Physical and Thermophysical Properties of 1-Hexyl-1,4-diaza[2.2.2

Article pubs.acs.org/jced

Physical and Thermophysical Properties of 1‑Hexyl-1,4diaza[2.2.2]bicyclooctanium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Łukasz Marcinkowski,† Adam Kloskowski,*,† and Jacek Namieśnik‡ †

Department of Physical Chemistry and ‡Department of Analytical Chemistry, Faculty of Chemistry, Gdansk University of Technology, , Narutowicza Str.11/12, Gdansk 80-233 Poland ABSTRACT: Owing to their unique properties, ionic liquids (ILs) have become a real alternative to organic compounds. Nowadays, ILs are commonly used in a number of areas such as chemical synthesis and separation techniques. However, ionic liquids continue to have a limited applicability in industry because the information on their physical and chemical properties is not readily available. This paper presents, for the first time, the results of studies on 1-hexyl-1,4-diaza[2.2.2]bicyclooctanium bis(trifluoromethylsulfonyl)imide ([HDABCO][NTf2]) ionic liquid which is relatively novel and not well researched. A wide range of properties, important from a point of view of potential applications of [HDABCO][NTf2], was examined. Parameters such as density, phase transition temperatures, long-term thermal stability, polarizability, hygroscopicity, and McReynolds constants have been determined.

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INTRODUCTION Room-temperature ionic liquids (RTILs) are organic compounds with relatively low melting points (< 373 K). Strong molecular interactions, which are typical for salts, are responsible for their negligible vapor pressures, the latter being one of the essential reasons for interest in this class of compounds. Owing to their unique physicochemical properties, ionic liquids are more often applied in areas where they have the potential to be an alternative to organic solvents. This applies mostly to their use in industrial processes as a medium for chemical reactions (also as catalysts),1 in separation processes2−5 and as a heat transfer media.7 On the other hand, the chemistry of ionic liquids as a group has not been investigated well enough to enable designing the structure of ionic liquids with complete predictability of their behavior in relation to specific applications. Regarding the availability of physicochemical data, ionic liquids based on the imidazolium, ammonium, or pyridinium cation have by far been best described in the literature.8 As data on such liquids is readily available, they are also a basis for developing theoretical models related to the impact of structure (i.e., the chain length of alkyl substitutes in the cation, and the type of anion) on the properties of the ionic liquids.9 On the other hand, a considerable number of reports have been published on the synthesis of novel ionic liquids based on new types of cations, for example, N-morpholinium, 1,4-diaza[2.2.2]bicyclooctanium, which are supposed to have a low environmental impact and are readily biodegradable.10 With respect to the physicochemical properties of novel ILs, this area is now a terra incognita in © 2014 American Chemical Society

the published studies. An additional impediment to the comprehensive assessment of the usefulness of ionic liquids in specific applications is the dispersal of data among papers of different scope. The investigations described in this paper were intended to provide a comprehensive set of the physicochemical parameters of the studied ionic liquid (Figure 1). Because there are no literature reports containing physicochemical description of [HDABCO][NTf2] the studies described in this paper are unique. A number of parameters such as melting point, freezing point, and decomposition temperature, long-term thermal stability, density, hygroscopicity, polarizability, and McReynolds constants (as a measure of polarity) were determined.

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EXPERIMENTAL SECTION Materials. [HDABCO][NTf2] > 99 % (CAS No. 89825650-3) was purchased from IoLiTec Ionic Liquids Technologies (Germany). The McReynolds compounds of analytical grade (CAS No.), that is, benzene (9072-35-9), 1-butanol (71-36-3), 2-pentanone (107-87-9), nitropropane (108-03-2), pyridine (110-86-1), and the n-alkane series: n-pentane, n-hexane, nheptane, n-octane, n-nonane, n-decane, n-undecane, n-hexadecane, were obtained from Sigma-Aldrich (Germany). Methanol (67-56-1) and chloroform (67-66-3) were HPLC grade (SigmaReceived: March 15, 2013 Accepted: February 6, 2014 Published: February 14, 2014 585

dx.doi.org/10.1021/je400252p | J. Chem. Eng. Data 2014, 59, 585−591

Journal of Chemical & Engineering Data

Article

viscosities in the range from 0.2 mPa·s to 20000 mPa·s. The temperature was controlled internally with a precision of ± 0.05 K within the range from 298.15 K to 338.15 K. All measurements were performed three times. Decomposition Temperature. Decomposition temperature was determined using a Netzsch TG209 thermogravimetric analyzer (Germany). The starting temperature of decomposition (Tstart) and the onset temperature (Tonset) were found using Proteus software from Netzsch. Tstart is the starting temperature of decomposition of the sample. Tonset is found graphically as the abscissa of the point of intersection of the baseline and a tangent to the decomposition curve in the course of decomposition (Figure 3). The experiments were performed at a constant heating rate of 10 K·min−1 under atmosphere of argon (100 mL·min−1) and air (100 mL·min−1). The measurements were performed in aluminum oxide (Al2O3) pans. All procedures in the sample preparation step, such as weighing or transferring to the pans, were performed in a glovebox under nitrogen atmosphere in order to prevent water absorption by the ionic liquids. Three measurements were performed to verify the repeatability of the procedure. Because of the shape of TGA curves during decomposition, the obtained data were also subjected to differential analysis (DTG). Next, determination of the long-term thermal stability of [HDABCO][NTf2] was performed. For this purposes, 15 h long TGA measurements of samples of ionic liquid (11−18 mg) were performed at different temperatures ranging between 523 K and 583 K (at 10 K intervals). Melting Temperature. The phase transition temperatures were determined by differential scanning calorimetry using a Netzsch DSC Phoenix 204 apparatus, while the obtained results were processed using Netzsch Proteus software. The measurements were performed under argon atmosphere (flow rate 20 mL·min−1). Samples of ionic liquid were dispensed into Al2O3 open pans. The DSC apparatus was calibrated using indium 99.999 % (429.7485 K) as standard. To verify the possibility of water absorption by the sample, the measurement was performed three times. Between the measurements, the sample was kept at a temperature of 373 K for 3 h under argon flow. The obtained results overlapped within the limits of measuring accuracy, which showed that the samples had not absorbed water. The phase transition temperatures were determined by alternately cooling and heating the sample within the temperature range from 200 K to 360 K at a rate of 10 K· min−1. The upper temperature was set based on the rheological evaluation of a liquid’s properties, that is, the temperature range in which the ionic liquid is in the liquid phase. The freezing and melting points were determined as the onset of phase transition. Polarizability. Polarizability was determined by measuring the values of refractive index corresponding to the yellow line of sodium (λ = 589 nm). The experiments were performed using the Abbe NAR-1T Liquid refractometer (Atago, USA) equipped with a thermostat for controlling the temperature with an accuracy of ± 0.1 K. The instrument precision on the nD scale was 0.0002. The measurements were performed in the temperature range from 313 K to 333 K. Hygroscopicity. Hygroscopicity was measured by placing a sample of ionic liquid and a sample of hygrostatic saturated solution of Mg(NO3)2·6H2O (ca. 50 % relative humidity at room temperature) in a desiccator. The desiccator was kept in a thermostatted water bath at 295 ± 1 K. The water content in

Figure 1. Chemical structure of [HDABCO][NTf2].

Aldrich, Germany). Deionized water was obtained using a Millipore water purification system. The gases, namely, nitrogen (purity, 99.999 %) and argon (purity, 99.999 %), were purchased from Linde (Poland). It is well-known that water content has a significant effect on the properties of ionic liquids.11 Having this in mind, the purchased liquid was predried. The procedure based on purging the thermostatted (50 °C) ionic liquid with an inert gas (N2) was applied. Prior to being introduced into the ionic liquid, nitrogen was passed through the Super Clean Gas Purifier moisture trap (Supelco, Germany) to ensure the minimum water content of the gas. The predrying procedure was continued (determination of water content at 24 h intervals) until a stable minimum content of water had been achieved. The final water content was determined at the level of 84.3 ± 1.7 ppm (average of three parallel measurements). Measurements were performed by Karl Fischer coulometric titration using a Metrohm 831 KF Coulometer. The total duration of the drying procedure was 72 h. To minimize the possibility of sample contamination with water, the sample was kept under a dry nitrogen atmosphere. The concentration of chloride ions, which remain after the synthesis, is another significant parameter affecting the properties of ionic liquids.11 The concentration of chloride ions in the investigated ionic liquid was measured by using a Dionex ICS 3000 ion-exchange chromatography system (Germany) equipped with a conductometric detector. The determined concentration of chloride ions was 0.0019 %, which indicated a high purity of the sample. Density, Viscosity, and Surface Tension Measurements. The densities of [HDABCO][NTf2] were measured using an Anton Paar DMA 5000 densimeter with a thermostatted system based on a Peltier unit, with a precision of 5.0· 10−6 g·cm−3 and standard uncertainty better than 3.5·10−5 g· cm−3. The applied temperature was in the range from 313.15 K to 333.15 K, with an accuracy of 0.01 K. The sample of ionic liquid was injected into the measurement cell using a gastight syringe to avoid water absorption. Measurements of surface tension were performed using Tensiometer Krüss K 10ST with a Wilhelmy plate. All measurements were performed in nitrogen atmosphere to avoid absorption of water. Samples were kept in a closed measuring cell with a volume of about 10 cm3, and the temperature was controlled with accuracy of ± 0.01 K within the range from 313.15 K to 333.15 K. The standard uncertainty of the measurements was in range 0.03 mN·m−1 to 0.11 mN·m−1. Viscosity measurements were carried out in the SVM 3000 viscometer (Austria). This apparatus ensures precision of ± 0.1 % and reproducibility < 0.35 % for 586



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but with different cations, that is, 1-butyl-3-methylimidazolium [BMIM] and 1-hexyl-3-methylimidazolium [HMIM] is presented in Figure 2.

the subsamples was determined by Karl Fischer coulometric titration after (1, 5, 15, 35, 65, 150, and 300) h of incubation. McReynolds Constants. The polarity of [HDABCO][NTf2] was determined based on the values of the McReynolds constant. To this end, two packed columns were prepared, one packed with a support coated with the ionic liquid, and the other one with squalane (CAS No. 111-01-3) as a reference phase. The GC columns were made of stainless steel tubes with L = 1 m and i.d.= 2.1 mm. The columns were washed sequentially with 2 mL of each dichloromethane, methanol, water, concentrated hydrochloric acid, water, methanol, and dichloromethane. Next, m-xylene solution of perhydropolysilazane (PSZ) (20 %) was passed through the column, followed by a heat treatment at 673 K for 2 h to form a silica layer. After cooling, the capillary was rinsed with dichloromethane and dried.12 The suitable packing of Chromosorb W HP-DMCS 80/100 mesh solid support (Sigma-Aldrich, Germany) was prepared by using a rotary evaporator in order to ensure a uniform distribution of sorbents on the column surface. The ionic liquid solution was prepared in methanol, while squalane was dissolved in chloroform. The support was weighed before and after the coating process. To avoid the interactions due to residual adsorption on the carrier, both the ionic liquid and squalane were coated in an amount equal to 40 % of the support weight. The experiments were carried out using the Agilent 7890 A gas chromatograph equipped with a FID detector (Palo Alto, USA). The injector and detector temperatures were set at 523 K. For a split ratio of 50:1 and the injected sample volume of 1 μL, the requirement of infinite dilution was fulfilled. Nitrogen was used as a carrier gas, with a volumetric flow rate of 0.02 L·min−1. The chromatographic analysis was performed at a constant temperature of 393 K.

Figure 2. Comparison of densities of [HDABCO][NTf2] and imidazolium-based ionic liquids at different temperatures. ●, [HDABCO][NTf2], ◇, [HMIM][NTf2],13 □, [BMIM][NTf2].14

Considering that the [NTf2] anion has a rather large volume, one would expect the presence of a spatial obstacle, hindering closeness between the anion and the cation, thereby also causing a decrease in the density of the ionic liquid. This effect may be observed when comparing the two imidazolium cationbased liquids; the liquid with the longer alkyl chain (C6H13) has much lower densities. Interestingly enough, in the case of (HMIM), the densities practically overlap with the results obtained for [HDABCO][NTf2] in the considered range of temperature. Moreover, it is worth noting that [HDABCO] has a lower molecular weight compared to [HMIM]. Although the [HDABCO] cation has a single alkyl substitute, which partially limits access of the anion, it is far from being a significant spatial obstacle. Practically, the entire fragment of the molecule with a positive charge is “accessible” for electrostatic interactions. This observation is also supported by the relatively high values of viscosity. Viscosity equal 390 mPa·s at 313.15 K locate [HDABCO][NTf2] among ionic liquids with higher viscosities.15 However, direct comparison of the viscosity values with others found in the literature is difficult due to different water content and methodology applied. Finally, eq 1 was used for calculating the value of the thermal expansion coefficient αp.

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RESULTS AND DISCUSSION Density, Viscosity, Surface Tension. The values of measured properties of [HDABCO][NTf2] and the corresponding temperatures are shown in Table 1. Table 1. Density (ρ), Viscosity (η), and Surface Tension (γ) of [HDABCO][NTf2] with Standard Uncertainties at the 313.15 K to 333.15 K Temperature Range (p = 0.1 MPa)a

a

T

ρ

u(ρ)

η

u(η)

γ

u(γ)

K

g·cm−3

g·cm−3

mPa·s

mPa·s

mN·m−1

mN·m−1

313.15 318.15 323.15 328.15 333.15

1.35515 1.35093 1.34672 1.34251 1.33829

0.00008 0.00012 0.00006 0.00006 0.00011

389.3 190.8 138.1 107.0 87.6

0.29 0.13 0.15 0.12 0.09

35.62 35.33 35.05 34.83 34.51

0.03 0.07 0.10 0.09 0.11

αp = −

Standard uncertainty u is u(T) = 0.01 K.

⎛ ∂ ln ρ ⎞ 1 ⎛ ∂ρ ⎞ ⎜ ⎟ = −⎜ ⎟ ⎝ ⎠ ⎝ ∂T ⎠ p ρ ∂T p

(1)

−3

where ρ is density in g·cm ; and T is temperature in K. The obtained average value of αp for the applied temperature range equals (−6.26 ± 0.01)·10−4 K−1, which is slightly lower compared to the values calculated for the [BMIM] (−6.4 ± 0.3)·10−4 K−1 and [HMIM] (−6.73 ± 0.005)·10−4 K−1 cations. Decomposition Temperature. Figure 3 shows thermal decomposition curves for [HDABCO][NTf2] which are based on the measurements in argon and air. The starting and onset temperatures for thermal decomposition under argon atmosphere were 583 K and 643 K, respectively. It was observed that the gas type had a minor effect on the decomposition process. Both the starting and onset temperatures of decomposition were approximately 5 K higher for samples heated in the air. In

The measurements were performed for the temperature range in which the ionic liquid is in the liquid phase. In the cooling step, [HDABCO][NTf2] becomes overcooled and is able to remain in the liquid phase for a long time. Therefore it was assumed that the temperature range below the melting point is not representative. For the investigated temperature range, the densities of [HDABCO][NTf2] were more than 30 % higher compared to the density of water. Moreover, a slight decrease in density with increasing temperature was observed. For comparative purposes, a compilation of the obtained results and literature data for ionic liquids based on a common anion, 587



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Figure 5. Isothermal decomposition of [HDABCO][NTf2] in the temperature range 523 K to 583 K under argon flow.

Figure 3. Thermal decomposition curves for [HDABCO][NTf2] obtained by TGA, with marked Tstart and Tonset temperatures. Solid line, argon; dashed line, air.

exposures to higher temperature might be expected. Based on the obtained results, it can be expected that the practical range of handling [HDABCO][NTf2] covers temperatures below 523 K. Taking into account the shape of the obtained curves and literature data, it was assumed that decomposition is a pseudofirst-order reaction.17 The reaction rate in its integrated form is described by the following equation:

respect to thermal resistance, the obtained results placed [HDABCO][NTf2] in the vicinity of average values for other ionic liquids whose onset temperatures ranged between 523 K and 773 K.16 Interestingly, for the investigated temperature range, the sample became almost entirely decomposed in the air, while under argon atmosphere, a ca. 10 % residue remained. Moreover, for decomposition under argon atmosphere, there are three different fragments in the fastest decomposition area of the curve, which are marked with tangent lines. Thus differential analysis of TG data was also performed (DTG). The DTG results are shown in Figure 4.

ln α = k·t + ln α0

(2)

where α denotes the mass of the ionic liquid (mg), α0 denotes the mass of the ionic liquid at t = 0, k is the reaction rate for decomposition (min−1), t is time (min−1). A logarithmic function was fitted to the data sets, resulting in a very good fit as described by the values of coefficient of determination (R2) above 0.979 and low standard deviations of slope (see Table 2). Table 2. Reaction Rate Constants (k) for Thermal Decomposition of [HDABCO][NTf2] in the Temperature Range 523.15 K to 583.15 K, and the Corresponding Values of the Coefficient of Determination (R2) T/K 523.15 543.15 553.15 563.15 573.15 583.15

Figure 4. Thermal decomposition curves for [HDABCO][NTf2] in argon obtained by TG and DTG analysis. Dashed line, TG; solid line, DTG.

k·104/min−1

R2

± ± ± ± ± ±

0.989 0.998 0.999 0.996 0.998 0.979

0.487 3.444 5.835 10.86 16.53 26.2

0.002 0.005 0.007 0.03 0.03 0.2

To find the value of activation energy (Ea) for thermal decomposition, a plot was prepared by using the Arrhenius equation (Figure 6):

The shape of the TG and DTG curves indicates a three-step decomposition of [HDABCO][NTf2] under argon atmosphere. The identification of decomposition products would require a more detailed analysis; however, such studies have not been carried out. Next, the long-term thermal stability studies on [HDABCO][NTf2] were conducted. Figure 5 shows the results of TG analysis performed for 15 h in the temperature range 523 K to 583 K. The results confirmed the general property of ionic liquids for which the practical decomposition temperature is significantly lower than Tonset. This is of particular importance when ionic liquids are used as solvents in chemical reactions or as stationary phases in gas chromatography, where longer

⎛ −E ⎞ k = A exp⎜ a ⎟ ⎝ RT ⎠

(3)

The values of the pre-exponential term and activation energy, calculated from the values of the abscissa and the slope (R2 = 0.998), were 6.7·10−10 and 138 kJ·mol−1, respectively. Melting Temperature. The results obtained by means of DSC during sample heating are shown in Figure 7. The shape of DSC scan during heating represents one of three types described in the literature,14 and it is typical for 588



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Polarizability. The refractive index measurements and the calculated values of polarizability and molar refraction in the temperature range 313 K to 333 K are presented in Table 3. The refractive indices are the mean values of three runs. The values of polarizability α and molar refraction Rm were calculated using eqs 4 and 5, respectively: α=

3R m 4πNA

Rm =

(4)

n2 − 1 M n2 + 2 ρ

(5)

where n is the refractive index, M is the molar weight of the ionic liquid in g·mol−1, ρ denotes density at the measurement temperature (g·cm−3), and NA is the Avogadro number. For comparative purposes, the values of molar refraction and polarizability for typical solvents have also been listed in Table 3. The molar refractions and polarizabilities of the investigated ionic liquid are much higher compared to other solvents, which indicates that the value of the electronic component of polarizability is high. Hygroscopicity. Water content is known to have an essential effect on most properties of ionic liquids.11 Although the presence of water may be a serious problem in some applications, water content must be determined in every case. From a practical point of view, the knowledge of the hygroscopic properties of ionic liquids is also important; it directly affects the procedures for their use. For strongly hygroscopic liquids, such as imidazolium ionic liquids with halide anions,20 any contact with atmospheric air must be avoided. The process of water absorption by [HDABCO][NTf2] was examined by placing glass vials filled with an ionic liquid (0.5 mL of each sample) in an environment with 50 % relative humidity. The interfacial (Il-gas phase) area was equal 1.54 cm2. The curve depicting water sorption by the investigated ionic liquid is typical for sorption processes. Initially, a rapid increase in the amount of absorbed water was observed. During the first hour, the content of water in the sample increased almost 4-fold from 86 to 368 ppm. During the next four hours of exposure, the water content increased 15-fold compared to the initial value, reaching 1350 ppm. Finally, the equilibrium amount of water was found to be nearly 8000 ppm. On the basis of the obtained results, it can be concluded that [HDABCO][NTf2] is moderately hygroscopic compared to imidazolium-based ionic liquids (with halide anions); the water saturation process in [HDABCO][NTf2] is much shorter, ca. 100 h.21 McReynolds Constants. In many cases, determination of polarity by means of such parameters as polarizability or dipole

Figure 6. The Arrhenius plot for thermal decomposition of [HDABCO][NTf2] in the temperature range 523 K to 583 K under argon flow.

Figure 7. DSC scan for [HDABCO][NTf2] depicting a sequence of glass-transition (Tg), cold crystallization (Tcc) and melting transition (Tm). Dashed line, cooling; solid line, heating.

amorphous compounds.18 In Figure 7, three events corresponding to different phase transitions are depicted (from left to the right), that is, glass-transition, cold crystallization, and melting. Melting (309.0 ± 0.85 K) and cold crystallization (273 ± 1.6 K) points were found as the onset of the corresponding peaks. The glass-transition temperature was determined as a midpoint of the heat capacity change (222 ± 5 K). An analysis of the DSC curve obtained in the cooling mode shows no clear evidence of the phase transition of the ionic liquid. A comparison of the enthalpy of the cold crystallization (9.0 ± 0.7 J·g−1) and melting (11.0 ± 1.0 J·g−1) transitions indicates that overcooled [HDABCO][NTf2] has an amorphous structure (∼80 %).

Table 3. The Values of Refractive Index (navg) Determined for [HDABCO][NTf2], and the Calculated Values of Polarizability (α) and Molar Refraction (Rm) at 313.15 K (p = 0.1 MPa). Standard Uncertainty (u) for navg and Combined Uncertainties (uC) for α and Rm Are Also Included ρ

u(ρ)

Rm

uC(Rm)

α

uC(α)

K

navg

u(navg)

g·cm−3

g·cm−3

cm3·mol−1

cm3·mol−1

[1023cm3]

[1023cm3]

313.15 water (298)b ethanol (298)b benzene (298)b

1.4379

0.0002

1.35515

0.00008

92.38 3.574 12.961 26.289

0.04

3.66

0.015

T

a

1.4

Standard uncertainty (u) is u(T) = 0.01 K. bValues calculated from eq 5 using the published values of refraction index.19 589



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I = 100Z +

100[log t R′ (i) − log t R′ (z)] log t R′ (z + 1) − log t R′ (z)

(6)

where Z is the number of carbon atoms for n-alkane with retention time tr′(z); tr′(i) is a reduced retention time for the test compound; tr′(z) denotes a reduced retention time for the n-alkane eluted before the test compound, and tr′(z + 1) denotes a reduced retention time for the n-alkane eluted after the tested compound. 4. Calculation of ΔI: (7)

ΔI = aX + bY + cZ + dU + eS

where weight coefficients a, b, c, d, and e characterize the contribution by a given test compound and X, Y, Z, U, and S are partial ΔI values for these test compounds (all characterizing the stationary phase relative to squalane). The following symbols were adopted for the five investigated compounds: benzene (a = 1), 1-butanol (b = 1), 2-pentanone (c = 1), nitropropane (d = 1), pyridine (e = 1). A set of McReynolds constants was obtained by subtracting the values of Kovats indices obtained for the tested compounds on both chromatographic columns. The produced results are compiled in Table 5.

Figure 8. Water uptake in the samples of [HDABCO][NTf2] exposed to 50 % relative humidity at a temperature of 296 K.

moment is insufficient for describing the force and type of interactions with different types of compounds. Therefore, the polarity of [HDABCO][NTf2] was characterized using McReynolds constants. This method is often employed for assessing the polarity of stationary phases in gas chromatography. Generally, the procedure is based on determining retention times for compounds which represent different types of molecular interactions by using columns packed with a support coated with the tested compound and squalane as a reference (nonpolar phase). This enables the determination of specific (in relation to their interactions) properties of the compound, which is a stationary phase in the chromatographic column, as well as its overall polarity. Information on the compounds used is provided in Table 4.

Table 5. Kovats Indices (I) and McReynolds Constants (ΔI) Obtained at a Temperature of 393 K

*

Table 4. Molecular Interactions of the Investigated McReynolds Compounds compound benzene

1-butanol 2-pentanon

nitropropane

pyridine

compound

Is

IHDABCO

ΔI

benzene 1-butanol 2-pentanone nitropropane pyridine

677.08 665.19 662.40 675.86 698.22

961.82 999.08 1174.48 1245.12 1222.50

284.74 333.89 512.08 569.26 524.28

lit* (X) (Y) (Z) (U) (S)

674 600 630 664 724

Literature values of Kovats indices determined at 393 K.22

The constants determined for squalane are in good agreement with literature data. In the case of McReynolds constants, it is customarily assumed that ΔI values below 100 denote nonpolar compounds; the values in the range 100 to 400 and above 400 indicate moderately polar and highly polar compounds, respectively. For [HDABCO][NTf2], the average ΔI value was 444, which shows that the investigated ionic liquid is highly polar. Moreover, it is mainly electron donor and acceptor interactions which contribute to the total polarity of [HDABCO][NTf2]. The contribution of disperse and protonacceptor interactions is much lower; it is at the level typical for compounds with moderate polarity.

effect weak base, π−π interactions related to weak dispersion forces and polarizability character of the phase some acidic properties indicates the hydrogen-bonding ability of the phase intermediate polarity compound relates to the polarizability and part of the dipolar character of the stationary phase strongly polar compound with no proton donor capability related to the electron donor, electron acceptor, and dipolar character of the phase strong proton acceptor and polar molecule indicates the acidic character of the phase

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CONCLUSIONS From a point of view of the practical use of ionic liquids, it is essential to determine their basic physicochemical and thermodynamic parameters. In the present paper we compiled a number of critical parameters, which characterize [HDABCO][NTf2] ionic liquid. It has been demonstrated that [HDABCO][NTf2] has moderate thermal resistance compared to other ionic liquids, but it can still be used in many potential applications. This ionic liquid is not much different from other ionic liquids in regards to the values of density and polarizability. [HDABCO][NTf2] is characterized by high polarity and shows an ability to generate specific interactions with compounds containing a carbonyl group (ketones) and compounds having nitrogen atoms in their structure, which indicates its applicability as a solvent/

The procedure for finding the values of McReynolds constant is as follows: 1. Determination of retention times (t r) for tested compounds and n-alkanes (C5−C11 and C16) 2. Determination of adjusted retention times (tr′) by subtracting the retention time of nonretained compound (t0) from the retention times obtained for each compound. 3. Calculation of Kovats indices from the following relationship: 590



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nisms and Temperatures of Ionic Liquids. Thermochim. Acta 2007, 465, 40−47. (18) Widmann, G. DSC of Amorphous Materials. Thermochim. Acta 1987, 112, 137−140. (19) CRC Handbook of Chemistry and Physics, 54th ed.; CRC Press: Cleveland, 1973. (20) Erdmenger, T.; Vitz, J.; Wiesbrock, F.; Schubert, U. S. Influence of Different Branched Alkyl Side Chains on the Properties of Imidazolium-Based Ionic Liquids. J. Mater. Chem. 2008, 18, 5267− 5273. (21) Cuadrado-Prado, S.; Domínguez-Pérez, M.; Rilo, E.; GarcíaGarabal, S.; Segade, L.; Franjo, C.; Cabeza, O. Experimental Measurement of the Hygroscopic Grade on Eight Imidazolium Based Ionic Liquids. Fluid Phase Equilib. 2009, 278, 36−40. (22) Rotzsche, H. Stationary Phases in Gas Chromatography. Journal of Chromatography Library; Elsevier: Amsterdam, 1991.

extractant for such groups of compounds. The results of thermogravimetric analyses indicate that a more detailed investigation into the mechanism of thermal decomposition of [HDABCO][NTf2] would be highly recommended.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +48-58-347-2110. Fax: +48-58-347-2694. Notes

The authors declare no competing financial interest.

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REFERENCES

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