Journal of Molecular Liquids 175 (2012) 33–37

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Journal of Molecular Liquids

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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

⁎ Corresponding author. Tel.: +91 291 2720857; fax:E-mail address: rjsengwa@rediffmail.com (R.J. Sengw

0167-7322/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.molliq.2012.08.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 May 2012Received in revised form 24 July 2012Accepted 12 August 2012Available online 25 August 2012

Keywords:Ionic conductivityStatic permittivityDipolar liquidsBinary mixturesAmides

The direct current ionic conductivity σdc of sixteen dipolar liquids having broad range of static permittivity (εs ~2to 170), and their binary mixtureswith amides as one of the constituent over the entire composition rangewereinvestigated at 30 °C. It has been observed that theσdc values of pure dipolar liquids increasewith the increase oftheir εs values. The σdc values of the dipolar liquids having εs value less than 50 are found in the range of 10−5 to10−7 S/cm. Further an increase of two orders of magnitude in σdc is observed with the increase of εs value from50 to 170. Comparatively low σdc value of 1,4‐dioxane (3.06×10−10 S/cm) is found due to its weak dipolarbehaviour. The binary mixtures consisting of nearly equal ionic conductivity constituents show either a smalldeviation in their σdc values from the ideal behaviour or a linear behaviour with concentration variation. Asignificant deviation in σdc values from the ideality has been found for the binary mixtures when the mixtureconstituents have a difference ofmore than one order ofmagnitude in their pure liquid state ionic conductivities.Itwas revealed that theσdc values of such binarymixtures are governedby the constituent having high ionic con-ductivity 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 prepa-ration of electrolyte solutions [1–5], synthesis of solid and gel-typepolymer electrolytes by solution–cast technique [6–8], and as thecosolvents for ionic liquids [9,10]. The direct current (dc) ionic conduc-tivityσdc of the electrolytematerials is the key parameter for their use inthe design of energy storage rechargeable batteries, supercapacitorsand electrochromic devices [1–10]. The viscosity η and static permittiv-ity 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 puresolvents also have significant σdc value, which is mainly due to thefree ions and electrons generated from the residual traces and thermallyactivated ionic impurities [11–16]. The investigations on σdc values ofpure dipolar liquids and their binary mixtures provide the informationabout the structural conformations and suitability in use of engineeringand industrial applications [12–16].

Among the various dielectric parameters, the characterization of εsvalues over the entire composition range of binarymixtures of the dipo-lar liquids helps in design of a suitable mixed solvent of required solvat-ing power [17–19]. Themolecular interactions, dipolar ordering, and thestructures of the amides molecules with different dipolar liquids overthe entire composition range of the binary mixtures were extensivelyinvestigated by the εsmeasurements [18–26]. Amides have a large liquidstate range, high polarity and strong solvating power, due to which

+91 291 2649465.a).

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these are mostly used as solvents in the electrolyte preparations [1–3].To the extent of our comprehension, the ionic conductivity data of am-ides in pure liquid state and mixed with various dipolar liquids werescarcely reported. Theσdc values of various analytical grade pure dipolarsolvents and their binarymixtures, with amide as one of the constituent,have been explored in the present paper in view of their detailed electri-cal characterization (dielectric polarization and electrical conductivity).Themain focus of this work is on the amides as one of the constituent ofthese mixed solvents.

2. Experimental

Analytical grade dipolar liquids, given in Table 1 with their abbrevi-ations, were purchased from various manufacturers and used as re-ceived. The mass fraction purity of these chemicals, as reported by themanufacturers, is greater than 0.99. Water used in this study is ofMillipore double distilled deionized. Binary mixtures of different liq-uids, with amides as one of the constituent, were prepared by volumeconcentrations over the entire mixing range at room temperature andsimultaneously byweightmeasurements themole fractions of themix-ture constituents were determined.

Agilent 4284A precision LCR meter and Agilent 16452A dielectrictest fixture were used for the measurement of capacitances C0 and CPwithout and with sample, respectively, and parallel resistance RP withsample under the influence of alternating current (ac) electric field of1 MHz linear frequency, f. All themeasurementsweremade at constanttemperature 30 °C using water circulatory bath with a Thermo-HaakeDC10 temperature controller.

Table 1Values of static permittivity εs, ionic conductivity σdc, viscosity η and the σdc·η product of different dipolar liquids at 30 °C.

Dipolar liquids εs εs σdc η σdc·ηExpt. Lit.a (S/cm) (mPa·s) (S/cm mPa·s)

N-methylformamide (NMF) 169.76 167.80 (35 °C) 7.70×10−4 1.73 1.33×10−3

Formamide (FA) 106.14 105.21 (35 °C) 4.52×10−4 3.18 1.44×10−3

Water (W) 76.53 76.60 7.77×10−5 0.798 6.20×10−5

Dimethylsulphoxide (DMSO) 46.05 46.80 (25 °C) 1.81×10−5 2.00 3.62×10−5

Ethylene glycol (EG) 39.84 39.95 2.72×10−6 16.10 4.38×10−5

N,N-dimethylacetamide (DMA) 37.72 36.63 (35 °C) 7.00×10−6 0.86 6.02×10−6

N,N-dimethylformamide (DMF) 36.55 37.65 (25 °C) 3.99×10−6 0.77 3.07×10−6

Ethyl alcohol (EA) 23.87 23.60 5.64×10−6 0.949 5.35×10−6

Acetone (Ac) 20.05 20.70 (25 °C) 2.06×10−6 0.306 6.30×10−7

2-methoxyethanol (ME) 17.18 17.58 (25 °C) 8.58×10−6 1.167 1.00×10−5

2-ethoxyethanol (EE) 13.91 13.26 (35 °C) 1.77×10−6 1.316 2.33×10−6

2-butoxyethanol (BE) 9.57 9.27 (35 °C) 5.40×10−7 1.982 1.07×10−6

2-(2-methoxyethoxy)ethanol (MEE) 16.36 13.77 (35 °C) 2.64×10−6 2.404 6.35×10−6

2-(2-ethoxyethoxy)ethanol (EEE) 14.46 12.58 (35 °C) 1.53×10−6 2.869 4.38×10−6

2-(2-butoxyethoxy)ethanol (BEE) 10.36 9.63 (35 °C) 5.66×10−7 3.388 1.97×10−6

1,4-Dioxane (Dx) 2.26 2.20 3.06×10−10 1.108 3.39×10−10

a The literature values of εs [18,21–23 and references cited therein].

20 40 60 80 100 120

10-6

10-4

10-2

εs

a

FA

W

DMSO

DMAEA

EGDMFAc

σ dc (

S/cm

)

8 10 12 14 16 18

10-7

10-6

10-5

10-4

b

EEE

EE

BEE

BEMEE

ME

Fig. 1. Static permittivity εs dependent dc ionic conductivity σdc of different dipolar liquidsat 30 °C.

34 S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37

The real ε′ and imaginary ε″ parts of the complex dielectric functionε*(ω) of the pure liquids and binary mixtures at electric field of angularfrequency ω=2πf were determined by using the equation

ε� ωð Þ ¼ ε ′−jε″ ¼ αCP

C0−j

1ωC0RP

� �ð1Þ

where α is the correction coefficient of the dielectric test fixture.The complex ionic conductivityσ*(ω)was determined by the relation

σ⁎ ωð Þ ¼ σ ′þ jσ ″ ¼ jωε0ε� ωð Þ ¼ ωε0ε″þ jωε0ε ′: ð2Þ

The ε′ value of a dipolar liquid at 1 MHz was described by its staticpermittivity εs, whereas the ε″ values were used for the determina-tion of the real part of ionic conductivity σ ′=ωε0ε″. In the static per-mittivity frequency regime, the σ ′ values of the dipolar liquids wereequal to their σdc (direct current (dc) or static ionic conductivity)values and the σ″(ω) varies linearly with frequency according tothe 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 liquidsat 30 °C are recorded in Table 1. The experimental εs values of theseliquids were found in good agreement with the literature values[18,21–23 and references cited therein].

3. Results and discussion

Fig. 1 shows the variation of σdc versus εs values of pure dipolarliquids at 30 °C. It is found that the σdc values of the dipolar liquidson logarithmic scale increase linearly with the increase of εs from~20 to 110 (Fig. 1a). Fig. 1b shows that the σdc values of the homolo-gous series of 2-alkoxyethanols and 2-(2-alkoxyethoxy)ethanols alsoincrease linearly with the increase of their εs values (9bεsb20) i.e. theσdc value decreases with an increase of molecular size. It is found thatthe 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 saturateswhen 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), whichis mainly due to its non-polar behaviour (εs=2.26). The observed σdc

values of DMF, NMF and DMSO are in good agreement with thosefound in the literature [12,14]. The η values of these dipolar liquids,taken from the literature [33], are presented in Table 1, and the sameare 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 afixed temperature. This result inferred that the different dipolar liquids

do not hold the Stokes–Einstein model (σdc·η=a constant) at fixedtemperature, which is in agreementwith the pure dipolar liquids inves-tigated 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 constitu-ent. From the comparative inspection of the σdc plots, it is found thatthe binary mixtures have the non-ideal behaviour of their σdc valueswith concentration variation when the mixture constituents ionicconductivity differ by one or more than one order of magnitude.The σdc values of an ideal binary mixture vary linearly from one con-stituent ionic conductivity value to the other with concentration var-iation [34]. The non-ideal behaviour of σdc values against constituentconcentration of some of the binary mixtures suggests that the

0.0 0.2 0.4 0.6 0.8 1.010-6

10-5

10-4

10-3 c

Ac+NMFAc+FAAc+DMAAc+DMF

σ dc (

S/cm

)

XDx; XDMSO; XAc

10-5

10-4

10-3 b DMSO+NMFDMSO+FADMSO+DMADMSO+DMF

10-9

10-7

10-5

10-3 a

Dx+NMFDx+FADx+DMADx+DMF

Fig. 2.Mole fraction concentration dependentσdc of (a) Dx+amides, (b) DMSO+amidesand (c) Ac+amides binarymixture 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) onthe data points show the range over which the high σdc constituent governs the mixtureconductivity. The straight lines over the full range of data points represent the linear fit ofσdc values with X.

0.0 0.2 0.4 0.6 0.8 1.010-6

10-5

10-4

10-3 c

W+NMFW+FAW+DMAW+DMF

σ dc (

S/cm

)

XEA; XEG; XW

10-5

10-4

10-3 b EG+NMFEG+FAEG+DMAEG+DMF

10-5

10-4

10-3a EA+NMF

EA+FAEA+DMAEA+DMF

Fig. 3. Mole fraction concentration dependent σdc of (a) EA+amides, (b) EG+amidesand (c) W+amides binary mixture at 30 °C. The straight lines drawn from joining theσdc 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 mix-ture conductivity. The straight lines over the full range of data points represent the linearfit of σdc values with X.

35S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37

heterogeneous molecular interactions influence the normal Browniandynamics of relaxing molecular dipoles, which controls the ionic mo-tion [13].

The Dx+amides mixtures have higher σdc values from the idealbehaviour (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 differ-ence in ionic conductivity of Dx and the amides (Table 1). The en-hancement in σdc from ideal behaviour of binary mixtures suggeststhat the hetero-molecular interactions facilitate the coupling betweenthe Brownian translation diffusion of ions and the rotational diffusionof molecules (the Debye relaxation), which results in increase of theirions' mobility [13,14]. The excess static permittivity and effectiveKirkwood correlation factor study of various binary mixtures con-firmed the decrease in a number of effective parallel aligned dipolesdue to heterogeneous H–bond interactions [18–24]. But the increaseof σdc values from ideality reveals that the ions conductive paths inthe H–bonded hetero-molecular structures of binary mixtures increaseby and large besides the decrease of effective parallel aligned dipoles inthe dipolar liquids mixtures. In case of DMSO+amides (Fig. 2b), theDMSO+NMF mixture shows a large enhancement in σdc values fromthe ideal behaviour, whereas DMSO+FA, DMSO+DMA and DMSO+DMF have a comparatively small deviation in their σdc values fromthe ideality. Similarly, in case of Ac+amides (Fig. 2c), the Ac+NMFand Ac+FA mixtures show the higher σdc values from the ideality,whereas these values of Ac+DMA and Ac+DMF vary linearly withthe mixture constituent concentration. These comparative σdc valuesof amide mixed Dx, DMSO and Ac binary mixtures inferred that theNMF and FA mixed dipolar liquids have more active ion conduction

behaviour as compared to that of the DMA and DMF mixed in thesame dipolar liquids.

Fig. 3 also confirms the enhancement of σdc values from ideality forEA+NMF, EA+FA, EG+NMF, EG+FA, W+NMF and W+FA mix-tures, whereas W+DMA and W+DMF mixtures have decrease intheir σdc values. The decrease of σdc values from ideal behaviour in-ferred that the water interactions with DMA and DMF produce morehindrance to the translational diffusion of the ions. The mixtures ofDMA and DMFwith EA and EG have ideal behaviour of their concentra-tion dependent σdc values. From the comparative study of Figs. 2–5, it isfound that σdc values of most of the binary mixtures have almost linearbehaviour over the broad concentration range of high ionic conductivityconstituent (NMF and FA), and shows an abrupt change in the low ionicconductivity constituent rich-concentration region. Such changes werenoticed in the concentration range ~0.6 to 0.7 of the cosolvents in theseamide mixed binary mixtures (except W+DMA andW+DMF). Theseresults reveal that the σdc values of the binarymixtures are governed bythe comparatively higher ionic conductivity constituent, i.e., NMF andFA over the broad range of the mixing.

Fig. 4 depicted the variation of σdc values with mole fraction con-centration of 2‐alkoxyethanol+amides mixtures. It is found that theconcentration dependent σdc values of the mixtures of homologousseries molecules with amides also deviate from the ideal behaviour.Similar type of behaviour of the σdc values with concentration varia-tion was also found for the mixtures of the homologous series of2-(2-alkoxyethoxy)ethanols mixed with FA (Fig. 5a). Fig. 5b and cshows the concentration dependent σdc values of binary mixturescontaining different amide molecules. For these mixtures, it is also

0.0 0.2 0.4 0.6 0.8 1.010-7

10-5

10-3 c BE+FABE+DMABE+DMF

σ dc (

S/cm

)

XME; XEE; XBE

10-6

10-5

10-4

b EE+FAEE+DMAEE+DMF

10-5

10-4

a ME+FAME+DMAME+DMF

Fig. 4. Mole fraction concentration dependent σdc of (a) ME+amides, (b) EE+amidesand (c) BE+amides binary mixture at 30 °C. The straight lines drawn from joining theσdc values at X=0 and 1 represent the ideal behaviour. The straight lines (in blackcolour) on the data points show the range over which the high σdc constituent governsthe mixture conductivity.

0.0 0.3 0.6 0.910-6

10-5

0.0 0.3 0.6 0.910-6

10-4

cNMF+DMANMF+DMF

σ dc (

S/cm

)

XFA; XNMF

10-5

10-4

10-3 b

FA+NMFFA+DMAFA+DMF

10-6

10-5

10-4

a

FA+MEEFA+EEEFA+BEE

DMF+DMA

XDMF

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 at30 °C. The straight lines drawn from joining the σdc values at X=0 and 1 represent theideal behaviour. The straight lines (in black colour) on the data points show the rangeover which the high σdc constituent governs the mixture conductivity.

36 S. Choudhary, R.J. Sengwa / Journal of Molecular Liquids 175 (2012) 33–37

observed that the mixture having a large difference in their constitu-ents 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 becauseof the mixture constituents have the same order of their ionic con-ductivity. Small variation in the σdc values of pure amides andcosolvents (Figs. 2–5 at X=0 or 1) inferred that the ionic conductiv-ity of a pure dipolar liquid may vary with the time of measurementsor the same dipolar liquid when obtained from different manufac-turers. This observation confirms that the ionic traces in analyticalgrade dipolar liquid may also vary due to environmental contact ofthe sample during their synthesis and measurements.

4. Conclusions

The σdc values of dipolar liquids increasewith the increase of their εsvalues. Most of the binary mixtures of dipolar liquids having amide asone of the constituent confirm the increase of σdc values from theideal behaviour, which is more pronounced when the conductivity dif-ference of mixtures constituents is large. Over the broad compositionrange of the studied binary mixture, their σdc values are governed bythe high ionic conductivity constituent (NMF and FA). Nearly equalionic conductivity constituents of the binary mixtures of dipolar liquidsobey the ideal behaviour of their σdc values with compositionconcentration.

Acknowledgements

The authors are grateful to theDepartment of Science andTechnology,NewDelhi for providing the financial assistance through research project

Nos. SR/S2/CMP-09/2002 and SR/S2/CMP-0072/2010. One of the authorsSC is thankful to the CSIR, New Delhi for the award of Research Associatefellowship.

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