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Ionic conduction in binary mixtures of dipolar liquids
Journal of Molecular Liquids 175 (2012) 33–37
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Ionic conduction in binary mixtures of dipolar liquids Shobhna Choudhary, R.J. Sengwa ⁎ Dielectric Research Laboratory, Department of Physics, J N V University, Jodhpur–342 005, India
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 24 July 2012 Accepted 12 August 2012 Available online 25 August 2012 Keywords: Ionic conductivity Static permittivity Dipolar liquids Binary mixtures Amides
a b s t r a c t The direct current ionic conductivity σdc of sixteen dipolar liquids having broad range of static permittivity (εs ~2 to 170), and their binary mixtures with amides as one of the constituent over the entire composition range were investigated at 30 °C. It has been observed that the σdc values of pure dipolar liquids increase with the increase of their εs values. The σdc values of the dipolar liquids having εs value less than 50 are found in the range of 10−5 to 10−7 S/cm. Further an increase of two orders of magnitude in σdc is observed with the increase of εs value from 50 to 170. Comparatively low σdc value of 1,4‐dioxane (3.06×10−10 S/cm) is found due to its weak dipolar behaviour. The binary mixtures consisting of nearly equal ionic conductivity constituents show either a small deviation in their σdc values from the ideal behaviour or a linear behaviour with concentration variation. A significant deviation in σdc values from the ideality has been found for the binary mixtures when the mixture constituents have a difference of more than one order of magnitude in their pure liquid state ionic conductivities. It was revealed that the σdc values of such binary mixtures are governed by the constituent having high ionic conductivity over a broad composition range. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dipolar liquids used as solvent play an important role in the preparation of electrolyte solutions [1–5], synthesis of solid and gel-type polymer electrolytes by solution–cast technique [6–8], and as the cosolvents for ionic liquids [9,10]. The direct current (dc) ionic conductivity σdc of the electrolyte materials is the key parameter for their use in the design of energy storage rechargeable batteries, supercapacitors and electrochromic devices [1–10]. The viscosity η and static permittivity εs of a dipolar solvent rule the σdc values of the electrolyte solutions, which are also influenced by the nature of the salt [4]. But the pure solvents also have significant σdc value, which is mainly due to the free ions and electrons generated from the residual traces and thermally activated ionic impurities [11–16]. The investigations on σdc values of pure dipolar liquids and their binary mixtures provide the information about the structural conformations and suitability in use of engineering and industrial applications [12–16]. Among the various dielectric parameters, the characterization of εs values over the entire composition range of binary mixtures of the dipolar liquids helps in design of a suitable mixed solvent of required solvating power [17–19]. The molecular interactions, dipolar ordering, and the structures of the amides molecules with different dipolar liquids over the entire composition range of the binary mixtures were extensively investigated by the εs measurements [18–26]. Amides have a large liquid state range, high polarity and strong solvating power, due to which
⁎ Corresponding author. Tel.: +91 291 2720857; fax: +91 291 2649465. E-mail address: [email protected] (R.J. Sengwa). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.08.011
these are mostly used as solvents in the electrolyte preparations [1–3]. To the extent of our comprehension, the ionic conductivity data of amides in pure liquid state and mixed with various dipolar liquids were scarcely reported. The σdc values of various analytical grade pure dipolar solvents and their binary mixtures, with amide as one of the constituent, have been explored in the present paper in view of their detailed electrical characterization (dielectric polarization and electrical conductivity). The main focus of this work is on the amides as one of the constituent of these mixed solvents.
2. Experimental Analytical grade dipolar liquids, given in Table 1 with their abbreviations, were purchased from various manufacturers and used as received. The mass fraction purity of these chemicals, as reported by the manufacturers, is greater than 0.99. Water used in this study is of Millipore double distilled deionized. Binary mixtures of different liquids, with amides as one of the constituent, were prepared by volume concentrations over the entire mixing range at room temperature and simultaneously by weight measurements the mole fractions of the mixture constituents were determined. Agilent 4284A precision LCR meter and Agilent 16452A dielectric test fixture were used for the measurement of capacitances C0 and CP without and with sample, respectively, and parallel resistance RP with sample under the influence of alternating current (ac) electric field of 1 MHz linear frequency, f. All the measurements were made at constant temperature 30 °C using water circulatory bath with a Thermo-Haake DC10 temperature controller.
34
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
Table 1 Values of static permittivity εs, ionic conductivity σdc, viscosity η and the σdc·η product of different dipolar liquids at 30 °C. Dipolar liquids
εs Expt.
εs Lit.a
σdc (S/cm)
η (mPa·s)
σdc·η (S/cm mPa·s)
N-methylformamide (NMF) Formamide (FA) Water (W) Dimethylsulphoxide (DMSO) Ethylene glycol (EG) N,N-dimethylacetamide (DMA) N,N-dimethylformamide (DMF) Ethyl alcohol (EA) Acetone (Ac) 2-methoxyethanol (ME) 2-ethoxyethanol (EE) 2-butoxyethanol (BE) 2-(2-methoxyethoxy)ethanol (MEE) 2-(2-ethoxyethoxy)ethanol (EEE) 2-(2-butoxyethoxy)ethanol (BEE) 1,4-Dioxane (Dx)
169.76 106.14 76.53 46.05 39.84 37.72 36.55 23.87 20.05 17.18 13.91 9.57 16.36 14.46 10.36 2.26
167.80 (35 °C) 105.21 (35 °C) 76.60 46.80 (25 °C) 39.95 36.63 (35 °C) 37.65 (25 °C) 23.60 20.70 (25 °C) 17.58 (25 °C) 13.26 (35 °C) 9.27 (35 °C) 13.77 (35 °C) 12.58 (35 °C) 9.63 (35 °C) 2.20
7.70 × 10−4 4.52 × 10−4 7.77 × 10−5 1.81 × 10−5 2.72 × 10−6 7.00 × 10−6 3.99 × 10−6 5.64 × 10−6 2.06 × 10−6 8.58 × 10−6 1.77 × 10−6 5.40 × 10−7 2.64 × 10−6 1.53 × 10−6 5.66 × 10−7 3.06 × 10−10
1.73 3.18 0.798 2.00 16.10 0.86 0.77 0.949 0.306 1.167 1.316 1.982 2.404 2.869 3.388 1.108
1.33 × 10−3 1.44 × 10−3 6.20 × 10−5 3.62 × 10−5 4.38 × 10−5 6.02 × 10−6 3.07 × 10−6 5.35 × 10−6 6.30 × 10−7 1.00 × 10−5 2.33 × 10−6 1.07 × 10−6 6.35 × 10−6 4.38 × 10−6 1.97 × 10−6 3.39 × 10−10
a
The literature values of εs [18,21–23 and references cited therein].
The real ε′ and imaginary ε″ parts of the complex dielectric function ε*(ω) of the pure liquids and binary mixtures at electric field of angular frequency ω = 2πf were determined by using the equation εðωÞ ¼ ε′−jε″ ¼ α
CP 1 −j ωC 0 RP C0
ð1Þ
where α is the correction coefficient of the dielectric test fixture. The complex ionic conductivity σ*(ω) was determined by the relation σ ⁎ðωÞ ¼ σ ′ þ jσ ″ ¼ jωε 0 εðωÞ ¼ ωε0 ε″ þ jωε0 ε′:
ð2Þ
The ε′ value of a dipolar liquid at 1 MHz was described by its static permittivity εs, whereas the ε″ values were used for the determination of the real part of ionic conductivity σ ′ = ωε0ε″. In the static permittivity frequency regime, the σ ′ values of the dipolar liquids were equal to their σdc (direct current (dc) or static ionic conductivity) values and the σ ″(ω) varies linearly with frequency according to the relation σ ″(ω) = ωε0εs with a slope of + 1 (in log–log scale) [15,27–32]. The measured εs and σdc values of the pure dipolar liquids at 30 °C are recorded in Table 1. The experimental εs values of these liquids were found in good agreement with the literature values [18,21–23 and references cited therein].
do not hold the Stokes–Einstein model (σdc·η = a constant) at fixed temperature, which is in agreement with the pure dipolar liquids investigated over broad temperature range [14]. Figs. 2–5 present the mole fraction concentration dependent σdc values of various binary mixtures with amides as one of the constituent. From the comparative inspection of the σdc plots, it is found that the binary mixtures have the non-ideal behaviour of their σdc values with concentration variation when the mixture constituents ionic conductivity differ by one or more than one order of magnitude. The σdc values of an ideal binary mixture vary linearly from one constituent ionic conductivity value to the other with concentration variation [34]. The non-ideal behaviour of σdc values against constituent concentration of some of the binary mixtures suggests that the
a
10-2
FA W
10-4 DMSO EA DMA DMF EG
Fig. 1 shows the variation of σdc versus εs values of pure dipolar liquids at 30 °C. It is found that the σdc values of the dipolar liquids on logarithmic scale increase linearly with the increase of εs from ~ 20 to 110 (Fig. 1a). Fig. 1b shows that the σdc values of the homologous series of 2-alkoxyethanols and 2-(2-alkoxyethoxy)ethanols also increase linearly with the increase of their εs values (9 b εs b 20) i.e. the σdc value decreases with an increase of molecular size. It is found that the highly polar NMF (εs = 169.76) has comparatively high σdc value (7.7 × 10 −4 S/cm) but the increase is not proportional to its εs value (Table 1). This observation reveals that the σdc value almost saturates when εs value of the dipolar liquid exceeds higher than 100. The σdc value of 1,4‐dioxane is very low (3.06 × 10−10 S/cm) (Table 1), which is mainly due to its non-polar behaviour (εs = 2.26). The observed σdc values of DMF, NMF and DMSO are in good agreement with those found in the literature [12,14]. The η values of these dipolar liquids, taken from the literature [33], are presented in Table 1, and the same are used for the determination of σdc·η product. The σdc·η product (Table 1) of these dipolar liquids was not found to be constant at a fixed temperature. This result inferred that the different dipolar liquids
σdc (S/cm)
3. Results and discussion
10-6
Ac
10-4
20
40
60
80
100
120
b ME
10-5 EE
MEE
10-6
BE
EEE BEE
10-7 8
10
12
εs
14
16
18
Fig. 1. Static permittivity εs dependent dc ionic conductivity σdc of different dipolar liquids at 30 °C.
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
a
10-3
35
a
10-3
EA+NMF EA+FA EA+DMA EA+DMF
10-5 10-4
10-7
Dx+NMF Dx+FA Dx+DMA Dx+DMF
10-5
b
σdc (S/cm)
10-3
DMSO+NMF DMSO+FA DMSO+DMA DMSO+DMF
10-4
b
10-3
σdc (S/cm)
10-9
EG+NMF EG+FA EG+DMA EG+DMF
10-4 10-5
10-5
c
10-3 10-4
10-6
10-4
Ac+NMF Ac+FA Ac+DMA Ac+DMF
10-5
0.0
0.2
c
10-3
W+NMF W+FA W+DMA W+DMF
10-5
0.4
0.6
0.8
1.0
XDx; XDMSO; XAc
10-6
0.0
0.2
0.4
0.6
0.8
1.0
XEA; XEG; XW
Fig. 2. Mole fraction concentration dependent σdc of (a) Dx+amides, (b) DMSO+amides and (c) Ac+amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity. The straight lines over the full range of data points represent the linear fit of σdc values with X.
Fig. 3. Mole fraction concentration dependent σdc of (a) EA+amides, (b) EG+amides and (c) W+amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity. The straight lines over the full range of data points represent the linear fit of σdc values with X.
heterogeneous molecular interactions influence the normal Brownian dynamics of relaxing molecular dipoles, which controls the ionic motion [13]. The Dx + amides mixtures have higher σdc values from the ideal behaviour (the straight line drawn by joining the pure liquids σdc values at X = 0 and 1) (Fig. 2a), which is mainly due to a large difference in ionic conductivity of Dx and the amides (Table 1). The enhancement in σdc from ideal behaviour of binary mixtures suggests that the hetero-molecular interactions facilitate the coupling between the Brownian translation diffusion of ions and the rotational diffusion of molecules (the Debye relaxation), which results in increase of their ions' mobility [13,14]. The excess static permittivity and effective Kirkwood correlation factor study of various binary mixtures confirmed the decrease in a number of effective parallel aligned dipoles due to heterogeneous H–bond interactions [18–24]. But the increase of σdc values from ideality reveals that the ions conductive paths in the H–bonded hetero-molecular structures of binary mixtures increase by and large besides the decrease of effective parallel aligned dipoles in the dipolar liquids mixtures. In case of DMSO + amides (Fig. 2b), the DMSO+ NMF mixture shows a large enhancement in σdc values from the ideal behaviour, whereas DMSO + FA, DMSO + DMA and DMSO+ DMF have a comparatively small deviation in their σdc values from the ideality. Similarly, in case of Ac + amides (Fig. 2c), the Ac + NMF and Ac + FA mixtures show the higher σdc values from the ideality, whereas these values of Ac + DMA and Ac+ DMF vary linearly with the mixture constituent concentration. These comparative σdc values of amide mixed Dx, DMSO and Ac binary mixtures inferred that the NMF and FA mixed dipolar liquids have more active ion conduction
behaviour as compared to that of the DMA and DMF mixed in the same dipolar liquids. Fig. 3 also confirms the enhancement of σdc values from ideality for EA + NMF, EA + FA, EG + NMF, EG + FA, W + NMF and W + FA mixtures, whereas W+ DMA and W + DMF mixtures have decrease in their σdc values. The decrease of σdc values from ideal behaviour inferred that the water interactions with DMA and DMF produce more hindrance to the translational diffusion of the ions. The mixtures of DMA and DMF with EA and EG have ideal behaviour of their concentration dependent σdc values. From the comparative study of Figs. 2–5, it is found that σdc values of most of the binary mixtures have almost linear behaviour over the broad concentration range of high ionic conductivity constituent (NMF and FA), and shows an abrupt change in the low ionic conductivity constituent rich-concentration region. Such changes were noticed in the concentration range ~0.6 to 0.7 of the cosolvents in these amide mixed binary mixtures (except W+ DMA and W + DMF). These results reveal that the σdc values of the binary mixtures are governed by the comparatively higher ionic conductivity constituent, i.e., NMF and FA over the broad range of the mixing. Fig. 4 depicted the variation of σdc values with mole fraction concentration of 2‐alkoxyethanol + amides mixtures. It is found that the concentration dependent σdc values of the mixtures of homologous series molecules with amides also deviate from the ideal behaviour. Similar type of behaviour of the σdc values with concentration variation was also found for the mixtures of the homologous series of 2-(2-alkoxyethoxy)ethanols mixed with FA (Fig. 5a). Fig. 5b and c shows the concentration dependent σdc values of binary mixtures containing different amide molecules. For these mixtures, it is also
36
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
a
a
ME+FA ME+DMA ME+DMF
10-4
10-4 10-5
10-5
EE+FA EE+DMA EE+DMF
10-4
10-5
b
10-3
σdc (S/cm)
b σdc (S/cm)
FA+MEE FA+EEE FA+BEE
10-6
10-4 FA+NMF FA+DMA FA+DMF
10-5 10-6
c
10-3
NMF+DMA NMF+DMF
BE+FA BE+DMA BE+DMF
c
10-4 10-5
10-6
10-7
DMF+DMA
10-5
0.0
0.2
0.4
0.6
0.8
1.0
10-6
XME; XEE; XBE
0.0
0.3
0.6
0.9
XDMF 0.0
0.3
0.6
0.9
XFA; XNMF
Fig. 4. Mole fraction concentration dependent σdc of (a) ME + amides, (b) EE + amides and (c) BE + amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X = 0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity.
Fig. 5. Mole fraction concentration dependent σdc of (a) FA+2-(2-alkoxyethoxy)ethanols, (b) FA+amides and (c) NMF+amides and DMF+DMA binary mixture in the inset at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity.
observed that the mixture having a large difference in their constituents σdc values significantly deviate from the ideal behaviour (FA + DMA, FA + DMF, NMF + DMA and NMF + DMF). But the σdc values of FA + NMF and DMF + DMA mixtures vary linearly because of the mixture constituents have the same order of their ionic conductivity. Small variation in the σdc values of pure amides and cosolvents (Figs. 2–5 at X = 0 or 1) inferred that the ionic conductivity of a pure dipolar liquid may vary with the time of measurements or the same dipolar liquid when obtained from different manufacturers. This observation confirms that the ionic traces in analytical grade dipolar liquid may also vary due to environmental contact of the sample during their synthesis and measurements.
Nos. SR/S2/CMP-09/2002 and SR/S2/CMP-0072/2010. One of the authors SC is thankful to the CSIR, New Delhi for the award of Research Associate fellowship.
4. Conclusions The σdc values of dipolar liquids increase with the increase of their εs values. Most of the binary mixtures of dipolar liquids having amide as one of the constituent confirm the increase of σdc values from the ideal behaviour, which is more pronounced when the conductivity difference of mixtures constituents is large. Over the broad composition range of the studied binary mixture, their σdc values are governed by the high ionic conductivity constituent (NMF and FA). Nearly equal ionic conductivity constituents of the binary mixtures of dipolar liquids obey the ideal behaviour of their σdc values with composition concentration. Acknowledgements The authors are grateful to the Department of Science and Technology, New Delhi for providing the financial assistance through research project
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Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Ionic conduction in binary mixtures of dipolar liquids Shobhna Choudhary, R.J. Sengwa ⁎ Dielectric Research Laboratory, Department of Physics, J N V University, Jodhpur–342 005, India
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 24 July 2012 Accepted 12 August 2012 Available online 25 August 2012 Keywords: Ionic conductivity Static permittivity Dipolar liquids Binary mixtures Amides
a b s t r a c t The direct current ionic conductivity σdc of sixteen dipolar liquids having broad range of static permittivity (εs ~2 to 170), and their binary mixtures with amides as one of the constituent over the entire composition range were investigated at 30 °C. It has been observed that the σdc values of pure dipolar liquids increase with the increase of their εs values. The σdc values of the dipolar liquids having εs value less than 50 are found in the range of 10−5 to 10−7 S/cm. Further an increase of two orders of magnitude in σdc is observed with the increase of εs value from 50 to 170. Comparatively low σdc value of 1,4‐dioxane (3.06×10−10 S/cm) is found due to its weak dipolar behaviour. The binary mixtures consisting of nearly equal ionic conductivity constituents show either a small deviation in their σdc values from the ideal behaviour or a linear behaviour with concentration variation. A significant deviation in σdc values from the ideality has been found for the binary mixtures when the mixture constituents have a difference of more than one order of magnitude in their pure liquid state ionic conductivities. It was revealed that the σdc values of such binary mixtures are governed by the constituent having high ionic conductivity over a broad composition range. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dipolar liquids used as solvent play an important role in the preparation of electrolyte solutions [1–5], synthesis of solid and gel-type polymer electrolytes by solution–cast technique [6–8], and as the cosolvents for ionic liquids [9,10]. The direct current (dc) ionic conductivity σdc of the electrolyte materials is the key parameter for their use in the design of energy storage rechargeable batteries, supercapacitors and electrochromic devices [1–10]. The viscosity η and static permittivity εs of a dipolar solvent rule the σdc values of the electrolyte solutions, which are also influenced by the nature of the salt [4]. But the pure solvents also have significant σdc value, which is mainly due to the free ions and electrons generated from the residual traces and thermally activated ionic impurities [11–16]. The investigations on σdc values of pure dipolar liquids and their binary mixtures provide the information about the structural conformations and suitability in use of engineering and industrial applications [12–16]. Among the various dielectric parameters, the characterization of εs values over the entire composition range of binary mixtures of the dipolar liquids helps in design of a suitable mixed solvent of required solvating power [17–19]. The molecular interactions, dipolar ordering, and the structures of the amides molecules with different dipolar liquids over the entire composition range of the binary mixtures were extensively investigated by the εs measurements [18–26]. Amides have a large liquid state range, high polarity and strong solvating power, due to which
⁎ Corresponding author. Tel.: +91 291 2720857; fax: +91 291 2649465. E-mail address: [email protected] (R.J. Sengwa). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.08.011
these are mostly used as solvents in the electrolyte preparations [1–3]. To the extent of our comprehension, the ionic conductivity data of amides in pure liquid state and mixed with various dipolar liquids were scarcely reported. The σdc values of various analytical grade pure dipolar solvents and their binary mixtures, with amide as one of the constituent, have been explored in the present paper in view of their detailed electrical characterization (dielectric polarization and electrical conductivity). The main focus of this work is on the amides as one of the constituent of these mixed solvents.
2. Experimental Analytical grade dipolar liquids, given in Table 1 with their abbreviations, were purchased from various manufacturers and used as received. The mass fraction purity of these chemicals, as reported by the manufacturers, is greater than 0.99. Water used in this study is of Millipore double distilled deionized. Binary mixtures of different liquids, with amides as one of the constituent, were prepared by volume concentrations over the entire mixing range at room temperature and simultaneously by weight measurements the mole fractions of the mixture constituents were determined. Agilent 4284A precision LCR meter and Agilent 16452A dielectric test fixture were used for the measurement of capacitances C0 and CP without and with sample, respectively, and parallel resistance RP with sample under the influence of alternating current (ac) electric field of 1 MHz linear frequency, f. All the measurements were made at constant temperature 30 °C using water circulatory bath with a Thermo-Haake DC10 temperature controller.
34
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
Table 1 Values of static permittivity εs, ionic conductivity σdc, viscosity η and the σdc·η product of different dipolar liquids at 30 °C. Dipolar liquids
εs Expt.
εs Lit.a
σdc (S/cm)
η (mPa·s)
σdc·η (S/cm mPa·s)
N-methylformamide (NMF) Formamide (FA) Water (W) Dimethylsulphoxide (DMSO) Ethylene glycol (EG) N,N-dimethylacetamide (DMA) N,N-dimethylformamide (DMF) Ethyl alcohol (EA) Acetone (Ac) 2-methoxyethanol (ME) 2-ethoxyethanol (EE) 2-butoxyethanol (BE) 2-(2-methoxyethoxy)ethanol (MEE) 2-(2-ethoxyethoxy)ethanol (EEE) 2-(2-butoxyethoxy)ethanol (BEE) 1,4-Dioxane (Dx)
169.76 106.14 76.53 46.05 39.84 37.72 36.55 23.87 20.05 17.18 13.91 9.57 16.36 14.46 10.36 2.26
167.80 (35 °C) 105.21 (35 °C) 76.60 46.80 (25 °C) 39.95 36.63 (35 °C) 37.65 (25 °C) 23.60 20.70 (25 °C) 17.58 (25 °C) 13.26 (35 °C) 9.27 (35 °C) 13.77 (35 °C) 12.58 (35 °C) 9.63 (35 °C) 2.20
7.70 × 10−4 4.52 × 10−4 7.77 × 10−5 1.81 × 10−5 2.72 × 10−6 7.00 × 10−6 3.99 × 10−6 5.64 × 10−6 2.06 × 10−6 8.58 × 10−6 1.77 × 10−6 5.40 × 10−7 2.64 × 10−6 1.53 × 10−6 5.66 × 10−7 3.06 × 10−10
1.73 3.18 0.798 2.00 16.10 0.86 0.77 0.949 0.306 1.167 1.316 1.982 2.404 2.869 3.388 1.108
1.33 × 10−3 1.44 × 10−3 6.20 × 10−5 3.62 × 10−5 4.38 × 10−5 6.02 × 10−6 3.07 × 10−6 5.35 × 10−6 6.30 × 10−7 1.00 × 10−5 2.33 × 10−6 1.07 × 10−6 6.35 × 10−6 4.38 × 10−6 1.97 × 10−6 3.39 × 10−10
a
The literature values of εs [18,21–23 and references cited therein].
The real ε′ and imaginary ε″ parts of the complex dielectric function ε*(ω) of the pure liquids and binary mixtures at electric field of angular frequency ω = 2πf were determined by using the equation εðωÞ ¼ ε′−jε″ ¼ α
CP 1 −j ωC 0 RP C0
ð1Þ
where α is the correction coefficient of the dielectric test fixture. The complex ionic conductivity σ*(ω) was determined by the relation σ ⁎ðωÞ ¼ σ ′ þ jσ ″ ¼ jωε 0 εðωÞ ¼ ωε0 ε″ þ jωε0 ε′:
ð2Þ
The ε′ value of a dipolar liquid at 1 MHz was described by its static permittivity εs, whereas the ε″ values were used for the determination of the real part of ionic conductivity σ ′ = ωε0ε″. In the static permittivity frequency regime, the σ ′ values of the dipolar liquids were equal to their σdc (direct current (dc) or static ionic conductivity) values and the σ ″(ω) varies linearly with frequency according to the relation σ ″(ω) = ωε0εs with a slope of + 1 (in log–log scale) [15,27–32]. The measured εs and σdc values of the pure dipolar liquids at 30 °C are recorded in Table 1. The experimental εs values of these liquids were found in good agreement with the literature values [18,21–23 and references cited therein].
do not hold the Stokes–Einstein model (σdc·η = a constant) at fixed temperature, which is in agreement with the pure dipolar liquids investigated over broad temperature range [14]. Figs. 2–5 present the mole fraction concentration dependent σdc values of various binary mixtures with amides as one of the constituent. From the comparative inspection of the σdc plots, it is found that the binary mixtures have the non-ideal behaviour of their σdc values with concentration variation when the mixture constituents ionic conductivity differ by one or more than one order of magnitude. The σdc values of an ideal binary mixture vary linearly from one constituent ionic conductivity value to the other with concentration variation [34]. The non-ideal behaviour of σdc values against constituent concentration of some of the binary mixtures suggests that the
a
10-2
FA W
10-4 DMSO EA DMA DMF EG
Fig. 1 shows the variation of σdc versus εs values of pure dipolar liquids at 30 °C. It is found that the σdc values of the dipolar liquids on logarithmic scale increase linearly with the increase of εs from ~ 20 to 110 (Fig. 1a). Fig. 1b shows that the σdc values of the homologous series of 2-alkoxyethanols and 2-(2-alkoxyethoxy)ethanols also increase linearly with the increase of their εs values (9 b εs b 20) i.e. the σdc value decreases with an increase of molecular size. It is found that the highly polar NMF (εs = 169.76) has comparatively high σdc value (7.7 × 10 −4 S/cm) but the increase is not proportional to its εs value (Table 1). This observation reveals that the σdc value almost saturates when εs value of the dipolar liquid exceeds higher than 100. The σdc value of 1,4‐dioxane is very low (3.06 × 10−10 S/cm) (Table 1), which is mainly due to its non-polar behaviour (εs = 2.26). The observed σdc values of DMF, NMF and DMSO are in good agreement with those found in the literature [12,14]. The η values of these dipolar liquids, taken from the literature [33], are presented in Table 1, and the same are used for the determination of σdc·η product. The σdc·η product (Table 1) of these dipolar liquids was not found to be constant at a fixed temperature. This result inferred that the different dipolar liquids
σdc (S/cm)
3. Results and discussion
10-6
Ac
10-4
20
40
60
80
100
120
b ME
10-5 EE
MEE
10-6
BE
EEE BEE
10-7 8
10
12
εs
14
16
18
Fig. 1. Static permittivity εs dependent dc ionic conductivity σdc of different dipolar liquids at 30 °C.
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
a
10-3
35
a
10-3
EA+NMF EA+FA EA+DMA EA+DMF
10-5 10-4
10-7
Dx+NMF Dx+FA Dx+DMA Dx+DMF
10-5
b
σdc (S/cm)
10-3
DMSO+NMF DMSO+FA DMSO+DMA DMSO+DMF
10-4
b
10-3
σdc (S/cm)
10-9
EG+NMF EG+FA EG+DMA EG+DMF
10-4 10-5
10-5
c
10-3 10-4
10-6
10-4
Ac+NMF Ac+FA Ac+DMA Ac+DMF
10-5
0.0
0.2
c
10-3
W+NMF W+FA W+DMA W+DMF
10-5
0.4
0.6
0.8
1.0
XDx; XDMSO; XAc
10-6
0.0
0.2
0.4
0.6
0.8
1.0
XEA; XEG; XW
Fig. 2. Mole fraction concentration dependent σdc of (a) Dx+amides, (b) DMSO+amides and (c) Ac+amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity. The straight lines over the full range of data points represent the linear fit of σdc values with X.
Fig. 3. Mole fraction concentration dependent σdc of (a) EA+amides, (b) EG+amides and (c) W+amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity. The straight lines over the full range of data points represent the linear fit of σdc values with X.
heterogeneous molecular interactions influence the normal Brownian dynamics of relaxing molecular dipoles, which controls the ionic motion [13]. The Dx + amides mixtures have higher σdc values from the ideal behaviour (the straight line drawn by joining the pure liquids σdc values at X = 0 and 1) (Fig. 2a), which is mainly due to a large difference in ionic conductivity of Dx and the amides (Table 1). The enhancement in σdc from ideal behaviour of binary mixtures suggests that the hetero-molecular interactions facilitate the coupling between the Brownian translation diffusion of ions and the rotational diffusion of molecules (the Debye relaxation), which results in increase of their ions' mobility [13,14]. The excess static permittivity and effective Kirkwood correlation factor study of various binary mixtures confirmed the decrease in a number of effective parallel aligned dipoles due to heterogeneous H–bond interactions [18–24]. But the increase of σdc values from ideality reveals that the ions conductive paths in the H–bonded hetero-molecular structures of binary mixtures increase by and large besides the decrease of effective parallel aligned dipoles in the dipolar liquids mixtures. In case of DMSO + amides (Fig. 2b), the DMSO+ NMF mixture shows a large enhancement in σdc values from the ideal behaviour, whereas DMSO + FA, DMSO + DMA and DMSO+ DMF have a comparatively small deviation in their σdc values from the ideality. Similarly, in case of Ac + amides (Fig. 2c), the Ac + NMF and Ac + FA mixtures show the higher σdc values from the ideality, whereas these values of Ac + DMA and Ac+ DMF vary linearly with the mixture constituent concentration. These comparative σdc values of amide mixed Dx, DMSO and Ac binary mixtures inferred that the NMF and FA mixed dipolar liquids have more active ion conduction
behaviour as compared to that of the DMA and DMF mixed in the same dipolar liquids. Fig. 3 also confirms the enhancement of σdc values from ideality for EA + NMF, EA + FA, EG + NMF, EG + FA, W + NMF and W + FA mixtures, whereas W+ DMA and W + DMF mixtures have decrease in their σdc values. The decrease of σdc values from ideal behaviour inferred that the water interactions with DMA and DMF produce more hindrance to the translational diffusion of the ions. The mixtures of DMA and DMF with EA and EG have ideal behaviour of their concentration dependent σdc values. From the comparative study of Figs. 2–5, it is found that σdc values of most of the binary mixtures have almost linear behaviour over the broad concentration range of high ionic conductivity constituent (NMF and FA), and shows an abrupt change in the low ionic conductivity constituent rich-concentration region. Such changes were noticed in the concentration range ~0.6 to 0.7 of the cosolvents in these amide mixed binary mixtures (except W+ DMA and W + DMF). These results reveal that the σdc values of the binary mixtures are governed by the comparatively higher ionic conductivity constituent, i.e., NMF and FA over the broad range of the mixing. Fig. 4 depicted the variation of σdc values with mole fraction concentration of 2‐alkoxyethanol + amides mixtures. It is found that the concentration dependent σdc values of the mixtures of homologous series molecules with amides also deviate from the ideal behaviour. Similar type of behaviour of the σdc values with concentration variation was also found for the mixtures of the homologous series of 2-(2-alkoxyethoxy)ethanols mixed with FA (Fig. 5a). Fig. 5b and c shows the concentration dependent σdc values of binary mixtures containing different amide molecules. For these mixtures, it is also
36
S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37
a
a
ME+FA ME+DMA ME+DMF
10-4
10-4 10-5
10-5
EE+FA EE+DMA EE+DMF
10-4
10-5
b
10-3
σdc (S/cm)
b σdc (S/cm)
FA+MEE FA+EEE FA+BEE
10-6
10-4 FA+NMF FA+DMA FA+DMF
10-5 10-6
c
10-3
NMF+DMA NMF+DMF
BE+FA BE+DMA BE+DMF
c
10-4 10-5
10-6
10-7
DMF+DMA
10-5
0.0
0.2
0.4
0.6
0.8
1.0
10-6
XME; XEE; XBE
0.0
0.3
0.6
0.9
XDMF 0.0
0.3
0.6
0.9
XFA; XNMF
Fig. 4. Mole fraction concentration dependent σdc of (a) ME + amides, (b) EE + amides and (c) BE + amides binary mixture at 30 °C. The straight lines drawn from joining the σdc values at X = 0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity.
Fig. 5. Mole fraction concentration dependent σdc of (a) FA+2-(2-alkoxyethoxy)ethanols, (b) FA+amides and (c) NMF+amides and DMF+DMA binary mixture in the inset at 30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in black colour) on the data points show the range over which the high σdc constituent governs the mixture conductivity.
observed that the mixture having a large difference in their constituents σdc values significantly deviate from the ideal behaviour (FA + DMA, FA + DMF, NMF + DMA and NMF + DMF). But the σdc values of FA + NMF and DMF + DMA mixtures vary linearly because of the mixture constituents have the same order of their ionic conductivity. Small variation in the σdc values of pure amides and cosolvents (Figs. 2–5 at X = 0 or 1) inferred that the ionic conductivity of a pure dipolar liquid may vary with the time of measurements or the same dipolar liquid when obtained from different manufacturers. This observation confirms that the ionic traces in analytical grade dipolar liquid may also vary due to environmental contact of the sample during their synthesis and measurements.
Nos. SR/S2/CMP-09/2002 and SR/S2/CMP-0072/2010. One of the authors SC is thankful to the CSIR, New Delhi for the award of Research Associate fellowship.
4. Conclusions The σdc values of dipolar liquids increase with the increase of their εs values. Most of the binary mixtures of dipolar liquids having amide as one of the constituent confirm the increase of σdc values from the ideal behaviour, which is more pronounced when the conductivity difference of mixtures constituents is large. Over the broad composition range of the studied binary mixture, their σdc values are governed by the high ionic conductivity constituent (NMF and FA). Nearly equal ionic conductivity constituents of the binary mixtures of dipolar liquids obey the ideal behaviour of their σdc values with composition concentration. Acknowledgements The authors are grateful to the Department of Science and Technology, New Delhi for providing the financial assistance through research project
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