Acoustic Parameters of Glycine, α-Alanine and β-Alanine
in Aqueous and Aqueous D-Glucose Solutions At 298.15 K.
Sanjibita Das and Upendra N. Dash*
Dept. of Chemistry, I.T.E.R, Siksha ‘O’ Anusandhan
Deemed to be University, Bhubaneswar-751030, Odisha (India)
*Corresponding Author E-mail: dr.upendranath.dash@gmail.com
ABSTRACT:
The acoustical parameters of glycine, α-alanine
and β-alanine have been measured in aqueous and aqueous D-Glucose
solutions at 298.15K. The molar sound velocity (R), molar compressibility (W),
free length (Lf), free volume (Vf), internal pressure (πi),
relaxation time (τ), ultrasonic attenuation (α/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, D-Glucose, amino acid, 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 the solution properties of the solutions consisting
of polar and non-polar components finds applications in industrial and
technological processes. In the present investigation, we have evaluated the
acoustic parameters such as the molar sound velocity(R) , molar compressibility
(W), free length (Lf), freevolume (Vf),internal pressure (πi),relaxation
time (τ), ultrasonic attenuation (α/f2) and van der-
Waals constant (b) at 298.15 K for the solutions of amino acids in water and
water + D-Glucose mixtures where the mass percentage of D-Glucose was varied
from 5 to 20 % with 5% increments. The results are discussed in the light of
molecular interactions.
MATERIALS AND METHODS:
All chemicals used were of AnalaR grades. Conductivity
water (Sp. cond. ~ 10-6 S cm-1) was used to prepare
solutions of D-Glucose (5, 10, 15 and 20 wt%) and the solutions were used on
the same day. The solutions of glycine, α-alanine and β-alanine were
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× 10-3 CP. The amino acid
content in the solutions varied over a range of 0.01 to 0.08 mol dm-3
in all the solvents.
Theoretical Aspects:
From the ultrasonic velocity (U),density (d) and
viscosity (η) data, the following parameters have been calculated.
(1)Molar sound velocity3 (R)
: R = M d-1 U1/3
Where, M is the effective molecular weight
(M = Σ mi xi ), in which mi and xi
are the molecular weight and the mole fraction of the individual constituents,
respectively.
(2) Molar Compressibility 4 (W):
According to Wada,
W= M d-1Ks-1/7
Where, W is a constant called Wada’s
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 isentropic compressibility by Jacobson’s empirical relation Lf
= KІ Ks1/2
Where, KІ is the Jacobson’s constant
which is temperature dependent and is obtained from the literature3.
(4) Free Volume (Vf): Suryanarayan et al. 6
obtained a formula for free volume in terms of the ultrasonic velocity
(U) and the viscosity of the liquid (η) as
Vf =( MU/Kη)3/2
where, M is the effective molecular weight (M = Σ
mi xi ),in which mi and xi are the
molecular weight and the mole fraction of the individual constituents,
respectively. K is a temperature independent constant which is equal to 4.28×109
for all liquids.
(5) Internal pressure (πi) :According
to Suryanarayan7, internal pressure is given by
πi = bІRT
(Kη/U)1/2 (d2/3/ M7/6)
where, bІ is the packing factor of
liquid which is equal to 1.78 for close packed hexagonal structure and 2 for
cubic packing. For many liquids bІ is equal to 2. KІ
is a dimensionless constant having a value of 4.28 × 109
independent of temperature and nature of liquid.
(6) Relaxation time8 (τ):τ
= 4η/3dU2
where, the symbols have their usual meaning.
(7) Ultrasonic Attenuation 9 (α/f2)
:α/f2 = 4π2τ /2U
(8) van der Waals constant10: van
der Waals constant (b) also called co-volume in the van der Waals equation
is given by the formula
b = M/d[1-(RT/MU2){1+(MU2/3RT)}1/2-1]
Where, R is the gas constant , M is the
molecular weight.
RESULTS AND
DISCUSSION:
From the measured values of the ultrasonic velocity
and density of the solutions of glycine, α-alanine and β-alanine in
aqueous and aqueous D-Glucose solutions reported earlier,11
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 for all the amino acids in all the
solvents 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, πi
due to electrostatic field of ions is given by eqn5.
Suryanarayan et. al6 showed that
the free energy of activation, ΔG is almost equal to the cohesive energy,
πi Vm. The result indicates that ΔG increases
with concentration and D-Glucose content in the mixed solvent. Positive values of
πi 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 the thermodynamic
variables that describe the liquid system of fixed composition. 12,13
It is seen that free volume varies irregularly with
solute concentration in water but decreases in aqueous D- glucose solutions. As
observed, internal pressure changes in a manner opposite to that of free
volume. The decrease of Vf (or increase of πi)
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/ disorderliness decreases with concentration and D-Glucose content.
This implies that the frictional force exerted by different layers of liquid
increases with concentration and D-Glucose content. As the frictional force
increases, ultrasonic absorption increases16. In the present case,
ultrasonic absorption or attenuation varies irregularly with concentration and
D-Glucose content.
CONCLUSION:
From the ultrasonic velocity and density values of the
solutions of amino acids in aqueous and aqueous D-Glucose solutions, the
acoustic parameters like molar sound velocity, molar compressibility, free
volume, free length, internal pressure, ultrasonic attenuation have been
calculated at 298.15 K. 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 system concerned.
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