Surface Tension of Tricyanomethanide- and Tetracyanoborate-Based

Article pubs.acs.org/jced

Surface Tension of Tricyanomethanide- and Tetracyanoborate-Based Imidazolium Ionic Liquids by Using the Pendant Drop Method Thomas M. Koller,† Michael H. Rausch,†,‡ Kaija Pohako-Esko,§ Peter Wasserscheid,§ and Andreas P. Fröba*,†,‡ †

Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Paul-Gordan-Straße 6, D-91052 Erlangen, Germany ‡ Department of Chemical and Biological Engineering, Institute of Engineering Thermodynamics (LTT), University of Erlangen-Nuremberg, Am Weichselgarten 8, D-91058 Erlangen, Germany § Department of Chemical and Biological Engineering, Institute of Chemical Reaction Engineering, University of Erlangen-Nuremberg, Egerlandstraße 3, D-91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The surface tension of nine tricyanomethanide ([C(CN)3]−)- and tetracyanoborate ([B(CN)4]−)-based ionic liquids (ILs) carrying a homologous series of the 1-alkyl-3methylimidazolium ([alkyl-MIM]+) cations [EMIM]+ (ethyl), [BMIM]+ (butyl), [HMIM]+ (hexyl), [OMIM]+ (octyl), and [DMIM]+ (decyl) was measured with the pendant drop method in the temperature range between (283 and 353) K at atmospheric pressure with an estimated uncertainty of 2 % (k = 2). For the probed ILs, the surface tension decreases with increasing temperature and increasing alkyl chain length of the cation. Smaller values for the [B(CN)4]−-based ILs compared to the [C(CN)3]−-based ILs having the same cation were observed. The measured surface tensions agree with the limited number of experimental data found in the literature for the two IL families. A simple prediction based on the surface tension measured at 293 K and the temperature dependence of density showed good agreement with the measured temperature-dependent data. In comparison to other [alkyl-MIM]+-based ILs with anions of varying molecular size, the fairly large surface tensions of the ILs investigated in this study could be attributed to the strong charge delocalization in their relatively small anions.

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INTRODUCTION Ionic liquids (ILs) represent a new class of working fluids in chemical and energy engineering, for example, in carbon capture processes,1 electrolyte applications,2 or catalytic reactions.3 Their low viscosities4−6 and large solubilities for carbon dioxide7−9 make tricyanomethanide ([C(CN)3]−)- or tetracyanoborate ([B(CN)4]−)-based ILs promising candidates for such applications. To design, for example, stable IL membranes used for gas separation, reliable data for the surface tension (ST) of ILs are required.10 Yet, there is still a scarce experimental data situation in literature for the ST of ILs in general11,12 and especially for the two IL families studied here.4,5,9,13−16 The accurate measurement of the ST is not trivial and cannot be realized for all conceivable ILs, which motivated the development of corresponding prediction methods.10,17,18 For this, reliable experimental data for a manageable amount of structurally diversified ILs are required. Most of the measurements reported in literature, however, are restricted to ambient temperature conditions.11,12 Furthermore, the sample characteristics such as purity are not specified and measurement © XXXX American Chemical Society

uncertainties are underestimated or even ignored in context with some of the published data. All this impedes the interpretation of deviations between ST data for the same ILs outside combined uncertainties if given as well as the development of sound prediction models. In our previous study,14 the pendant drop (PD) method was successfully used for the accurate determination of the ST for 21 ILs at ambient temperature with an expanded uncertainty (k = 2) of less than 1 %. Systematic variations in the chemical nature of cation and anion allowed for a detailed analysis of structure−property relationships. In the present study, the ST of nine [C(CN)3]−- and [B(CN)4]−-based ILs carrying homologous [alkyl-MIM]+ cations with varying alkyl sidechain length was measured for temperatures between (283 and 353) K at atmospheric pressure using a modified PD setup. Besides a comparison with available experimental data and a Received: March 31, 2015 Accepted: July 22, 2015

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DOI: 10.1021/acs.jced.5b00303 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Specification of the Studied IL Samples

a b

IL

source

[EMIM][C(CN)3] [BMIM][C(CN)3] [HMIM][C(CN)3] [OMIM][C(CN)3] [EMIM][B(CN)4] [BMIM][B(CN)4] [HMIM][B(CN)4] [OMIM][B(CN)4] [DMIM][B(CN)4]

IoLiTec IoLiTec IoLiTec IoLiTec synthesis synthesis synthesis synthesis Synthesis

−1

molecular weight M/g·mol 201.24 229.29 257.34 285.39 226.05 254.10 282.16 310.21 338.26

initial mole fraction purity > > > >

0.98 0.98 0.98 0.98

purification method none none none none elution elution elution elution elution

final mole fraction purity

> > > > >

0.99 0.99 0.99 0.99 0.99

analysis method for purity

water mass fraction wa

analysis method for water content

ICb ICb ICb ICb NMRc NMRc NMRc NMRc NMRc

1.14·10−3 7.57·10−4 1.71·10−3 7.46·10−4 6.02·10−4 9.10·10−4 5.57·10−4 5.59·10−4 5.29·10−4

KFCTd KFCTd KFCTd KFCTd KFCTd KFCTd KFCTd KFCTd KFCTd

The combined expanded uncertainties Uc are between Uc(w) = 0.2·w for w = 5·10−4 and Uc(w) = 0.05·w for w = 2·10−3 (level of confidence = 0.95). Ion chromatography. cNuclear magnetic resonance spectroscopy. dKarl Fischer coulometric titration

certainty (k = 2) of the water content measurements is estimated to be between 20 % and 5 %, corresponding to water mass fractions ranging from 5·10−4 to 2·10−3. Complete specification of the IL samples is summarized in Table 1. Surface Tension Measurements by the Pendant Drop Method. Surface (or rather, interfacial14) tensions at the ionic liquid/air interface were obtained by the PD method. This method is commonly used for measuring the ST of ILs.12,14,19,20 In this study, a universal surface analyzer (OEG, SURFTENS universal) was used, where the geometrical profile of a PD is compared with the theoretical drop profile. The latter is given by the Laplace equation for curved interfaces exposed to a gravitational field according to21

simple predictive method, the results are interpreted regarding structural and intermolecular influences.

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EXPERIMENTAL SECTION Materials and Sample Preparation. All [C(CN)3]−based ILs were provided by IoLiTec Ionic Liquid Technologies, Heilbronn, Germany. For the synthesis of all [B(CN)4]−-based ILs performed in our laboratories, the halide salts 1-ethyl-3methylimidazolium chloride ([EMIM]Cl), 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), and 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl) were purchased from Solvent Innovation GmbH (now Merck KGaA) and washed with acetone before use. In addition, 1-methylimidazole distilled prior to usage as well as 1-bromooctane and 1-bromodecane used as received were purchased from Merck KGaA. By alkylation on 1-methylimidazole with the corresponding 1bromoalkane, 1-octyl-3-methylimidazolium bromide ([OMIM]Br) and 1-decyl-3-methylimidazolium bromide ([DMIM]Br) were synthesized. Potassium tetracyanoborate (K[B(CN)4]) was obtained from Merck KGaA and used as received. Then, all [B(CN)4]−-based ILs were synthesized from their corresponding halide salts by an ion exchange reaction. Equimolar amounts of a 0.5 mol·kg−1 aqueous solution of the halide salt and K[B(CN)4] were suspended in distilled water under rigorous stirring at ambient temperature for 24 h. The formed upper aqueous phase was decanted while the lower IL phase was diluted with dichloromethane and washed with distilled water several times. After drying of the IL-rich phase over magnesium sulfate, dichloromethane was removed from the IL product under vacuum atmosphere. For the [C(CN)3]−-based ILs, the sample purities specified by IoLiTec were obtained by ion chromatography. The purities of the [B(CN)4]−-based ILs were verified by 1H nuclear magnetic resonance (NMR) analysis (JEOL, ECX +400 spectrometer) with dimethyl sulfoxide-d6 (DMSO-d6) as solvent. The corresponding NMR spectra including the peak integrals can be found in the Supporting Information. Before use, all samples were dried at about 323.15 K on a vacuum line (50 Pa) over a period of at least 2 h. Furthermore, the samples were analyzed regarding their water contents by Karl Fischer coulometric titration (Metrohm, 756 KF coulometer). The reported values for the water mass fraction w of the ILs are averaged from the water contents determined directly before and after the ST measurements. The increase in the water mass fraction was in the range from 2·10−5 for [OMIM][B(CN)4] to 9·10−4 for [BMIM][B(CN)4]. The expanded relative un-

(ρ − ρair )gz ∂φ sin φ 2 = − − ∂s R σ x

(1)

The quantities in eq 1 are explained in Figure 1 which illustrates the shape of a drop in the x−z-plane. Here, φ is the

Figure 1. Schematic representation of the shape of a pendant drop with the quantities given in eq 1.

angle with respect to a tangent on point P and s is the arc length along the drop shape measured from the origin of the x−z coordinate system, that is, the apex of the droplet at point O. R is the radius of curvature relative to the apex, while g = 9.81 m·s−2 denotes the gravitational constant. ρ and ρair are the densities of the IL and the surrounding air phase. For the matching measured and calculated shape of the drop, the ST σ is evaluated by solving the differential eq 1 numerically. The main components of the PD drop instrument include an illumination screen, a CCD camera, a dosing system, and the software for data evaluation. Our previous measurements of the ST of ILs4,5,14,22,23 were performed inside an optical glass cell for photometry (Hellma, 402.000) at ambient conditions. Droplets pending on a capillary, which was supplied with B



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Figure 2. (a) Measurement cell. (b) Modified PD setup for the measurement of the surface tension of ILs as a function of temperature.

sample liquid via a syringe installed in the dosing system, were formed between screen and camera and then analyzed. For the measurements of the ST of ILs as a function of temperature, the available PD setup was modified as follows. A cuboid-formed and gold-coated measurement cell with four optical accesses and an inner volume of about 6 cm3 was used as measurement chamber. The outer dimensions of the cell shown in Figure 2a are 5 cm × 4 cm × 4 cm, corresponding to its length, width, and height, respectively. For temperature control, the cell was immersed inside a water basin positioned between screen and camera, see Figure 2b. Sealed copper tubes mounted between the outer surface of the cell windows and openings in the walls of the water basin allowed for optical access to the pendant drop. Water was circulated continuously around the measurement cell by a lab thermostat connected to the basin via two insulated flexible tubes. A calibrated Pt 100 Ω resistance probe directly contacting the outer surface of the cell was positioned at a distance of approximately 20 mm from the liquid drops. With this probe, temperatures are measured with an uncertainty (k = 2) of ± 20 mK. The upper inlet of the cell protruding from the water surface in the basin was covered with a sealing film to minimize humidity inside the cell. Syringes with cylinders and plungers made of polyethylene and polypropylene and manufactured by Henke Sass Wolf GmbH were filled with approximately 5 cm3 of the dried ILs. They were connected with stainless steel capillaries with an outer diameter of about 0.91 mm produced by MedChrom GmbH and fixed in the dosing system. After penetrating a capillary through the sealing film at the cell inlet, its lower end was adjusted in the line of sight of the optical system. Then, the basin was completely filled with water. After adjusting the water temperature by the thermostat, waiting periods of about 30 min were sufficient to reach constant temperature conditions at the measurement cell. Measurements were performed at temperatures between (283 and 353) K in steps of 10 K. After that, repetition measurements at 293 K were carried out to check the stability of the probed IL samples. The two data sets obtained for 293 K agreed within uncertainties for all ILs and were used for further data evaluation. The temperature stability during the measurements was within ± 0.05 K for T = (283 to 313) K and within ± 0.08 K for T = (323 to 353) K.

For an accurate determination of the ST by the PD method, a reliable calibration of the optical setup is necessary. Here, the two-step calibration method was used where the capillary is moved vertically by using a micrometer screw in the dosing system. With varying path lengths of the capillary from (1.5 to 3.5) mm in steps of 0.5 mm, the averaged calibration value in Pixel·mm−1 was determined and showed relative double standard deviations between (0.1 and 0.7) % for all reported measurements. Droplets were generated with the aid of a second micrometer screw at the dosing device by moving the syringe plunger relative to the syringe shell. To fulfill the working eq 1, the size of the formed droplets should be as large as possible. About 20 to 30 drop profiles were recorded for each droplet. After a first calibration, the procedure of drop formation and recording of the droplet profiles was performed for at least 5 different droplets for a given IL and temperature. Then, the calibration procedure was repeated to check the stability of the optical and mechanical system. For each data point, the associated calibration values agreed within their double standard deviations. One additional droplet was analyzed with the use of the newly determined calibration value. Furthermore, the temperature was recorded before and after the study of each analyzed droplet. For the evaluation of σ on the basis of eq 1, accurate temperature-dependent data for the density ρ of the investigated [C(CN)3]−- and [B(CN)4]−-based ILs are needed. Since the samples used here are not identical with those investigated in our previous studies, the density of selected samples was checked with the same vibrating U-tube density meter.4,5,24 Because of the agreement of the density results for the different samples within combined uncertainties of 0.02 % (k = 2), the densities employed in the present study were adopted from our previous investigations.4,5,24 They are represented as a function of temperature T according to ρcalc (T ) = ρ0 + ρ1T + ρ2 T 2

(2)

with the fit parameters ρ0, ρ1, and ρ2 given in the cited publications. Considering its negligible effect on the final σ value, the density of air which is in contact with the IL droplet C



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could be approximated by ρair = 0.0012 g·cm−3 in eq 1 for all temperatures. The ST values determined for the individual drop profiles recorded for each drop were averaged arithmetically in a first step. From the at least six average values obtained for different drops of a particular IL at a given temperature, the final ST value σ was averaged where each considered value has the same statistical weight. For estimating the uncertainty of the ST data on a confidence level of more than 95 %, the double standard deviations of the σ values obtained from the individual drop profiles for each drop were averaged over the at least six drops contributing to a particular data point. This value was always larger than the double standard deviation of the averaged σ data obtained for each single drop. On the basis of these calculations, the relative uncertainty Δσ·σ−1 of the present measurement results for the ST can be estimated to be 2 % (k = 2). It accounts for the uncertainties caused by the calibration and measurement procedure, the temperature stability, and the densities of the ILs and air. For each state, the reported temperature T represents the average of all individual temperatures noted before and after the measurement of the at least six individual droplets. The average of the double standard deviations of the recorded temperatures for each state and IL was found to be 0.08 K. Taking also the uncertainty of the used resistance probe into account, the expanded uncertainty for all reported temperatures is estimated to be 0.1 K (k = 2). The modified PD apparatus was checked by measuring the ST of the reference fluid water (σ ≈ (65 to 75) mN·m−1) in the temperature range from (283 to 343) K in steps of 10 K and that of diisodecyl phthalate (DIDP, σ ≈ 30 mN·m−1) in the temperature range from (288 to 308) K. The corresponding density values for data evaluation were taken from literature.25,26 For both fluids, good agreement between our results and reference data25,27 could be found within uncertainties with maximum relative deviations of 2.2 % for water and 1.2 % for DIDP.

Table 2. Surface Tension σ of [C(CN)3]−- and [B(CN)4]−based ILs Obtained by the Pendant Drop Method as a Function of Temperature T at 0.1 MPaa

RESULTS AND DISCUSSION First, the ST of the investigated [C(CN)3]−- and [B(CN)4]−based ILs as a function of temperature are presented. Then, the results are compared with available literature data and a simple empirical correlation. Finally, the effects of variations in the cationic alkyl side-chain length and in the anion type of imidazolium-based ILs on the ST are discussed from a molecular point of view. Summary of Surface Tension Data. The measured ST data for the investigated [C(CN)3]−- and [B(CN)4]−-based ILs at the studied temperatures and 0.1 MPa are summarized in Table 2 and Figure 3. For validation of the measured data, two independent measurement series were performed for [EMIM][C(CN)3] with samples from the same charge. The mean absolute deviation of the obtained data at comparable temperatures was 1.56 % and smaller than the estimated expanded uncertainty of 2 %. Here, the average values for σ and T from both measurement series are given in Table 2 and Figure 3. The measured ST data of the [C(CN)3]−- and [B(CN)4]−based ILs range from (29 to 52) mN·m−1, which is typical for ILs. The general trend for ILs that the ST decreases with increasing temperature is also observed here. The temperature dependence of the ST results for each IL can be represented well by a linear equation of the form

where T is the temperature in K, and σ0 and σ1 are the fit parameters. For the data correlation, the same statistical weights are assumed for all experimental data points given in Table 2. The resulting fit parameters as well as the root-mean-square (rms) deviation of the ST data from the fit are summarized in Table 3. The lower part of Figure 3 shows that the relative deviations of the experimental ST data from the fits on the basis of eq 3 are clearly smaller than the expanded uncertainty of 2 %. The rms deviations of the measured ST data relative to those calculated according to eq 3 were in the range between (0.18 and 0.59) %. For a given cation, the ST of the [B(CN)4]−based ILs tends to decrease somewhat more strongly with increasing temperature compared to the [C(CN)3]−-based ILs. Increasing the alkyl side-chain length for both anion types results by trend in a weaker decrease of σ with T. Comparison of Surface Tension Data with a Simple Prediction Method and Literature Data. In Figure 4, the experimental results obtained for each IL are compared with a simple prediction method and available literature data as a function of temperature. For the [C(CN)3]−-based ILs in the upper part of the figure and for the [B(CN)4]−-based ILs in its lower part, the predicted and literature ST data σ are related to the σcalc values calculated from the correlations according to eq 3. Dashed lines indicate the estimated expanded uncertainties of the ST results of 2 %.

T K

σ mN·m

T −1

[EMIM][C(CN)3] 283.18 51.1 293.25 50.4 303.24 49.39 313.10 48.62 323.08 47.83 333.17 47.19 343.09 46.55 353.36 44.68 [OMIM][C(CN)3] 283.28 40.07 293.21 39.03 303.32 38.38 313.21 37.65 323.12 36.89 333.10 36.09 343.16 35.31 353.03 34.89 [HMIM][B(CN)4] 283.27 41.99 293.29 41.32 303.27 40.40 313.29 39.39 323.26 38.60 333.20 37.61 343.18 37.00 353.25 35.86

K

σ mN·m

−1

[BMIM][C(CN)3] 283.39 51.0 293.33 49.23 303.19 48.44 313.08 47.34 323.07 46.40 333.02 45.55 343.42 44.87 353.37 44.03 [EMIM][B(CN)4] 283.10 49.66 293.26 48.72 303.35 47.57 313.29 46.45 323.20 45.67 333.09 44.87 343.12 44.04 353.45 43.40 [OMIM][B(CN)4] 283.23 38.64 293.32 38.01 303.30 36.81 313.27 36.18 323.23 35.19 333.18 34.15 343.21 33.44 353.28 32.41

T

σ

K

mN·m−1

[HMIM][C(CN)3] 283.23 44.44 293.35 43.55 303.33 42.88 313.26 42.09 323.25 41.49 333.18 40.80 343.11 39.91 353.53 39.07 [BMIM][B(CN)4] 283.23 45.23 293.23 44.64 303.30 43.65 313.24 42.62 323.24 41.71 333.20 40.81 343.22 39.87 353.30 39.08 [DMIM][B(CN)4] 283.19 35.44 293.24 34.74 303.28 33.67 313.26 33.07 323.17 32.05 333.17 31.37 343.13 30.60 353.24 29.98

a

The combined expanded uncertainties Uc are Uc(σ) = 0.02·σ, Uc(T) = 0.1 K, and Uc(p) = 3 kPa (level of confidence = 0.95).

σcalc(T ) = σ0 + σ1T

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D

(3)



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Figure 3. Surface tension σ of the studied ILs as a function of temperature T at 0.1 MPa and relative deviation of measured surface tensions σ from the σcalc data calculated according to eq 3: ◇, [EMIM][C(CN)3]; ○, [BMIM][C(CN)3]; □, [HMIM][C(CN)3]; △, [OMIM][C(CN)3]; ⧫, [EMIM][B(CN)4]; ●, [BMIM][B(CN)4]; ■, [HMIM][B(CN)4]; ▲, [OMIM][B(CN)4]; ▼, [DMIM][B(CN)4]. Lines in the upper part represent fits of the experimental data according to eq 3.

Figure 4. Relative deviation of predicted and experimental ST data σ for the studied ILs from the fit (σcalc) according to eq 3 as a function of temperature T. (a) [C(CN)3]−-based ILs: [EMIM][C(CN)3]: , eq 4; ◇, Domańska et al.;13 ◆, Martino et al.15 [BMIM][C(CN)3]: ···, eq 4. [HMIM][C(CN)3]: − − −, eq 4. [OMIM][C(CN)3]: ··, eq 4. (b) [B(CN)4]−-based ILs: [EMIM][B(CN)4]: , eq 4; ○, PD, Koller et al.; 5 ● , SLS, Koller et al.; 5 ▽ , Tong et al. 16 [BMIM][B(CN)4]: ···, eq 4. [HMIM][B(CN)4]: − − −, eq 4; □, PD, Koller et al.;4 ■, SLS, Koller et al.;4 ▼, Mota-Martinez et al.9 [OMIM][B(CN)4]: ··, eq 4; △, Kolbeck et al.14 [DMIM][B(CN)4]: ····, eq 4. The expanded uncertainty of the experimental ST data obtained in this study of 2% is given as dashed line.

Table 3. Coefficients of eq 3 and Root-Mean-Square (rms) Deviation of the Experimental Surface Tension Data from the Correlation for the Studied [C(CN)3]−- and [B(CN)4]−Based ILs σ0 IL [EMIM][C(CN)3] [BMIM][C(CN)3] [HMIM][C(CN)3] [OMIM][C(CN)3] [EMIM][B(CN)4] [BMIM][B(CN)4] [HMIM][B(CN)4] [OMIM][B(CN)4] [DMIM][B(CN)4] a

mN·m

σ1 −1

75.35 77.39 65.52 61.03 75.09 71.12 66.94 64.18 57.96

mN·m−1·K−1

rmsa

−8.524·10−2 −9.518·10−2 −7.459·10−2 −7.462·10−2 −9.049·10−2 −9.085·10−2 −8.773·10−2 −8.979·10−2 −7.964·10−2

0.59 0.57 0.18 0.32 0.40 0.21 0.26 0.30 0.34

reference fluids.26 For the present ST results, the predictions from eq 4 are shown as curves in Figure 4. Here, the [C(CN)3]−- and [B(CN)4]−-based ILs carrying the same cation are represented with the same type of line. For all studied ILs, the prediction method according to eq 4 suggests a slightly stronger temperature dependence of σ than the linear fits of our measured data. There is a trend that an increase in the alkyl side-chain length of the [C(CN)3]−- and [B(CN)4]−based ILs, which goes along with an increasing IL dynamic viscosity η,6 results in a better agreement between fit and prediction. This observation can be related to the fact that the McLeod−Sudgen prediction method28,29 was developed for fluids with relatively large viscosity. In consideration of the estimated uncertainty of the prediction, the empirical method provides a good description of the ST of the low-viscosity [C(CN)3]−- and [B(CN)4]−-based ILs for a limited temperature range between (283 and 323) K. In this range, agreement with the experiment within combined uncertainties is found. The water mass fractions of the samples investigated experimentally in this study and in literature are all below 2· 10 −3. Our previous work30 on mixtures of 1-ethyl-3methylimidazolium ethylsulfate ([EMIM][EtSO4]) and water revealed no significant influence of water on the ST for water mass fractions up to 0.11. Hence, the effect of slightly different water contents of the IL samples on the experimental ST values can be neglected in the following data comparison.

Standard percentage deviation of σ to the fit.

As a prediction method, a simple scheme based on the MacLeod−Sugden correlation28,29 was used. According to this scheme, the temperature dependence of the ST of the studied ILs is estimated from that of the density by ⎛ ρ (T ) ⎞ 4 ⎟⎟ σpred(T ) = σcalc(Tref ) ·⎜⎜ calc ⎝ ρcalc (Tref ) ⎠

(4)

at temperatures between (283 and 353) K. In eq 4, σcalc(Tref) and ρcalc(Tref) are the ST and density calculated according to eqs 3 and 2 at a reference temperature of Tref = 293.15 K. The proposed method predicts the ST of high-viscosity fluids with a typical uncertainty of less than 2 % (k = 2) tested for several E



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Experimental data for [C(CN)3]−-based ILs are only available in the case of [EMIM][C(CN)3]. Here, Domańska et al.13 obtained temperature-dependent data between (298 and 338) K with a tensiometer based on the Du-Noüy ring method and specified a very low uncertainty of 0.04 mN·m−1. They used a sample provided by IoLiTec with a mass fraction purity ≥ 98 %. Most of their data are within the uncertainty of our data with relative deviations below 2.1 %. A single value at 298.15 K was measured by Martino et al.15 with the capillary rise method. Reasons for the negative deviation of 4.1 % of their datum from our correlation cannot be provided due to the lack of specifications about sample purity, water content, and experimental uncertainty. For [EMIM][B(CN)4] and [HMIM][B(CN)4], we could determine the ST for samples provided by Merck KGaA (nominal purities > 99 %) by using both the PD method at ambient temperature and surface light scattering (SLS) at higher temperatures.4,5 While the data obtained from the PD measurements agree within combined uncertainties with our present data, distinctly smaller ST data were found by SLS. Though our SLS results imply quite large uncertainties due to the metrological challenge at the experimental conditions studied, the relative deviations between the SLS and the presented PD data of up to −15.3 % for [EMIM][B(CN)4] and −12.0 % for [HMIM][B(CN)4] are mostly outside combined uncertainties. The discrepancies may be attributed to the different sample sources. Further reasons could be ongoing sample decomposition at larger temperatures during the SLS experiments and/or the presence of traces of surface-active contaminants in the used SLS sample cell. Using a tensiometer based on the bubble pressure method, Tong et al.16 reported experimental data for [EMIM][B(CN)4] provided by Merck KGaA (mass fraction purity > 99 %). Their temperature-dependent data with a specified uncertainty of 0.2 mN·m−1 are in very good agreement with our data. The same holds for the single datum determined by Mota-Martinez et al.9 with an uncertainty of 0.3 mN·m−1 for a sample of [HMIM][B(CN)4] supplied by Merck KGaA (nominal purity > 99 %). Furthermore, Kolbeck et al.14 measured the ST of [OMIM][B(CN)4] at ambient temperature with the PD method and a specified uncertainty of 1 % (k = 2). For the investigated sample from IoLiTec (nominal purity > 99 %), a relative deviation of + 3.2 % compared to our correlation is found. Effect of Cation and Anion on Surface Tension of [C(CN)3]−- and [B(CN)4]−-Based ILs. In the following, the influences of the alkyl side-chain length in the cation and of the anion on the ST of the [C(CN)3]−- and [B(CN)4]−-based ILs will be discussed. This should be done within the framework of our qualitative analysis between the chemical structure and the surface tension for a comprehensive set of systematically varied ILs.14 For this, the ST data of selected [alkyl-MIM]+-based ILs carrying different anions at a temperature of 298.15 K are shown in Figure 5 as a function of the carbon number nC in the alkyl side chain of the cation. Besides the trigonal-planar [C(CN)3]− and tetrahedral [B(CN)4]− anions (indicated by diamonds), the smaller tetrafluoroborate ([BF4]−) and methylsulfate ([CH3SO4]−) (circles) as well as the larger perfluorinated bis(trifluoromethylsulfonyl)imide ([NTf2]−) and tris(pentafluoroethyl)trifluorophosphate ([FAP]−) (triangles) anions were chosen. For comparison, also the linear n-alkanes n-pentane (nC = 5), n-hexane (nC = 6), n-octane (nC = 8), ndecane (nC = 10), n-dodecane (nC = 12), and n-hexadecane (nC

Figure 5. Experimental (σ) and calculated (σcalc) surface tensions of [alkyl-MIM]+-based ILs carrying different anions and of n-alkanes at 298.15 K as a function of the carbon number nC in the alkyl side chain of the cation of the ILs or in the n-alkanes. References for the surface tension data and lines indicating trends for the different groups are discussed in the text.

= 16) are included in Figure 5 as squares. The molecular volume Vm (Vm = (M/ρ)/NA where NA is Avogadro’s constant) of the studied components with a given alkyl chain length increases in the order n-alkane < [BF4]− < [CH3SO4]− < [C(CN)3]− < [B(CN)4]− < [NTf2]− < [FAP]−. The ST data of the [C(CN)3]−- and [B(CN)4]−-based ILs (nC = 2, 4, 6, 8, 10) investigated in this study were calculated at T = 298.15 K according to eq 3. Data for [EMIM][BF4], [OMIM][BF4], [OMIM][FAP], and the [NTf2]−-based ILs with nC = 1, 2, 4, 6, 8, 10, and 12 measured at 298.15 K were adopted from our previous study.14 To extend the database for the [BF4]−- and [FAP]−-based ILs, ST data obtained with the PD method by Ghatee and Zolghadr31 for [EMIM][BF4] and [OMIM][BF4] and by Rebelo et al.32 for the 1-dodecyl-3methylimidazolium ([DoMIM]+)-based IL [DoMIM][BF4] were used. For the latter, their measured data obtained between 313 K and 343 K were extrapolated to 298.15 K according to eq 3. Furthermore, the σ values at 298.15 K for the [FAP]−-based ILs (nC = 2, 4, 6) determined by Součková et al.33 with the Wilhelmy plate method and for the [CH3SO4]−based ILs (nC = 1, 4, 6, 8) determined by Torrecilla et al.34 with the PD method were considered. ST data for the n-alkanes with nC = 5, 6, 8, 10, and 12 were taken from the Refprop database.25 For hexadecane (nC = 16), the ST value obtained by Rolo et al.35 using the Wilhelmy plate method was employed. The experimental uncertainties of all data are within the symbols in Figure 5. The ST of the [C(CN)3]−- and [B(CN)4]−-based ILs decreases with increasing nC. This is also valid for the other IL families depicted in Figure 5 and generally found in literature for [alkyl-MIM]+-based ILs.12 In our previous work,14 we studied the influences of the structures of as well as the intermolecular interactions between IL molecules on the corresponding ST values according to Langmuir’s principle.36 This principle implies that only the outer liquid surface region affects the ST. More details about Langmuir’s principle in connection with ILs as nonisotropic molecules forming nonisotropic interactions are given in ref 14. In agreement with the observations made by other authors,12,19,37 the alkyl side chains of the ILs are preferentially orientated toward the gas phase.14 By this, an outer aliphatic layer is formed at the surface above an ionic sublayer mainly consisting of the charged imidazolium rings and the anions. With increasing alkyl chain F



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delocalization in the bulky [NTf2]− or [FAP]− anions and their noncoordinating C−F bonds.14 The decrease of σ for the [C(CN)3]−- and [B(CN)4]−-based ILs with increasing nC is similar to that for ILs carrying the larger [NTf2]− and [FAP]− anions, but much weaker than that for the ILs with the smaller [BF4]− or [CH3SO4]− anions. The approximately linear decrease of σ for small nC values is illustrated in form of lines for the different anions in Figure 5. Taking into account the comparatively constant dispersive interactions at the outer aliphatic layer for ILs with a given cation, a stronger charge distribution in the anion seems to have a more pronounced effect on the ST of longer-chained ILs. The electrostatic potential acting on the surface may be reduced more strongly for smaller anions with more localized charges than for larger anions featuring a spatially broad charge delocalization. In consequence, a more pronounced charge distribution in the anion of ILs tends to go along with a weaker decrease of σ with increasing nC. For sufficiently long alkyl side chains in the [alkyl-MIM]+ cation of ILs, a second region with an approximately constant ST between about (25 and 30) mN·m−1 indicated by the dashed lines in Figure 5 can be found. The saturation of the ST may be attributed to the enrichment of the nonpolar aliphatic chains at the IL surface19 and corresponds with the behavior of the long-chained n-alkanes converging to the same ST range. The above discussion regarding the influence of the anion on σ also explains the different saturation characteristics for the studied IL groups. While for the groups with the small [BF4]− or [CH3SO4]− anions and the large [NTf2]− or [FAP]− anions the saturation (σ ≈ 30 mN·m−1) starts for nC values between 6 and 8, this plateau is not reached for the [C(CN)3]−- and [B(CN)4]−-based ILs at such nC values, probably due to still significant contributions from the ionic sublayer to the ST. This is supported by our previous study14 in which for several [OMIM]+-based ILs with various anions, the highest presence of charged groups at the surface could be evidenced for [OMIM][B(CN)4] by X-ray photoelectron spectroscopy.

length, the aliphatic layer more and more dominates the IL surface, forming a nonpolar segregated domain. The strength of the dispersive interactions between the alkyl chains in the aliphatic layer seems to increase with increasing chain length, which is illustrated by the increasing ST of the n-alkanes with increasing carbon number, cf. Figure 5. In contrast, the contributions from the ionic sublayer to the ST of ILs primarily induced by electrostatic interactions between the charged groups are gradually reduced. This behavior is because the distance between the charged ionic layer and the IL surface is increased, resulting in a reduced potential acting on the surface. The decreasing ST of ILs with a given anion with increasing nC indicates that the decreasing influence of the ionic sublayer on the ST with increasing alkyl side-chain length seems to be more pronounced than the increasing strength of the dispersive interactions between the alkyl chains in the outer layer. According to Figure 5, the anion associated with the [alkylMIM]+ cation affects the magnitude of σ of the IL. Larger electrostatic interactions in the ionic sublayer promote a larger ST for ILs with a given cation. This effect seems to be enhanced for smaller anions with easier access to the alkyl-rich surface compared to larger anions. According to Tokuda et al.38 and Freire et al.,39 the Lewis basicity of the anion mainly affects the ST of ILs. The Lewis basicity was found to be high for anions having locally large negative charges with an asymmetric distribution such as [CH3SO4]− and to be low for anions having symmetrically distributed low negative charges such as [NTf2]− or [FAP]−. Thus, a high Lewis basicity generally found for smaller anions corresponds with a high ST. This conclusion is in contradiction to the findings of Law and Watson.40 On the basis of their measurements on ILs with various anions, they related increasing σ values for ILs to increasing size of symmetrical anions. For relatively small to medium-sized symmetrical anions, Kolbeck et al.14 found the same trend between ST and molecular volume for [OMIM]+-based ILs carrying the anions [BF4]−, hexafluorophosphate ([PF6]−), and [B(CN)4]−. This behavior was attributed to increasing intermolecular van der Waals contributions in the bulk with increasing anion size as well as to anisotropic effects at the surface.14 Considering the ILs investigated in the present study, the slightly lower ST of the [B(CN)4]−-based ILs than those of the smaller [C(CN)3]−-based ILs having the same cation agree with the conclusions drawn by Tokuda et al.38 and Freire et al.39 The effect of Lewis basicity induced by the weaker charge delocalization over three cyano groups in the [C(CN)3]− anion compared to over four cyano groups in [B(CN)4]− seems to be stronger than that of the different anion sizes and the stronger van der Waals interactions given by the [B(CN)4]− anion. In comparison with the IL families with smaller and larger anions given in Figure 5, the [C(CN)3]−- and [B(CN)4]−-based ILs show relatively high ST values. Comparing the short-chained [EMIM]+-based ILs where the electrostatic contribution of the ionic sublayer to the ST is strongest, [EMIM][C(CN)3] and [EMIM][B(CN)4] show only slightly lower values than [EMIM][BF4] despite their weaker Lewis basicity. This behavior might be caused by the relatively easy access of the charged groups to the surface, especially for the smaller planar [C(CN)3]− anion, resulting in a more pronounced impact of Coulombic interactions on the ST. On the contrary, the ILs [EMIM][NTf2] and [EMIM][FAP] show distinctly smaller ST values compared to the [C(CN)3]−- and [B(CN)4]−-based homologues. This can be attributed to the large charge

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CONCLUSIONS The pendant drop method has been used for the determination of the surface tension of nine [C(CN)3]−- and [B(CN)4]−based ILs carrying [alkyl-MIM]+ cations with varying alkyl sidechain length for temperatures between (283 and 353) K at atmospheric pressure with an estimated expanded uncertainty of 2 %. For all studied ILs, the decreasing surface tension with increasing temperature could be represented well by linear fits. The measured results are in good agreement with literature and with predictions based on a simple empirical method. For ILs carrying the same cation, somewhat smaller surface tensions were found for the ILs based on the larger [B(CN)4]− anion. Compared to other [alkyl-MIM]+-based ILs with varying anions, the [C(CN)3]− and [B(CN)4]−-based ILs show relatively large surface tension values. This exceptional behavior of the ILs investigated in this study could be explained by the interplay between the relatively small molecular sizes of and the pronounced charge delocalization within their anions. By trend, the surface tension for ILs with a given anion decreases with increasing alkyl side-chain length in the cation. This could be attributed to a more strongly decreasing contribution from the ionic sublayer at the IL surface compared to the gradually increasing contribution of the aliphatic layer present at the surface. G



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ASSOCIATED CONTENT

S Supporting Information *

1

H nuclear magnetic resonance (NMR) spectra for the synthesized [B(CN)4]−-based IL samples. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00303. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel. +49-9131-85-29789. Fax +49-9131-85-29901. E-mail [email protected]. Funding

This work was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) by funding the Erlangen Graduate School in Advanced Optical Technologies (SAOT) within the German Excellence Initiative. In addition, financial support from the Seventh European Commission Framework Program for Research and Technological Development for the project “Novel Ionic Liquid and Supported Ionic Liquid Solvents for Reversible Capture of CO2” (IOLICAP Project No. 283077) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank IoLiTec Ionic Liquid Technologies, Heilbronn, Germany, for providing all [C(CN)3]−-based IL samples.

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REFERENCES

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Surface Tension of Tricyanomethanide- and Tetracyanoborate-Based

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