Indian Journal of Pure & Applied Physics
Vol. 46, December 2008, pp. 852-856
Molecular interaction study of two aliphatic alcohols with cyclohexane
R Thiyagarajan & L Palaniappan*
Department of Physics, SRM Engineering College, Chennai, Tamil Nadu
*School of Physics, Universiti Sains Malaysia, Pulau Penang 11800, Malaysia
E-mail: [email protected]
Received 11 July 2007, revised 24 September 2008; accepted 8 October 2008
Sound velocity, density and viscosity values have been measured at 303 K in the two binary systems of cyclohexane +
ethanol, 1-propanol. From these data, acoustical parameters such as adiabatic compressibility, free length, free volume and
internal pressure have been estimated using the standard relations. The results are interpreted in terms of molecular
interaction between the components of the mixtures. Observed excess value in all the mixture indicates that the molecular
symmetry existing in the system is highly disturbed by the polar alcohol molecules and dispersive type interactions are
existing in the systems.
Keywords: Ultrasonic velocity, Acoustic parameters, Dipolar and dispersive interactions
1 Introduction
In many industrial applications liquid mixtures,
rather than single component liquid system, are used
in processing and product formulations1,2. Liquid
mixtures consisting of polar and non-polar
components are of considerable importance in
industries such as petrochemical, pharmaceutical and
dye. The formation and destruction of azeotropes in
petrochemical industries3, the biological activity of
drug molecules4 and the activation energy of the
metabolic process5 basically depend on the type and
strength of the intermolecular interactions. Various
methods are available to identify these interactions
and ultrasonic study is one such more reliable and
commonly used study.
Thermodynamic and transport properties of liquid
mixtures have been extensively used6,7 to study the
departure of a real liquid mixture behaviour from
ideality. Further, these properties have been widely
used to study the intermolecular interactions between
the various species present in the mixture8,9.
As alcohols are highly polar, they can easily form
azeotropes of binary complexes. Ethanol is reported10
to form azeotropic mixture with cyclohexane at
337.8 K whereas 1-propanol at 347.7 K and the
separation is a major task in petrochemical industries.
In the present work, the measurement of ultrasonic
velocity, density and viscosity and computation of
related parameters at 303 K in two non-ideal binary
mixtures of cyclohexane+ethanol and cyclohexane+1propanol have been studied.
2 Experimental Details
The mixtures of various concentrations in mole
fraction were prepared by taking purified AR grade
samples at 303 K.
The ultrasonic velocity (U) in liquid mixtures have
been measured using an ultrasonic interferometer
(Mittal type, Model F-81) working at 2 MHz
frequency with an accuracy of ± 0.1 ms–1. The density
(ρ) and viscosity (η) are measured using a
pycknometer and an Ostwald’s viscometer
respectively with an accuracy of 3 parts in 105 for
density and 0.001 Nsm–2 for viscosity.
Using the measured data, the acoustical parameters
such as adiabatic compressibility (β), free length (Lf),
free volume (Vf) and internal pressure (πi) and their
excess parameters have been calculated using the
following standard expressions11:
β = (U 2ρ) –1
…(1)
L f = KT β½
…(2)
M U
V f =  eff 
 ηk 
 kη 
πi = bRT  
U 
3
2
…(3)
1
2
 23
ρ
 76
 M eff




…(4)
THIYAGARAJAN & PALANIAPPAN: MOLECULAR INTERACTION STUDY OF ALIPHATIC ALCOHOLS
A E = Aexp – Aid
…(5)
Aid = ∑ xi Ai
…(6)
where, KT is the temperature dependent constant
having a value 199.53×10–8 in MKS system, k is a
constant equal to 4.28×109 in MKS system,
independent of temperature for all liquids, b is the
cubical packing fraction taken as 2 for all the liquids,
R is the universal gas constant, T is the experimental
temperature, M eff = ∑ xi mi where, x is the mole
fraction and m is the molecular weight of ith
component and AE stands for excess property of any
given parameter, Aexp is the experimental value and Aid
is the ideal value.
3 Results and Discussion
The measured values of density (ρ), viscosity (η)
and sound velocity (U) for the two binary systems of
cyclohexane + ethanol and cyclohexane + 1-propanol
at 303 K are presented in Table 1.
The perusal of this Table shows that the density and
viscosity shows decreasing trend in both systems with
the increase in mole fraction of cyclohexane. As
regards sound velocity, ethanol shows a continuous
increase, whereas in 1-propanol system, it initially
decreases, exhibit a minimum at 0.2 mole fraction and
then continually increasing with increase in mole
fraction of cyclohexane.
As per Edward Peters12, a higher density or
viscosity of a component molecule is a reflection of
higher intramolecular interactions. So, among the
853
three components taken here, 1-propanol is having
higher intra interactions. A molecule having higher
intra interaction is also expected to show high degree
of intermolecular interactions. Thus among the two
binaries, in any given mole fraction, 1-propanol
records more velocity that reflects the existence of
more intermolecular interactions.
Comparing the two alcohols, ethanol is a good
solvent that can dissolve both the polar and nonpolar
components. The hydrophilic –OH group of ethanol
can dissolve the polar whereas the short hydrophobic
hydrocarbon group can dissolve the nonpolar13. But
1-propanol is not a good solvent as ethanol. As
regards cyclohexane, it exhibits conformation and
exists either in rigid chair form (minimum potential
energy) or in boat form (maximum potential energy)
and also in skew-boat form14, all can undergo rapid
interconversion at room temperature15. Due to the
presence of bowspirit-flagpole interaction in the boat
cyclohexane, breaking up of hydrogen bonds are
highly favoured in it16.
In the ethanol mixture, the cyclohexane is
completely dissolved and so no chances of hydrogen
bond ruptures and only the interaction with the
cyclohexane ring and the active groups of ethanol,
which are mostly dispersive in nature. The increase in
mole fraction of cyclohexane increases the net
dispersive interactions and hence the velocity
continuously increases as observed. The case of
1-propanol mixture is different due to the less
salvation tendency of 1-propanol. It is to be
remembered that the methyl group becomes more
nonpolar17 with increase in chain length. Further the
−OH group can dissolve preferably the polar
Table 1 — Values of density (ρ), viscosity (η) and ultrasonic velocity (U) at 303 K
ρ /kgm–3
η×103 /Nsm–2
U /ms–1
Mole fraction
of cyclohexane
ethanol
1-propanol
ethanol
1-propanol
ethanol
1-propanol
0.0000
780.5
795.6
0.983
1.634
1130.0
1193.0
0.1000
778.2
791.5
0.897
1.479
1133.5
1187.2
0.1980
775.3
786.8
0.882
1.382
1136.3
1182.0
0.3001
772.5
782.5
0.848
1.253
1145.8
1185.1
0.3971
770.8
778.8
0.835
1.126
1154.6
1187.6
0.5001
762.5
775.3
0.820
1.053
1163.7
1191.4
0.5981
754.7
771.0
0.804
0.942
1173.6
1193.6
0.7070
750.8
767.5
0.788
0.902
1184.6
1195.0
0.8041
744.2
764.4
0.772
0.835
1191.6
1198.6
0.9004
738.7
762.2
0.749
0.826
1209.7
1212.6
1.0000
767.7
767.7
0.800
0.800
1230.3
1230.3
854
INDIAN J PURE & APPL PHYS, VOL 46, DECEMBER 2008
component. So, in the lower mole fraction ranges the
added cyclohexane has practically no interaction with
the methyl or hydroxyl group of 1-propanol, thus
sound velocity seems to decrease. However, as the
mole fraction of cyclohexane increases, the hydrogen
bond rupture of the boat form is of considerable
extent and they leads to additional dipole type
interactions. This is quite in line with the observation
as the sound velocity at any given mole fraction is
more for 1-propanol system than in ethanol system.
Cyclohexane being non-polar the predominant
dispersive type interactions with temporary dipolar
type are existing as a net result of intermolecular
forces16 in both systems, but more in 1-propanol
system. The observed non-linear change in all these
measured parameters indicates the existence of
specific interactions18.
A reduction in density and viscosity with increase
in mole fraction of cyclohexane suggests that the
existing intermolecular interactions are weakening in
magnitude19. However, the increasing sound velocity
with increasing mole fraction of cyclohexane leads to
a notion that the system is getting more and more
compact, which is not true as the interactions due to
cyclohexane are dispersive in nature. This is indicated
by the existence of minimum sound velocity in 1propanol system. However, as cyclohexane mole
fraction is increased, sound velocity exhibits
increasing trend in 1-propanol system. This increasing
velocity on increasing mole fraction of cyclohexane
with decreasing density is somewhat a peculiar nature
and is mainly due to the conformation property of
cyclohexane. It is also to be noted that the density of
mixture falls below the pure component value.
Conversely, the density values observed at higher
mole fractions are found to be lower than the pure
cyclohexane. This supports the rupture of hydrogen
bonds; thereby the structure of cyclohexane is
collapsed leading to a density lower than that of pure
component.
The
calculated
parameters
of
adiabatic
compressibility (β), free length (Lf), free volume (Vf)
and internal pressure (πi) are listed in Table 2. This
table suggests that for both systems β and Lf are of
similar nature and in decreasing trend whereas, Vf and
πi are of opposite nature to each other. In general, a
continuously decreasing trend of β with increasing
mole fraction of cyclohexane in these systems lends
support to the idea that the systems are in a more
compressed state. The ruptured hydrogen bonds create
many new dipoles in the medium and thereby increase
the compactness. The reduction in Lf indicates that the
components are much more closer20. This confirms
the existence of specific interactions between the
components.
The perusal of 1-propanol system shows that β and
Lf are almost unaltered in intermediate mole fraction
range, i.e., the addition of cyclohexane in the mole
fraction range of 0.2 to 0.8 has very small influence
on compressibility and free length. This is clear
evidence that interactions of weak nature, that too in
very small magnitude, are existing in the system. As
the dipole moment value of cyclohexane is zero, it
can offer only dispersive type interactions, whereas 1propanol, of dipole moment 1.58D10, can exhibit
dipoles and can support induced dipoles. But, this
inducement of dipoles in cyclohexane seems to be
highly restricted, as β and Lf almost remain unaltered.
This may be attributed to the fact the dipole
inducement will be arrested by bowspirit-flagpole
interaction16. At higher mole fraction of cyclohexane,
because of its inherent compact structure, β and Lf are
decreasing.
Cyclohexane is a ring type molecule, whereas
alcohol is a linear molecule. The reduction of alcohol
reduces the straight chain type and the system is more
and more occupied by ring-structured molecules,
which occupies comparatively less space. In the two
components, one is non-polar and the other is strong
polar, attractive type interactions are feeble and so
molecules are not in a closed shield or cage-like
formation21. Each component still maintains their
identity and at the same time, the collective molecular
influence is reduced. Thus, free volume between the
components is in an increasing trend as the
cyclohexane mole fraction is raised, as is seen from
Table 2. The increase in free space between the
components offer a support to stabilize their
independent nature and hence mutual force
(attractive/repulsive) retains their original value, i.e.,
the interactive forces decrease.
To ascertain the exact type of interaction existing
between the components, respective excess
parameters have been calculated and for the present
systems, the perusal of Figs 1 and 2 indicates that βE
and LfE are positive over the entire mole fraction
range. The positive βE clearly suggests that weak
interactions are existing between the components of
the mixture. The trend of Figs 1 and 2 shows that βE
and LfE are increasingly positive as cyclohexane mole
fraction is increased. Thus, the strong dipolar
interactions are restricted more and more whereas the
THIYAGARAJAN & PALANIAPPAN: MOLECULAR INTERACTION STUDY OF ALIPHATIC ALCOHOLS
855
Table 2 — Values of adiabatic compressibility (β), free length (Lf), free volume (Vf) and internal pressure (πi) at 303 K
β×1010 /Pa−1
Lf×1011 /m
Vf×107 /m3mol–1
πi×10−8 /Pa
Mole fraction
of cyclohexane
ethanol
1-propanol
ethanol
1-propanol
ethanol
1-propanol
ethanol
1-propanol
0.0000
10.033
8.831
6.370
5.976
0.435
0.328
9.49
8.80
0.1000
10.001
8.963
6.310
5.973
0.565
0.401
8.19
7.98
0.1980
9.989
9.097
6.306
6.006
0.648
0.467
7.44
7.37
0.3001
9.860
9.099
6.265
6.016
0.774
0.572
6.67
6.69
0.3971
9.731
9.104
6.224
6.020
0.878
0.785
6.13
6.07
0.5001
9.684
9.086
6.208
6.012
1.004
0.983
5.58
5.61
0.5981
9.620
9.103
6.188
6.015
1.137
1.107
5.12
5.08
0.7070
9.491
9.124
6.146
6.025
1.298
1.109
4.70
4.78
0.8041
9.463
9.106
6.137
6.019
1.454
1.372
4.35
4.42
0.9004
9.250
8.922
6.067
5.957
1.552
1.488
4.01
4.36
1.0000
8.605
8.605
5.853
5.899
1.662
1.662
4.00
4.00
Fig. 3 — Mole fraction versus excess free volume at 303 K
Fig. 1 — Mole fraction versus excess adiabatic compressibility
at 303 K
Fig. 4 — Mole fraction versus excess internal pressure at 303 K
Fig. 2 — Mole fraction versus excess intermolecular free length
at 303 K
weak dispersive interactions are predominating and
are maximum around 0.8 mole fraction. As the
molecular symmetry is retained to a greater extent at
0.9 mole fraction, βE and LfE are found to decrease.
Further, it may be noticed from the Fig 2 that both
systems exhibit same magnitude of interaction at 0.7
mole fraction of cyclohexane, beyond which the LfE of
systems show reverse nature that reflects the existing
intermolecular interactions. However, VfE and πiE , for
the mixtures, are mostly negative (Figs 3 and 4) and at
higher mole fractions of cyclohexane, they become
positive in ethanol system. Such transition in sign at
higher cyclohexane mole fraction indicates that the
dispersive type interaction alone is existing, which is
weak in nature. Existence of such weak interactions
was confirmed by Palaniappan et al.22 in some
cyclohexane systems. Among the two alcohol systems
considered, a more systematic variation in VfE and πiE
is observed for ethanol system whereas it is not so for
1-propanol system. The presence of one more methyl
group in 1-propanol system produces more number of
856
INDIAN J PURE & APPL PHYS, VOL 46, DECEMBER 2008
induced dipoles in cyclohexane, provided chair form
of cyclohexane is available in the system as it is much
more stable than boat form16. Such restrictions are
reflected in the observed haphazard trend of VfE and
πiE in 1-propanol systems.
4 Conclusions
Weak dispersive type intermolecular interactions
are confirmed in the systems. Dipole inducement is
found to be more in 1-propanol system. Components
maintain their individuality in the mixture.
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