Article pubs.acs.org/jced
Temperature Dependence of the Relative Static Permittivity of Homologous Series of Liquid 1,n‑Dicyanoalkanes NC(CH2)nCN, n = 2 to 6 ́ Jolanta Swiergiel* and Jan Jadzẏ n
Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland ABSTRACT: The relative static permittivities of liquids belonging to the homologous series of 1,n-dicyanoalkanes NC(CH2)nCN, n = 2 to 6, were determined as a function of temperature. At the isothermal conditions, the relative static permittivity of the compounds under study shows a uniform and rather moderate decrease with an increase of the n number, with an exception of 1,2-dicyanoethane, n = 2, the high permittivity of which distinctly departs from that dependence.
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INTRODUCTION 1,n-Dicyanoalkanes are organic liquids of a relatively high permittivity, so they are important materials for many uses, among others, as the solvents for numerous ionic compounds.1−3 An especially interesting and important compound from that series is 1,2-dicyanoethane (succinonitrile) which is a well-known example of a molecular plastic crystal-forming compound. Due to its high polarity, liquid succinonitrile is able to dissolve various type of salts, and the transition to the plastic phase generates a solid ionic conductor with high ionic conductivity at ambient temperatures, similar to the conductivity of some commercial liquid electrolytes used, for example, in lithium batteries.4−8 Despite no questionable significance of the static dielectric properties of liquid dicyanoalkanes, the corresponding experimental data are hardly available in a wide temperature range of the compounds.9−13 In this work we present the results of the dielectric studies performed for the series of liquid 1,n-dicyanoalkanes in a wide temperature range. There are two main goals of the paper: (i) to show an influence of different number of −CH2− groups linking two strongly polar cyano-group CN on the permittivity of the compounds, and (ii) to study of the dynamics of ionic impurities in these polar liquids, based on the frequency and temperature dependence of the dielectric losses due to the residual conductivity in liquid dicyanoalkanes.
supplied by Aldrich, is presented in Table 1. As the traces of water are the main impurity strongly influencing the Table 1. Sample Information
a
source
mass fraction puritya
1,2-dicyanoethane 1,3-dicyanopropane 1,4-dicyanobutane 1,5-dicyanopentane 1,6-dicyanohexane
Aldrich Aldrich Aldrich Aldrich Aldrich
0.996 0.993 0.999 0.983 0.992
Given in the Aldrich Certificate of Analysis.
permittivity value, the liquid dicyanoalkanes were stored over the molecular sieves 4 Å (Aldrich) during more than one week before the measurements. The complex permittivity spectra were recorded with the use of an HP 4194A impedance/gain phase analyzer in the frequency range from 500 Hz to 5 MHz. The temperature of the measuring cell was controlled with a “Scientific Instruments” temperature controller, model 9700, within ± 2·10−3 K. The standard uncertainty for the permittivity determination u(εs) was 0.05. The details on the used experimental setup can be found in our recent paper.14
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RESULTS AND DISCUSSION As the dipolar relaxation of studied dicyanoalkanes occurs at the gigahertz region,10 the frequency of the electric stimulus used in our experiments corresponds to the static dielectric regime of the compounds. Then, the real, ε′, and imaginary, ε″,
EXPERIMENTAL SECTION The following 1,n-dicyanoalkanes of a general chemical formula (CH2)n(CN)2, were studied: 1,2-dicyanoethane, n = 2 (synonyms: butanedinitrile, succinonitrile); 1,3-dicyanopropane, n = 3 (pentanedinitrile, glutaronitrile); 1,4-dicyanobutane, n = 4 (hexanedinitrile, adiponitrile); 1,5-dicyanopentane, n = 5 (heptanedinitrile, pimelonitrile); and 1,6-dicyanohexane, n = 6 (octanedinitrile, suberonitrile). The compounds were used without purification. The purity of the compounds, as © 2012 American Chemical Society
chemical name
Received: September 3, 2012 Accepted: December 7, 2012 Published: December 14, 2012 128
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parts of the complex permittivity, ε* = ε′ − jε″, (j = −11/2), are the simplest possible; namely, the real part is equal to the relative static permittivity, ε′ = εS, and the dielectric losses ε″ are only due to the ionic conductivity and can be presented in the following general form: σ0,ion ε″(ω) = ε0(ω)α (1) where σ0,ion is the dc ionic conductivity, ω = 2πf denotes the angular frequency of the measuring electric field, f is the frequency, and ε0 = 8.85 pF·m−1 is the permittivity of free space. The exponent α is an empirical quantity which represents the slope of the linear dependence log(ε″) ∼ log(f). In the simplest case, when an ionic current fulfills the Ohm’s law, that is, the current density (j) is a linear function of the electric field strength (E), the exponent α is equal to 1.15 Figure 1 presents, as an example, the complex permittivity spectra recorded for 1,5-dicyanopentane in its static dielectric
Figure 2. Temperature dependences of the relative static permittivity of 1,n-dicyanoalkanes of different number n of CH2 groups in the molecules. The solid lines represent the best fit of eq 2 to the experimental data (full points). The open points represent the literature data: ▽, ref 10; and ○, ref 12.
determined in the megahertz region of the measuring electric field frequency. In Figure 2 the open points present the literature data: the open circlesthe permittivity (for n = 2 to 4) estimated from Figures 5 and 6 of the paper by Schwarz et al. (1970)12 and the open trianglesthe relative static permittivity of succinonitrile (n = 2), determined at three temperatures by Clemett and Davies in 1960.10 The solid lines in Figure 2 represent the best fit of the empirical equation: B C + 2 (2) T T to the experimental permittivity data. T is the absolute temperature. The values of the fitting parameters A, B, and C are presented in Table 3. Table 3 contains also the standard deviations, σ*, calculated with the formula: εS(T ) = A +
1/2 ⎛ ∑ (ε − εi ,calc)2 ⎞ i i ,exp ⎜ ⎟ σ* = ⎜ ⎟ nd − n p ⎝ ⎠
(3)
where nd and np denote the number of the experimental points and the number of the parameters, respectively. Figure 3 presents the relative static permittivity of 1,ndicyanoalkanes as a function of n, at the temperature of 333 K. As can be seen in the figure, the εS (n) dependence is a relatively weak with only the traces of an odd−even effect, that is, an alternation of the permittivity value in dependence on the length of the hydrocarbon chain (represented by n value) linking two CN groups in 1,n-dicyanoalkanes molecules. Very impressive is the polarity of succinonitrile, decidedly higher than that of the other members of the homologous series. Succinonitrile is known as a plastic crystal-forming compound, where the plasticity origins not from globular shape of the molecules, as usual, but results from the molecular internal rotation:4,16−19 the succinonitrile molecule can exist in three conformationsa nonpolar trans one (μt = 0) and two equivalent, highly polar gauche ones (μg = 5.8 D).13 The conformations are interrelated by a 120° rotation around the central CC bond. The high permittivity of succinonitrile certainly results from the enhanced dipole moment of the molecules of the compound, which is a consequence of a strong shift of the
Figure 1. Dielectric spectra of 1,5-dicyanopentane, the real (a) and imaginary (b) parts, recorded in the liquid phase of the compound.
regime. The spectra are similar for other dicyanoalkanes studied. The real part of the spectra (a) presents the relative static permittivity of dicyanopentane, and only in the low frequencies one observes an important increase of the permittivity due to the electrode polarization effects. As seen in panel b of the figure, the imaginary part of the complex permittivity spectra have a form of the strait lines indicating the ionic current as a sole source of the dissipation energy in the studied dielectric material. The relative static permittivity values resulting from the analysis of the real part of the complex permittivity spectra of dicyanoalkanes recorded at different temperatures are presented in Figure 2 and in Table 2. As it results from Figure 1a, the static permittivity of the studied compounds can be 129
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Table 2. Relative Static Permittivity (εS) of Liquid 1,nDicyanoalkanes of Different Number n, under Pressure of 1013 hPaa
Table 3. Values of the Parameters of eq 2 Corresponding to 1,n-Dicyanoalkanes of Different n and Standard Deviations σ* Calculated with eq 3
εS T/K 248.15 250.65 253.15 255.65 258.15 260.65 263.15 265.65 268.15 270.65 273.15 275.65 278.15 280.65 283.15 285.65 288.15 290.65 293.15 295.65 298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15 355.65 358.15 360.65 363.15 365.65 368.15 370.65 373.15 a
n=2
n=3 40.78 40.45 40.08 39.78 39.42 39.08 38.74 38.40 38.10 37.78 37.44 37.16 36.83 36.56 36.27 35.97 35.71 35.41 35.14 34.87 34.60 34.33 34.04 33.83 33.59 33.35 33.13 32.87 32.67 32.44 32.21
56.09 55.46 54.85 54.29 53.68 53.11 52.52 51.95 51.36 50.81 50.28 49.73 49.18 48.66 48.14 47.63 47.11
n=4
34.70 34.35 34.01 33.67 33.36 33.04 32.73 32.41 32.12 31.83 31.53 31.26 30.96 30.67 30.38 30.09 29.80 29.50 29.22 28.94 28.69 28.46 28.21 27.98 27.73 27.52 27.30 27.09 26.83 26.60 26.40
n=5
n=6
36.21 35.82 35.44 35.06 34.70 34.35 33.99 33.65 33.31 32.98 32.66 32.36 32.04 31.73 31.43 31.14 30.86 30.57 30.28 30.00 29.71 29.45 29.18 28.92 28.67 28.42 28.17 27.94 27.68 27.46 27.23 27.00 26.77 26.56 26.32
28.23 28.00 27.77 27.52 27.29 27.06 26.82 26.60 26.36 26.16 25.94 25.75 25.55 25.32 25.11 24.94 24.74 24.55 24.37 24.20 24.00 23.80 23.63 23.47 23.30
n
A
2 3 4 5 6
−38.941 1.625 −6.883 −2.957 0.034
σ*
C/K2
B/K 4
3.586·10 1.037·104 1.238·104 0.999·104 0.795·104
−1.400·10 −0.161·106 −0.226·106 −0.040·106 −0.066·106 6
0.0113 0.0201 0.0313 0.0101 0.0107
Figure 3. Relative static permittivity of 1,n-dicyanoalkanes as a function of n, at 333 K.
is interesting and important to analyze the imaginary part of the dielectric spectra, ε″(ω), of the compounds. An example of the spectra, recorded for 1,5-dicyanopentane, is presented in Figure 1b. As it was mentioned above, in the case of the static dielectric regime of a given compound, the slope of the linear dependence of ε″ vs frequency (in log−log scale) reflects a type of the dynamics of the ions in the medium. According to eq 1, the slope is represented by the exponent α. Although the nature and the concentration of the ions existing in liquid dicyanoalkanes are not known, they form the ionic conductivity background, and our conclusions will concern just that background. The result of the analysis of the imaginary part of the dielectric spectra of studied here dicyanoalkanes is presented in Figure 4. For the dc conductivity in the “neat” liquid dicyanoalkanes, σ0,ion, which is of the order of 10−5 S·cm−1,
Standard uncertainties u are u(εS) = 0.05, and u(T) = 0.01.
trans−gauche rotational equilibrium of the molecules toward to the strongly polar gauche conformation.13 As some of the compounds from the studied series of 1,ndicyanoalkanes are used as a medium for an ionic conduction, it
Figure 4. Temperature dependence of the exponent α of eq 1 resulting from the analysis of the dielectric losses vs frequency of the electric stimulus. 130
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the exponent α is nearly exactly equal to unity (0.99 ± 0.01), for all members of the homologous series of 1,n-dicyanoalkanes, independently of the polarity of compounds. The (background) ionic current fulfills the Ohm’s law quite perfectly what reflects the normal Brownian diffusional dynamics of ions in investigated liquid media.
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Mechanical Investigation of Solvent Effects on Conformational Equilibria of Butanedinitrile. J. Am. Chem. Soc. 2002, 124, 9318−9322. (19) Derollez, P.; Lefebvre, J.; Descamps, M.; Press, W.; Fontaine, H. Structure of succinonitrile in its plastic phase. J. Phys.: Condens. Matter 1990, 2, 6893−6903.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +48 61 86 95 162. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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