Indian Journal of Chemistry
Vol. 48A, April 2009, pp. 512-519
Structure and hydrogen bonding in binary mixtures of N,N-dimethylformamide
with some dipolar aprotic and protic solvents by dielectric characterization
R J Sengwa*, Vinita Khatri & Sonu Sankhla
Dielectric Research Laboratory, Department of Physics, JNV University, Jodhpur 342 005, India
Email: [email protected]
Received 28 July 2008; revised and accepted 24 February 2009
The low frequency limit static dielectric constant, optical frequency dielectric constant, excess dielectric constant, and
the Kirkwood correlation factor of the binary mixtures of N,N-dimethylformamide with water, ethyl alcohol, ethylene
glycol, glycerol, dimethyl sulphoxide, acetone and 1,4-dioxane over the entire composition range are investigated at 30oC.
The values of these functions emphasize strong N,N-dimethylformamide−water hydrogen bond interaction due to breaking
of water tetrahedral structure, whereas the N,N-dimethylformamide−alcohols complexations show a strong hydroxyl group
number dependence with enhancement in the structural ordering of alcohols. The dielectric functions of
N,N-dimethylformamide−water and N,N-dimethylformamide−1,4-dioxane mixtures reveal the similarity in the complexes,
although the N,N-dimethylformamide−water interactions are stronger. The excess functions confirm the weak hydrogen
bond interactions of N,N-dimethylformamide with dimethyl sulphoxide and acetone. The pronounced deviation in the
excess functions of the studied mixtures confirms the formation of a stable adduct of stoichiometric ratio 1:1 in these
systems.
Keywords: Hydrogen bonds, Solvent effects, Aprotic solvents, Protic solvents, Dielectric functions, Excess properties
IPC Code: Int. Cl.8 G01N31/00
The binary mixtures of dipolar aprotic and protic
solvents have current interest in biological, chemical,
pharmaceutical, technological, and laboratory
applications because mixed solvents manifest
different physicochemical properties as compared to
those of the pure constituents of the mixture1-6.
Amongst the physicochemical properties, dielectric
constant of the mixed solvents, which is sensitive to
molecular interactions specially H-bond formation,
enhances or controls most of the applications, and
hence precise dielectric characterization is important.
The dipolar solvent-solvent interactions are more
complex in mixed solvents than in their pure form,
and the characterization of these interactions is
challenging in the liquid systems in which the
molecules of mixture constituents have a large
number of possible atom-atom interactions.
Among the different solvents, the H-bonded
structures of amides and their mixtures, specially with
water and aliphatic alcohols, have an important role in
biological systems, because the hydrogen bonds are
important as a force governing the structure and
dynamics of chemical and biological systems. In the
case of the amides, the dipolar aprotic solvent,
N,N-dimethylformamide (DMF) is unable to engage
in N–H···O=C hydrogen bonding with dialkyl
substitution at nitrogen. However, with water and
alcohols, DMF forms the C–H···O and C=O···H type
H-bond interactions, which result in the complex
H-bonded network structures of these systems7,8.
DMF is also able to form the H-bonded network
structures with dipolar aprotic solvents. The dielectric
measurements have great potential for studying such
H-bonded network structures with dipolar ordering,
the strength of complexations and the stoichiometric
ratio of stable adduct, and also their dynamics in the
mixtures. Several investigators have studied the
structural properties of pure DMF and DMF mixed
dipolar solvents by dielectric measurements at fixed
frequency or over a broadband microwave frequency
region9–18. More precise dielectric studies on DMF
mixed solvents over the entire range of mole fractions
are needed to enhance their applications.
In continuation of the dielectric studies on
amide−solvent mixtures8–18, we report herein the
dielectric constants of DMF mixed with water, and
glass forming liquids, i.e., alcohols, and some
common polar aprotic bio-solvents over the entire
SENGWA et al.: DIELECTRIC CHARACTERIZATION OF H-BONDS IN BINARY MIXTURES
mole fraction range at 30oC. These investigations
provide information on dipolar ordering, strength of
H-bond complexation and stoichiometric ratio
corresponding to stable adducts in the DMF-dipolar
solvent mixtures with a view to gain deeper insight
into the mixed solvents H-bonded network structures.
The effect of the number of hydroxyl groups of the
cosolvent19,20 and the type of aprotic solvent for the
formation of complexes in the DMF mixtures have
also been studied on the basis of the comparative
dielectric parameters of the mixed solvents.
measurement of the cell without and with sample. The
measurement accuracy of the cell was ±0.3%,
estimated by calibration of the cell with the standard
liquids by using their accurate dielectric constant
values from the literature. The high frequency limit
dielectric constant, ε∞, was taken as the square of the
refractive index nD, which was measured with an
Abbe refractometer at the wavelength of sodium-D
light. The maximum measurement error in ε∞ values
was ±0.02%. All measurements were made at 30oC
and the temperature was controlled thermostatically
using the microprocessor based Thermo–Haake DC10
controller. The evaluated ε0 values of pure liquids
alongwith literature ε0 values6,10,17,21–27 are recorded in
Table 1, and the ε0m values of the mixtures against
DMF mole fraction xDMF are recorded in Table 2.
Materials and Methods
Grade reagent N,N-dimethylformamide (DMF) was
purchased from Loba Chemie, ethyl alcohol (EA) and
glycerol (Gly) were purchased from S D Fine
Chemicals, ethylene glycol (EG) and 1,4-dioxane
(Dx) were purchased from E Merck, and dimethyl
sulphoxide (DMSO) and acetone (Ac) were purchased
from Qualigens Fine Chemicals of India. Doubly
distilled deionized water (W) was also used as one of
the solvents. Binary mixtures of DMF with other
solvents were prepared at 16 volume concentration
over the entire composition range at room
temperature. Simultaneously, the mole fractions of the
mixture constituents were determined by weight
measurements.
The values of static dielectric constant, ε0, of pure
solvents and ε0m, of the ‘DMF-cosolvents’ binary
mixtures were determined by using capacitive
measurement method with a short compensation at
1 MHz. Agilent 4284A precision LCR meter and a
four terminal cell Agilent 16542A liquid dielectric
test fixture were used for the capacitance
Data analysis
The excess static dielectric constant ε 0E , for binary
mixture, is defined as9
ε 0E = (ε 0m − ε ∞ m ) − [(ε 01 − ε ∞1 ) x1 + (ε 02 − ε ∞2 ) x2 ]
… (1)
where x is the mole fraction and subscripts m, 1 and 2
represent the binary mixture and components 1 and 2
of the binary mixture, respectively. ε 0E is the
deviation evaluated after subtracting the effect of high
frequency limit dielectric constant ε∞ from ε0 values.
The evaluated ε 0E values along with ε0m and ε∞m
values of the binary mixtures versus DMF mole
fraction, xDMF are plotted in Figs 1 and 2.
Table 1―Experimental and literature values of static dielectric constant (ε0), dipole moment (µ), and
evaluated values of the Kirkwood correlation factor (g) of pure solvents at 30oC. [The xDMF values are the mole
fraction of N,N-dimethylformamide at maximum value of excess dielectric constant εE0 for the binary mixtures
of N,N-dimethylformamide with the corresponding solvent listed in first column]
ε0
Solvent
Expt
N,N-dimethylformamide
Water
Ethyl alcohol
Ethylene glycol
Glycerol
Dimethylsulphoxide
Acetone
1,4-Dioxane
36.55
76.53
23.87
39.84
41.17
46.05
20.05
2.26
a
µ (D)
g
xDMF
ε0E
4.09l
1.82m
1.82m
2.38n
2.56i
4.34l
2.93o
0.45p
1.01
2.88
2.73
2.35
2.61
0.99
1.11
1.01
0.46
0.46
0.46
0.54
0.53
0.53
0.46
−8.90
0.70
2.85
4.51
1.02
0.57
−3.34
Lit.
37.60a, 37.65b (25oC)
76.39c, 76.60d
23.60e, 23.87f
39.95g, 40.23h (25oC)
42.5i (25oC)
46.8e, j (25oC)
20.70e (25oC)
2.20k, 3.19g
Ref 10, bRef 17, cRef 21, dRef 22, eRef 6, fRef 23, gRef 24, hRef 25, iRef 26 , jRef 10, kRef 27, lRef 15,
Ref 28, nRef 29, oRef 30, pRef 31
m
513
INDIAN J CHEM, SEC A, APRIL 2009
514
Table 2―Static dielectric constant (ε0m) of the binary mixtures of DMF-cosolvent at various
mole fractions of DMF (xDMF) at 30oC
xDMF
DMF-W
0.000
0.016
0.034
0.055
0.078
0.104
0.134
0.169
0.209
0.258
0.317
0.389
0.481
0.601
0.764
1.000
ε0m
76.53
75.12
73.62
72.50
70.80
68.92
67.00
64.53
61.46
59.14
55.83
52.43
48.57
44.55
40.59
36.55
DMF-DMSO
0.000
46.05
0.062
45.98
0.125
45.66
0.189
45.03
0.253
44.32
0.317
43.99
0.383
43.38
0.449
42.99
0.515
42.16
0.582
41.60
0.650
40.85
0.719
40.08
0.788
39.39
0.858
38.55
0.929
37.75
1.000
36.55
xDMF
ε0m
DMF-EA
xDMF
ε0m
DMF-EG
xDMF
ε0m
DMF-Gly
0.000
0.051
0.104
0.158
0.215
0.274
0.334
0.397
0.463
0.530
0.601
0.674
0.751
0.830
0.913
1.000
0.000
0.049
0.099
0.152
0.207
0.265
0.324
0.386
0.451
0.519
0.590
0.664
0.742
0.824
0.910
1.000
0.000
0.063
0.127
0.191
0.256
0.321
0.386
0.453
0.519
0.586
0.654
0.722
0.791
0.860
0.930
1.000
23.87
24.65
25.39
26.22
27.05
27.80
28.78
29.63
30.57
31.39
32.16
32.98
33.96
34.72
35.57
36.55
DMF-Ac
0.000
20.05
0.063
21.32
0.127
22.50
0.192
23.52
0.256
24.72
0.322
25.78
0.387
27.03
0.453
28.07
0.520
29.16
0.587
30.34
0.655
31.45
0.723
32.49
0.791
33.62
0.860
34.65
0.930
35.72
1.000
36.55
The Kirkwood correlation factor, g, of the pure
liquid is determined by the following expression28,
(ε 0 − ε ∞ ) (2ε 0 + ε ∞ )
4π N d
g µ2 =
9kT M
ε 0 (ε ∞ + 2) 2
… (2)
where µ is the dipole moment, d the density of liquid
at temperature T, M the molecular weight, k the
Boltzmann constant and N the Avogadro’s number.
The µ values15,26,28–31, used in Eq. (2), along with the
evaluated g values of the pure solvents, are listed in
Table 1. In the present study we have used ε∞= nD2 ,
and the literature µ values of the dipolar solvents
(gas-phase or in non-polar solvent) for the evaluation
of pure polar solvent g values.
39.84
40.23
40.71
41.01
41.24
41.35
41.42
41.38
41.23
40.98
40.56
39.93
39.36
38.57
37.69
36.55
41.17
41.62
41.78
42.85
43.32
43.47
43.54
43.48
43.36
43.11
42.42
41.59
40.55
39.32
38.37
36.55
DMF-Dx
0.000
2.26
0.074
3.78
0.146
5.38
0.218
7.46
0.288
9.51
0.358
11.45
0.426
13.43
0.494
15.82
0.560
18.32
0.626
20.84
0.690
23.26
0.754
25.76
0.817
28.47
0.879
31.00
0.940
33.65
1.000
36.55
In binary mixtures of polar solvents, the effective
averaged angular Kirkwood correlation factor, geff, of
unlike molecules is evaluated from the modified
Kirkwood equation32,33 based on volume fraction
mixture law

µ2 d
4 π N  µ12 d1
φ1 + 2 2 φ2  g eff

9 k T  M1
M2

=
(ε 0m − ε ∞m ) (2ε 0m + ε ∞m )
ε 0m (ε ∞m + 2) 2
… (3)
where φ1 and φ2 are the volume fractions of liquids 1
and 2, respectively, and ε0m and ε∞m are the measured
values of static dielectric constant and high frequency
limiting dielectric constant of the binary mixture. For
SENGWA et al.: DIELECTRIC CHARACTERIZATION OF H-BONDS IN BINARY MIXTURES
75
W
EA
EG
Gly
45
40
30
ε0m
ε0m
(a)
(a)
60
515
20
DMSO
Ac
Dx
10
30
0
2.2
(b)
(b)
2.1
2.1
W
EA
EG
Gly
1.9
1.8
ε∞m
ε∞m
2.0
1
(c)
2.5
(c)
0
0.0
-1
E
-2.5
W
EA
EG
Gly
-5.0
-7.5
0.0
0.2
0.4
0.6
0.8
1.0
xDMF
of DMF (xDMF) for DMF-cosolvents. [(○) DMF-W; () DMF-EA;
(∆) DMF-EG; and (∇) DMF-Gly binary mixtures at 30oC. In (a)
and (b), lines are smooth joining through the data points, whereas
in (c) lines are non-linear fits. For clarity, error bars are not
indicated. Error bars are smaller than the size of the symbols.
Dotted horizontal line in (c) is to see the deviation].
φ1 = 1 and φ2 = 0, Eq. (3) reduces to Eq. (2) with the g
value of pure liquid 1, and vice versa. The evaluated
geff values of the DMF−cosolvent binary mixtures are
plotted against the volume fraction of DMF, φDMF, in
Figs 3 and 4.
Assuming that the Kirkwood correlation factor for
the molecules of liquids 1 and 2 are g1 and g2 in the
mixture, and contribute to geff values proportional to
their pure liquid g values, then the Kirkwood
correlation factor Eq. (2), for the binary mixture on
the basis of simple volume mixture law, can be
written as11:

µ2 d g
4 π N  µ12 d1 g1
φ1 + 2 2 2 φ2  g f

9 k T  M1
M2

(ε 0 m − ε ∞ m ) (2ε 0 m + ε ∞ m )
ε 0m (ε ∞ m + 2) 2
-2
DMSO
Ac
Dx
-3
-4
0.0
0.2
0.4
0.6
0.8
1.0
xDMF
Fig. 1―Plots of (a) ε0m, (b) ε∞m, and, (c) ε 0E against mole fraction
=
ε0
E
DMSO
Ac
Dx
1.9
1.8
5.0
ε0
2.0
… (4)
Fig. 2―Plots of (a) ε0m, (b) ε∞m, and, (c) ε 0E against mole fraction
of DMF (xDMF) for DMF-cosolvents at 30oC. [() DMF-DMSO;
(∆) DMF-Ac; and (○) DMF-Dx].
where gf is the corrective Kirkwood correlation factor
(also called deviation in Kirkwood correlation
parameter) for a binary mixture. The evaluated gf
values of the DMF-cosolvent binary mixtures, at
different volume fractions of DMF, φDMF, are plotted
in Figs 3 and 4.
Results and Discussion
Table 1 shows that ε0 values of the pure polar
solvents are in good agreement with the reported
literature values. The plots of ε0m and ε∞m values
against DMF mole fraction, xDMF (Figs 1 and 2) are
more or less non-linear, and confirm the molecular
interactions between the DMF and cosolvent
molecules of the binary mixtures in the present study.
The non-linear behaviour indicates that the use of
‘approximated’ dielectric constant values by simple
molar additive equations will lead to erroneous results
in the dielectric constant dependent applications of the
mixed solvents.
INDIAN J CHEM, SEC A, APRIL 2009
516
2.80
1.05
2.45
W
EA
EG
Gly
1.00
geff
2.10
geff
(a)
1.10
(a)
1.75
0.95
0.90
1.40
0.85
1.05
0.80
1.12
DMSO
Ac
Dx
1.05
(b)
(b)
1.10
W
EA
EG
Gly
1.08
1.00
0.95
1.04
gf
gf
1.06
0.90
DMSO
Ac
Dx
1.02
0.85
1.00
0.98
0.80
0.0
0.2
0.4
0.6
0.8
1.0
0.0
φDMF
0.2
0.4
0.6
0.8
1.0
φDMF
Fig. 3―Plots of (a) geff, and, (b) gf against volume fractions of
DMF, (φDMF) for DMF-cosolvents at 30oC. [(○) DMF-W; ()
DMF-EA; (∆) DMF-EG; (∇) DMF-Gly].
Fig. 4―Plots of (a) geff, and, (b) gf against volume fractions of
DMF (φDMF) for DMF-cosolvents at 30oC. [() DMF-DMSO; (∆)
DMF-Ac; (○) DMF-Dx].
Figure 1 shows that the concentration dependent
ε0m values of the DMF-dipolar protic solvents deviate
from ideality, which confirms the formation of
complexes between DMF and dipolar protic solvent
molecules mainly through H-bonds. The non-linear
behaviour of the ε∞m values also confirms the change
in the electronic polarization caused by unlike
molecular interactions in these solvents29,30. The nonzero values of the excess dielectric constant ε 0E of the
mixed solvents is the experimental evidence of the
formation of hydrogen bond complexes and the
strength of H-bond connectivity between the unlike
molecules in the mixed solvents9,11,17,18. The ε 0E
values of DMF mixed with W, EA, EG and Gly were
found to be non-zero over the entire concentration
range and these values range from the large negative
for DMF-W to positive for DMF-alcohols. The
change in structures of the mixtures constituents due
to variation in dipolar alignments and subsequent
H-bond formation is responsible for the divergent
signs for the observed ε 0E values of these systems9,18.
The correlation of the molar concentration of DMF,
xDMF to pronounced maximum positive or negative
ε 0E values is evaluated by fitting the ε 0E (xDMF) data to
the polynomial equation using Origin® non-linear
curve fitting tool. The xDMF values alongwith the
maximum ε 0E values are recorded in Table 1.
The magnitude of concentration dependent negative
ε values of DMF-W mixtures and its maximum
value position on the mole fraction scale of DMF i.e.,
xDMF ~ 0.46 (Table 1) is in good agreement with the
results reported earlier9. The concentration of
maximum deviation in ε 0E values corresponds to the
stoichiometric ratio of stable adduct formation with
comparatively higher range dipolar ordering9,17,18,24.
The maximum deviation in ε 0E values of DMF-W at
xDMF ~ 0.46 is attributed to the formation of 1:1
H-bonded complexes between DMF and W moleE
0
SENGWA et al.: DIELECTRIC CHARACTERIZATION OF H-BONDS IN BINARY MIXTURES
cules, probably through C=O···H, which also supports
the conclusion drawn from the thermodynamical
properties7, FTIR34,35 measurements, and molecular
simulations study8,36,37 on DMF-W mixtures. The
negative ε 0E values of DMF-W mixtures suggest that
the DMF molecules coalesce and break the tetrahedral
structure of water, resulting in decrease in effective
number of dipoles contributing to the polarization of
DMF-W mixtures.
The positive ε 0E values of DMF-alcohols mixtures
over the entire concentration range are in the order:
DMF-Gly > DMF−EG > DMF-EA as shown in Fig. 1
and Table 1, which is according to the descending
number of −OH groups of the aliphatic alcohol
molecules. The positive ε 0E values of DMF−alcohols
suggest that the DMF molecules are built-in via
H-bonding into the H-bonded structures of pure
alcohols, which results in the net increase in effective
number of dipoles contributing to the mixtures
dielectric polarization. The EA molecules have
H-bonded polymeric chain-like structures with a switch
over type of molecular reorientation throughout the
chain38. The comparative small positive ε 0E values of
DMF-EA mixtures suggest that the C–H···O hydrogen
bonds are formed due to switching of H-bonded EA
molecules. EG molecules have −OH groups at both
ends with a gauche conformation, which is a result of
intramolecular hydrogen bonding, and probably
because only one −OH group of EG molecule
can interact with neighbouring molecules through
H-bond29. The comparative increase in ε 0E values of
DMF-EG mixture as compared to the DMF-EA
mixture suggests that the DMF molecules built-in via
H-bonding into EG structure results in the breakup of
EG intramolecular H-bond association. This promotes
the parallel dipolar ordering and net increase in
effective number of dipoles contributing to the
polarization of the DMF-EG mixture. The higher
positive ε 0E values of DMF-Gly confirm the
comparative large increase in the net effective number
of dipoles due to DMF complexation with Gly. The
DMF molar concentrations corresponding to a
pronounced maximum value of excess dielectric
constant ε 0E (max) is nearly 0.5 (Table 1) indicating the
formation of a stable adducts in the stoichiometric ratio
1:1 in all the investigated mixtures.
517
The non-linear behaviour of ε∞m values of DMFpolar protic solvent mixtures (Fig. 1) indicates the
change in electronic polarization of the mixtures
constituents, which is caused by H-bond
complexation28,29. The magnitude of non-linearity of
the ε∞m values of DMF-polar protic solvents mixtures
has the same order as ε0m values. This suggests that
the change in electronic polarization due to unlike
molecules H-bond interactions is governed by the
strength of H-bond complexations in these mixtures.
Figure 2 shows the ε0m and ε∞m values of DMF
mixtures with polar aprotic solvents i.e., DMSO, Ac
and Dx. In the case of DMF-Dx mixtures, the
deviation in ε0m from ideality is very small and does
not appear in the ε0m(xDMF) plots of DMF-DMSO and
DMF-Ac mixtures. The ε 0E values of DMF-Dx
mixtures are comparatively large and negative over
the entire concentration range, whereas these values
are positive but comparatively small in the case of
DMF-DMSO and DMF-Ac mixtures. From
comparative results, it is inferred that in the DMF-Dx
mixture, the H-bonded interactions between unlike
molecules are comparative strong whereas DMF-Ac
and DMF-DMSO have very weak interactions. From
these results, it can be concluded that in the DMF-Ac
and DMF-DMSO mixtures, the mixture constituents
mix almost ideally. However, the significant negative
ε 0E values of DMF-Dx mixture indicate that this is
always not true for mixtures of all polar aprotic
solvents. Further, in the case of DMF-Dx, the xDMF
corresponds to a pronounced maximum ε 0E value at
~ 0.46, suggesting the formation of a stable adduct of
stoichiometric ratio 1:1, which is exactly same as in
the case of DMF-W mixture. The magnitude of ε 0E of
DMF-Dx mixture is about half of that of ε 0E of
DMF-W mixtures, which suggests that the DMF-W
H-bond complexes are about two times stronger than
the H-bond complexes formed between DMF and Dx
molecules in the DMF-Dx mixture. The H-bond
interaction strength of the DMF with DMSO
molecules is very weak over the entire molar
concentration range and has almost the same ε 0E
values over a wide middle concentration range
(Fig. 2). The almost linear behaviour of ε∞m(xDMF)
values of the DMF-polar aprotic solvent mixtures
(Fig. 2b) confirm that the H-bond complexation
518
INDIAN J CHEM, SEC A, APRIL 2009
between these aprotic solvents does not affect or has a
very small effect on their electronic polarization.
Table 1 shows that the g values of the pure liquids
EA, EG, Gly and W are much higher than unity and
confirm the existence of H-bonded multimers with
parallel dipolar alignments28. The order of g values of
the alcohols is: EA > Gly > EG, and these values are
lower than the g value of the pure water molecules.
The higher magnitude of g value confirms the
stronger strength of H-bonded structures with large
range dipolar ordering of these pure liquid alcohols39.
The unity g value of DMF confirms its non selfassociative molecular behaviour in the pure liquid
state. Monohydric alcohol (EA) has a comparative
higher g value and the dihydric alcohol (EG) has
lower g values. The order of g values of EA, EG and
Gly are not according to the number of −OH groups
in these molecules, and may be because of the nearly
equal ε0 values of EG and Gly (Table 1). However,
the ε 0E values of DMF-alcohols increase with the
increase of the number of −OH groups, which
suggests that the H-bonded molecular structures of the
DMF with alcohols, are governed by the number of
−OH groups of the alcohol molecule. Figure 3 shows
that the geff values of the mixed solvents vary in the
range of g values of the mixtures′ constituents.
However, the non-linear behaviour of geff values of
DMF-W and DMF-alcohols mixtures confirms the
significant change in dipolar ordering in these mixed
solvents due to hetero molecules H−bond interactions.
Further, it is found that DMF-W mixture has positive
deviation in geff values, whereas all the DMF-alcohols
mixtures have negative deviations, which also
supports the change in sign of the deviation of ε 0E
values of DMF-W and DMF-alcohols mixtures from
their ideality. In mixtures the deviation of gf values
from unity also confirms that the structures of the
mixture constituents change significantly in the
investigated mixtures. The unity gf values of the
DMF-EA indicate that the H-bond interaction
between DMF and EA molecules changes their pure
liquid structures in such a way that the net structural
variation compensates the dipolar ordering in the
mixture.
Similar to DMF, the pure polar aprotic solvents
DMSO and Dx also have g values nearly unity (Table
1), which indicates the absence of H-bond molecular
interactions in their pure liquid state. The g value of
Ac is slightly higher than unity, which indicates the
presence of very weak H-bonded interactions between
the Ac molecules in their pure liquid state. The
non-linear behaviour of geff values and the deviation
of gf values from unity also support the formation of
H-bonded complexes between DMF and the polar
aprotic solvents used in the present study. Further, the
magnitude of gf values shows that the DMF-Dx has
stronger H-bonded structures as compared to the
DMF-Ac and DMF-DMSO complexations.
Conclusions
The precisely measured concentration dependent
values of dielectric parameters of DMF mixed with
different polar solvents have been reported. The
dielectric study attempts to explain the hydrogen bond
formation and the variation in pure solvent structures
over the entire concentration range in DMF-polar
protic and DMF-polar aprotic solvents. The number
of −OH groups of aliphatic alcohols as well as water
molecules has an influence on the H-bond complexes
properties and dipolar ordering of DMF molecules.
The dielectric parameters values of DMF with aprotic
polar solvents show that the DMF molecules form
weak hydrogen bonds with Ac and DMSO, but the
strength of the H-bonds formation of DMF with Dx is
comparatively strong. Further, the H-bond complexes
between DMF and Dx molecules are similar to the
DMF and water molecules complexes, but the
strength of H-bond in the DMF-Dx complexes is
nearly half of that in the DMF-W complexes.
Acknowledgement
The authors thank the University Grants
Commission, New Delhi, for a project grant under
which the work was carried out and also the
Department of Science and Technology, Government
of India, New Delhi, for providing experimental
facilities.
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