INTRODUCTION:
The measurement of ultrasonic velocity
provides qualitative information about the nature and strength of molecular
interaction in solutions. The study of solution properties of the solutions
consisting of polar and non- polar compounds finds applications in industrial
and technical processes. In the present investigation, we have evaluated the
acoustic parameters such as molar sound velocity (R), molar compressibility
(W), free length (Lf), free volume (Vf), internal
pressure (pi), relaxation time (t), ultrasonic attenuation (a/f2) and van der Waals constant (b) at 298.15K for the
solutions of glycine in water + SB, water + SBr, water + N, mixtures, where the
mass percentage of SB, SBr, and N, was varied from 0.1 to 0.3% with 1%
increments. The results are discussed in the light of molecular interactions.
MATERIALS AND METHODS:
All chemicals used were of AnalaR grades.
Conductivity water (Sp and ~10-6 Scm-1) was used to prepare solutions of
SB, SBr and N (.1, .2, .3 wt%) and the solutions were used on the same day. The
solution of glycine was prepared on the molal basis and conversion of molality
to molarity was done by using the standard expression1 using the
density values of the solutions determined at 298.15K. Solutions were kept for
2 hours in a water thermostat maintained at the required temperature accurate
to within ± 0.1K before use for density measurements.
Density measurements were done by using a specific gravity bottle (25ml
capacity) as described elsewhere2.
At least five observations were taken and
differences in any two readings did not exceed ± 0.02%. An ultrasonic interferometer (model No.F-81, Mittal
enterprises, New Delhi) operating at a frequency of 2MHz and overall accuracy
of ± 0.5 m/s was used for the velocity
measurement at 298.15K only. Viscosity measurements were made by using an
Ostwald’s Viscometer (25 ml capacity) in a water thermostat whose temperature
was controlled to ± 0.05K. The
values of viscosity so obtained were accurate to within ± 0.3 x 10-3 CP. Glycine content
in the solutions varied over a range of 0.006 to 0.08 mol dm-3 in
all the hydrotropic agents.
Theoretical Aspects:
From the ultrasonic velocity (U), density
(d), and viscosity (h) data, the
following parameters have been calculated.
(1) Molar sound velocity3(R): R
= 
d-1 U1/3
where, is the effective molecular
weight ( = ĺmi xi), in which mi
and xi are the molecular weight and the mole fraction of individual
constituents, respectively.
(2) Molar compresibility4 (W):
According to Wada, W=d-1 K S-1/7,
where, W is a constant called Wada1s constant or molecular
compressibility which is independent of temperature and pressure.
(3) Intermolecular free length5 (Lf):
It is the distance between the surfaces of the molecules. It can be calculated
using insentropic compressibility by Jacobson’s empirical relation Lf
= KI Ks1/2, where KI is the
Jacobson’s constant which is temperature dependent and is obtained from the
literature3.
(4) Free
Volume (Vf): Suryanarayan et. al6 obtained a formula for
free volume in terms of the ultrasonic velocity (U) and the viscosity of the
liquid (h) as Vf = (U/Kh)3/2 where is the effective molecular
weight (=ĺmi xi),
in which mi and xi are molecular weight and the mole
fraction of the individual constituents, respectively, K is temperature
independent constant which is equal to 4.28 x 109 for all liquids.
(5) Internal Pressure
(pi): According to
Suryanarayan7, internal pressure is given by, pi = bI RT (Kh/U)1/2 (d2/3/1/6), where bI
is the packing factor, which is equal to 1.78 for close packed hexagonal structure
and 2 for cubic packing. For many liquids bI is equal to 2. KI
is a dimensionless constant having a value of 4.28 x 109,
independent of temperature and nature of liquid.
(6) Relaxation time8 (t) : h = 4t /3dU2 where the symbols have their
usual meanings.
(7) Ultrasonic Attenuation9 (a/f2): a/f2=4p2t/2U.
(8) van der Waals
constant10 : van der Waals constant (b) also called co-volume in van
der Waals equation is given by the formula
b=/d[1-(RT/U2){1+U2/3RT)}1/2-1]
Where, R is the gas constant,
is the effective molecular
weight.
RESULTS AND DICCUSSION:
From the measured values of the ultrasonic
velocity and density of the solutions of glycine in aqueous SB, SBr and N
solutions as reported earlier11, the values of the molar sound velocity
(R) evaluated by means of eqn.(1) are given in Table 1.
As observed, the molar sound velocity
increases with increase in concentration of the solutions of glycine in all the
hydrotropic agents studied. This type of behavior is similar to that observed
earlier 8,9.
It is known that when a solute dissolves in
a solvent some of the solvent molecules are attached to the ions (generated
from the solute) because of ion-solvent interactions. Since the solvent
molecules are oriented in the ionic field (i.e., electrostatic fields of ions)
the solvent molecules are more compactly packed in the primary solvation shell
as compared to the packing in the absence of the ions. This is the reason, why
the solvent is compressed by the introduction of ions. Thus the electrostatic
field of the ions causes compression of the medium giving rise to a phenomenon
called electrostriction. Since the solvent molecules are compressed, they do
not respond to any further application of pressure. So the solution becomes
harder to compress; i.e., the compressibility decreases and internal pressure
increases. Hence isentropic compressibility as well as internal pressure
describes the molecular arrangement in the liquid medium. The increase in
internal pressure pi due to electronic field of ion is given by
eqn(5).
Suryanarayan et. al6 showed that
the free energy to activation, ΔG is almost equal to the cohesive energy, piVm. Positive values of pi indicate
the presence of some specific interactions between unlike molecules in the
components.
Free Volume, Vf is the effective
volume accessible to the centre of a molecule in a liquid. The structure of a
liquid is determined by strong repulsive forces in the liquid with the
relatively weak attractive forces providing the internal pressure which held
the liquid molecules together.
The free volume seems to be conditional by
repulsive forces, whereas the internal pressure is more sensitive to attractive
forces. These two factors together uniquely determine the entropy of the
system. Thus, the internal pressure, free volume and temperature seem to be
thermodynamic variables that describe the liquid system of fixed composition12,13.
It is seen that free volume decreases with
increase in concentration of the solutions of hydrotropic agents. As observed,
the internal pressure changes in a manner opposite to that of free volume. The
decrease of Vf (or increase of pi)
indicates the formation of hard and/or tight solvation layer around the ion14,15.
The fractional free volume (Vf/V)
is a measure of disorderliness due to increased mobility of the molecules in a
liquid. It is observed that mobility decreases with concentration. This implies
that the frictional force exerted by different layers of liquid increases with
concentration and the hydrotropic agent contents. As the frictional force
increases, ultrasonic absorption increases16. In the present case,
ultrasonic absorption or attenuation increases with concentrations of the
hydrotropic agents content.
CONCLUSION:
From the ultrasonic velocity and density
values of the solutions of glycine in aqueous solutions of hydrotropic agents;
the acoustic parameters like molar sound velocity, molar compressibility, free
volume, free length, internal pressure, and ultrasonic attenuation have been
calculated at 298.15K. The results show that the specific ion-ion, ion-solvent
and solvent-solvent interactions play an important role for explaining the
acoustic parameters. However, any deviation from the usual behavior is probably
due to characteristics structural changes in the systems concerned.
ACKNOWLEDGEMENTS:
One of the authors (SP) is very much
thankful to the President of the Governing Body, and the Principal, U.N. (Auto)
College, Adashpur, Odisha for sanction of the leave for doing research work in
, I.T.E.R., S ‘O’ A University, Bhubaneswar
REFERENCES:
1. Robinson,
R.A. and Stokes, R.H., Electrolyte Solutions, Butterworths Scientific
Publication, London, 1955:p.30
2. Das,
U.N., and Supkar. S. Acoustic Letters, 1992:16, 135.
3. Nikam,
P.S., etal, Asian Journal of Chem. 1994:6, 237.
4. Rath
D. C and Samal, K, Acoustic Letters 1988: 29(12)49.
5. B
Jacobson, Acta Chem. Scand. 6, 1952: 1485.
6. C.V.
Suryanarayan, Indian J. Pure Appl. Phys., 27, 1989:751.
7. Dash
U.N. et al, Ultra Science 2005: 15(1)1.
8. a) Dash
U.N. et al, Indian J. Chem. Technol 2004:178(11).
b) S.
Das and U.N. Dash J. Chem. Pharm. Res., 4(1),2012 : 754-762.
9. Maharatha
D. et al, Researcher 2011:6,3(2).
10. Ali
A. and Nain, A.K., J. Pure Appl. Ultrason. 22,2000:10.
11. S.
Pattnaik and U.N. Dash J.Chem. Pharm. Res., 4(9),2012:4364-4369.
12. S.Tlirumaran
and K.J.Sabu, Indian J.Pure Appl. Phys 47, 2009:87-96.
13. S.
Prabakar and K. Rajagopal, J. Pure Appl. Ultrason. 27, 2005:41-48.
14. V.K.
Syal, S.Chauhan, R.Gautam, Ultrasonics, 36, 1998:619.
15. S.
Singh et al Indian J. Pure Appl. Phys. 629, 1977:
16. T.M.
Aminabhavi, Indian J.Technol. 30, 1992:303.