Acoustic Parameters of Glycine in Aqueous
Solutions of Surfactants at 298.15 K
Smruti Pattnaik1* and Upendra N. Dash2
1U.N. Auto. College, Adashpur
2Dept. of Chemistry, I.T.E.R., Siksha ‘O’
Anusandhan Deemed to be University, Bhubaneswar-751030, Odisha
*Corresponding Author E-mail: dr.upendranath.dash@gmail.com
The acoustical parameters of
glycine have been measured in aqueous solutions of surfactants, such as sodium
cetrimide, sodium dodecylsulphate(SDS), and Triton-X(100) (TX-100) at 298.15K.
The 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) values have been calculated from the experimental data. These
parameters are used to discuss the molecular interactions in the solutions.
KEYWORDS: Acoustical parameters,
Surfactants, Glycine, Ultrasonic Velocity, Compressibility.
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 continuation of our work on
determination of acoustic and thermodynamic parameters of the solutions of
amino acids in the presence of hydrotropic agents,1,2 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
+ cetrimide, water + SDS, water + TX-100, mixtures, where the mass percentage
of cetrimide, SDS, and TX-100, was varied from 0.1 to 0.3% with 0.1%
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 cetrimide, SDS and TX-100 (0.1, 0.2 & 0.3 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 expression3 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 elsewhere4. 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 surfactants.
Theoretical Aspects:
From the ultrasonic velocity (U), density
(d), and viscosity (h) data, the following parameters
have been calculated.
(1) Molar sound velocity5 (R): R
= 
d-1 U1/3 where, is the effective molecular weight ( = åmi xi), in
which mi and xi are the molecular weight and the mole
fraction of individual constituents, respectively.
(2) Molar compresibility6 (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 length7(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
literature7.
(4) Free Volume (Vf):
Suryanarayan et. al8 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 (=åmi 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
Suryanarayan8, 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)
: t
= 4h/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 and 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 cetrimide, SDS and
TX-100 solutions, 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 surfactants studied. This type of behavior is similar to that
observed earlier 10,11. It is of interest to note that the acoustic
parameters including the sound velocity decrease in the solutions of
surfactants as follows:
TX-100 > Cetrimide > SDS
It follows that the neutral
surfactant has a greater contribution toward the molecular interactions
followed by cationic surfactant cetrimide and in turn the anionic surfactant
SDS.
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).
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 surfactants contents. As the frictional force increases,
ultrasonic absorption increases17. In the present case, ultrasonic
absorption or attenuation increases with concentrations of the surfactant
contents.
CONCLUSION:
From the ultrasonic velocity and density
values of the solutions of glycine in aqueous solutions of surfactants; the
acoustic parameters like molar sound velocity, molar compressibility, free
volume, free length, internal pressure, and ultrasonic attenuation. 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 characteristic 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
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