High-temperature dehydration behavior and ionic conduction of 2,5-dimethylanilinium chloride monohydrate

Article abstract of DOI:10.1016/j.jallcom.2016.02.110

The 2,5-dimethylanilinium chloride monohydrate compound is obtained by slow evaporation at room temperature. This material is characterized by DSC, X-Ray powder diffraction, Raman and impedance spectroscopy technique measured in the 2.10²-5.10<

Full text of DOI:10.1016/j.jallcom.2016.02.110

Accepted Manuscript
High-temperature dehydration behavior and ionic conduction of 2,5-dimethylanilinium
chloride monohydrate
Sameh Guidara, Habib Feki, Younes Abid
PII:
S0925-8388(16)30378-4
10.1016/j.jallcom.2016.02.110
JALCOM 36728
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 January 2016
Revised Date: 11 February 2016
Accepted Date: 12 February 2016
Please cite this article as: S. Guidara, H. Feki, Y. Abid, High-temperature dehydration behavior and ionic
conduction of 2,5-dimethylanilinium chloride monohydrate, Journal of Alloys and Compounds (2016),
doi: 10.1016/j.jallcom.2016.02.110.
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ACCEPTED MANUSCRIPT
High-temperature dehydration behavior and ionic
conduction of 2,5-dimethylanilinium chloride
monohydrate
Sameh Guidara*, Habib Feki, Younes Abid
Laboratoire de Physique Appliquée (LPA), Université de Sfax. Faculté des Sciences 3000,
BP.1171, Sfax, Tunisie.
*Corresponding author Tel: +216 23865145, Email: sameh_guidara@yahoo.fr
Abstract
The 2,5-dimethylanilinium chloride monohydrate compound is obtained by slow evaporation at
room temperature. This material is characterized by DSC, X-Ray powder diffraction, Raman and
impedance spectroscopy technique measured in the 2.102 - 5.106 Hz frequency and 292–422 K
temperature ranges. The calorimetric study has revealed three endothermic peaks at 355 K, 392 K
and 403 K which defines four successive phases denoted I, II, III and IV. The first peak
corresponds to water escape from the crystal. After heating above 355 K, the compound
dehydrates and the crystal space group changes from non-centrosymmetric to centrosymmetric
symetry. The activation energy responsible for dielectric relaxation extracted from the modulus
spectra is found to be almost the same as the value obtained from temperature variation of dc
conductivity for phases I and IV. These results indicate that the transport is through ion hopping
mechanism. The influence of the dehydration process on the compound conductivity was also
discussed.
Keywords: Calorimetric study, Dehydration, Impedance spectroscopy, Dielectric properties.
1. Introduction
Semi organic materials have attracted intensive interests owing to their potential applications in
data storage, signal processing, sensing, rewriteable optical data storage, etc. [1-3]. These
materials are not only very important for having the potential for a large range of optical
properties such as efficient luminescence and second-order non-linear optical activity [4-7], but
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also favorable for studying the structure–property relationship [8]. Furthermore, few electrical
and dielectrical studies of these compounds have been reported. This work is a continuation of
our interest in 2,5-dimethylanilinium derivatives. Our last three published papers have been
devoted to the X-ray diffraction and vibrational studies as well as DFT calculations of nonlinear
optical compounds, 2,5-dimethylanilinium chloride monohydrate [9], 2,5-dimethylanilinium
dihydrogen phosphate [10] and bis 2,5-dimethylanilinium sulfate [11]. As an extension of our
searches for exploring new properties concerning hybrid salts, we report a detailed investigation
of the thermal, electrical and dielectrical behavior of the 2,5-dimethylanilinium chloride
monohydrate compound. The crystal structure of this compound was elucidated by W. Smirani et
al. involving water molecules [12]. In order to determine the effect of the water molecules escape
on ionic conduction, we have made many investigations. The techniques used include differential
scanning calorimetry (DSC), X-ray powder diffraction, impedance and Raman spectroscopy at
several temperatures. This combination of techniques allowed us to characterize in detail physical
properties of this compound and correlated them with the structural changes in the crystal
accompanying the dehydration process.
2. Experimental procedure
The title compound was synthesized from an ethanolic solution of 2,5-dimethylaniline and
hydrochloric acid in equimolar amounts. The obtained solution was left to stand at room
temperature until formation of colorless single crystals of 2,5-dimethylanilinium chloride
monohydrate (2,5-DACM) [9]. Structural information at room temperature has been mentioned
elsewhere [12]. In summary, the title compound crystallizes in the monoclinic system with the
non centrosymmetric space group P21 (Z=2) with a = 7.529(3)Å, b = 7.452(2)Å, c = 9.032(4)Å
and β = 102.830(4)°. From single crystal XRD data, it is observed that the asymmetric unit of the
title compound is built up from one chloride anion Cl-, one 2,5-dimethylanilinium cation [2,5-
(CH3)2C6H3NH3]+ and one water molecule. The crystal packing is determined mainly by N-
H…O, N-H…Cl and O-H…Cl hydrogen bonds. The thermal behavior of this compound was
investigated by differential scanning calorimetry using a DSC (Q-100) TA Instrument in the 250-
450 K temperature range with scanning rate 5°C/min under N2 atmosphere. X-ray powder
diffraction pattern was recorded in a Philips PW 1710 diffractometer operating with copper
radiation Kα = 1.5418 Å for the 2θ angular range of 5ꢀ60°. Unit cell parameters of the
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synthesized compound were refined by Rietveld method from powder data. The impedance
spectroscopy was performed on pellet disks of about 5 mm diameter and 1.5 mm thickness. The
electrical impedances were measured at a frequency ranging from 200 Hz to 5 MHz with a
NOVOCONTROL system integrating ALFA dielectric interface. The measurements were carried
out at temperatures in the range 292–422 K.
3. Results and discussion
3.1. Calorimetric study
The DSC measurement of 2,5-DACM were performed on heating a sample (2.9 mg mass) with
scanning rate 5 °C/min in the 250-450 K temperature range. The thermal analysis result is
reported in Fig. 1. This unambiguously shows the existence of three heat anomalies at T1=355 K,
T2=392 K and T3=403 K, respectively, which defines four successive phases denoted I, II, III and
IV. Peak 1 describes a process which is associated with water molecules escape from the crystal.
Interestingly, this dehydration is also observed in a quite similar compound, 2,3-
dimethylanilinium hydrogen sulfate monohydrate, at nearly the same temperature [13].
According to visual observation of the dehydration crystal on heating stage, the white product
covered the whole surface in the early stage of dehydration.
3.2. Structural analysis
In order to determine the structural changes in the crystal accompanying the dehydration process,
we have performed X ray powder measurement. The sample was heated up to 370 K and then
was cooled to room temperature. The obtained XRD pattern is shown in Fig 2. The Rietveld
refinements data performed using FULLPROF program [14] revealed that the 2,5-
dimethylanilinium chloride (2,5-DAC) crystallized in the monoclinic structure with the
centrosymmetric P21/m space group with a = 18.845Å, b = 5.203Å, c = 17.041Å and β = 91.157°
with a reliability factor χ2=3.68. The comparative graphs of X ray powder patterns of both
hydrate and dehydrate compound are given in Fig. 3. Described above changes are due to the
dehydration process corresponding to water vanishing from the crystal. The water escape was
preceded by a weakening of the hydrogen bonds in which the water molecules were involved.
3.3. Temperature evolution of the Raman spectra
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It is of interest to observe the possible changes in the Raman bands corresponding to
characteristic vibrations of the molecules at different temperatures, so we have undertaken a
Raman study between 298 and 358 K. It is necessary to mention that the detailed assignment of
the IR and Raman bands at room temperature is given in our previous work [9]. The recording of
the Raman spectra versus temperature (Fig. 4) in the range 3600-2700 cm-1 shows that the most
significant change is related to the vanishing of the intense band observed at 3390 cm-1 assigned
to stretching O-H of water molecule which is related to the removal of water from the crystal
lattice and confirms the above conclusion.
3.4. Dielectrical study
The study of the dielectric properties is an important source for valuable information about the
mechanisms of conduction in the materials and the origin of the dielectric relaxation. The
complex dielectric function is expressed as:
ε* = ε’ - jε”
(1)
where ε’ and ε” are the real and imaginary part of the dielectric constant, respectively. The
temperature dependence of the relative dielectric constant of the ε’r and ε”r (200 Hz – 220 kHz)
of the title compound are shown in Fig. 5. These spectra present a prominent dielectric peak at
T=362 K associated to the dehydration process corresponding to the peak centered at T1 = 355 K
in the thermogram. This temperature does not change with increasing frequency, suggesting that
the sample does not present any type of relaxation in the frequency range. The dielectric constant
decreases with the increase in frequency, which is a general characteristic of a dielectric material.
This is due to the absence of dipolar and ionic polarizations in the material at a higher frequency
[15].
3.5. Impedance spectroscopy
The complex impedance spectra of title compound are shown in Fig. 6. In the studied frequency
range and for all measured temperature excepting dehydration temperature region, one relaxation
peak is observed revealing that the single semicircular response is from grain interior and no
grain boundaries are involved, which indicates departure from the ideal Debye behavior [15]. In
the temperature range between 352-372K, the impedance spectra mainly consists of two parts: a
high frequency region due to the ionic conduction of 2,5-DACM pellet and a low-frequency
region characterized by a straight line due to a surface layer effect. A careful analysis of the
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thermal evolution of the complex impedance spectra shows that there is an increase of the value
of (-Z”) in each phase, which is in good agreement with the calorimetric study. As temperature
increases, the radius of the arc corresponding to the bulk resistance of the sample decreases for
each phase, indicating an activated thermal conduction mechanism. It is worth to note that the
material shows an unusual response from hydrated to dehydrated state. A deviation from the
Arrhenius behavior was observed surrounding the dehydration process with the impedance
spectra increasing due to the heat-induced removal of inter-layer water. This process
corresponding to the loss of water is accompanied by an important decrease of conductivity. Such
behavior was observed in many other compounds [16,17].
3.6. Complex modulus analysis
Modulus formalism is generally used to analyze dielectric relaxation. The complex electric
modulus M is defined by the reciprocal of the complex permittivity ε* (M* = 1/ε*) [18] and
corresponds to the relaxation of the electric field in the system when the electric displacement is
constant. So from the electric modulus, we can represent real dielectric relaxation process
admitted as:
M∗ = Mꢀ + jM′′
(2)
where M’ and M” are the real and imaginary parts of complex modulus.
Fig. 7 represents the real M’ variation against angular frequency of 2,5-DACM at some
temperature. In the low frequency region, the magnitudes of M’ tend to zero and coincide with
each other and at high frequencies, all the curves attain to asymptotic form and reach a constant
value, which confirms a negligibly contribution of electrode effect. In the other frequency region,
the value of M’ has followed a continuous dispersal with increase in frequency which may be
caused by short range mobility of charge carriers.
In order to determine some characteristic parameters of the charge carriers such as their activation
energy and relaxation frequency, numerical simulations of the modulus spectra are interesting.
Bergman has proposed:
ꢁ"
ꢂꢃꢄ
M"(ω) =
(3)
β
ꢋ ꢍ
β
ꢉꢊβ
ω
ω
ꢂꢃꢄ
(
)
ꢅꢆβ ꢇꢈ
ꢋꢌβꢈ
ꢋꢇꢈ
ω
ω
ꢂꢃꢄ
where M”max and ωmax is the peak maximum and peak angular frequency of imaginary part of the
modulus, respectively. β is the well-known Kohlrausch parameter. This parameter characterizes
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the degree of non-Debye behavior which decreases with the increase in the relaxation time
distribution (0 ≤ β ≤ 1). Fig. 8 represents the frequency dependence of M”(ω) recorded at
T=342K with fitting. The normalized imaginary part M”/M”max as a function of angular
frequency is shown in Fig. 9. The plots show a slightly asymmetric peak at each temperature.
This peak shifted towards higher relaxation frequencies with increasing temperature for each
phase indicating a correlation between the motions of mobile charge carriers [19].
Frequency relaxation fꢎ = ωꢂꢃꢄ was determined from fitting M” using the above equation, which
ꢏπ
is thermally activated and could be expressed as:
ꢑ
ꢃ
fꢎ = fꢐexp ꢈ− ꢒ ꢔꢋ
(4)
ꢓ
where Ea is the activation energy and f0 is a pre-exponential factor and kB is the Boltzmann’s
constant. A plot between Ln(fp) and 1000/T is shown in Fig. 10, which is well described by
Arrhenius equation. A change of the curve slope is noted at T1, T2 and T3, which confirms the
DSC study.
Fig. 11 shows the imaginary part of the electrical modulus M′′ as a function of the real part M′ at
T=342K. In the studied frequency range, single relaxation peak was observed which is associated
with grain effect.
In order to distinguish whether the short range or long range movement of charge carries is
dominant in a relaxation process [20] we have presented the combined plot of M” and –Z” versus
frequency at T=342K in Fig. 12. The separation of peak positions of M” and -Z” suggests that the
relaxation process is dominated by the short range movement of charge carriers and departs from
the ideal Debye type behavior while the frequencies coincidence suggests the long range
movement of charge carriers is dominant [21-23].
3.7. Electrical conductivity
Electrical conductivity in the material is due to the ordered motion of weakly bound charged
particles under the influence of an electric field and it is a thermally activated process. It is one of
the significant properties of the materials to be characterized depending on the nature of charge
carriers dominating the conduction process such as electrons/holes or cations/anions.
3.7.1. AC Conductivity
The phenomenon of the conductivity dispersion is generally analyzed using jonscher’s universal
power law [24].
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σꢕꢖ ω = σꢗꢖ + Aωꢘ
(5)
( )
with σdc is the value of the conductivity at low frequency, A is a factor dependent on temperature
and s is an exponent function of temperature (0<s<1). The exponent s represents the degree of
interaction between mobile ions and the environments surrounding them.
The frequency variation of the ac conductivity, σac(ω) for 2,5-DACM compound at T=342K is
shown in Fig. 13 (inset). The conductivity results are fitted by the above equation (eq. 5) and
presented with solid line. At high frequencies, the conductivity shows dispersion pattern which is
a characteristic of ωs, whereas the frequency independent plateau region at low frequencies
corresponds to DC conductivity of the material.
The hopping frequency ωh of the charge carriers which represents the crossover frequency from
dc to dispersive conductivity region at ω > ωh can be calculated by the following expression:
ωꢙ = ꢈσꢚꢛꢋꢅ/ꢘ
(6)
ꢜ
The hopping frequency is temperature dependent and it obeys the Arrhenius equation:
ꢑ
ꢃ
ωꢙ = ωꢐexp ꢈ− ꢒ ꢔꢋ
(7)
ꢓ
where ω0 is the pre-exponential of hopping frequency and Ea the activation energy for the
hopping frequency [25]. Fig. 14 shows the variation of Ln(ωh) versus (1000/T). It shows an
Arrhenius-type behavior. The activation energies obtained from straight’s slopes line segments
are EaI=3.14 eV, EaII=4.74 eV and EaIV=0.75 eV.
Conductivity master curves for several temperatures for the title compound are shown in Fig. 13.
AC conductivity can be expressed by a function with a non-dimensional frequency by the
following scaling from:
σ
(ω) = F ꢈωω ꢋ
(8)
ꢃꢛ
σ
ꢚꢛ
ꢝ
In this scaling process, the ac conductivity axis is scaled by σdc and the frequency axis by ωh, σdc
is a parameter obtained from the fit of the conductivity as a function of angular frequency.
Scaling the conductivity spectra in this way at different temperatures merges on a single curve
which implies that the relaxation dynamics of charge carriers is independent of temperature.
3.7.2. DC Conductivity
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DC conductivity data are plotted in Arrhenius format as Ln(σdcT) versus (1000/T) (Fig. 10)
described by:
ꢑ
ꢃ
σꢗꢖT = Bexp ꢈ− ꢒ ꢔꢋ
(9)
ꢓ
where B is a pre-exponential factor, kB the Boltzmann’s constant and Ea is the thermal activation
energy of ion migration.
Fig. 10 identified four regions. Regions (I), (II) and (IV) correspond to an Arrhenius type
–
behavior. The activation energy calculated from linear fit to the data points is Ea I = 2.82 eV in the
hydrated compound, Ea II = 0.48 eV and Ea IV = 0.62 eV for dehydrated compound. Activation
energies values obtained from the analysis of modulus, conductivity data and hopping frequency
are close for phases I and IV. These results indicate that the transport is through ion hopping
mechanism in the investigated sample at these phases. In order to determine which ion is
responsible for the hopping mechanism of the title compound in its hydrated form, let’s have a
look into the structure arrangement. So, we have presented in Fig. 15 the atomic arrangement
projection along the a axis of the title compound. From the single crystal XRD data, the crystal
packing can be described as a typical layered organization. A projection of such a layer shows
that the Cl- anions are linked to the water molecules by O—H···Cl hydrogen bonds to form
infinite corrugated chains along the b direction (Fig. 15). These chains are themselves connected
+
via N—H···O and N—H···Cl hydrogen bonds originating from NH3 groups, so as to built
inorganic layers spreading around the (a,b) plane [12]. From the structure disposition and since
the chloride anion is 3.48 times lighter than the 2,5-dimethylanilinium cation, we can deduce that
the conductivity in this sample in its hydrated form is may be assured by the contribution of the
movements of anionic parts along the b-axis.
4. Conclusion
The 2,5-dimethylanilinium chloride monohydrate compound was studied at various temperatures
with the help of DSC, XRD, Raman and impedance measurements. DSC curve shows an intense
peak at about 355K which corresponds to water escape from the crystal. After heating above
355K the compound dehydrates and the crystal space group changes from non-centrosymmetric
(P21) to centrosymmetric (P21/m) space group, while maintaining the same system. Impedance
spectroscopy was used to characterize the electrical and dielectrical behavior of this material in
its both hydrated and dehydrated forms as a function of frequency at various temperatures. It
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makes clear that during the heating process and near the dehydration temperature, the radius
semicircles of the Cole-Cole spectra increase significantly, which corresponds to the loss of
water, reflecting the perturbation of the ion transport mechanism at this temperature range. The
near value of activation energies obtained from the modulus, conductivity data and hopping
frequency confirm that the transport is through ion hopping mechanism in the sample for phases I
and IV. From the hydrated compound structure, the transport is probably dominated by the
motion of anionic parts “Cl-” along the “b” direction.
Acknowledgment
The authors thank the Unity of Raman Spectroscopy at Faculty of Sciences of Sfax, Tunisia for
Raman measurements.
.
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0
-2
-4
-6
endo
-0,4
-0,6
-0,8
355
403
392
385
390
395
T(K)
400
405
280
300
320
340
360
380
400
420
T(K)
Figure 1: Differential scanning calorimetry diagram of 2,5-DACM compound.

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Figure 2: X-ray powder diffraction pattern of 2,5-DAC.

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Figure 3: X-Ray powder diffraction of 2,5-DACM and 2,5-DAC measured at room
temperature.

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υ
(OH)
358K
328K
298K
3600
3400
3200
3000
2800
wavenumber (cm-1)
Figure 4: Temperature evolution of the Raman spectra in the 3600-2700 cm-1 wavenumbers
range.

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200 Hz
532 Hz
105
104
103
102
101
1040 Hz
4950 Hz
9670 Hz
46 kHz
113 kHz
220 kHz
εr'
280
300
320
340
360
380
400
420
440
T(K)
200 Hz
532 Hz
106
105
104
103
102
101
100
10-1
10-2
1040 Hz
4950 Hz
9670 Hz
46 kHz
113 kHz
220 kHz
εr''
280
300
320
340
360
380
400
420
440
T(K)
Figure 5: Temperature dependence of ε’r and ε”r for 2,5-DACM before and after dehydration.

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Figure 6: The Cole–Cole plots at different temperature of 2,5-DACM before and after
dehydration.

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Figure 7: Angular frequency dependence of M′ at some temperatures of 2,5-DACM
compound.

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1.0
0.8
0.6
0.4
0.2
0.0
342K
fit
β = 0.7717
103
104
105
106
107
ω
(rad.s-1)
Figure 8: Angular frequency dependence of M" at T=342K for 2,5-DACM.

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1.0
0.8
0.6
0.4
0.2
0.0
322K
332K
342K
352K
362K
372K
102
103
104
105
106
107
ω
(rad s-1)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
392K
402K
412K
422K
103
104
105
106
107
ω
(rad s-1)
Figure 9: Angular frequency dependence of M" at several temperatures.

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0
-5
24
20
16
12
8
Ln(
σ.T)
Ln(fp)
-10
-15
-20
-25
E=0.62 eV
E=2.82 eV
E=0.48 eV
E=2.76 eV
E=0.65 eV
4
III
II
I
IV
0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1000/T (K-1)
Figure 10: Dependence of Ln(σdcT) and Ln(fp) on reciprocal temperature.

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T= 342 K
0.020
0.015
0.010
0.005
0.000
M''
0.00
0.01
0.02
0.03
0.04
0.05
M'
Figure 11: The Variation of M″ as a function of M′ at T=342K.

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Figure 12: The M″ and (-Z″) spectra at T= 342 K of 2,5-DACM.

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Figure 13: Conductivity master plot in the 292-422 K temperature range.
(inset): Angular frequency dependence of the AC conductivity at T=342K fitted using
Jonscher’s law.

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20
15
10
5
E=3,14 eV
E=4,74 eV
E=0,75 eV
0
II
I
IV III
-5
2.4
2.6
2.8
3.0
3.2
3.4
1000/T (K-1)
Figure 14: Temperature dependence of the hopping frequency.

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Figure 15: Projection along the a axis of the atomic arrangement of 2,5-DACM [12].

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Highlights
►The calorimetric study of 2,5-DACM has revealed three endothermic peaks.
► The dielectrical properties were studied using the impedance measurements.
► The X ray powder diffraction has been performed.
► The mobility of the charge carriers was reported and discussed.