolium rings are in perpendicular orien-

tation to the surface. Introduction of the methyl group at C-2 position in imidazolium is

expected to favor the orientation with imidazolium perpendicular to surface and hence

higher surface tension than the one with ring parallel to the surface. Within this ori-

entation, substitution of C2-H of imidazolium by CH3 group, besides increasing their

orientational order, will lead to increased van der Waals interactions among the cation

rings at the surface [36], that in turn will over dominate the decreased coulombic inter-

actions of imidazolium rings with their counter anions [37].

Straight line fit parameters of equation 2.1 from Figure 2.1 were used to calculate

thermodynamic variables for the RTIL/air interface. Thus, while the intercept gives the

surface energy (Es), the slope of the straight line fits gives surface entropy (Ss) [43].

Assuming that surface tension vanishes at critical temperature (Tc), the ratio of the Es

to Ts gives Tc. The resulting thermodynamic surface characteristics are tabulated in Ta-

ble 2.1. The surface entropies of the RTILs seem low in comparision to those reported

49

Table 2.1: Values of Surface Tension (γ), Surface Entropy (Ss) and Surface Energy (Es) for[BMIM][BF4], [BMIM][PF6] and [BDMIM][BF4].

Ionic Liquid γ at 298 K Ss Es TcmJm−2 mJm−2K−1 mJm−2 K

[BMIM ][BF4] 49.2 0.0404 61.29 1577.17[BMIM ][PF6] 48.0 0.0343 58.25 1698.34

[BDMIM ][BF4] 50.8 0.0471 64.86 1 377.13

for conventional organic solvents, thereby indicating a high surface organization of con-

stituents at the RTIL surface. The slopes of the fit lines in Figure 2.1 are a measure

of sensitivity of prevailing surface interaction to temperature. As evident from these

values, surface interactions decrease with temperature in the order [BDMIM][BF4] >

[BMIM][BF4] > [BMIM][PF6]. This order is also an evidence for the higher value of

γ for [BDMIM][BF4] in comparison to [BMIM][BF4] is on account of stronger van der

Waals interactions in the former. These results well prove the predictions of Law et al.

[44], that the structure of cation and its orientation at surface are mainly responsible for

surface properties of RTILs. The calculated values of critical temperature show that ther-

modynamically [PF]−6 combination with imidazolium cation is more stable than its [BF]−4

analogue and introduction of additional alkyl chain leads to thermodynamic instability

in dialkylimmidazolium based RTILs.

Conductivity

Figure 2.3 depicts the variation of specific conductance (κ) with temperature for the

investigated RTILs. Conductivity increases linearly with temperature and is in the or-

der [BDMIM][BF4] < [BMIM][PF6] < [BMIM][BF4], in conformity with the trend

expected on the basis of ion mobility. We used the reported values of density and co-

efficient of viscosity [15–17] for calculation of molar conductance (Λm) and Walden

product (Λm.η) for [BMIM][BF4] and [BMIM][PF6]. The resulting values are plotted

as a function of temperature in Figure 2.3. It is worth to mention that the κ and Λm

we found are significantly greater in magnitude than the values reported for these RTILs

in earlier studies [15–17]. This perhaps is on account of open to atmosphere conditions

50

Figure 2.3: (A) Specific conductance (κ) of [BMIM][BF4], [BDMIM][BF4] and [BMIM][PF6]RTILs as a function of Temperature (B) Walden Product (Λm.η) as a function of Temperature.Density values and η were taken from references, 8-10.

of the conductivity cell contents during our measurements which will allow RTILs to

absorb moisture because of their hygroscopic nature. Interestingly the Walden product

is not a constant but a function of temperature that shows an exponential decay with

temperature. This temperature dependency of Walden product is unexpected in light of

earlier reports [20, 45], which say that viscosity is the major force in impeding the mo-

tion of RTIL ions, while the dissociation for neat RTILs is independent of temperature.

Our observations and besides the ion diffusion experiments [18], however indicate that

the ionicity of RTILs should be temperature dependent. From electrical conductance

point of view, RTILs can be viewed as an equilibrium mixture of dissociated ions (ions

free to conduct-solute) dissolved in the associated part (associated ion clusters not able

to conduct-solvent) of RTIL, as per the equilibrium

A+B− A+ +B− (2.2)

The equilibrium concentrations depend on the dissociation constant of the above equilib-

rium (Kdis) and hence the free energy of dissociation (∆Godis) of RTIL as per equation,

Kdis = exp

(−∆Go

dis

RT

)(2.3)

Greater the value of ∆Godis, smaller is the extent of dissociation. Increasing temperature

increases the extent of dissociation and hence ionicity of RTIL and at the same time

decreases the viscosity of the solvent. Thus the increase in molar conductance with

51

temperature can be attributed to both these factors. Since κ is a measure of the number

of ions/volume, it can be proposed that,

κ ∝ Kdis = exp

(−∆Go

dis

RT

)(2.4)

Similarly the viscosity will be an exponential function of temperature as per the relation.

η = ηoexp

(EaRT

)(2.5)

Plots of ln(κ) and ln(η) vs. 1/T gave slope values which clearly indicate that with in-

crease of temperature the increase in former is less than the decrease of later. A similar

variation has also been reported by Hayamizu et al. [18] and is perhaps the reason for de-

cay of Walden product with temperature for the RTILs. These observations imply that in

case of RTILs with increase of temperature, their Λm does not increase as expected from

the extent of decrease in their η by the imposed variations in temperature. Hence it can

be infered that in RTILs, else than viscosity, there are some additional forces that impede

the motion of their constituent ions. It is in this context that deviations from Walden rule

have recently led to the use of fractional Walden rule for RTILs [46], according to which

for RTILs,

Λm × ηα = Constant (2.6)

thus a logarithmic plot of Λm vs. η should give a straight line with slope α. The experi-

mental data when plotted in the form of equation 2.6, gave value of α equal to 0.76 for

both [BMIM][BF4] and [BMIM][PF6]. Interestingly we found that for both these RTILs,

this value matches the ratio of activation energy for Λm and activation energy for viscous

flow as obtained from the Arrhenius plots for these two observables. Such correlation

has also been recently reported by Schreiner et al. [47]

2.2.4 Effect of cosolvent

Surface tension

Figure 2.4 depicts the variations in γ with change in volume fraction of RTIL in ACN.

Curvature of the traces indicates nonideality of the mixtures. To make the effect more

clearly visible we calculated the γ-deviations for the mixture using equation

∆γ = γ − (XILγIL +XACNγACN) (2.7)

52

Figure 2.4: (A) γ and (B) ∆γ (calculated using equation 2.7) as a function of composition inacetonitrile for [BMIM][BF4] and [BMIM][PF6].

The results plotted in Figure 2.4B clearly demonstrate that mixing of RTIL with ACN

is nonideal and deviations from ideal behavior are more pronounced for [BMIM][PF6]

than for [BMIM][BF4]. This may be ascribed to more structured nature of [BMIM][PF6]

than [BMIM][BF4]. IR spectra recorded for different dilutions (with ACN) of the two

RTILs are shown in Figure 2.5. As evident the C-H vibrations of the imidazolium ring

Figure 2.5: FTIR absorption spectra in the range 2500-3500 cm−1 at changing dilutions withacetonitrile for (A) [BMIM][PF6] and (B) [BMIM][BF4].

remain unchanged, implying that for cation-anion pairs in imidazolium based RTILs,

the interactions are predominantly couloumbic in nature. However, a slight blue shift is

noticed in case of CH3- group vibrations of the side chain. This perhaps is a result of

increased freedom of the said groups on account of dilution in RTIL.

53

Conductivity

Figure 2.6 depicts the κ values as a function of volume fraction of RTIL in ACN. For

both [BMIM][BF4] and [BMIM][PF6], the traces pass through maxima. Using the den-

sity values of the RTILs and ACN the corresponding molar conductances and mole frac-

tions were calculated. The molar conductance vs. mole fraction of RTILs is plotted in

Figure 2.6B. The feature of nonideality observed in surface tension plots is also seen in

Figure 2.6: (A) κ (specific conductance) and (B) Λ (molar conductance) as a function of com-position in acetonitrile for [BMIM][BF4] and [BMIM][PF6].

these conductivity plots. The behavior of molar as well as specific conductance -an initial

increase followed by a decrease- has been reported earlier [48, 49] and can be explained

in terms of variations in structural order of RTIL upon addition of cosolvent.

According to Dupont [50], pure imidazolium based RTILs form an extended hydro-

gen bonded network due to self aggregation of monomeric cations under the influence

of anions. Because of this extended hydrogen bonded systems, imidazolium based

RTILs are highly structured and consequently regarded as supramolecular fluids. Signa-

tures of structural organization in the investigated RTILs were clearly observed in their

UV-visible absorption and emission spectra. Figure 2.7 shows the absorption spectra

recorded for the pure [BMIM][PF6] and [BMIM][BF4].

The long tail of the absorption band has been attributed to the presence of a large

number of varied size supramolecular aggregates that are energetically different and

characterized by specific absorption maxima [51–53]. Direct evidence for presence of

54

Figure 2.7: UV-visible absorption spectra for pure (A) [BMIM][PF6] and (B) [BMIM][BF4]ionic liquids.

such supramolecular aggregates has been reported earlier [54, 55]. The tail is more pro-

nounced in case of [BMIM][PF6] than [BMIM][BF4] which indicates the more structured

nature of former.

More interesting features were observed in the emission spectra recorded for pure [BMIM]

[BF4] and [BMIM] [PF6], as shown in Figure 2.8. The emission spectra show two max-

ima, one in lower and other in higher wavelength range. The position and relative in-

tensity of both the maxima were found to be excitation wavelength dependent. At lower

excitation wavelengths, the maxima in the low wavelength region of emission spectrum

is dominating. As the excitation wavelength increases, the maxima at higher wavelength

becomes more intense while the maxima in lower wavelength range in emission spec-

trum decreases in intensity and finally disappears. Another interesting observation is that

beyond a particular range, the upper wavelength range maxima of emission spectrum

shows a significant red shift with increase of excitation wavelength.

Paul et al. in their reports [52, 56] have proved that that the short wavelength emis-

sion in imidazolium based RTILs is due to the monomeric form of the imidazolium

cation while excitation and relaxation of varied sized and energetically different asso-

ciated forms of the constituent ions are responsible for the tail in absorption and long

wavelength components in their emission spectra. The aggregates are expected to have

their specific absorption and emission maxima. With changing excitation wavelengths,

different species get excited and hence different emission behavior is observed. How-

ever theory of emission from photoexcited assembly of variedly energetic species [57]

55

Figure 2.8: PL emission spectra at changing excitation wave lengths recorded for pure (A)[BMIM][PF6] (B) [BMIM][BF4].

demands that emission from such a photoexcited assembly will always be from the least

energetic specie and hence excitation wavelength dependence of maxima in emission

spectrum of RTILs is unexpected. Short fluorescence lifetimes of excited species and

minimal interactions among energetically different aggregates that reduce the chances

of energy transfer among these species in RTILs is perhaps the reason for the excita-

tion wavelength dependence of their emission spectra [51]. The shape and intensity of

two component excitation wave length dependent photoluminescence (PL) spectrum, as

shown in Figure 2.8, thus also attests the presence of supramolecular aggregates in the

investigated RTILs.

Marked variations were observed in the absorption and emmision spectra of RTILs on

dilution with ACN. Decrease in tail length of the absorption spectra upon dilution with

ACN as shown in Figure 2.9, indicates the breakage of supramolecular aggregates in

RTILs. However, presence of absorption tail up to high dilution limits implies the pres-

ence of supramolecular structures (probably of smaller size) in these solutions.

Emission spectra recorded for varied RTIL/cosolvent compositions, as shown in Figure

2.10 also lead to similar conclusions. As per earlier reports [51, 53], the high wavelength

emissions are due to supramolecular aggregates while the low wavelength emissions

come from monomeric forms of RTILs. As seen in Figure 2.10, the relative emission on

account of monomeric forms (low wavelength hump) increases on successive dilutions

with ACN in comparison to that from aggregated forms, clearly indicating the transition

of aggregated forms of RTIL into monomeric forms. Normalizing the emission spectrum

56

Figure 2.9: UV-visible absorption spectra at changing dilutions with acetonitrile for (A)[BMIM][PF6] and (B) [BMIM][BF4].

with respect to concentration, as shown in Figure 2.10B also indicate that addition of

ACN breaks the network of supramolecular aggregates in RTILs. The relative emission

Figure 2.10: PL emission spectra at excitation wave length of 330 nm recorded for[BMIM][BF4] at changing dilutions with acetonitrile (A) as recorded spectra (B) spectra nor-malized with respect to concentration.

of monomeric forms to aggregate forms changes from dominating aggregated forms in

pure RTIL to dominating monomeric forms in dilute states. However, an interesting fea-

ture worth to be underscored is that the emission from the aggregates does not vanish

even at high dilutions, thereby indicating the presence of associated structures in RTILs

at these dilution limits. The positions of maxima in the traces in Figure 2.6 and the

emission spectra of Figure 2.10 hence imply that the structural order of RTILs is re-

tained up to high dilutions (RTIL volume fraction = 0.3), beyond which the RTIL exists

as conventional electrolyte which is strongly associated. This is in agreement with earlier

57

reports based on absorption spectra and kinetic measurements [58] and dielectric relax-

ation spectroscopy measurements [59] on N-alkylimidazolium based RTILs.

The curved behavior observed in traces of Figure 2.4 and 2.6 clearly demonstrate the

nonideal mixing of investigated RTILs with ACN. Thus it may be proposed that the ad-

dition of ACN leads to structural distortions of RTILs and its use as cosolvent for RTIL

should always be carried out after the extent of structural distortions is known. These

studies in turn will establish the dilution limit up to which the cosolvent can be added

without affecting the essential ordered solvent characteristics of RTILs.

2.2.5 Conclusion:

Through conductivity, surface tension and spectroscopic (UV-visible, FTIR and Pho-

toluminescence) measurements, it is established that both increase in temperature and

addition of cosolvent lead to decrease in coulombic as well as van der Waals interactions

and hence structural disorder in N-alkylimidazolium RTILs. Thus use of these two op-

tions to decrease the viscosity of RTILs for electrochemical investigations, will always

be at the cost of loss in structural order at the interface and in the bulk of RTIL. Cosol-

vent addition preserves the structural aspects up to a certain limit of dilution (Vil = 0.3,

for acetonitrile), beyond which RTILs behave as strongly associated electrolytes wherein

structure specific effects of RTILs on any phenomena of interest cannot be expected.

58

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